a comparative study of the growth of wild oats (avena fatua l. and a. ludoviciana dur.) and of...

THURSTON, JOAN hl. (1959). Ann. appl. Biol. 47 (4), 716-739

A COMPARATIVE STUDY OF THE GROWTH OF WILD OATS (AVENA FATUA L. AND A. LUDOVICIANA DUR.)

AND OF CULTIVATED CEREALS WITH VARIED NITROGEN SUPPLY

BY JOAN M. THURSTON Rothamsted Expmkental Station, Harpenden, Herts.

Growth analysis of wild oats ( A v w fatua and A. Iudoericiana) grown in pots with different levels of nitrogen supply showed many similarities to spring barley, winter oats and winter wheat.

Small differences that could affect competition between wild oats and cereals occurred mainly in the seedlings. Wild oat seedlings were smaller than the corresponding cultivated cereals in total dry weight, total nitrogen content, leaf area and number of shoots. However, very young wild oat plants had higher net assimilation rates than the cultivated cereals and soon caught up and passed them. The difference in net assimilation rate did not persist, and in the later stages of growth differences in dry-matter production depended mainly on differences in leaf area. Another important difference between wild oats and cultivated cereals was that 98-100 yo of the wild oat seeds and none of the crop seeds were dormant z months after harvest.

Ear emergence in wild oats spread over a longer period, the range of ear heights was greater and the tallest ears were taller than in the corresponding cultivated cereals. Assimilation in the ear appeared to account for less of the total dry matter of the plants of wild and cultivated oats than of wheat. The wild oats produced more seeds per plant than the cultivated cereals, but the rooo-grain weight, and hence the total dry weight of seeds, was lower in the weeds than in the crop.

Addition of nitrogen to the soil affected the growth of the wild oats in the same ways as the cultivated cereals; they took up the same amount of nitrogen per plant as winter oats and winter wheat but more than spring barley.

It is concluded that wild oats are most susceptible in the seedling stage to competition from the crop and that nitrogenous fertilizer applied to an infested field is unlikely to alter the balance between the yields of crop and of wild oats.

INTRODUCTION The pot experiments described here were undertaken to study the development of wild oat plants throughout their life cycle in greater detail than is possible in the field and to compare them with cultivated cereals grown under similar conditions with varied nitrogen supply. It was hoped that this might throw some light on the competition between wild oats and cultivated cereals in infested fields, and whether the competition is altered by adjusting the nitrogen status of the soil.

A comparison of the two species of wild oats was required as well as of wild oats and crop plants. Most seeds of Avena fatua in Britain germinate in spring though some seedlings appear in autumn, whereas A. ludoviciana germinates in the winter (Thurston, 1951b, 1954). Two experiments were therefore made. Exp. A was sown in spring (mid-March) with spring barley var. Spratt Archer BNj8 for com-

Growth of wild oats and nitrogen supply 7'7 parison and Exp. B was sown in autumn (end of October) with winter oats var. Grey Winter and winter wheat var. Squareheads Master 1314 for comparison with the two species of wild oats.

For convenience, the inflorescences of all species at all stages of development are referred to as ears.

M ~ H O D S The soil was clay-with-flints from Rothamsted Farm, low in available nutrients, put through a 4 in. screen to remove the larger stones. In Exp. A soil was used alone but in Exp. B 9 parts soil were mixed with I part sand by weight. The nitrogen content of the soil in Exp. A was 0.12 % total nitrogen and that for Exp. B came from the same site. The pots were glazed with a drainage-spout at the bottom, those used in Exp. A holding 7 lb. (z$ kg.) of soil and those used in Exp. B being of similar diameter and three times as deep.

A basal dressing of phosphate and potmh was given at the same rate per pound of soil in the two experiments. Each pot in Exp. A received 0.55 g. K,HPO, in 25 mi. of solution and in Exp. B 1-65 g. K,HPO, in 50 ml. mixed with the soil before the pot was filled.

Three levels of nitrogen were used. Treatment ON had no added nitrogen. Treat- ments IN and zN consisted of sulphate of ammonia in solution, zN being double the amount of IN. Exp. B received only one dressing, given when the pots were filled, at 1-5 g. in 50 ml. for IN and 3.0 g. in 50 ml. for zN. Exp. A in the small pots received one-third of these amounts when the pots were filled, but by the end of May even the plants receiving the higher rate showed symptoms of nitrogen deficiency so the doses were repeated, IN receiving a further 0.5 g. and zN receiving a further 1-0 g. of sulphate of ammonia in 25 ml. of solution per pot.

Ripe wild oat seeds were collected on Rothamsted farm in the harvest before each experiment started: the seeds of A. fatua used in Exp. A came from Long Hoos in 1948 and for Exp. B from the adjacent Hoosfield in 1949. Both samples consisted of a mixture of types fA and f B (Thurston, 1957). The seeds of A. Zzldoviciana came from Broadbalk in both years, and consisted entirely of type 1A (Thurston, 1957).

To overcome dormancy, the seeds of both species of wild oats were soaked in tap water for an hour, then the softened lemmas and pales were removed and the imbibed grains were pricked with a needle. The seeds were then left to germinate on moist filter-paper in a glasshouse at about 60" F. As soon as the embryos began to grow, the seedlings were distributed equally between the pots and planting out was continued until every pot contained ten plants. Seeds of A. fatua were planted out at 3-4 days from sowing in Exp. A and 3-5 days in Exp. B, but A. ludoviciana took 7-13 days in Exp. A and 6-10 days in Exp. B. The barley in Exp. A was treated in the same way as the wild oats but germination was more uniform and ten plants were put into each pot on the third day after sowing. In Exp. B, twenty seeds of wheat or cultivated oats were sown directly in the pots on the day that the first wild oat seeds were planted out and the resulting plants were thinned to ten per pot after 18 days.

7'8 J O A N NI. THURSTON The pots stood on trucks kept in a glasshouse at night and during wind, heavy

rain or frost, but in a bird-proof cage on fine days. Exp. A had four blocks harvested on different dates, and Exp. B with the longer

growing period had eight blocks. Each block had two pots of each nitrogen level for each species. Treatments were randomized within blocks and block differences were not significant.

In both experiments, ten plants were allowed to grow in each pot to give enough material for analysis at the first harvest, then the number was reduced to five uniform plants per pot. The final harvest in both experiments was when all seeds were ripe, and the intervening harvests were at intervals of 2-4 weeks during the period of most active growth of the plants.

To collect the wild oat seeds as they ripened and shed, the ears were enclosed in porous Cellophane bags as soon as the stalks had finished elongating. The barley in Exp. A was not bagged as there was no risk of seed-shedding, but in Exp. B the wheat and cultivated oats were bagged so that all species should be treated alike. Both experiments remained in the glasshouse after bagging.

Pots were watered with rain water or demineralized water as often as required to keep the soil moist. Occasional outbreaks of mildew (Erysiphe graminis) were con- trolled by dusting with flowers of sulphur, and the panicles were dusted with DDT to control aphih before the bags were put on.

The method of harvesting was the same for all except the final harvest in both experiments. Pots were unrandomized and the appearance of all plants recorded, the numbers of shoots, leaves and, in the later harvests, ears were counted and the maximum height from ground level was measured. Two random plants, or one random leaf lamina from each of the five plants when they were large, had their leaf area measured by blue-printing. The leaf area per plant was calculated from the ratio of area to fresh weight of the sample leaves and the total fresh weight of laminae. The leaf laminae of all plants were separated from the sheaths, and stems were cut off at ground level. Roots were washed free of soil, and laminae, stems including leaf-sheaths, and roots were dried at 100' C. After weighing, replicates were bulked, and ground ready for nitrogen analysis.

All the leaves were dead by thefinal harvest, so were not measured or analysed separately. A subsample of the seeds of all treatments in Exp. B was taken for germination testing and the remainder were dried and analysed in the same way as the rest of the plant material.

Table I shows the dates of the various operations in the two experiments. Germination was tested in a cool glasshouse with fluctuating temperatures, from

25 September 1950 to 26 January 1951. Four plastic tanks ( IS x 9 x 10 in.) were filled with tap water to within 2 in. of wooden grids on which were laid filter-paper wicks covered with a double sheet of filter-paper. The filter-paper in each tank was marked out into twelve rectangles each 24 x 4 in. giving forty-eight such 'plots' in all. Tanks A and B contained one set of replicates and tanks C and D the other. Treatments were randomized independently in the two sets. The hygroscopic awns of the wild oats were removed to prevent the seeds from moving out of their

Growth of wild oats and nitrogen supply 7'9

positions on the filter-paper, but the lemmas were not removed. All sets of wild oats were put up in duplicate, pricked and unpricked, to get estimates of dormancy and viability. The dry seeds were pricked through the lemmas before the test started, but many seeds remained ungerminated and were pricked again on 21 November 1950. Pricking is now known to be more effective when the seeds are thoroughly moistened.

Nitrogen in the dried and ground material was determined by the micro-Kjeldahl short method using no salicylic acid. Replicates were bulked before sampling for analysis.

RESULTS (I) Appearance of plants

Given sufficient nitrogen, barley was paler green than wild oats and wheat was darker green than wild or cultivated oats, all of which were greyish green.

Nitrogen-deficiency symptoms were similar in all species. Growth was retarded. Leaves turned pale green, then yellow, and began to die.

In Exp. A, the ON plants of A. fatua and barley were already slightly paler than those with added nitrogen by 14 April, and in barley the added nitrogen had already increased the size of the leaves. By the first harvest barley plants differed according to the amount of nitrogen given, ON having greenish-yellow leaves many of which were dying, IN being pale green with a few of the older leaves starting to die and 2N being darker green sturdy plants with wider leaves than the others. Deficiency symptoms were less severe in A. fatua ON than in the corresponding barley and there was little difference between IN and 2N. Symp- toms intensified rapidly until when the second dressing of nitrogen was given on 5 May, barley ON seemed likely to die and even 2N had some yellow leaves, while A.fatua at IN was beginning to turn pale. A. Zudwiciana, which germinated later and had made smaller plants, remained green in all treatments up to this stage. By the second harvest all species were obviously improved by both levels of added nitrogen, though the differences were greatest in barley and least in A. ludoviciana. At the third harvest the effects of nitrogen were still prominent in all species, though the barley was beginning to ripen and its leaves were dying in all treatments.

720 J O A N M. THURSTON In Exp. B, the tips of the leaves of A. fatua but not of the other species were

slightly damaged by cold winds while the plants were out of doors in the daytime in mid-December. No treatment differences were seen at the first harvest nor on 7 February. The first signs of chlorosis in ON appeared in wheat at the second harvest, but definite symptoms did not develop until a fortnight later, when new growth in wheat ON plants was slightly paler than in IN and 2N and the older leaves were yellow and dying. Symptoms were less severe in A. fatua and oats at ON and A. ludoviciana still showed none. On 4 April the ON plants in all species showed marked deficiency symptoms and the 2N plants were all healthy, but the species differed in response to IN, wheat showing chlorosis, cultivated oats being slightly less affected and A. fatua and possibly A. ludoviciana showing only very slight symptoms. At the third harvest nitrogen responses were very obvious in all species and they persisted to the sixth harvest. Wheat appeared to be more severely affected by nitrogen deficiency than wild or cultivated oats. By the seventh harvest the leaves were beginning to die as the plants ripened, ON dying faster than IN or 2N in all species and wheat quicker than wild or cultivated oats. A. fatua leaves died earlier than the corresponding A. ludoviciana and cultivated oats retained yellowish-green leaves in ON until after the seventh harvest.

( 2 ) Dry wezght of plants (Table 2) In Exp. A, A. ludoviciana, which is winter germinating in the field, was signifi-

cantly below A. fatua at first, but the difference in dry weight disappeared by the final harvest. In Exp. B, where A . ludoviciana received its full growing period, the dry weights of the two species of wild oats never differed significantly. The fact that A. ludoviciana took from 4 to 9 days longer than A. fatua to germinate might have contributed to its backwardness in the early stages of Exp. A, but in Exp. B it seems to have made no difference. This might be because the first harvest in Exp. A was only 6 weeks after sowing, whereas in Exp. B it was 8 weeks.

The crops had more dry weight than the wild oats at the start of both experiments, probably because the crop seeds are larger. The wild oats caught up and passed the barley before the second harvest of Exp. A and remained ahead, though the difference in ON at the third and fourth harvests was not significant. In Exp. B, the crops were ahead for the first two harvests but the wild oats began to catch up in early April and there was no significant difference between species at the third harvest. After that, the crops never got ahead of the wild oats again in treatment ox. Oats withstood the nitrogen deficiency better than wheat and at the sixth harvest the difference between them was significant.

The full effect of additional nitrogen was seen in both experiments after the wild oats had caught up with the crop. In Exp. A it increased the wild oats more than the barley, although the barley responded to nitrogen at the first harvest and the wild oats did not until the second harvest. In contrast, in Exp. B the crops benefited more than the wild oats from added nitrogen, oats responding more than wheat to the lower level. At the third harvest the distinction was between plants with and without added nitrogen, the difference between I N and zN developing

Growth of wild oats and nitrogen supply 721 later-by the end of April for both crops, mid-May for A. ludoviciana and the end of May for A. fatua. In Exp. A, where A. ludoviciana plants were smaller than A. fatua, the difference between IN and 2N developed later in A. ludoviciana.

TABLE 2. Total dry weight in g . per ten plants on successive dates

Species Avena fatuu

A. ludvviciana

Spring barley

S.E. f

(a) Wild oats and spring barley

Total dry weight (g. per ten plants) at harvest

c A , Treatment I 2 3 4.

ON 2'2 8-7 I 1.3 7'7 IN 2'6 33.8 50'1 49'9 2N 3.1 52'9 92'1 78.9 ON 1 '4 6.9 8.9 90 IN 1.6 28-9 49'5 51-2 zN 1.3 33'0 83.2 71'4 ON 2'8 4'6 7'7 7'1 IN 5'5 21.7 35'6 31.6 zN 6.1 45'3 613 58.2

0.16 I -6 2.8 3'7 -

(b) Wild oats and winter cereals

Total dry weight (g. per ten plants) at harvest

% distribution of total dry weight at seventh harvest

A I c

Species Treatment I 2 3 4 5 6 7 8 Ears StemsLeaves Roots Avma fatua ON 0.35 6-8 29 44 52 65 66 55 18 53 7 22

IN 0.38 6.4 37 66 82 IOO 1x1 IIO 14 56 9 21 zN 0.35 6.3 54 68 90 146 137 165 16 55 10 19

A . ludovieiana ON 0.33 6 3 30 43 51 64 65 66 23 51 5 21 IN 0.35 6.1 46 6r 78 113 1 1 1 111 26 51 5 18 2N 0.33 5.4 42 71 98 144 150 160 25 49 7 I9

Winter oats ON 0.67 8.3 31 39 54 73 72 68 13 58 8 21 IN 0.65 8.7 46 68 97 122 134 147 13 61 9 17 2N 0.59 9.1 48 93 126 161 190 197 14 63 8 15

Winter wheat ON 0.85 11-1 34 36 44 50 70 61 9 60 6 25 IN 0.90 9'3 44 59 72 94 125 118 9 64 7 20 2N 0.80 9-6 51 82 105 147 x81 184 10 65 7 18

S.E. ? - 0.03 0.6 3-9 3.7 5.5 6.6 5.3 6.2 - - - - Maximum dry weight for each treatment of each species in heavy type.

* The lower yields and higher S.B. of the final harvest compared with the third harvest are due to pieces of dry leaves breaking off the plants. In order to reduce damage, each pot was harvested when it was ripe, instead of waiting for all treatments to ripen. The biggest differences are in the wild oats at 2N, where most damage was noticed.

Cultivated oats with added nitrogen continued to grow to the end of Exp. B, whereas wheat stopped increasing in dry weight about mid-June, and both wild oats about the end of May. This agrees with observations in the field, where wild oats ripen and shed their seeds before the crops in which they are growing are ready to cut.

722 J O A N M. THURSTON

( 3 ) Area of leaf lamina per plant (Table 3 ) At the start of both experiments, the crops had greater leaf area per plant than

the wild oats, but later the wild oats caught up and passed both wheat and barley. Cultivated oats remained ahead of wild oats (except for A.futua at the third harvest) and reached the highest leaf area per plant of any of the species used.

TABLE 3 . Leaf area per plant

(Only leaves with part of the lamina still green were measured)

Species Avena jatua

A , 1udoViCinna

Barley

S.B.

Treatment AveMfatua ON

IN ZN

A. ludoviciana ON IN 2N

Winter oats ON IN Z N

Winter wheat ON IN zN

S.B. F -

( a ) Wiid oats and spring barley

Leaf area (cm.a per plant) at harvest *

I 1\

Treatment I 2 3 ON I9 43 IN 35 I73 2N 36 308

34 I55 176

ON I4 44 20 IN 18 211 111

2N 18 338 I37 ON 29 25 IN 56 96 2N 67 180

I 2 26 55

- I I8 9

( b ) Wild oats, winter wheat and winter oats

Leaf area (cm.s per plant) at harvest * ,

I 2 3 4 5 6 7'4 87 222 297 258 23 1

6.8 92 223 zqz 218 164 6.7 86 434 413 4'7 332 7'0 80 538 576 564 415

I 2.6 I08 266 228 252 212 12'1 I22 442 478 537 474 12'0 119 517 661 772 716 I 3.8 7 5 5 220 146 161 I 18 15'5 149 467 311 314 297 14'1 161 612 484 544 426 0.7 I 0 30 21 15 16

9.0 91 419 418 452 365 7'7 80 617 658 618 485

7

7

229 287 81 I44 237

262 370 I02 I93 273 17

111

111

Maximum area for each species at each level of nitrogen in heavy type.

In Exp. B, wheat reached its maximum leaf area at the third harvest and this was followed by a sharp decline due to death of tillers (Table 5) . A. ludoviciana had its maximum leaf area at the fourth harvest, A. futua between the fourth and fifth and cultivated oats at the fifth. The drop in leaf area after reaching the maximum was less abrupt in the oats than in wheat. Additional nitrogen did not affect the Ieaf area in any species until after the second harvest and in cultivated oats the

Growth of wild oats and nitrogen supply 723 difference between IN and 2N did not appear until after the third harvest. The two wild oats were similar in leaf area except at 2N in the third, fourth and fifth harvests, when A. fatua exceeded A. Zudoviciana.

In Exp. A, A. fatua exceeded A. Zudoviciana only at the third harvest, by which time the leaves of the earlier-ripening A. Zudoviciana had started to die.

(4) Number of Zeaves per plant (Table 4) Most of the changes in number of leaves per plant with species, time and nitrogen

treatment were similar to those in total leaf area, but in Exp. B the maximum number of leaves was generally reached before the maximum total area and had begun to fall by the time the maximum area was reached. Leaf size, therefore, had an important effect on total leaf area. In both wild oats and crop the first leaves were

TABLE 4. Mean number of Zeaves per plant (a) Wild oats and spring barley

Before harvest Harvest - I 7 April 14 April I 2 3

Mean of three levels of nitrogen

species Mean of three

S.E.

Avena fatua 2'0 3'0 6.6 20 I 0 A. Zudoviciana 1'1 2'4 7'4 32 I 2 Barley 2'2 3'4 6.6 16 5

ON I '7 2.8 5'2 I 0 5 IN 1-8 2.9 7'5 25 I 0 2N I 43 3.1 7'8 33 I 2

0.1 0.06 0'2 1'1 0.4 -

(b) Wild oats, winter oats and winter wheat Harvest

Mean of three levels of nitrogen

S.E. 2

Mean of four species

S.E.

r ~~

I 2 3 4 5 6 7 Avena fatua 3.2 12 22 18 15 16 10

A. Zudoviciuna 2.8 17 24 21 19 13 8 Winter oats 41 23 35 32 29 26 16 Winter wheat 5'9 33 23 15 I3 10 8

- 0.06 0.5 0.8 0.7 0-2 0-2 0-2

ON 40 22 16 15 13 9 6 IN 41 21 27 22 21 17 I I

2N 3'9 20 34 28 24 22 I5 - 0.05 0.4 0.7 0.6 0.2 0.2 0.2

(c) Wild oats, spring barley, winter oats and winter wheat Mean maximum number of leaves per plant

Exp. A. Maximum at harvest 2

Exp. B. Maximum at harvest 3, except *, where maximum was at harvest 2

Avmu fatua A. ludoviciana Barley Avena fatua A. ludoviciana Winter oats Winter wheat

ON 9

16 5

13 I7* 22* 36*

IN 2N

ZI 31 36 44 17 25

23 28 2 5 30 37 46 33* 33

724 J O A N M. THURSTON small, the area of successive leaves increasing to a maximum and then decreasing, the flag-leaf being one of the shorter leaves although wider than the leaves of the seedling.

In treatment ON of Exp. B, wheat developed many more leaves than any of the oats, but they were small and the maximum leaf area was less than in wild and cultivated oats. Unlike wheat and barley, cultivated oats had more leaves than wild throughout Exp. B. Except at the first harvest, A. Zudoviciana had slightly more leaves than A.futua at all levels of nitrogen in both experiments, but the differences were scarcely significant and were not reflected in the leaf areas.

( 5 ) Number of shoots per plant (Table 5 ) The shoot numbers showed the same trends as the number of leaves, with crops

above wild oats at first and below later, except for winter oats which always had more shoots than the wild oats. The date of the maximum recorded shoot number coincided with the maximum leaf-number, except in A. ludoviciana with additional

TABLE 5 . Number of living shoots per plant and number of ears at harvest (a) Wild oats and spring barley

Mean number of shoots per plant at harvest

Species Treatment Avena fatua ON

IN 2N

A. ludoviciana ON I N 2N

Spring barley ON IN 2N

14. iv. 49 I

1'0 I .6 1'0 3'0 1'0 3 '0 1'1 2'7 1'1 3'3 1'0 3'4 1'0 1'0 I '4 2.8 1.6 3 '2

2

r5 5'7 8.1

4-6 10.4 11.9

3'9 1.1

63

3

4-0 3 '7 2.7 4'9

1-1

5'6 I '0

I '9 3.1

Mean number of ears

per plant at harvest 4

1 '7 2.8

1.8 3'2

I -8 3'2

1'1

I '0

1'0

S.E. 2 0' I 0 2 0.7 0 3 0' I

(b) Wild oats, winter oats and winter wheat

Mean number of shoots per plant at harvest Mean no. of 7- * , ears per plant

Species Treatment I 2 3 4 5 6 7 at harvest 8 A v m fatua ON 1.0 2.7 43 4.2 3.2 2.9 1.3

IN 1.0 3.2 6.3 5.6 4-7 3.8 2.9 2N 1'0 2'9 7'1 5'7 4'4 q 3 3'5

A. ludmiciana ON 1.0 4.2 6.6 5-7 4-7 2.7 1-5 IN 1.0 4.0 7.2 8'2 6.8 4'2 2.3 2N 1.0 3.8 7.1 7 8 6.8 5.0 3.9

Winter oats O N 1.5 5-0 7 5 7-0 6.3 4.3 2.3 IN 1 . j 5.0 IZT 1o.j 8.9 6.6 3-9 2N 1.4 5'3 1 5 ' 0 15'4 9.8 7'5 5'7

Winter wheat ON 2.5 7'4 4.5 2.9 2.0 1.5 1-7 IN 2.7 6.9 6.7 4-4 3.5 2.7 2.9 2N 2-4 6.2 8.0 6.6 4'4 3.6 3.6

S.E. f 0.1 0.4 0.4 1.0 0.3 0.4 0 2

1'0

3'2

1-6 2.4 3 '0

2'0

3-8

4'6 6.6 1'2

2'3 3'5 0 .3

Maximum number of shoots for each species at each level of nitrogen in heavy type.

Growth of wild oats and nitrogen supply 725 nitrogen, where the number of shoots continued to rise after the number of living leaves had begun to fall.

A. ludoviciana showed its characteristic high shoot-number and flat habit of the young plant (Thurston, 1951 b) whether it germinated in spring or in autumn, and regardless of the nitrogen status. Unlike the other species, the number of shoots formed by A. ludoviciana was scarcely affected by the amount of nitrogen supplied. However, additional nitrogen delayed the death of shoots and treatments IN and zN reached their maximum shoot-number about a fortnight later than ON.

Both wild oats produced more shoots in treatment ON in Exp. B than in Exp. A, perhaps because they had more soil and hence more available nitrogen, rather than because the growing period was longer. This is borne out by the absence of a corresponding increase in treatments IN and 2N.

In both experiments, and at all levels of nitrogen, A. fatua showed its characteristic upright habit with fewer shoots than A. Zudoviciana.

Barley showed an increase in shoot-number with added nitrogen by the thirtieth day, but the wild oats not until the first harvest. A. ludoviciana was the last species in Exp. A to show a significant difference between IN and 2N, because the extra nitrogen affected the survival of the shoots but not their formation. The effect of additional nitrogen in prolonging the life of shoots was also seen in both wild oats in Exp. B, but 2N showed very little advantage over IN. Cultivated oats responded more than either of the wild oats to added nitrogen.

Wheat produced shoots at a different time from all the oats. Tillering started and reached its peak and death of shoots began earlier in wheat. The addition of IN delayed the peak of shoot production but did not increase the number of shoots, whereas 2N increased the number of shoots as well.

(6) Number of ears per plant (Table 5 ) Only about 50% or less of the total shoots eventually produced ears. The only

exception was barley in treatment ON where most of the plants had only one shoot which carried a poor spike, Apart from this, the percentage of shoots which gave rise to ears in Exp. A was characteristic of the species rather than the treatment, although in all species the lowest percentage was in treatment IN where the nitrogen stimulated shoot production but seemed too little to enable the shoots to mature. A. bdoviciana with most shoots produced only the same number of ears as A. fatua and barley. In contrast, in Exp. B there was a trend in all species to- wards a higher percentage of flowering shoots where additional nitrogen was given, and differences between species were smaller. A. fatua responded most to IN and only a little more to 2N. Cultivated oats, in spite of its many shoots per plant, produced a higher percentage of ears than A. ludoviciana.

(7) Ear emergence (Table 6 ) In Exp. A, ears began to emerge at approximately the same date (20 June) in

both wild oats at all levels of nitrogen and in barley at ON, a week later than barley with added nitrogen.


TABLE 6 . Ear emergence and maximum haght of culms

Avena fatua A. ludoericiana Barley

S.E. F

.4oena fattra A. ludoviciana Winter oats Winter wheat

S.E. &

Days from sowing to Duration of ear j o "/b ear emergence emergence in days -r---h-7 OX I N 2N ON IN 2N

Exp. A. Spring sown IOO 103 103 7 18 19* 1 0 0 IOI 103 1 1 10 16* 1 0 3 96 97 7 I 3 I4

-Y----, VJ i--

- I

Exp. B. Autumn sown

Z I ~ 217 218 7 22 22 209 Z I O 211 7 10 I I

207 210 212 9 10 I I

213 214 214 S 7 7 - ---v---J I 3

Maximum height of culms (cm.) Y ON IN zN

66 11s 120

73 108 103 47 85 87

3

142 138 162

130 140 138 112 11s 127 L-

3

126 147 IS2

50 ?& ear emergence is the stage at which half the ears have begun to emerge. Duration of ear emergence is the period from the start of emergence of the first ear to the start of

An ear was counted as emerging when at least one spikelet and not just the awn could be seen

* Ear emergence continued after Cellophane bags had been put on the older ears and regular

Maximum height of culms = distance from soil level to tip of awn of top-most spikelet when ear

emergence of the last ear.

projecting from the end or side of the enveloping leaf-sheath.

ear counts had stopped.

is held erect, in tallest culm from each pot at harvest.

Although the first ears to emerge in Exp. B were A . fatua with added nitrogen on 13 May, emergence proceeded slower than in the other species and consequently the date of 50% ear emergence was later. A. fatua in treatment ON showed a delay until 18 May in the start of ear emergence similar to that of barley in Exp. A. Ear emergence in A . ludoviciunu was similar to cultivated oats at ON, except that it continued longer in the latter which produced more ears. Ear emergence in wheat started on 26 May about 2 weeks after the first oats, finishing at about the same time as A . ludoviciana and after A. fatua, but the total number of ears was the same in wheat as in wild oats. In treatment ON the number of ears per plant was so low that species differences were less obvious than in the treatments with added nitrogen.

In both wild oats in both experiments, and in oats and wheat in Exp. B, treatment ON reached 50% ear emergence more quickly than either IN or 2N, though most of the differences were scarcely statistically significant. In contrast, the barley in treatment ON of Exp. A almost stopped growing just before ear emergence with the result that ears emerged very slowly indeed and this treatment, in spite of its few ears, took a week longer to reach 50% emergence than either IN or 2N.

Spring sown A . ludoviciana reached 50% ear emergence at the same time as A . fatua and later than barley with added nitrogen, but when it was sown in autumn, nearer to its normal germination time, A . Zudoviciuna reached 50 % ear emergence about a week before A. fatua. Winter oats came into ear at the same time as

Growth of wild oats and nitrogen supply 727 A. ludoviciana and winter wheat slightly later, though in IN and 2N wheat reached 50% ear emergence before A. fatua.

In Exp. B, ear emergence of A. fatua with added nitrogen continued for 22 days, compared with 7-11 days for the other species. This would be important in any control measure applied to the developing ears, as with an age range of 3 weeks it would be impossible to get all the ears at the appropriate stage of development at any one time. In Exp. A, although fewer ears were produced, the duration of ear emergence in A. fatua at IN or 2N was only slightly less. There was some indi- cation of a long period of ear emergence in A. ludoviciana at 2N in Exp. A, but exact figures were not obtained as regular ear-emergence counts were discontinued when the Cellophane bags were put on the older ears.

Comparing the dates of sowing and ear emergence of wild oats in the two experiments, a difference of 5 months in sowing date is followed by a difference of only I month in ear emergence, and of one week in the relative dates of ear emergence of the two species. The date of germination of the wild oat seeds in the field probably has only a fairly small effect on the date of ear emergence.

(8) Heights of tallest culms at harvest (Table 6 ) As the ears had to be enclosed in Cellophane bags before the seeds ripened, culm

heights were not easily measured. However, the tallest culm in each pot was found, straightened and measured after the bags and seeds were removed. Height of culms on a plant of wild oats varied more than in wheat or barley. Cultivated oats varied more in height than the other two crops but less than wild oats.

In Exp. A, A. fatua was significantly taller than A. ludoviciana only in treatment 2N and in Exp. B only at ON and zN, but not in IN because of a low value for both pots of A. fatua in that treatment. Both wild oats were taller than wheat and barley at all levels of nitrogen although these were long-strawed varieties. A. fatua at ON and zN and A. ludoviciana at 2N were taller than cultivated oats. Nitrogen level therefore had very little effect on the relative heights of crop and wild oats, though the higher rate of nitrogen application brought A. ludoviciuna above cultivated oats.

(9) Seed production (Table 7 ) The number of ears per plant and the Iooo-grain weight were of similar magni-

tude in both experiments, but the numbers of seeds per ear and hence the total seeds and total dry weight of seeds were much greater in Exp. B with the larger pots and longer growing season than in Exp. A. There was no significant difference between the numbers of seeds produced by the two species of wild oats in Exp. B, but in Exp. A, A. ludoviciana fell behind A. fatua, especially at 2N.

The Iooo-grain weight was little affected by nitrogen, although some effects were statistically significant. All crops had a higher Iooo-grain weight than the corresponding wild oats and A. ludoviciana was consistently higher than A. fatua. The mean number of seeds per ear fluctuated less in the crops than in the wild oats with changing levels of nitrogen. This was expected in wheat and barley with spike inflorescences, but not in cultivated oats with panicles like wild oats. Cultivated

47 App. BioL 47


TABLE 7. Number, weight and nitrogen content of ripe seeds at jinal harvest

Species

Avena fatua

A. ludaticiana

Barley

S.E. k

Auena fatua

A . ludm'&na

Winter oats

Winter wheat

S.E.

Seeds Per Seeds Dry weight

Treatment fence plant 1000 seeds inflores- Per (g.1 of

(a) Wild oats and spring barley OK 1 5 16 13.8 IN 55 92 11'0 2N 58 162 11-5

ON 11 I 1 23'4 IN 42 75 17.7 2N 32 102 I 5.6 ON I 0 I 0 27' I IN I9 35 31.4 2N 22 72 30'3

3 3 0.6

(b ) Wild oats, winter wheat and winter oats

ON 1 I 4 114 12.3 IN 5 0 156 I 3.0 2N 74 279 I 1.4 ON 63 99 20'1

IN 75 I79 18.6 ZN 88 26 I I 9.0 ON 38 74 24 I IN 35 159 24' I 2N 37 246 23'9 ON 37 42 34'3 IN 33 74 39'2 2N 37 I 26 347

7 I 0 0.9

Total dry weight of seeds (g.) per plant

0'22 1.01 1.85 0.26 1.33 1.58 0.28 1.09 2.17 0.07

1-41 2.03 3-16 1.98 3'32

1'79 3'82 5.88 1 '44 2.89 4'35 014

4.96

Total nitrogen

seed (mg.1 Per

023 0.13 0.16 0.45 024 0.26 0.47 0' 37 0.36 -

0.14 0.17 014 0.24

0.23 032 0.30 0.29 0'5.5 0'59 0'59

0'22

-

oats responded to added nitrogen by producing more ears rather than by increasing the number of seeds per ear, whereas wild oats did both, an increase in nitrogen always giving more ears per plant and usually increasing the mean number of seeds per ear. However, in Exp. A, A. fatua at 2N had no more seeds per ear than at IN, and in A. ludoviciana at 2N in that experiment and A. fatua at IN in Exp. B, the additional ears were so small that the mean number of seeds per ear fell with added nitrogen.

Wild oats produced approximately twice as many seeds per plant as wheat or barley, except at ON in Exp. A where all species bore so few seeds that the differences were not significant. Cultivated oats had fewer seeds per plant than A. futua at ON and 2N but never differed significantly from A. Zudoviciam, although the total in cultivated oats was made up of numerous ears with comparatively few seeds each and in A . ludoviciana by fewer ears with more numerous seeds.

In total dry weight of seeds per plant, cultivated oats exceeded both wild oats at IN and 2N, but barley exceeded the wild oats only at 2N. Wheat exceeded A. fatua at IN and zN but was significantly below A. Zudoviciana at all levels of nitrogen. The addition of nitrogen, therefore, generally increased the yield of the crops more

Growth of wild oats and nitrogen supply 729 than that of the wild oats when the plants were not in competition with each other.

The main differences in seed production between wild oats and cultivated cereals are those which would be expected from deliberately selecting the crops for large, even grain size and ears of similar height and age for heavy yield and easy harvesting, contrasted with natural selection in wild oats for many seeds produced over a long period to allow the species to multiply and spread in spite of control measures.

(10) Viability and dormancy of seeds in Exp. B The wheat seeds from all three treatments were'Ioo% viable and the cultivated

oats 98-99 yo. A. fatua germinated 94-99 % after pricking but A. ludoviciana only 79-81 % ; most of the failures were among the second seeds of the spikelets.

In A. fatua 99-1oo% of the viable seeds were dormant and in A. ludoviciana 98- 99 %. The high percentage of dormant seeds is usual in freshly collected samples of A. fatua from pots or field plots, but the first seeds of the spikelets of A. ludoviciam are not usually dormant and the proportion of dormant seeds in the whole sample is about 50% (Thurston, 1951b; 1957). The high temperatures experienced in the glasshouse are thought to be responsible for the first seeds being dormant in this test.

Neither viability nor dormancy were affected by the nitrogen levels at which the parent plants were grown.

(I I) Net assimilation rate (Tables 8, 9) Net assimilation rate ( E ) was estimated from the total dry weights and leaf areas

by the method of Gregory (1926). In Exp. A, E was estimated for a 5-week period of tillering and vegetative growth (25 April to 30 May) followed immediately by a +-week period covering culm elongation and the beginning of ear emergence (30 May to 26 June). Estimates for such long periods probably differ from the true means (Williams, 1946), but the biases are likely to be similar for the different species.

In Exp. B, E was estimated for four periods:

(2) 7 Mar. to 18 Apr. 6 weeks Maximum shoot number, 19 Dec. to 7 Mar. I I weeks Tillering (1)

beginning of death of older leaves

(3) 18 Apr. to I May 2 weeks Culms developing, non- flowering shoots dying

(4) I May to 15 May 2 weeks Up to beginning of ear emergence

E was not estimated for seedlings before tillering began. In the first period of both experiments, E was higher for wild oats than for the

crops, though the wild oats differed less from cultivated oats than from wheat or barley, and the difference between A. fatua and cultivated oats was scarcely significant. The higher E of young wild oats compared with crop plants of the same age

47-2


Species Avena fatua

A. Iud~~iCiana

Barley

TABLE 8. Net assimilation rate (g./m.Z/week) (a) Wild oats and spring barley

Mean

Mean S.E.

S.E. of species mean Mean of all species

S.E. of nitrogen mean

Species Avma fatua

.-I. ludoviciann

Winter oats

Winter wheat

Treatment ON I N zN

ON I N 2N

ON IN 2N

ON IN 2N

Interval between harvests - 1-2 2-3 44 18 73 25 79 43 65 28 40 18 70 33 60 57 57 36 14 45 44 66 69 38 42 50

6 I 1

3 6

32 27 62 41 69 46

3 6

14ean

Mean

Mean

Mean


Interval between harvests

S.E.

S.E. of species mean Mean of all species

S.E. of nitrogen mean

Weighted mean over whole period 1-3

32 52 63 49 30 54 59 47 28 54 55 45

5 3

30 53 59

3

Weighted mean

, over whole Treatment 1-2

ON 18 IN 16 2N 18

'7 O N 17 IN 17 2N 16

17 ON 16 I N 16 2N 16

16 ON 16 I N I 3 2N 13

I4

0.7 0.4

ON 17 I N 15 2N 16

0.4

2-3 25 24 30 26 26 32 26 28

25 24 23

21

21

2 1

21

21

3 2

23 25 25

I

3-4 28 35

25 29 18 26

24 18 24 38 27

5 19 29 I8 6

3

I1

20

24 26

3

4-5 8

18 19 15 18

24 21

21

31 29 23 28 26 22 2 1

23 8

5 20 22 22

4

period 1-5 20 20 21

20

21 22 20

21

19 20 21

20

I 8 17 I8

17 0.9 0 '5

19 20 20

0.4

73 I Growth of wild oats and nitrogen SUPPLY

TABLE 9. Leaf area duration and increase in dry weight after ear emergence (i.e. fromffth to eighth harvest) in Exp. B (Means of three levels of nitrogen for each species)

Mean rate of Mean leaf increase in

area duration Mean increase in dry weight per (m.l/weeks/ dry weight unit leaf area

Species 10 plants) (g./xo plants) (g./m.'/week) Avena fatua 1.58 35'0 22 A. ludoviciana 1'32 36.7 28 Winter oats 1.95 44'9 23 Winter wheat 1'29 47'5 37

must be an important factor in enabling the wild oat seedlings, which at first are smaller than those of the cultivated cereals, to catch up with the crops in size and dry weight.

In the second period of Exp. A, the mean for barley was above that of both wild oats, though only the difference between A. fatua and barley was significant, and in individual treatments it was only at IN that barley significantly exceeded wild oats. In Exp. B, the mean of E for both crops remained below the wild oats for the second period but in the third and fourth periods there was no significant difference between species means.

After ear emergence, part of the dry-matter production amounting to 20-30% of the dry weight of the grain of wheat or barley is attributable to photosynthesis by the ears (Watson, 1952), so that E calculated in the usual way is no longer a measure of the photosynthetic efficiency of the leaves. There is evidence from Exp. B that ears of wheat contributed a higher proportion of the assimilation of the whole plant than did those of the Avena species. The integrated leaf area per plant over the period between ear emergence and maturity (leaf area duration, Table 9) was less for wheat than for wild or cultivated oats, but in spite of this the dry-matter increment was greatest for the wheat, and consequently the mean rate of increase of dry matter per unit leaf area (last column, Table 9) was much higher for wheat than for oats. As the photosynthetic efficiency of the leaves of wheat before ear emergence as measured by E (Table 8 b) was usually less than for wild or cultivated oats, it is reasonable to suppose that the same continued to be true after ear emergence. If so, the apparent higher rate of dry-matter production per unit leaf area after ear emergence implies that assimilation by the ears relative to that by the leaves must have been greater for wheat than for oats; for cultivated oats it was about the same as for wild oats.

The effect of nitrogen on E in any particular period was similar in all species. In Exp. A, additional nitrogen increased E, though there was little difference between IN and zN. In Exp. B, the mean value for E over the whole period from the first to the fifth harvest was unaffected by nitrogen level, but this was because E was higher for ON than for IN or zN during the long first period, although in the subsequent three shorter periods E was lower in ON, as in Exp. A.

732 J O A N M. THURSTON In Exp. A, the only significant differences in E within species caused by nitrogen

were in the first period, and between ON and 2N in A. ludoviciana in the second period. The only significant difference between IN and zN was with barley in the first period.

In the first period of Exp. B, E for ON exceeded IN in A. futua and both IN and 2N in wheat. Although these differences and two involving wheat and either A. fatua or A. ludwiciana in the second period are statistically significant it is doubtful if they are of practical importance. Table 8 b shows that there are no clear trends in individual values of E in the different periods, and some of the biggest differences are obviously due to anomalous results, e.g. wheat, ON, third period.

(12) Uptake of nitrogen and its distribution in the plant (Tables 10, I I)

In both experiments and in all species the total nitrogen in the plant reached its maximum early (second harvest in Exp. A, third harvest in Exp. B) and thereafter remained almost constant. Breakage of parts of plants made brittle by the second application of nitrogen accounted for some decrease in Exp. A, especially in A.fatua 2”. In treatment ON, all species in Exp. A and both wild oats in Exp. B were rather slow in reaching their highest nitrogen content compared with plants receiving added nitrogen.

The second dressing of nitrogen in Exp. A was given 9 days after the first harvest, by which time the plants had begun to differ in nitrogen content according to the amount supplied, but had not taken up their full amount of nitrogen.

The maximum nitrogen content of spring-sown plants was recorded after I I weeks’ growth, but in autumn-sown plants the total nitrogen was still rising after 184 weeks. However, the maximum was reached by mid-April in the autumn- sown plants, but not until the end of May in spring-sown plants. In both experiments the maximum total nitrogen was reached long before the maximum dry weight.

TABLE 10. Total nitrogen, mg. per plant (a) Wild oats and spring barley

Distribution of nitrogen between parts of the plant

* Whole plant, harvest

Avena jatua OK I N 2N

Mean A. ludoviciana ON

IN 2N

Mean Barley ON

IN 2N

Mean

I 2 3 4 3.7 6.7 7.2 6.9 9‘3 31-6 30.8 28.6

11.5 68.1 61.6 50.6 8.2 3 5 3 33’2 2 8 7

3.2 5.7 6.9 9.8 6.1 36.0 33.4 33.2 5.8 65-7 66-9 59-8

3’6 4’7 7‘3 7’1 10’9 24’5 26-3 22.4 18.0 56.4 49.0 45-1 ro.8 28-2 27’5 249

5’0 35‘8 35’7 343

Harvest z Harvest 4 c--h--7& Leaf Stem Root Top Root Seed

1.8 1-0 3’9 1.6 1.5 3.8 11.6 6.4 13.6 6.3 10’2 12.1 30.2 13-2 24‘7 14.0 1 1 . 0 25.6 145 6.9 14 1 7’3 7’6 13’8 2.2 1-1 2.4 2.2 2.6 5-0

31.6 16.0 18.1 19.1 13.8 26.9 16.6 8-1 11.2 94 8.3 16.5 1.4 1.4 1-9 1.5 0.9 4.7 7.9 8.4 8.2 5.3 4‘0 13.1

16.7 15-0 24’7 9‘7 9.8 25.6 8.7 8.3 11.6 5.5 49 145

15’9 7‘1 13.0 7’0 8.5 17’7

Growth of wild oats and nitrogen supply

TABLE 10 (cont.)


Whole plant, harvest

I 2 3 4 5 Avena fatua ON 3.4 26.0 33.6 35.4 35.0

IN 3.5 27.4 73.6 72.0 67.4 zN 2.9 27.5 128.3 116.9 108.1

Mean 3.3 27'0 78.5 74'8 70'2 A. ludouicicma ON 3-2 262 37.4 38.7 37.1

IN 3.1 26.6 77'1 72-4 71.3 2N 3.1 241 115.9 121.4 110'8 Mean 3'1 256 76.8 77'5 73'1

Winter oats ON IN 2N Mean

Winter wheat ON IN 2N Mean

5.8 29.7 5'9 35'9 5'4 38.5 5'7 34'7 8.3 35'3 8.8 41.9 8.4 43'7 8.5 40'3

45'5 35'2 76.0 78.4

118.2 1322. 799 74'8 38.9 35'1 71'7 76.0

123.8 125.2 781 78.8

36.7 81.8 128.2 82'2 36.8 73'1

126.3 78'7

Avenafatua ON I N 2N

Mean A. Ludaviciana ON

IN 2N

Mean Winter oats ON

IN ZN

Mean Winter wheat ON

IN 2N

Mean

73 3

7

8 30.1 62.8 96.9 63'3 40-2 68.9

111'5

73'5 42.8 84.3

I 20'7 82'6 38.7 75'6

124'3 795

Distribution of nitrogen between parts of the plant

Harvest 3 Harvest 6 Harvest 8 --- Leaf Stem Root Leaf Stem Root Top Root Seed 15-9 8.1 9.6 8.3 18.7 8.6 9.6 4.1 16.4 41'4 15.6 16.6 17'4 34.8 13.8 25'3 X1.8 25'7 69.9 27.8 30.6 27-8 64.6 19.1 41.5 15.6 39.8 42-4 17'2 189 17'8 39'4 13'8 25-5 1o.5 273 15.5 y 5 12-4 7.0 20'2 8-7 yo 7-4 23-8

63.3 29.2 23.4 27-7 64.3 15.7 34.9 15.6 61.0 39'4 189 185 175 4 ~ 7 12'6 21.3 10'9 41-3 1 7 2 9.9 18.4 8-8 20.8 9-4 10.6 7.7 24s 40.7 19.7 15.6 21.7 43.5 14.9 23.6 13.0 47.7 64'4 3 5 2 18.6 35.9 67'9 16.5 32.4 16.0 72'3 40.8 21.6 17'5 22'1 44'1 13'6 22'2 12'2 48.2 15.9 11.4 11.6 8.5 17.6 7.3 10.1 6.0 22.6 37.3 18.7 15.7 22.7 39.5 10.5 23.3 7.8 ~5 62.8 36.8 24'2 33.6 66.5 17-0 37.8 12-9 73.6 38.7 2w3 17'2 21.6 41.2 11.6 23'7 8.9 46.9

A 3

39'5 17.9' 19'7 17.7 43'5 13'3 20'0 9.8 39'1

Analysis by micro-Kjeldahl short method using no salicylic acid. As the replicate samples were bulked before analysis, standard errors cannot be calculated.

During the period of approximately constant total nitrogen, the nitrogen in the leaves decreased sharply, with a corresponding increase in nitrogen in the stem fraction (which included dead leaves and developing ears). The total nitrogen in the roots also tended to decrease in this period although in two sets of plants in Exp. A and five sets of plants in Exp. B the maximum was reached after the second or third harvest. By the final harvest of Exp. A, all the roots except A. ludoviciana

734 J O A N &I. THURSTON

TABLE I I . Uptake of nitrogen compared with amount supplied (a) Total nitrogen supplied, mg. per pot

In soil (approx.) 3000 8000 Exp. A Exp. B

Added in IN 212 3'8 Added in zN 424 636

(b) Total nitrogen taken up (mg. per pot of five plants), and yo of nitrogen supplied ON IN 2N - -

Increase of Increase of yo of total N yo of total N yo of

Total PI: soil N due to added N due to added N Exp. Species taken up taken up treatment taken up treatment taken up

A A. fatua 36 1'2 I 2 2 58 305 72 A. ludoviciana 49 I .6 131 62 286 67 Spring barley 37 1'2 95 45 245 58

B A. fatua 178 2'2 190 60 464 73 A . ludoviciana 20 I 2.5 '85 58 406 64

Winter wheat '95 2.4 186 58 437 69 Winter oats 228 2.8 194 61 434 68

Highest recorded total N used for each species in each treatment (see Table 10). Increase due to I N = (total for ~N)-(total for ON). Increase due to zN = (total for zN)-(total for ON).

ON were below their nitrogen content at the second harvest and in Exp. B by the sixth harvest all species in all treatments were below their third harvest nitrogen content.

In spite of the striking similarities in nitrogen content of wild oats and corresponding plants of cultivated cereals, there were certain differences. In Exp. B the total nitrogen in the crops at the first harvest was higher than in the wild oats although no species had begun to show differences due to added nitrogen. By the second harvest the differences in total nitrogen between weeds and crop were less but the crops contained more nitrogen at IN and 2N than at ON whereas the wild oats did not. The difference between the two levels of added nitrogen did not appear until the third harvest, when both crops and wild oats showed it and the differences between species were small. Exp. A with quicker growth and fewer harvests did not show these early differences. The first harvest in Exp. A corresponds to a stage between the second and third harvests of Exp. B, with all species responding to added nitrogen, but only barley showing a higher uptake from 2N than from IN.

When the uptake of nitrogen is compared with the amount supplied (Table 11) the similarity between wild oats and cultivated cereals shows again, both in the small amount of nitrogen extracted from the soil and in the higher percentage taken up when more is available, whether from increasing the volume of soil or adding sulphate of ammonia. However, differences between species were small. Barley took up the available nitrogen less than the wild oats or the winter cereals, and at 2N, A. fatua consistently took up more than A. ludoviciana.

Growth of wild oats and nitrogen supply 73 5 Throughout Exp. A and at the first harvest of Exp. B, the visual signs of nitrogen

deficiency corresponded to the nitrogen analyses of the plants, but at the second harvest of Exp. B, the analysis showed more difference in nitrogen content between treatments than the appearance of the plants indicated, wheat showing only slight deficiency symptoms in ON and the three oats none at all. Symptoms developed quickly between the second and third harvests, all species except A. ludoviciana being paler in colour and weaker in growth, with fewer shoots, in treatment ON than in IN or 2N by 23 March. The symptoms reflected the nitrogen content in all treatments of all species at and after the third harvest.

DISCUSSION The growth analysis of the two species of wild oats in these two experiments showed how similar they are in many respects to cultivated cereals. However, there were small differences that may be very important in determining the nature and severity of competition between wild oats and cereal crops in the field. In the early stages wild oat seedlings of both species were smaller than those of the corresponding crops in total dry weight, total nitrogen content, leaf area and number of shoots. However, very young wild oats had higher net assimilation rates than the cultivated cereals and soon caught up and passed the crop plants. The difference in net assimilation rate did not persist, and over the greater part of the growing season the net assimilation rates of cultivated cereals and wild oats were alike. Differences in the amount of dry matter produced after the earliest stages of growth depended almost entirely on differences in leaf area.

The initial differences indicate that the crop can exert its greatest competitive effect against wild oats while the latter are seedlings. Leaf area is very important in this competition because it is directly concerned in assimilation and because over- shadowing of one plant by another affects assimilation indirectly. The young wild oat plants depend on their high net assimilation rate to overcome their initial dis- advantage of size. Of the three varieties of cultivated cereals tested, winter wheat should compete best with wild oats because it reached its maximum shoot number and leaf area soonest.

The number of shoots per plant and the percentage of shoots which flowered were both characteristic of the species. Cultivated oats had more shoots per plant than any other species in these experiments. Barley had the highest percentage of flowering shoots in Exp. A and A. fatua in Exp. B. A. ludoviciana with the highest number of shoots per plant had slightly fewer flowering shoots per plant than the other species in both experiments and consequently the lowest percentage of flowering shoots.

Ear emergence was spread over a longer period in A. futua than in the crops. A. ludoviciana also tended to have a long period of ear emergence. Wild oat ears started to emerge earlier than those of wheat and oats, but barley given sufficient nitrogen started before either of the wild oats. Ear emergence in barley without added nitrogen was very slow as the plants were nearly dead by that time.

Both wild oats sown in spring reached 50% ear emergence later than barley.

736 J O A N M. THURSTON Autumn-sown A. fatua reached 50 yo ear emergence later than either winter wheat or winter oats, but A. ludoviciana coincided with winter oats and was slightly ahead of winter wheat.

The dates of ear emergence of wild oats relative to the crops in which they are growing are important agriculturally, because control treatments are often suggested for application to the young wild oat ears to prevent seed formation. The success of such a treatment in controlling wild oats and the amount of damage done to the crop both depend on the extent to which the ears are protected by the leaf-sheaths and on the relative maturity of crop and weed seeds on the date of treatment. The long period of ear emergence in wild oats, suggests that treatment on only one date would be unlikely to catch all wild oat seeds at a vulnerable stage. The relative heights of culms of crop and wild oats will also affect the results of applying chemi- cals. Although the tallest ears of wild oats were always taller than those of the corresponding crop plants (even though they were long-strawed varieties), the difference in height between the tallest and shortest ears was greater in wild oats and many of the smaller ears would be hidden by the surrounding crop. Over- shadowing of the small ears would decrease their assimilation, but there is some evidence that assimilation in the ears is less important in oats than in wheat and barley.

The projection of the tallest ears of wild oats above the crop facilitates the loca- tion and removal of isolated plants before they can shed seeds.

The Iooo-grain weight and total weight of seed produced by the crops were higher than those of the wild oats, although the wild oat seeds were weighed enclosed in their substantial lemmas and paleas, with the awns attached, whereas the wheat seeds had no enveloping structures. The number of seeds per ear also fluctuated less with changes in nitrogen supply in the crops, including cultivated oats, than in the wild oats. Wild oats, in contrast, produced more seeds per plant than the crops. The differences in shoot height, ear size, Iooo-grain weight and duration of ear emergence are probably the result of continued selection of the crops for heavy-yielding seed-heads of uniform height and ripeness, whereas natural selection of the wild oats has favoured the multiplication and spread of those producing the greatest number of seeds.

A very important difference between wild oats and the crops lay in the 98-100% dormant seeds of the weeds and none in the crop seeds when tested 2 months after harvest. Dormancy enables the weeds to withstand one or more years in which they cannot set seed, and is the main reason why wild oats are so troublesome in agriculture.

In general, wild oats benefited from nitrogen added to the soil in the same ways and to about the same extent as the crops. This is in line with observations on Rothamsted farm at the end of the second world war, on the continuous winter wheat plots of Broadbalk which had become infested with A. ludoviciana and the continuous spring barley plots of Hoosfield infested with A. fatua. T h e wild oats in the good crops on the plots receiving sufficient nitrogen were tall and robust, while those in the short thin crops on the plots receiving inadequate nitrogen

Growth of wild oats and nitrogen supply 737 were much smaller and weaker, but on all plots the tallest wild oat ears showed above the surrounding crop. Field observations and pot experiments therefore agree in suggesting that the addition of nitrogen is unlikely to affect the ratio of wild oats to crop, and the pot experiments indicate the reasons, i.e. that the added nitrogen does not begin to affect the growth of the crop until after the seedling wild oats have begun to catch up, and that thereafter the crop and wild oats respond alike to the nitrogen supplied.

Both wild oat species took up about the same amount of nitrogen per plant as the crops in Exp. B at all levels of nitrogen. In Exp. A, barley took only 75-83 % of the amount extracted from the soil by A. Zudoviciana. At ON the uptake of nitrogen by A. futuu was like that of barley but at IN and 2N it was higher, like A. Zudoviciaaa. A wild oat plant would therefore take as much nitrogen out of the soil as a crop plant, but if the crop was spring barley at a medium to high level of nitrogen the wild oats might take up more nitrogen per plant than the crop. A. Zudoviciana is rarely found in spring barley owing to the relative times of germination and sowing, but if it did occur it would probably take up more nitrogen per plant than the crop whatever the level of nitrogen.

The effect on wild oats of deficiency of a major element, nitrogen, can now be compared with the effect of lack of a minor element, manganese (Thurston, 1951 a). As with manganese, wild oats needed less additional nitrogen than the crop plants to reduce the severity of leaf symptoms. Unlike manganese deficiency, however, nitrogen deficiency affected both wild oats to the same extent as the crops tested, the reduction in total dry weight at ON compared with 2N being 87-89 % in Exp. A and 59-6374 in Exp. B. Manganese deficiency affected seed production in wild oats differently from cultivated oats; wild oats showed less reduction in number of seeds per plant and more in manganese content per seed than cultivated oats. In contrast, the response to nitrogen deficiency in wild oats was like that of the three crops; all showed similar percentage reduction in seeds per plant from zN to ON, and the nitrogen content per seed was unaffected by the amount of nitrogen supplied (Table 7 ) although it varied with species. The anomalous values for all three species in treatment ON of Exp. A are probably due partly to scarcity of material for analysis, but the higher Iooo-grain weight of the wild oats in ON also contributed to this result. Wild oats were well below all three crops in total nitrogen content per seed, due mainly to the lower Iooo-grain weight of the wild oats.

The figures for the net assimilation rate of wheat and barley during the main growing period are similar to those reported by Watson (1952) and Heath & Gregory (1938). The increase of net assimilation rate with added nitrogen in Exp. A is similar to that found by Gregory (1926) and Watson (1952), and the decrease in net assimilation rate in Exp. B due to added nitrogen, followed by an absence of any nitrogen effect, is anomalous.

The increase in shoots and ears per plant and in nitrogen content of grain in the two pot experiments is in line with field observations on wheat (Watson, 1939). Watson also found that the Iooo-grain weight of wheat was depressed by applying nitrogen to the soil, but in these experiments the three crops showed hardly any

738 J O A N M. THURSTON effect of nitrogen on Iooo-grain weight and the two wild oat species showed a depression of Iooo-grain weight with added nitrogen in Exp. A only.

The results with the two wild oat species and three crops in these two pot experiments agree with the conclusion of Heath & Gregory (1938) that over most of the growing period the net assimilation rate is constant. They note, however, that high initial values have been recorded in Sudan grass (Ballard & Petrie, 1936) and mangolds and sugar beet (Watson, 1952) and conclude that most of the high values come from favourable weather. Exp. B shows the opposite effect, that of winter conditions depressing the net assimilation rate of all species at the outset, with a marked rise in spring to a maximum in late May. As the effect of seasonal conditions was similar in wild oats and crops, this factor seems unlikely to affect the competition between them.

Assimilation in the ear may account for the high net assimilation rate of wheat in the last period of Exp. B. Photosynthesis in the ear of barley can account for 19-28% of the dry weight of the grain, but assimilation in the ears of wild and cultivated oats appeared to contribute much less of the total dry matter of the plant.

Watson, Thorne & French (1958) found that, in contrast to pot experiments, about one-quarter of the total nitrogen in barley in the field was taken up after ear emergence began. They ascribed the difference to the restriction of the volume of soil available to the roots in a pot. The results of the two pot experiments with barley and four other species reported here agree with those of the pot experiments of other workers, but it is possible that in the field the uptake of nitrogen might continue after ear emergence.

The assistance of Dr D. J. Watson in planning the experiments and presenting the results is gratefully acknowledged.

REFERENCES

BALLARD, L. A. T. & PETRIE, A. H. K. (1936). Physiological ontogeny in plants and its relation to nutrition. I. The effect of nitrogen supply on the growth of the plant and its parts. Aust. J. exp. Biol. med. Sci. 14, 135.

GREGORY, F. G. (1926). The effect of climatic conditions on the growth of barley. Ann. Bot., Lond., 40, I .

HEATH, 0. V. S. & GREGORY, F. G. (1938). The constancy of the mean net assimilation rate and its ecological importance. Ann. Bot. Lond., N.S. 2, 81 I .

THURSTON, J. M. (1951a). A comparison of the growths of wild and of cultivated oats in manganese-deficient soils. Ann. appl. Biol. 38, 289.

THURSTON, J . M. (1951b). Some experiments and observations on the germination of wild oat (Avena fatua and A. ludoviciana) seeds in soil and the emergence of seedlings. Ann. appl. Biol. 38, 812.

THURSTON, J. M. (1954). A survey of wild oats (Awena fatua and A . ludoviciana) in England and Wales in 1951. Ann. appl. Biol. 41, 619.

THURSTON, J . M. (1957). Morphological and physiological variation in wild oats (Awena fatua L. and A. ludoviciana Dur.) and in hybrids between wild and cultivated oats. J . agric. Sci. 49, 259.

WATSON, D. J. (1936). The effect of applying a nitrogenous fertilizer to wheat at different stages of growth. J . agric. Sci. 26,391.

Growth of wild oats and nitrogen supply 739 WATSON, D. J. (1939). Field experiments on the effect of applying a nitrogenous fertilizer

WATSON, D. J. (1952). The physiological basis of variation in yield. Adwanc. Agron. 4, 101. WATSON, D. J., THORNE, G. N. & FRENCH, S. A. W. (1958). Physiological causes of differ-

ences in grain yield between varieties of barley. Ann. Bot., Lond., N.S., 22, 321. WILLIAMS, R. F. (1936). Physiological ontogeny in plants and its relation to nutrition.

11. The effect of phosphorus supply on the growth of the plant and its parts. Aust. J. exp. Biol. med. Sci. 14, 167.

WILLIAMS, R. F. (1946). The physiology of plant growth with special reference to the con- cept of net assimilation rate. Ann. Bot., Lond., N.S. 10, 41.

to wheat at different stages of growth. J. agric. Sci. 29, 379.

(Received 9 February 1959)

a comparative study of the growth of wild oats (avena fatua l. and a. ludoviciana dur.) and of...

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