J. Sci. Food Agric. 1981, 32, 339-346

Effects of Drought on Yield and on Grain and Malt Characters in Spring Barley

Alan G. Morgan and Timothy J. Riggs

Plant Breeding Institute, Maris Lane, Trumpington, Cambridge CB2 2L Q

(Manuscript received 17 June 1980)

The effects on yield and on grain and malt characters, of drought stress applied at different stages of grain development and ripening were studied in three varieties of spring barley grown in a glasshouse. Differences between variety means were found for most of the characters studied, but only for wort filtration time was there a significant interaction between variety and irrigation treatment. Grain yield was reduced by all the treatments involving drought stress and the reduction was greatest when stress was applied at heading and maintained for at least 14 days. A similar response was found for the number of ears per plant. Number of grains per main-shoot ear was reduced by drought stress applied at heading but not when the stress was applied from 32 days after heading until harvest. Grain size was significantly reduced by all treatments. Raw-grain characters known to be correlated with malt extract were found to be affected by the treatments: grain nitrogen content, barley extract viscosity and the rate of sedimentation of barley flour in ethanol were all increased by drought stress, the degree of response varying with the length and timing of the period of drought. Malt extracts were reduced by drought stress whether this was applied early or late in grain development. Malting loss and germinative energy were reduced, and wort filtration time increased by late stress. =-Amylase activity in the germinating grain was unaffected by any of the treatments.

1. Introduction

Maltsters in the UK use approximately 20% of the national barley crop of 9-lox lo6 t1Y2 and pay a premium to farmers for grain of acceptable quality from recognised malting varieties. The major criterion of acceptability for malting, apart from variety, is grain nitrogen content which should not usually be higher than 1.8 % dry matter (DM). The nitrogen content of the grain can to some extent be controlled by the level and timing of fertiliser application, but the rainfall pattern during the growing season is often the major determinant of both the time of application of the fertiliser and its availability to the plant.2

Grain appearance and size are also criteria for suitability for malting. The grain should be plump, well-filled and thin-skinned; shrivelled, poorly-filled grain is normally low in carbohydrate, high in protein, and produces low malt extracts. Good grain size appears to be largely a result of favourable rainfall and other environmental influences. Surveys of quality in the 1974, 1975 and 1976 seasons suggested that the proportion of small grains was even more variable between years and between regions than nitrogen content.2 Local weather conditions, causing variation between districts in grain nitrogen content and grain size, are principally responsible for the fact that a large proportion of the production of barley from malting varieties is not suitable for malting.2

Shortage of water during initial periods of crop development may reduce yield and leave the grain shrivelled when ripe.3-5 Other characters affected by drought stress, and important in determining suitability for malting, are the germinative energy of the grain and the relative proportions of starch, nitrogen and /3-glucan.6-10 Characters influencing malting quality may be differentially affected

0022-5142/81/0400-0339 $02.00 0 1981 Society of Chemical Industry

339

340 A. G . Morgan and T. J. Riggs

according to the period during grain development over which the drought extends. Thus the relative influence of these characters on the quality of the malt may vary with environment, and a study of this variation could contribute to our understanding of the complexity of interacting factors involved in malting quality. From an agronomic point of view it would help to know the period of growth at which the yield and quality of the crop may be most affected by drought stress.

This experiment was designed to investigate the effects of drought, occurring at different stages after heading, upon yield and its components and upon the constitution of the grain. Samples of grain were malted and the relationships between measured grain characteristics and malting per- formance determined. Three varieties of barley were used and the drought stresses were applied under controlled conditions in the glasshouse.

2. Experimental 2.1. Design and experimental treatments Plants were grown in a glasshouse in plastic pots 12.5 cm in diameter and containing approximately 1.5 kg washed grit. Pre-germinated seeds were sown at two per pot on 3 March 1977 and the plants were grown in natural light throughout the experiment. Between heading and harvest the noon temperature ranged from 13 to 34°C (mean 24°C).

A split-plot randomised block design was used with three replications. Each of three benches in the glasshouse constituted a replication and carried eight trays each containing 72 pots. An auto- matic irrigation system allowed the supply of nutrient solution to be controlled independently for each tray. Pairs of adjacent trays constituted main-treatment plots each receiving one of the four irrigation treatments described later. Main-treatment plots were divided into sub-treatment plots each consisting of 48 pots of one of the three spring barley varieties used in the experiment.

In unstressed periods each tray was flooded in sequence for a period of approximately 15 min in the early morning and again in the late afternoon. The four irrigation treatments were: (i) irrigation from sowing to heading (approximately 0.5 cm of awn protruding above the auricle of the flag leaf) followed by drought stress until harvest ; (ii) drought stress beginning at heading but with irrigation resumed from 14 days after heading until harvest: (iii) irrigation until 32 days after heading followed by drought stress until harvest; and (iv) irrigation from sowing to harvest (control).

Prior to the main experiment, a pilot experiment was carried out to determine the permanent wilting point of the three varieties. Two seeds were sown in each of 12 pots for each variety as described previously and plants were supplied with culture solution until heading. The pots plus plants were then weighed immediately after irrigation. Culture solution was subsequently withheld and the pots weighed twice each day. At each weighing watering was resumed on one of the pots of each variety. The permanent wilting point was taken as the percentage decrease in the weight of pot plus plants at the point when a resumption of watering failed to revive the plants. This was found to be similar for each variety and therefore the mean value of 13.7% was used during the main experiment.

For drought stress the culture solution was withheld until the percentage decrease in the weight of a sample of pots and plants was close to the permanent wilting point. A single irrigation of culture solution was then given and the cycle repeated for the duration of the droughting period. There were 21, five and seven such cycles in treatments 1, 2 and 3, respectively, and the drought stresses applied in treatments 1 and 2 were started on 25 May. The period from heading until cessation of grain growth on the main shoots of plants in treatment 4 was 3&35 days.

The culture solution used was similar to that described by Rhodes and Mathers,ll except that the minor element constituent, molybdic acid, was replaced by 0.165 g of the more soluble ammonium molybdate [(NH4)6 Mo7024*4H20]. To restrict grain nitrogen content, ammonium nitrate was omitted from the solution after the five to six leaf stage.

The three spring barley varieties used (Proctor, Maris Mink and Lofa Abed) were chosen to represent a range in potential malting quality. They have life-cycles of similar length. Thus when stress was applied in any one main-treatment the plants of all three varieties would be at approxim- ately the same stage of development.

Drought, yield, grain and malt characters in spring barley 341

2.2. Measurements and analytical procedures The number of ears and the number of grains on the main-shoot ear were recorded at harvest, after which the grain was threshed by hand. Total grain dry weight for each sub-plot was determined and the percentages of grain retained on 2.2 and 2.5 mm sieves were recorded; 1000-grain weight was measured on the fraction retained on the 2.2 mm sieve, and subsequent analyses were also conducted on this fraction after storage for 16 weeks to ensure the breaking of dormancy.

The relative levels of acid-soluble j3-glucan in the grain were determined by a modification of the falling-time method described by Morgan and Gothard:12 samples of 1.25 g flour were extracted with 20 ml acid solution of pH 1.5 for 4 h at 40°C; the solution was then cooled to 20°C and allowed to settle for 20 min before measurement of the falling-time of a 5.5 mm diameter steel ball in a 6 mm precision-bore glass tube containing the extract. High falling-time is generally associated with poor malting quality.l3

a-Amylase was measured by a modification of the method of Smith14 using an extract of grain which had been germinated for 3 days in a Petri dish at 17°C. Exactly 2.5 g of milled grain was added to 20 ml extracting solution and incubated at 30°C for 1 h. After filtration, 0.5 ml of filtrate was added to 50 ml extracting solution before being assayed. Germinative energy was determined by counting the number of grains which had germinated after 3 days when 100 grains were incubated at 17°C in a Petri dish containing 4 ml of deionised water.

Sedimentation tests were conducted using a modification of the method of Palmer:15 a 1 g sample of grain was milled in a water-cooled Janke and Kunkel mill (type A105) for 5 s and 0.1 g of the fraction of flour which passed through a 250 pm sieve was used; this was shaken with 2 ml of a 70% ethanol solution initially at 0°C and then allowed to settle for 15 min at room temperature; turbidity of the solution was then measured using an EEL colorimeter fitted with an OGRI filter. High turbidity is generally associated with good malting quality.l5

Micro-malting was performed using the procedures described by Gothard et al. :16 samples of 20 g were steeped at 17°C using the following regime-5 h steep, 19 h air-rest, 5 h steep, 19 h air-rest, 5 h steep; a period of 90 h was then allowed for modification in 150 mm x 24 mm boiling tubes; the green malts were kilned for 48 h at temperatures of 50°C for 24 h, 65°C for 8 h and 80°C for 16 h. Roots and shoots were then removed and the malts reweighed to obtain the malting loss.

Analyses of the malts for cold water extract (CWE), hot water extracts (HWE) and wort filtration rates were performed as described by Gothard et a1.l’ Grain nitrogen contents (% DM) and the soluble nitrogen contents of 5 ml of wort were measured using a Kjeldahl method.l*

3. Results 3.1. Yield, yield components, and grain characters Results are presented as mean values, averaged over varieties, for each level of water supply (Tables 1 and 3) and as variety means averaged over treatments (Tables 2 and 4). Interaction between varieties and water supply was not significant for any character except wort filtration time; only main effects of the treatments will be discussed.

The number of ears per plant was reduced by all the treatments involving drought when compared with the fully irrigated control treatment (Table 1). Only treatments 1 and 2 significantly reduced the number of grains on main-shoot ears. Yield was reduced by all the drought treatments but the earlier the stress was applied after heading the greater was the reduction of yield. Grain from the stressed plots was thinner than that from the control plots as illustrated by the 1000-grain weights and sieve fractions.

Falling-time was increased significantly by all the drought treatments; the largest increase (42 % above the control) was associated with treatment 1. Sedimentation reading was reduced by drought stress and particularly by stress applied late in grain development (treatment 3). The difference between sedimentation readings for treatments 1 and 2 suggests that the drought stress applied in treatment 1 had its greatest effect after 14 days from heading. Grain nitrogen content was increased by drought stress, particularly when this occurred early in grain development. Thus the effects of

342 A. G . Morgan and T. J. Riggs

drought on grain composition were to increase falling-time and grain nitrogen content, and to reduce sedimentation reading-all these effects are associated with poor malting quality.l3,15,19

Differences between varieties for yield components were small, although significant for number of ears and 1000-grain weight (Table 2). Lofa Abed gave the highest yield and Proctor the lowest.

Table 1. Main treatment means for plant and grain characters ~

Irrigation treatment

Character 1 2 3 4 (Control) S.e.d.a

Number of ears per plant Number of grain on main-shoot ear Yield (g) Grain retained on 2.2 mm sieve (%) Grain retained on 2.5 mm sieve (%) 1000-grain weight (g)* Grain nitrogen (%) Sedimentation reading Falling-time (s)

4.3 5.0 6.2 8 .4 0.35 13.4 15.5 23.8 25.6 1.82

89.3 87.2 93.4 98.7 1.51 70.8 84.8 81.4 91.6 1.80 38.4 38.8 39.3 40.7 0.41

18.7 23.7 19.4 26.9 0.95 13.3 10.7 11.0 9.4 0.38

421 461 493 542 19.3

1.86 1.82 1.74 1.68 0.029

a Standard error of differences between means. 6 This grain was the fraction retained by a 2.2 mm sieve.

Table 2. Variety means for plant and grain characters

Variety

Character Proctor Maris Mink Lofa Abed S.e.d.

Number of ears per plant Number of grain on main-shoot ear Yield (g) Grain retained on 2.2 mm sieve (%) Grain retained on 2.5 mm sieve (%) 1000-grain weight (g) Grain nitrogen (%) sedimentation reading Falling-time (s)

5.9 18.5

93.2 76.9 38.3

1.75 30.1 8 .5

459

6.2 17.9

95.0 82.8 40.2

15.2 11.3

47 1

1.80

6.8 19.8

95.7 86.7 40.8

21.2 13.4

508

1.77

0.41 1.68

16.61 1.14 1.70 0.30 0.025 0.80 0.32

Table 3. Main-treatment means for malting quality characters

Irrigation treatment

Character 1 2 3 4 (Control) S.e.d.

Germinative energy a-Amylase (arbitrary units) Malting loss (g) Loglo wort filtration time Permanently soluble nitrogen (%) Cold water extract (%) Hot water extract (%) Adjusted extract (%)

96.4 98.8 96.3 98.3 0.52 4925 4178 4883 4853 187.3

6.96 8.22 7.26 8.34 0.213 1.728 1.618 1.759 1.623 0.0445 0.592 0.563 0.498 0.489 0.0164

15.4 17.2 16.8 17.4 0.33 64.9 68.1 68.3 70.4 0.85 67.9 70.8 70.3 71.9 0.85

Drought, yield, grain and malt characters in spring barley 343

Table 4. Variety means for malting quality characters

Variety

Character Proctor Maris Mink Lofa Abed S.e.d.

Germinative energy a-Amylase (arbitrary units) Malting loss (g) Loglo wort filtration time Permanently soluble nitrogen (%) Cold water extract (%) Hot water extract (%) Adjusted extract (%)

97.5 5168

7.62 1.576 0.552

17.4 69.8 71.9

97.1 5363

7.73 1.838 0.523

16.6 67.7 70.2

97.1 0.44 4003 163.1

7.71 0.157 1.632 0.0353 0.531 0.0161

16.0 0.22 66.2 0.79 68.5 0.80

Lofa Abed also had the largest fraction of grain retained on a 2.5 mm sieve. The largest differences between varieties for the raw-grain characters were, as expected, for the tests used to predict malting quality. Proctor had the highest sedimentation reading and the lowest falling-time. Maris Mink had a low sedimentation reading but an intermediate falling-time.

3.2. Malt characters All the characters measured, except a-amylase activity, were affected by drought stress (Table 3) . Germinative energy was reduced by 2 % when the drought stress occurred late in grain development but drought during the 14 days following heading did not affect this character. Drought occurring late in grain development also reduced malting loss, increased wort filtration time and reduced CWE. For all these characters the differences between the mean values for treatments 1 and 2 suggested that the drought stress applied in treatment 1, from heading to harvest, had its greatest effect after 14 days from heading. In contrast, the content of permanently soluble nitrogen in the wort was greatly increased by early drought stress, as was the content of nitrogen in the raw grain (Table 1).

Because of the negative relationship between HWE and grain nitrogen content (%) (Table 5), it was necessary to adjust HWE results for differences in grain nitrogen before comparing them. The general equation :l9

Adjusted extract = HWE + 11 [Grain nitrogen content (%) - 1.51

was used. The effects of drought stress on HWE and adjusted extract were similar (Table 3): treat- ment 1 caused a large decrease, whereas the reductions caused by treatments 2 and 3 were much less

Table 5. Correlation coefficients between grain and malt characters (d.f. = 34)

Loglo wort Permanently Malting filtration soluble Cold water Hot water Adjusted

a-Amylase loss time nitrogen extract extract extract

Yield -0.27 Grain retained on -0.27

2.2 mm sieve (%)

2.5 mm sieve (%) Grain retained on -0.30

1000-grain weight -0.41 Grain nitrogen 0.08 Sedimentation reading - 0.06 Falling-time 0.40

0.34 -0.18 0.49 -0.23

0.54 -0.23

-0.43 0.11 -0.27 0.25

0.33 -0.65 -0.33 0.22

-0.62 0.32 -0.62 0.39

-0.60 0.35

-0.47 0.04

0.02 0.51 0.26 -0.75

0.54 -0.47

0.30 0.12 0.38 0.21

0.37 0.21

0.02 0.08

0.53 0.48 -0 .55 -0.31

-0.71 -0.66

0.34-Significant at 5% level; 0.42-significant at 1 % level; 0.53-significant at 0.1 % level.

344 A. G. Morgan and T. J. Riggs

pronounced. Treatment 1 also caused a large reduction in CWE, as did treatment 3, but treatment 2 did not reduce CWE significantly from the value recorded for the control. It is notable that there was significant variation in adjusted extract, over treatments, suggesting that variation in grain nitrogen content was not the only factor involved in the response of HWE to drought stress.

No differences were detected between varieties for germinative energy, malting loss and perman- ently soluble nitrogen in the wort (Table 4). The lowest filtration time and highest extracts were recorded for Proctor. Maris Mink had a very high filtration time but gave intermediate values for CWE, HWE and adjusted extract. Lofa Abed produced low CWE and HWE and showed signific- antly lower cr-amylase activity than the other two varieties. The relative HWE results for the three varieties were in agreement with normal malting experience of these varieties.

4. Discussion

Drought stress was first applied on 25 May, 83 days after sowing (treatments 1 and 2). At this time the total number of tillers is normally declining20, 21 and tiller survival may be particularly sensitive to environmental stress. The large effect on the number of ears per plant (Table 1) of a period of drought occurring at and shortly after heading, supports the findings of Day et aL20

Anthesis normally occurs in barley while the ear is still partially enclosed in the flag-leaf sheath. In this experiment the first drought stresses were applied shortly before anthesis but too late to affect the number of spikelets initiated; nevertheless, the mean number of grains per ear on the main shoot was much reduced by stress applied at heading (Table 1). This contrasts with the findings of Day et ~ 1 . ~ 0 that mean number of grains per ear was not much reduced by drought stress during this period, but these authors did not measure main shoots and tillers separately. The reduced grain number observed here might be due to increased spikelet death or to incomplete fertilisation after anthesis4, 22 or to abortion of developing grains.

Gallagher et aL23 noted that the mean weight per grain in Proctor spring barley was relatively stable in a large number of experiments. They speculated that about 70% of final grain dry weight in a crop which suffered a severe drought while the grain was filling could have been supplied by translocation from the stem. Such compensatory translocation would allow the plant to fill its grains despite adverse conditions. Day et u Z . , ~ O however, reported that mean grain mass was most affected by drought in the grain-filling period and that when yield is limited by drought, the percentage decrease in mean grain mass can be as great as the decrease in the other components of grain yield. In the experiment reported here 1000-grain weights were measured on a sieved fraction of grain and would not be expected to show the full range of variation in grain size. However, the sieved fractions indicated that drought stress resulted in a higher proportion of small grains (Table 1). The reduced yield associated with drought stress seems, however, to have been primarily due to reductions in ear number and the number of grains per ear.

In terms of grain development, drought stress in treatment 2 was applied during the period of endosperm cell division. In treatment 3 the stress was applied towards the end of the cell-expansion phase22 whilst in treatment 1 drought was applied during both phases. The grain and malt characters most affected by drought during the cell-division phase were percentage grain nitrogen and per- centage soluble nitrogen in the wort. Although the percentage nitrogen levels were higher in grain from stressed plants there were no significant differences between treatments in the absolute amount of protein in the grain. Thus, the increased percentage grain nitrogen was apparently the result of reduction in carbohydrate content. Reduction in the number of endosperm cells would limit the potential for carbohydrate accumulation during grain filling. Characters affected by drought during the cell-expansion phase were sedimentation reading, germinative energy, malting loss and wort filtration time. Sedimentation behaviour is a reflection of endosperm structure,24 and Brocklehurstz2 has shown that in poorly-filled grains of wheat fewer small starch granules are laid down during the cell-expansion phase. The changes in endosperm structure associated with drought during this period are likely to be responsible for the observed reduction in germinative energy and presumably a low enzyme activity during malting. This would result in poor modification and hence the higher permanently soluble nitrogen and wort viscosity found here.

Drought, yield, grain and malt characters in spring barley 345

Table 6. Correlation coefficients between grain characters (d.f. = 34)

Yield (A) Grain retained on 0.91

Grain retained on 0.96 0.98

1000-grain weight (D) 0.55 0.73 0.64

2.2 mm sieve (%) (B)

2.5 mm sieve (%) (C)

Grain nitrogen (E) -0.71 -0.68 -0.72 -0.36 Sedimentation reading (F) 0.16 0.18 0.13 -0.27 -0.39 Falling-time -0.11 -0.07 -0.14 0.21 0.46 -0.66

A B C D E F

Table 7. Correlation coefficients between malt characters (d.f. = 34)

or-Amylase (A)

Loglo wort filtration time (C) 0.15 -0.18 Malting loss (B) -0.08

Permanently soluble 0.11 -0.18 -0.12 nitrogen (D)

Cold water extract (E) 0.22 0.56 -0.31 -0.38 Hot water extract (F) 0.33 0.49 -0.29 -0.30 0.73 Adjusted extract 0.41 0.47 -0.24 -0.18 0.69 0.97

A B C D E F

Falling-time, CWE and HWE were equally affected by the early or late drought treatments but were further affected by the extended drought stress in treatment 1. The increase in falling-time as a result of drought stress supports the findings of others7-10,15 that drought stress increases the viscosity of raw-grain extracts caused by compounds such as P-glucans.

Although there was a highly significant difference in a-amylase activity between Lofa Abed and the other varieties (Table 4) the differences between treatments were not significant (Table 3). Others workers25- 26 have also found little environmental influence on this character.

The relationships between the various grain and malt characters are given in Tables 5-7. Adjust- ment of HWE for grain nitrogen content reduced the highly significant negative correlation between these characters to a non-significant level (Table 5) . The predictive tests, falling-time and sedimenta- tion reading were strongly correlated with HWE and with CWE. There was also a strong negative relationship between sedimentation reading and wort filtration time; both these characters were most influenced by the later drought stress, which reduced the sedimentation reading and increased wort filtration time (Tables 1 and 3). Sedimentation reading was not strongly correlated with grain nitrogen (Table 6)-this is in agreement with the findings of Palmer and Harveyz4 that sedimentation rate was not greatly influenced by grain nitrogen but principally by the ease with which the starch granules could be released from the protein matrix during milling.

Falling-time was not correlated with wort filtration time, showing that the viscosity-increasing agents in the wort are different in kind and/or independent in quantity from those extracted from the raw grain for the falling-time test.

In this experiment plants were grown in pots in a partially controlled environment. This offered the advantage of observing the response of the crop to a drought stress at a particular stage of development: the periods of water stress could be precisely timed because root volume was limited and the available water supply was quickly exhausted when irrigation was withheld. However, the cyclic application of drought, alternating with a single irrigation treatment, is clearly different from the pattern of stress that is likely to occur in the field.20 Nevertheless, the trends observed in this experiment for yield and for grain and malt characters in response to drought stress are perhaps more clearly apparent and more easily interpreted than would be the case for a field-grown experi- ment. The results confirm in some detail that drought stress affects many of the grain characters that influence the quality of malt. The extent of this effect depends upon the duration of the stress and the period of grain development at which it occurs.

346 A. G. Morgan and T. J. Riggs

Acknowledgements The authors are grateful to the Home-Grown Cereals Authority for financial support and to Dr E. J. M. Kirby, Mr D. B. Smith and Dr F. G. H. Lupton for helpful advice.

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