Ann. appl. B id . (1995). 127:561-571 Printed in Great Britain 561

Effect of light environment during soil disturbance on germination and emergence pattern of weeds

By PETER KRYGER JENSEN Danish Institute of Plant and Soil Science, Department of Weed Control and

Pesticide Ecology, Flakkebjerg, DK-4200 Slagelse, Denmark (Accepted 20 December 1995)

Summary This paper describes results from experiments which investigated the effects

of light intensity during soil disturbance on germination and emergence pattern of weeds. Different emergence patterns were demonstrated for seeds which are instantly flash induced compared to seeds which are induced to germinate by integrating a weak light signal over a period of time. A reduced and delayed emergence is achieved after a disturbance in darkness compared to a soil disturbance in daylight. The increased emergence after soil disturbance in day- light is due to additional plants originating from seeds placed at a soil depth in the pots where daylight cannot penetrate and induce seeds to germinate, but which are induced during the short exposure period. A close relationship between soil disturbance intensity and number of weed plants emerging was found in field experiments with shallow harrowings. It was also shown that a portion of the increased number of seedlings arising when soil disturbance is carried out in daylight, compared to soil cultivation in darkness, originates from seeds germinating from deeper soil layers, resulting in a deeper average germination depth.

Key words: Photostimulation, tillage intensity, emergence pattern, field emer- gence, germination depth, Matricaria inodora, Stellaria media, Chenopodium album

Introduction Light as an environmental factor influencing seed dormancy and germination in field

conditions was first demonstrated by Sauer & Struik (1964) and by Wesson & Wareing (1969). It required a further 20 years before the effect of light intensity during the cultivation process was shown to influence weed germination and emergence in the field (Hartmann & Nezadal, 1990). Since then there have been a number of investigations under field conditions where the effect of cultivations carried out at day and at night have been compared (Jensen, 1992; Kuhbauch, Gerhards & Klumper, 1992; Ascard, 1994; Scopel, Ballare & Radosevich, 1994). Two factors determine why the light intensity during cultivations influence the germination and the final number of emerged weeds. The first is the extreme light sensitivity of a proportion of the soil seedbank, which means that these seeds are induced to germinate if exposed to a few milliseconds of full daylight (Scopel, Ballare & Sanchez, 1991; Van Der Woude, 1985). The other important factor is light penetration in soil. The intensity of daylight transmitted through the surface soil layers decreases rapidly and light intensity in 0 1995 Association of Applied Biologists

562 PETER KRYGER JENSEN

a few millimetres depth in normal soil types is too low to induce germination in even the most light sensitive seeds (Bliss & Smith, 1985; Woolley & Stoller, 1978; Kasperbauer & Hunt, 1988; Mandoli et ul . , 1990). The potential emergence depth of most common weed species is below this depth (Chancellor, 1964; Froud-Williams, Chancellor & Drennan, 1984). A proportion of the seeds which both before and after a cultivation are situated below the depth where light can penetrate the soil, will be directly exposed to the light conditions prevailing at the soil surface during the cultivation process when a tine or plough ‘opens up’ the soil. Light induction in this way is an either/or action whereas induction of seeds in undisturbed soil is a gradually advancing process, where the effect of red light on the phytochrome equilibrium is counteracted by the thermal dark reversion process (Toole, 1973). The position of seeds in the soil determines whether the content of physiologically active phytochrome, Pf,, increases and consequently after a time interval reaches the threshold value where germination starts. This means that one would expect different emergence patterns of seeds which are induced to germinate after a flash induction compared to seeds induced by integrating a longer lasting low intensity irradiation, as hypothesised by Tester & Morris (1987) and Mandoli et al. (1990) and shown by Jensen (1992).

This paper includes an investigation demonstrating this difference in emergence pattern for two weed species. It also describes field experiments examining the influence of cultivation intensity on germination and emergence of Chenopodium album L. and the influence of light environment during soil tillage on potential depth from which emergence may occur.

Materials and Methods

Pot experiment Seeds of Matricuria inodora L. and Stellaria media L. were collected from plots in the

field in 1990 and were divided into perforated plastic bags with 200 seeds mixed with 50 g sterilised soil in each. The plastic bags were buried at a depth of 300 mm in the field in December 1990. One litre pots were filled with a soi1:peat mixture (2: 1 w/w) containing all necessary nutrients to within 20 mm of the top before the germination test was carried out. The germination test was initiated in the late evening on 9 April 1991, when the plastic bags were taken from the field at night and handled in darkness using starlight amplifiers. This equipment amplifies the low background radiation during the night to a level where it is possible to see by the human eye. Half of the pots were sown in darkness, where the content of the bags (seeds in sterilised soil) were dispersed on the top of the subsoil, and covered with either 4 or 8 mm of sterilised soil. The other half of the pots were sown the same way the following morning in daylight. The experiment included 12 replicates. All pots were placed outside on a table and sub-irrigated by an automatic watering system. The emergence pattern was followed by regular counting of seedlings.

Field experiments Field experiments were carried out at the Department of Weed Control in Flakkebjerg

in 1992-1994, either in April or in September. The soil type at this site is a sandy loam. A seedbed harrow with a distance between the tines of 100 mm, and equipped with a rotovating crumbler, was used in all experiments. The harrow was mounted on a tractor with a wheel width of 2.5 m, which enabled the tractor to straddle the plots of 2.5 x 10 m. The tractor speed was 6 km h-’ (1.7 m s-l) in all operations, and a harrowing depth of 30-40 mm was

Germination of weeds in relation to light and soil tillage 563

used. The harrowing treatments were normally followed by a rolling. Light intensity was measured with a LI-190SA Quantum sensor connected to a LI-1000 datalogger, (LI- COR, 4421 Superior Street, PO Box 4425, Lincoln, Nebraska 68504, USA) measuring photosynthetic active radiation, PAR, (400-800 nm) in the range above 0.0001 pE m-2 s-'. Daylight treatments were carried out when PAR exceeded 1000 pE m-* s-l and night- or dark treatments were carried out when PAR was below 0.0001 pE rnp2 s-'.

In all field experiments, emerged plants were killed with glyphosate (RoundupR, Mon- santo-Searle) prior to the experimental treatments. Three experiments on cultivation intensity were carried out in April 1993 (trial A) and 1994 (trial B & C ) in a field where seeds of C. album L. were sown the preceding September. A randomised complete block design with eight replicates per treatment was used. Counts of emerged seedlings in the field experiments were done using 4 X 0.25 m-2 quadrats per replicate.

A fourth field experiment investigating germination depth of seedlings was carried out in September 1992 in a field infested primarily with S . media. Germination depth was deter- mined in the experiment by digging up 25 seedlings per replicate in selected treatments and measuring the length of the hypocotyl from the seed to the soil surface. A randomised complete block design with four replicates per treatment was used.

Statistics Data from field experiments examining number of harrowings in daylight were fitted to

an exponential function:

SL = X - Y * EXP (- Z * No. of harrowings) (1) in which SL is number of emerged seedlings, X is the asymptotic maximum of seedlings at infinite number of harrowings, (X - Y) is the intercept or the number of seedlings emerged at zero harrowings, and Z determines the slope of the curve. Data were also fitted to a simple linear regression function (2). The two models were compared with respect to their accuracy in describing the experimental data by an F-test:

(RSS2 - RSSl)/(DF2 - DF1) RSSl/DFl

in which RSS and DF is residual sum of squares and degrees of freedom and the numbers 1 and 2 refer to the above mentioned models.

F =

Results and Discussion

tnteraction between light regime during soil disturbance and seed placement in the soil

Pot experiment Figs 1 and 2 show the number of emerged seedlings in the pot experiment as a function

of time. It is evident from the figures that the assessments should have continued longer, especially with M . inodora. It was clear, however, that light during the sowing operation stimulated the emergence of seedlings. This is most easily interpreted from the results with S. media. In both sowings in the light the number of emerged seedlings were similar, and greater than those which emerged from sowings in the dark. Light-sensitive seeds which were exposed to light during the short sowing operation germinated very uniform and rapidly. The emergence rate of seeds sown in darkness at 8 mm depth was slower and there

564 PETER KRYGER JENSEN

0 10 20 30 50 Days after sowing

Fig. 1. Emergence pattern of Sfellark media after sowing at two soil depths and in different light conditions. Prior to sowing. the seeds were stored for four months in a field at a depth of 300 mm. The seeds were taken from the field at night and handled in darkness until the sowing in pots, where the sceds were covered with either 4 or 8 mm sterilised soil. Vertical bars represent LSD ( P = 0.05). Sowing conditions: Light, 4 mm depth; -I-- Light, 8 mm depth; -4- Dark, 4 Inn1 depth; -0- Dark, 8 mm depth.

was a lower maximum emergence. These seeds had at no moment during preparation received light, and any seed with a light requirement therefore, can only have been induced to germinate by light penetrating the soil. Investigations on light transmittance in soil, reviewed by Tester & Morris (1987), showed that this is very unlikely to happen to any great extent in normal soil types at 8 mm depth. One could argue that the seedlings emerging in this treatment originated from a non-light requiring fraction of seeds but this requires further investigation.

Seeds sown in darkness at 4 mm depth followed quite a different emergence pattern. This can be explained as a combination of the emergence rate of seedlings in the dark sown treatment 8 mm, with additional emergence of seedlings originating from seeds induced to germinate by light penetrating the soil. Whereas emergence had almost ceased in the other treatments at the last assessment date, there was still vigorous emergence in this treatment. This indicates that seed placed at this depth integrated the weak light signal and, after some time, individual seeds exceeded the threshold content of active phytochrome, Pfr, which triggers their germination. Since differences between the dark treatments started at about 24 days after sowing, it required up to 2-3 wk or more after the sowing before the seeds reached their threshold levels of Pfr and started germination from this depth.

Consequences for field conditions Under field conditions, seeds might be placed more or less uniformly at all depths, and

a gradient of responses will be obtained. The light intensity from penetrating light decreases with increasing soil depth, and the time when the critical Pfr content is achieved will increase

Germination of weeds in relation to light and soil tillage 565

90-

80 -

70-

,” 40- 0

B 30-

0 10 20 30 40 50 Days after sowing

Fig. 2. Emergence pattern of Matricaria inodora after sowing at two soil depths and in different light conditions. Prior to sowing, the seeds were stored for four months in a field at a depth of 300 mm. The seeds were taken from the field at night and handled in darkness until the sowing in pots, where the seeds were covered with either 4 or 8 mm sterilised soil. Vertical bars represent LSD ( P = 0.05). Sowing conditions: -+- Light, 4 mm depth; -*- Light, 8 mm depth; -#- Dark, 4 mm depth; -0- Dark, 8 mm depth.

until a certain depth is reached, where the effect of light on the phytochrome system is neutralised by the thermal reversion process. Light sensitive seeds placed below this critical depth can only be stimulated to germinate if induced during a soil disturbance which provides an adequate amount of light photons to enable them to reach the critical P, content. The expected effect of carrying out soil tillage operations in the field under different light conditions is therefore much more diffuse than under controlled conditions. After a soil cultivation in darkness one would expect germination of seeds without a light require- ment, and shallow placed seeds with a light demand which can be fulfilled from light penetrating the soil. If the same cultivation was carried out in the field in daylight one would expect an additional fraction of germination originating from seeds which during the short moment where the implement has ‘opened up’ the soil, exposing them directly to the prevailing light conditions, have received sufficient light for germination to be triggered. Furthermore, after a cultivation in daylight, one would expect immediate induction of a proportion of the shallow placed seeds. Therefore in the field a lower total germination and emergence of weed plants, and a delay in emergence time of some of the weeds, is expected after a soil cultivation in darkness, compared to the same treatment in daylight. This is in fact what has been found in investigations in recent years (Jensen, 1992; Kuhbauch et al., 1992; Freiburghaus & Hani, 1993). In the investigation by Jensen (1992), there was approximately 3 days’ difference between 50% emergence after sowing in daylight and after night sowing.

Relationship between cultivation intensity and weed germination The fact that light induction of weed seeds during the cultivation process is a major factor

566 PEIER KRYGER JENSEN

No. of plants

1 A.

No. of plants ,800

700

600

500

400

300

200

100

0

B.

0 5 10 15 20 25 No. of harrowings

Fig. 3 . Observed values and predicted curve for the relationship between number of harrowings in daylight and emergence of Chenopodium album plants per m - 2 in three field trials. 3A represents one trial in 1993, 3B and 3C represents two trials in 1994. 0 represents daylight treatments, * dark treatments.

Germination of weeds in relution to light and soil tillage 567

No. of plants

400 1 I

C.

0 , , , , , / , / , l , ) , , , , , l , , , . , l , , , , , (

0 5 10 15 20 25 No. of harrowings

Fig. 3-continued.

influencing the germination of weeds after a soil disturbance raises the following question. If a proportion of the seeds are light induced by a single disturbance, then one would expect that a second disturbance with the same intensity would induce the same proportion of the remaining light-sensitive seeds. That is, with an increasing number of shallow soil dis- turbances in daylight, an exponential relationship between number of disturbances and germinating seeds would be expected, whereas a constant germination would be expected with increasing numbers of disturbances in darkness.

This hypothesis was tested in three experiments where 0-16 shallow harrowings were carried out either in daylight or at night. Increasing number of cultivations at night was only included in one of the experiments. These experiments were carried out in a field where seeds of C . album L. were sown the preceding year. The observed results, and the predicted relationship, are shown in Fig. 3 . The parameter estimates and confidence limits of the predicted relation are shown in Table 1, which also includes the actual intercept values and the intercept values as a percent of the asymptotic maximum.

The exponential model reduced the RSS significantly ( P < 0.05) compared to the linear model for all three experiments. The intercept values as a percent of the asymptotic value expresses the predicted emergence of plants at zero harrowings as a percent of emerged plants at an infinite number of harrowings. This value ranged between 5% and 10%. If the asymptotic value is interpreted as all seeds potentially capable of germinating and emerging, then it can be concluded that approximately 90-95 % of the potentially germinable seeds in these experiments needed a soil disturbance in order to break dormancy and germinate.

As the seedbed quality varies greatly between treatments with zero or two harrowings, the difference in emergence could reflect situations where the soil cultivation changed some other factors of importance for germination and establishment of plants than light exposure, such as soil aeration (Roberts, 1972; Karssen & Hilhorst, 1992) and humidity (Roberts & Potter, 1980). However, it is believed that the relationship obtained in Fig. 3 is principally

568 PETER KRYGER JENSEN

Table 1 . Parameter estimates with asymptotic standard error in parenthesis and intercept oalues ( I ) for rhe predicted relation between harrowing intensity in daylight and emergence of Chenopodium album plants. I / X denotes the number of plants at zero harrowings in percent of the asymptotic maximum at infinite number of harrowings. A , B and C refers to

Fig. 3 Exp . X Y Z i I/X

A 153.5 136.9 0.14 16.6 10.8 (31.2) (29.2) (0.06)

B 653.2 591.9 0.13 61.3 9 .4 (117.3) (109.3) (0.05)

C 370.5 354.4 0.09 16.1 4.3 (87.5) (81.7) (0.04)

1

2

3

13

14

15

16

17

18

19 I >20

0 10 20 30

No. of plants

Fig. 4 . Effect of light intensity during soil disturbance on germination depth of weeds in the field. The Figure shows the mean length of the hypocotyl from the seed to the soil surface. 150 plants were assessed in each treatment. Stellaria media was the dominating species. Std dev = 1.43. H 2 Harrowings in daylight; Ul 2 Harrowings in darkness.

Germination of weeds in relation to light and soil tillage

1

2

3

4

5

6

569

h

E E v

5 2 10

8 11

€

-0

.- 0-

.9 12

8 13

14

15

16

18

19

>20 I

0 5 10 15 20 25 No. of plants

Fig. k o n t i n u e d . U 5 Harrowings in daylight; Ql 5 Harrowings in darkness.

a matter of an increasing number of seeds being light-induced by increasing harrowing intensity. It is predicted that four, five or seven (Figs 3A, 3B and 3C respectively) harrowings with this implement would induce germination in approximately 50% of the potentially germinable seeds. With another implement, or at another speed with the implement used, one would assume the same curve, but with different ;lumbers of disturbances to achieve the same emergence pattern.

There was an increase in the number of emerged plants between zero and four cultivations in the dark whilst thereafter the number was constant (Fig. 3C). Again the increase in emergence after a few cultivations could reflect situations where the soil cultivation changed some other factors of importance for germination and establishment of plants rather than light exposure alone.

This relationship between soil disturbance intensity and weed emergence, exemplified by the results with C. album, suggests that the result of experiments comparing cultivations in light and darkness is highly dependent on the chosen cultivation intensity and implement type.

570 PETER KRYGER JENSEN

Relationship between light environment during disturbance and germination depth The difference in germination and emergence between disturbances in daylight and in

darkness should in theory be due to deeper placed seeds induced to germinate by the short light flash when the tine opens the soil in daylight. This was tested in an experiment where two and five harrowings were carried out either in daylight or at night, and where the length of the hypocotyl from the seed to the soil surface was measured (Fig. 4). The figure shows the frequency distribution of hypocotyls from 2.5 plants per replicate. A chi-square (continuity adjusted) test was performed with data divided in two groups according to whether hypocotyl length was above or below 6 mm length. The division in groups has no theoretical foundation but is based on an evaluation of the results in Fig. 4. The test confirmed that both for two and five cultivations there was a significant ( P = 0.01 respectively P = 0.05) deviation from the expected frequency if light conditions did not influence germination depth. The distribution follows that theoretically expected with more plants with long hypocotyls (deep germination depth) in the daylight cultivated plots, and a significant difference ( P = 0.0.5) in mean hypocotyl length between day and night cultivated treatments. Mean hypocotyl length after two and five cultivations respectively were 7.2 mm and 7.7 mm in daylight treatments and 6.0 mm and 5.9 mm after night cultivation.

Conclusions A relationship between soil disturbance and seed germination in the field has been shown

in many investigations. I t is, however, only recently that light has been ascribed a role in this relationship. This paper elucidates some of the effects which different light intensities during the cultivation process exert on light-sensitive seeds in the soil. The results raise some questions related to the time of soil tillage operations in integrated weed control strategies. The effect of the delay in seedling emergence time when soil disturbance is carried out in darkness has to be quantified in relation to competition in crop situations. The effect of delaying the emergence time of the weeds relative to that of the crop on direct weed control measures in the crop needs to be ascertained.

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(Received 25 July 1995)