Germination of grain amaranth (Amaranthus hypochondriacus × A. hybridus): effects of seed quality, temperature, light, and pesticides

European Journal of Agronomy ELSEVIER

European Journal of Agronomy8 (1998) 127-135

Germination of grain amaranth (Amaranthus hypochondriacus x A. hybridus): effects of seed quality, temperature, light, and pesticides W. Aufhammer *, D. Czuczorova, H.-P. Kaul, M. Kruse University of Hohenheim, Institute for Crop Production and GrasslandResearch, Fruwirthstrafle23, D-70599 Stuttgart, Germany Accepted 23 July 1997

Abstract

Seven experiments were conducted in incubators to determine the effects of several factors on the germination of two amaranth cultivars. Investigated factors were year of harvest, crop type of the mother plant, seed position on the mother plant, stage of maturity, temperature, light, and seed dressing. Percentage germination and germination speed were recorded. Most effects appeared in interaction with cultivars. Percentage germination was above 80% with temperatures higher than 16°C, The germination speed was closely related to temperature and decreased with decreasing temperature. Light and even short illumination inhibited germination and slowed it down at temperatures below 25°C, presoaking accelerated germination. Seed dressing with the fungicide Dichlofluamid had the desired effect on fungi, but the suceptibility of genotypes to the insecticide Imidachloprid differed. The portion of dormant seeds increased during the course of seed ripening. Seed storage of more than 1 year decreased the percentage germination. An early harvest of homogenous and dense amaranth crops is recommended for amaranth seed production. More research is needed concerning presowing treatments of amaranth seeds. © 1998 Elsevier Science B.V.

Keywords: Grain amaranth; Germination; Seed quality; Seed dressing; Environmental conditions

1. Introduction

The high nutritional value of amaranth grains (Amaranthus spp. L.) supports recent interest in using species from this genus as grain crops all over the world (Kauffman, 1992). Depending on the soil and weather conditions, problems concerning field emergence lead to a high risk in amaranth cultivation (Aufhammer et al., 1995). The quality of very small seeds (grain weight: 0.6-0.8 mg) and the conditions for germination should therefore be * Corresponding author. Tel: +49 711 459 2381; Fax: +49 711 459 2297; e-mail: [email protected] 1161-0301/98/$19.00 © 1998ElsevierScienceB.V. All rights reserved. PII S1161-0301(97)00049-X

'optimal' to minimize the risk in establishing amaranth crops. The seedling emergence rate of amaranth is crucial in establishing productive crops under field conditions (Putnam, 1990). A high percentage germination is a prerequisite for a high percentage of emergence and, finally, for high grain yield. Sufficiently high plant densities are also important, as are uniform development and maturation of individual crop plants. These factors play a major role in determining harvest dates and in reducing grain losses before and at harvest (Aufhammer et al., 1995). Seed quality strongly influences the field emergence of amaranth. The conditions of seed ripening

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and the length of storage period seem to have a very specific influence on the germination of A. hypochondriacus. Three months after harvest, Hirose et al. (1991) measured a higher percentage germination for seeds than 15 months after harvest. In contrast, Kwack and Kang (1985) found a higher percentage germination with 1-year-old seeds in comparison with 2- and 3-year-old seeds. After harvest, a high percentage of the seeds were dormant. Bartolini and Hampton (1989) examined the percentage germination of A. cruentus from peak flowering to harvest. It did not exceed 25% at any time, and the dormancy could not be broken by prechilling or KNO3. In the first period of storage, dormancy seems to break down, later on the seeds are aging and their viability is decreasing. Other factors influencing seed quality are the growth conditions for and the shape of the mother plant, the stage of maturity and the seed's position on the inflorescence of the mother plant (Gray and Thomas, 1982). Seed dormancy of A. retroflexus is increasing as the age of the mother plant increases (Kigel et al., 1979). Due to the length of the vegetation period of amaranth, a reasonable sowing date in Germany is mid-May, when the mean air temperature is about 12°C. For A. hypochondriacus, Hirose et al. (1991) measured more than 80% germination, provided that the temperature was above 19°C. Consequently, the percentage and speed of emergence of A. hypochondriacus seedlings are best at temperatures of between 20 and 35°C (Webb et al., 1987). As this range of temperatures is hardly reached during May in Germany, high rates of field emergence are unusual (Aufhanmler et al., 1995; Kruse, 1996). The authors recommended the time to emergence to be as short as possible to reduce the risk of upsilting and crusting of the soil surface before emergence. Both processes prevent most of the seeds from emerging (National Academy of Sciences, 1984). Webb et al. (1987) and Aufhammer et al. (1994) stated that all field conditions affecting field emergence should be favourable to overcome the effect of temperature. A shallow sowing depth is especially important. However, shallow sowing is limited by the fact that the germination of amaranth is inhibited by light at temperatures below 25°C (Kwack and Kang, 1985; Gutterman et al., 1992).

The present study was conducted to investigate methods to overcome problems in establishing amaranth crops. As a first step, the reaction of modern, high-yielding cultivars from USA to temperature and light on percentage germination and on germination speed of amaranth was examined. The uneven maturation of seeds (plant development, occurence of branching) as well as seed dormancy are problems. To examine this, the effects of the length of storage period, stage of maturity, shape of the mother plant and the position of the seed on the mother plant, which have to be taken into account when producing seeds, were determined. Additionally, the possibilities to enhance seed quality by seed treatments (dressing, pre-soaking) were checked.

2. Materials and methods

2.1. Experimental conditions and measured traits Seven factorial experiments (I-VII) were conducted in incubators in 1995 and 1996, each one with four replications (Table 1). They included the amaranth cultivars K 343 and K 432, which are selected crossings between A. hypochondriacus and A. hybridus bred in the USA. Seeds were collected from field experiments conducted on the experimental farm Ihinger Hof near Stuttgart in southwest Germany between 1992 and 1996. After harvest, the seeds were ventilated until the water content was below 16% (w/w) and then stored in a refrigerator at 4°C. Germinations were varried out in 90-mm-diameter Petri dishes, on which three filter papers (Schleicher and Scht~ll 595) were placed, in four replicates. The papers were saturated with distilled water and 50 seeds placed on each Petri dish. The Petri dishes were closed and placed into incubators at the appropriate temperature (cf. Table 1). With the exception of experiment II, in which light was an investigated factor, the incubators were kept dark, and seeds were illuminated every day for about 10 min only during the counting of the seedlings. After counting, the filter papers were re-saturated with distilled water. The first counting was carried out 48 h after the start of each experiment and subsequently every 24 h. A seed was considered to

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Table 1 Design and details of experiments Experiment

I

II

III

IV

V

VI

VII

Cultivar

K432 K343 1992

K343 K432 1994

K432 K432 1994

K343 K343 1994

K432 K432 1995

K343 K343 1995

K432

Normal

Normal

Normal

Normal

Normal

Seed position All mixed on mother plant

All mixed

All mixed

All mixed

Normal Bushy Small All mixed

Seed fraction All mixed

All mixed

All mixed

All mixed

13°C 16°C 35°C

7°C 13°C 35°C

Short light

Short light

Short light

None

None

4 September 1996

4 September 1996

Year of harvest

1994 Shape of Normal mother plant

Temperature

Light conditions Seed treatment Start of experiment

7°C 13°C 10°C 35°C 13°C 16°C 19°C 35°C Short light Shortlight Permanent light Dark None None 10 August 4 September 1995 1995

Main inflorescence Main inflorescence Otop branches •middle •bottom branches Harvested on Grains scattered Harvested on • 2 September • after harvest • 2 September • 12 September • after ventilation • 12 September • 23 September threshed grains •23 September 13°C 7°C 13°C

Short light Short light

None None None Presoaked Euparen Gaucho 9 October 4 September 8 February 1995 1996 1995

be g e r m i n a t e d w h e n its radicle h a d g r o w n longer t h a n 3 m m . A f t e r counting, the g e r m i n a t e d seeds were r e m o v e d f r o m the Petri dishes. I f fungi were visibly g r o w i n g on the surface o f one o f the seeds, this was r e c o r d e d , a n d the seed was r e m o v e d i m m e d i a t e l y to a v o i d fungi g r o w i n g all over the Petri dish a n d d i s t u r b i n g the experiment. T h e species o f the fungi were n o t specified. A l l experiments, except e x p e r i m e n t V, were c a r r i e d o u t over 14 days. D u r i n g the first 2 weeks o f e x p e r i m e n t V, a l m o s t no seeds germinated, so this e x p e r i m e n t was t e r m i n a t e d after 18 days. The r e c o r d e d p a r a m e t e r s were: (1) the p e r c e n t a g e g e r m i n a t i o n on each Petri dish at the e n d o f the e x p e r i m e n t a n d (2) the g e r m i n a t i o n index.

1996

The

l a t t e r was defined a c c o r d i n g to E d w a r d s

(1934) as

(~ x N3 G e r m i n a t i o n i n d e x - i= 1 (Ni) i=1

w h e r e T~ = t h e n u m b e r o f d a y s after the start o f the experiment, N ~ = t h e n u m b e r o f seeds germin a t e d o n d a y Ti, a n d c = the length o f the experim e n t m e a s u r e d in days. This index is r e c o m m e n d e d b y N i c h o l s a n d H e y d e c k e r (1968) for m e a s u r i n g the speed o f germination. A n analysis o f v a r i a n c e was carried o u t to

130

W. Aufhammer et al. / European Journal of Agronomy 8 (1998) 127-135

examine the main effects and interactions of the investigated factors on these two parameters. Comparisons of means were made using Fisher's least significant difference (LSD) test at the 0.05 probability level.

2.2. Experimental design An overview of the design of the experiments is given in Table 1. The temperature in the incubators was kept constant during the duration of the experiments. Light conditions were varied in experiment II by keeping Petri dishes in the light during the whole experiment ('permanent light'), or putting them into a box and counting them only at the end of the experiment ('dark'). Therefore, the germination index could not be calculated on the 'dark' level. In experiment III, the seeds were presoaked to accelerate germination as described by Mandal and Basu (1987) for wheat. The seeds were watered for 2 h at 25°C in aqua dest. and redried afterwards by ventilation. Previously, 2 h was found to be the optimum pretreatment duration. Other important problems of crop establishment are the heavy infestation of the seed coat with fungi in many seed lots and the attack by insects of the seedlings after emergence (Copeland and McDonald, 1995). From our experience, we know that the former prevents seed germination, and the latter very often kills the seedlings, even when the very small seedlings are only slightly damaged. To prevent this, the seeds were dressed with Euparen (active ingredient: Dichlofluamid) or Gaucho (active ingredient: Imidachloprid) in experiment IV. The dressing was applied by spraying the seeds with an aqueous solution of 1% Euparen or 50% Gaucho, respectively. An indication of the dose on a per-seed or a per-weight basis was not possible. The factors concerning the conditions of seed production were tested in experiments V-VII. In experiment V, seeds of three different field experiments were used. These experiments were carried out on the experimental station Ihinger Hof in 1995, but due to different crop densities together with different soil conditions and sowing dates, they resulted in plants with a very different shape. Optimal soil and weather conditions after sowing

resulted in a homogenous crop with a density of 34 plants m-2 in an experiment described by Kaul et al. (1996). These plants were indicated as 'normal'. During another experiment (described by Kruse, 1996), which was sown a few days later, the crops could not emerge before a long period of rain. The crops had a density of about 18 plants m -2, but the plant distribution was not uniform. Consequently, the plants of this experiment were very 'bushy', with many branches and presumably with an inhomogenous seed maturity. A third (unpublished) experiment was conducted on a poor soil with a very high crop density (570 plants m-2). The plants were very 'small' with small inflorescences. This should have resulted in a homogenous maturity within the inflorescence. The stage of maturity was included in experiment V by collecting the seeds, which were scattered after shaking the plants at the time of harvest or after ventilation, respectively. These fractions were compared with the seeds threshed out after ventilation. The fractions should represent different stages of maturity. Another method to separate the seeds by the stage of maturity was chosen in experiments VI and VII, i.e. the harvest at different dates. As seed dormancy is built up during maturity, the first plants were harvested when a scattering of seeds could be observed (2 September). Subsequent harvests were carried out at 10-day intervals. The plants were separated into main inflorescence and branches to obtain information on the development of seed quality as affected by the seeds' position on the mother plant (experiment VI). The lowest ten branches of the main inflorescence were cut and divided into three parts: top, middle and bottom. These fractions were tested in experiment VII.

3. Results

3.1. Cultivars The quality of the seedlots used in the experiments depended on the year of harvest and on the cultivar. Significant effects of the factor cultivar on the percentage germination were found in all experiments in which it was tested. Furthermore,

W. Aufhammer et al. / European Journal of Agronomy 8 (1998) 127-135

131

many interactions of cultivar with other factors were detected (see below).

Table 3 Germination (%) and germination index (d) as affected by the interaction cultivar x harvest date in experiment VI

3.2. Mother plant and stage o f maturity

Parameter

Percentagegermination Germination index

The fact that the environmental conditions for the mother plant had a specific influence on the germination was confirmed by the significant interaction between the cultivar and the year of harvest in experiment I (Table 2). The figures also show that storage of amaranth seeds longer than 2 years reduced their viability. The plant shape also had an influence on the germination of K 432 seeds. In experiment V, the seeds borne on inhomogenous crops with bushy plants showed a significantly lower percentage germination (55.3%) than those borne on plants with normal (74.6%) or small inflorescences (77.8%; LSDs~: 13.9%). However, this factor had no significant effect on the germination index. In experiment V, there was no significant effect of the seed fraction on germination. However, the method of shattering for dividing the seeds into fractions with different stages of maturity may not have been suitable for this purpose, but there was a significant effect of the date of harvest on germination experiments VI and VII. The interaction between harvest date and cultivar in experiment VI is shown in Table 3. The percentage germination decreased with increasing maturity. The differences between the cultivars presumably refer to different stages of development. The same effect was observed in experiment VII (data not shown). The percentage germination of seeds from the main inflorescence and of those from the branches differed significantly in experiment VI. Only 42% of the seeds from the main inflorescence were

Cultivar/ harvest date

K 432

Table 2 Germination (%) as affected by the interaction cultivar x year of harvest in experiment I

2 September 62.0 12 September 32.8 23 September 20.5 LSDs~

K 343

K 432

K 343

61.8 74.0 47.3

4.56 4.13 4.37

4.48 3.99 6.57

8.80

0.519

germinating, but 57% of those from the branches (LSDs~: 5.8%). The germination index did not differ. The part of the branches, on which the seeds were borne, did not have a significant main effect on germination, but there was an interaction with the harvest date (Fig. 1 ). Whereas the percentage germination of the seeds from the top and the middle part of the branches decreased with time, that of the seeds from the bottom part remained constant. The position of the seeds on the branch did not have any significant effect on the speed of germination, which corroborated the results of experiment VI. 3.3. Conditions during germination Fig. 2 underlines the importance of temperature for the germination of a m a r a n t h seeds independent

~oo

Ip ] osition of seeds on the branch

8o

6o

....

ILsD,, ~

40

2O

Cultivar

Year of harvest 1992

1994

32.8 16.2

69.0 57.7

o

September 2 nd

K432 K343 LSD5%

3.76

September 12'"

S e p t e m b e r 23'"

Fig. 1. Percentage germination as affected by the interaction date of harvest x position of seeds on the branch in experiment VII.

W. Aufhammer et aL / European Journal of Agronomy 8 (1998) 127-135

132 100 I

Table4 Germination(%) and germinationindex(d) as affectedby the interaction temperaturex light conditionsin experimentII

Temperature

80

Percentage germination

Temperature/ light conditions

13°C

35°C

13°C

35°C

Light Short light Dark LSD5%

7.0 56.0 74.8

89.3 88.0 74.8

4.98 2.26 --

2.75 2.26 --

60

o= '~ 40

20

K 432 1992

K 432 1994

K 343 1992

Germinationindex

Parameter

7.69

1.555

K 343 1994

Fig. 2. Percentage germination as affected by the interaction cultivar x temperature x year of harvest in experiment I.

of the length of the storage period. The germination of seeds harvested in 1994 was above 80%, provided that the temperature was >16°C. Differences between the cultivars occurred, especially at lower temperatures. The seeds stored for 3 years did not germinate at percentages higher than 60% (K 432) or 40% (K 343), respectively. There was also a strong influence of temperature on the speed of germination (Fig. 3). By increasing the temperature from 7°C to 19°C, the germination index decreased from 12.0 to 3.1 days (K 432) or 4.5 days (K 343), respectively. The temperature had a strong influence on the reaction of amaranth seeds to light, as shown by the interaction between both factors in experiment II (Table 4). There was 14r

no significant difference in the percentage germination between seeds exposed to 13°C and 35°C, when they germinated in the dark. A short time of illumination (about 10min) each day during the counting period reduced germination significantly when the temperature was 13°C. With this temperature and continuous illumination, the percentage was less than 10% of that in the dark. At the 35°C temperature level, this effect did not appear. The lower percentage germination in the dark at 35°C may be due to the mould appearing on 28.5% of the seeds of K 432, especially in the dark. With illumination, only 18% were moulded. The microclimate in the dark box into which the seeds were placed during the experiment may explain these moulding differences. The seeds of cultivar K 343 from 1994 were hardly infected by fungi. A similar light x temperature interaction was found on the germination index (Table 4). 3.4. Seed treatments

12 ~10 -o .E 8

4

0

K 432

K 343

Fig. 3. Germination index as affected by the interaction

cultivarx temperaturein experimentI.

In experiment IV, the seed dressing with the fungicide Euparen and the insecticide Gaucho affected germination significantly. Gaucho decreased and Euparen enhanced the germination index (Table 5). The percentage germination was influenced differently, depending on the genotype and temperature. The percentage germination of K 432 was not significantly affected by Gaucho, but enhanced by Euparen. In contrast, both treatments decreased the germination of K 343, but the effect of Euparen was stronger. With the infested seeds of K 432, the percentage attacked by fungi was reduced by the Euparen treatment from 7.3%

W. Aufhammer et al. / European Journal of Agronomy 8 (1998) 127-135 Table 5 Germination (%) as affected by the interaction cultivar × treatment and temperature × treatment, and germination index (d) as affected by treatment. All data from experiment IV Parameter Percentage germination

Percentage germination

Treatment K 432

K 343 7°C 13°C 35°C

None Gaucho Euparen LSDs~

55.8 40.3 49.8

47.8 53.5 59.5 5.93

11.5 56.0 85.5 4.8 50.5 8 5 . 8 6.5 65.5 91.7 7.26

Germination index

6.24 6.94 6.87 0.338

Table 6 Germination (%) as affected by the interaction cultivar × presoaking treatment in experiment III Cultivar

K 432 K 343 LSD5%

Treament Pre-soaking

None

67.8 64.7

77.8 62.0 6.87

to 0.3%. With the healthy seeds of K 343, no effect was found. From the results in Table 5, it can also be seen that the percentage germination was only significantly affected at 13°C. The interaction between temperature and seed dressing shows that the the germination index was increased significantly only at 35°C (data not shown). The pre-soaking treatment increased the speed of germination significantly (pre-soaked seeds: 3.7 days, untreated: 4.3 days; LSD5%: 0.27 days). Table 6 shows that the pre-soaking treatment decreased the percentage germination of the seeds of cultivar K 432, but not of K 343. These results were in agreement with former, unpublished experiments.

4. Discussion

All germinations were done between late summer and winter, i.e. not at the normal sowing time. If the amaranth seeds should have a 'biological clock' limiting their time of germination, this

133

could have biased the results. However, the invariable high percentages germination at 35°C indicate that environmental effects were much more important than an internal germination cycle. Figs. 2 and 3 confirm the strong influence of temperature on the germination of amaranth, which has been described in earlier studies using other species of Amaranthus (Oladiran and Mumford, 1985; Hirose et al., 1991; Gutterman et al., 1992). Experiment I additionally revealed the decrease of germination speed along a decreasing temperature range. Table 7 presents the functional relation between temperature and germination index, which differed between genotypes but not with the year of harvest. Years had no significant effect on the parameters of the equation. The strong relations underline the importance of temperatures above 16°C not only for obtaining a high percentage germination but also for speedy germination and thus for a low risk of emergence in the field. The observed reaction to light of A. hypochondriacus × A. hybridus crossings described here was the same as reported by Kwack and Kang (1985) with A. hypochondriacus and by Kendrick and Frankland (1969) and Gutterman et al. (1992) with A. caudatus. The inhibition of the germination by light at temperatures lower than 25°C seems to be characteristic for all Amaranthus species and cultivars. Due to limited possibilities of solving the emergence problems by control of the environmental conditions in the field, it is most important to harvest seedlots of high quality. In experiments VI and VII, it was shown that an early harvest can result in a high germinability. Seed dormancy seems to develop during the last stage of maturity. This is in contrast with the results of Bartolini and Hampton (1989) who found an increasing percenTable 7 Functional relationship between germination index (IND) and temperature (T) for the genotypes K 432 and K 343 in experiment I Cultivar

Equation

K 432

In(IND) =4.775 - 1.154 ln(T) r 2= 0.94 ln(IND) =4.499-1.003 In(T) r 2 = 0.96

K 343

134

~ Aufhammer et al. /European Journal of Agronomy 8 (1998) i2~135

tage germination from flowering to harvest in A. cruentus, but this did not exceed 25%. The physiological background of the dormancy does not seem clear as yet. The experiments of Hirose et al. (1991) showed that seeds were still dormant 3 months after harvest. However, according to the results of experiment I and those of Kwack and Kang (1985), a storage of more than 1 year lowers the seeds' germinability. The method of separating the dormant seeds from the non-dormant directly after harvest used in experiment V failed. Another factor of influence is the quality of the crop established to grow seeds. Dense, uniform crops result in a better germination than heterogenous ones with bushy plants (experiment V). This is confirmed by the finding that the seeds from the branches showed a lower germination rate than those from the main inflorescence. Bartolini and Hampton (1989) reported similar results. At all temperatures, the speed of germination was enhanced by pre-soaking (experiment III). Although the final percentage germination was not improved, this can be regarded as an initial step for reducing the emergence risk in the field. Other methods of pre-soaking, osmoconditioning (Hofmann, 1992), stratification (Copeland and McDonald, 1995), or the use of growth regulators (Gray and Thomas, 1982) should be examined. Kendrick and Frankland (1969) stated that the dormancy is a secondary photodormancy induced by prolonged exposure to white- and far-red light. In this case, a light pretreament could perhaps be useful. Usually, the germination of small seeds is stimulated by red light. We found the opposite reaction of amaranth to be due to the temperatures, which are much cooler than those at the plants' natural habitat. It is not possible to draw general conclusions about seed-dressing from our experiments because only two pesticides were tested, each at only one concentration and with an unknown dose on a per-seed or a per-weight basis. Furthermore, the cultivars differed in their susceptibility to infection. Using the applied concentration of Euparen, the infestation of the seeds by fungi was reduced. The fungicide did not reduce the germination of K 343, but enhanced that of K 432. A decrease of insect attack against amaranth seedlings in the field after

seed dressing with Gaucho was observed by Kt~bler (1995, pers. commun.). Depending on the genotype, the applied concentration of Gaucho had an adverse effect on germination. At low temperatures, both dressings had a negative effect on the speed of germination. All these effects show that seed germination of amaranth can be affected by chemical substances but that interactions with cultivar and environmental conditions have to be taken in account.

5. Conclusions The numerous interactions of the factor cultivar with other factors underline the great variability within a single Amaranthus species in reaction to germination conditions. A breeding program to obtain cultivars that are more suitable for central European conditions with low temperatures during the sowing season could be promising. Another possibility would be to develop cultivars with a shorter vegetation period. This would allow a later sowing at higher temperatures. The ambiguity of cultivars of this type has been discussed by Kruse (1996). The light inhibition of germination at temperatures below 25°C makes it impossible to sow amaranth seeds on top of the soil when the temperature is below 25°C, even if there is enough water for this method. This had been proposed by Aufhammer et al. (1994) as a possible solution to the emergence problems. Following our results, a solution for the problem of dormancy seems to be possible by using only dense, uniform crops without branches for seed production and harvesting the seeds very early. However, this will lower the seed yield and will entail higher threshing losses and higher drying costs. Presowing treatments of the seeds may result in seedlots of higher quality. However, more investigations are needed for specific recommendations on such treatments.

Acknowledgment The authors acknowledge the skilful technical support of Mrs A. Rakosiova and Mrs G. Bloos

W. Aufhammer et al. / European Journal of Agronomy 8 (1998) 12~135

and the useful suggestions o f an anonymous reviewer.

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