Euphytica 126: 123–127, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
123
Temperature effects on seed germination and expression of seed dormancy in wheat J.M. Nyachiro1,∗ , F.R. Clarke2 , R.M. DePauw2 , R.E. Knox2 & K.C. Armstrong3 1 Alberta
Agriculture, Food and Rural Development, Field Crop Development Centre, Lacombe, AB, Canada, T4L 1W8; 2 Agriculture and Agri-Food Canada, SPARC, Box 1030, Swift Current, SK, Canada, S9H 3X2; 3 Agriculture and Agri-Food Canada, ECORC, Ottawa, ON, Canada, K1A 0C6; (∗ Author for correspondence, E-mail:
[email protected]) Received 30 September 2001; accepted 28 February 2002
Key words: preharvest sprouting, seed dormancy, seed germination, temperature, Triticum aestivum, wheat
Summary Because preharvest sprouting decreases quantity and quality of wheat grain, researchers need effective protocols to assess response to preharvest sprouting conditions. The aim of this study was to determine which temperature gives the greatest difference in seed germination and expression of seed dormancy in 10 spring wheat genotypes. The genotypes were grown in the field near Swift Current, Saskatchewan in 2000 in a randomized complete block with four replicates. Seed samples were harvested at approximately 25% moisture content (wet weight basis) and dried to 12% moisture content with minimal after-ripening. Germination was under controlled environment at temperatures of 10, 15, 20 and 30 ◦ C in darkness. A weighted germination index (WGI) was calculated. The analysis of WGI, for each temperature, showed highly significant (p ≤ 0.01) genotype effects on germination. Most genotypes decreased in WGI (increased dormancy) as temperature was increased from 10 to 30 ◦ C. The greatest differences in seed germination tended to be at 15 ◦ C and 20 ◦ C. The level of seed dormancy depended on the genotype and germination temperature.
Introduction Insufficient seed dormancy can contribute to preharvest sprouting of wheat. This undesirable trait negatively impacts grain producers and the processing industry in terms of yield losses, reduced test weight and undesirable chemical changes that can decrease grain quality. Frequently, white-seeded wheat genotypes are the most affected by pre-harvest damage (MacKey, 1976) although some white-seeded genotypes have relatively high levels of pre-harvest sprouting resistance (McCaig & DePauw, 1992; Wu & Carver, 1999). Wheat genotypes vary in resistance to pre-harvest sprouting and in the duration of seed dormancy (McCaig & DePauw, 1992; Reddy et al., 1985). Cultivars lacking seed dormancy do not have the ability to stop hydrated seeds from sprouting. This trait sets in as seeds are developing, and is carried through to maturity to be manifested even when the seed re-imbibes
water. Grains removed from a maturing plant of a sprouting-resistant genotype and placed under suitable germination conditions, which includes water, oxygen and temperature, do not sprout readily. In contrast, grains removed from a maturing sprouting-susceptible genotype and placed in optimum germination conditions sprout readily (Walker-Simmons, 1987). Seed germination increases as temperature rises, for most crop species. However, some crop species have higher percentage germination at lower temperature as recognized by Harrington (1923) who determined that low temperature regimes break seed dormancy in wheat. Temperature is one of the important environmental factors influencing the induction of seed dormancy during seed development and expression of seed dormancy during germination (Beldorok, 1968; George, 1967; Olsson & Mattson, 1976; Strand, 1980; Vegis, 1964; Weisner & Grabe, 1972). Low temperature (10 ◦ C) during seed development can induce
124 Table 1. Summary of spring wheat genotypes used for temperature and sprouting study Genotype
Inflorescence
Sprouting response
Reference
Kleiber OS59-15-8-20 RL4137 Snabbe AC Karma HY344 AUS1408 Janz SC8019-R1 SC8021-V2
Awnless Awned Awnless Awnless Awned Awned Awned Awned Awnless Awnless
Dormant Dormant Dormant Susceptible Susceptible Susceptible Dormant Susceptible Dormant Dormant
(Stoy & Sundin 1975) (Czarnecki 1983) (Czarnecki 1983) (Stoy & Sundin 1975) (Knox et al., 1998) (DePauw et al., 1995) (Mares 1992) (Mares 1992) (DePauw & McCaig 1989) (DePauw & McCaig 1989)
high and prolonged dormancy while low temperature during germination breaks dormancy of freshly harvested seeds. Intermediate germination temperature (20 ◦ C) allows different genotypes to express the degree of seed dormancy, and high temperature (30 ◦ C) only allows seeds of genotypes with very low level or no dormancy to germinate. However, the effect of temperature on sprouting in a wide range of wheat genotypes grown in diverse environments remains largely unexplored. The objectives of this study were: (1) to determine dormancy variation in 10 spring wheat genotypes, (2) to determine the effects of varying temperature on the germination of whole wheat seed, and (3) to determine the optimum temperature regime at which there were wide differences in seed germination among a set of 10 spring wheat genotypes.
This sampling criteria was used in all plots to decrease differential dormancy attributed to grain physiological maturity (DePauw & McCaig, 1991; Paterson et al., 1989). The spikes were instantly dried in a forced air dryer at 37 ◦ C for 72 h to decrease the grain moisture content to about 12%. The spikes were threshed by hand and the seed samples were kept at –20 ◦ C until needed, so as to reduce decay of dormancy resulting from after harvest seed maturation. For each genotype, a sample of 200 seeds per replicate was counted and placed in a large Petri dish with 10 ml of 1% ‘No Damp’ to surfacesterilize the seed. The contents were shaken on a bench top horizontal shaker for 20 minutes. After 20 min of shaking, the excess No Damp solution was poured off and the seed samples were rinsed three times using double distilled H2 O and placed into sterile labeled Petri dishes with two filter papers (12.5 cm diameter).
Materials and methods
Temperature levels and seed germination
Genotypes and field cultivation
Fifty of the surface-sterilized seeds from each plot per genotype were placed into Petri dishes, with the crease facing down. To each Petri dish, 6 ml of double distilled water was added. This volume kept the filter papers uniformly soaked-wet without flooding. The Petri dishes were covered and incubated in a germination cabinet at 10 ◦ C and approximately 95% relative humidity with no light for 8 days. This protocol was followed for three other temperature levels 15, 20, and 30 ◦ C. Each plate was checked daily, and seeds with radicles of at least 2 mm long and signs of visible pericarp rupture were counted as germinated and removed from the Petri dishes. A weighted germination index as described by Reddy et al. (1985) was calculated with maximum weight given to the seeds germinating
Ten spring-type wheat genotypes (T. aestivum L.) that differed for dormancy (Table 1) were grown in the field near Swift Current, Saskatchewan. The genotypes were cultivated in a randomized complete block with four replicates. Each genotype per replicate was planted in two rows of 3-m plots of about 400 plants. Every plot was separated from the next plot by a row of spring-planted winter wheat that stayed vegetative during the crop season. A sample of 120 spikes from each plot was harvested from the main or primary tillers that had yellow peduncles with no chlorophyll in the glumes. At this stage the grains had approximately 25% moisture content on a wet weight basis.
125 early and less to those germinating late [Eq. 1]. W GI = {8 × n1 + 7 × n2 + ...... + 1 × n8 }/8 × N (1) where n1 , n2 , ...., n8 are the number of seeds that germinated on 1st, 2nd, and subsequent days until the 8th day, respectively; 8, 7, ...., 1 are the weights given to the seeds germinated on 1st , 2nd, and subsequent days until the 8th day. N is the total number of seeds placed for germination. Seed germination is inversely related to the degree of dormancy and should predict pre-harvest susceptibility of the genotypes (Hagemann & Ciha, 1984). Germination experimental design and data analysis Germination tests for each of the four temperatures were conducted as a randomized complete block with four replicates of the ten genotypes. One growth cabinet was consistently used and subsequent temperature treatments were set as desired. The analysis was performed using PROC GLM of SAS (SAS Institute, 1990), where replicates were considered as random while genotypes were assumed fixed. For each temperature level, genotype mean values of four replicates were compared using the least significant difference (LSD0.05 ). The same experimental materials and procedures were followed for seed samples tested for germination starting November 15, 2000 (Experiment 1), and on December 1, 2000 (Experiment 2). All statements of significance are at p ≤ 0.05 except where stated.
Results Most genotypes responded with decreasing WGI mean values (increasing dormancy), as temperature changed from 10 to 30 ◦ C in both Experiment 1 and 2 (Tables 2 and 3). Four genotypes HY344, Snabbe, Janz and AUS1408 were the exceptions showing an increase of 4 to 12% WGI mean values as temperature increased from 10 to 15 ◦ C in Experiment 1. These genotypes, except AUS1408, showed a similar response in Experiment 2, with Snabbe showing a further 4% increase in WGI as temperature was increased from 10 ◦ C to 20 ◦ C. In Experiment 1 (Table 2), the WGI identified HY344, Janz, Snabbe, AC Karma, and Kleiber as low in seed dormancy compared to the other genotypes at 10 ◦ C. The genotypes responded in a relatively similar pattern in Experiment 2 (Table 3). As the temperature
Table 2. Average weighted germination indices (WGI) of wheat genotypes germinated at 10, 15, 20, and 30 ◦ C at about 95% relative humidity in the dark in Experiment 1 Genotype
Temperature 10 ◦ C 15 ◦ C
20 ◦ C
30 ◦ C
Kleiber OS59-15-8-20 RL 4137 Snabbe AC Karma HY344 AUS1408 Janz SC8019-R1 SC8021-V2
0.73 0.62 0.53 0.76 0.76 0.77 0.38 0.73 0.49 0.64
0.61 0.22 0.35 0.85 0.76 0.87 0.42 0.76 0.48 0.50
0.11 0.02 0.06 0.64 0.32 0.61 0.16 0.28 0.10 0.15
0.02 0.02 0.04 0.15 0.11 0.32 0.02 0.05 0.002 0.06
Mean LSD (p < 0.05)
0.64 0.06
0.58 0.10
0.24 0.04
0.08 0.06
Table 3. Average weighted germination indices (WGI) of wheat genotypes germinated at 10, 15, 20, and 30 ◦ C at about 95% relative humidity in the dark in Experiment 2 Genotype
Temperature 10 ◦ C 15 ◦ C
20 ◦ C
30 ◦ C
Kleiber OS59-15-8-20 RL 4137 Snabbe AC Karma HY344 AUS1408 Janz SC8019-R1 SC8021-V2
0.67 0.56 0.42 0.74 0.76 0.79 0.60 0.73 0.50 0.60
0.54 0.27 0.30 0.83 0.74 0.84 0.48 0.75 0.43 0.50
0.19 0.06 0.13 0.77 0.54 0.80 0.35 0.42 0.13 0.35
0.01 0.03 0.03 0.15 0.08 0.40 0.18 0.08 0.03 0.06
Mean LSD (p < 0.05)
0.64 0.08
0.57 0.09
0.37 0.06
0.11 0.10
was increased from 10 to 15 ◦ C, the genotypes with low seed dormancy maintained low seed dormancy except Kleiber which tended to show some increase in dormancy. An increase in temperature to 20 ◦ C showed that most genotypes responded with relatively lower WGI mean values compared to the responses at 15 ◦ C for both Experiment 1 and 2. As temperature increased to 30 ◦ C, all genotypes showed a decrease in WGIs. However, most of the susceptible genotypes, e.g. HY344 and Snabbe, tended to have relatively
126 higher WGI mean values at 30 ◦ C compared to most of the dormant genotypes in both Experiment 1 and 2. AUS1408 was the exception in Experiment 2 having a relatively high value of 0.18 at 30 ◦ C. Maximum differences in dormancy between lines tended to occur at 15 or 20 ◦ C. For example in Experiment 1, the difference between AC Karma and SC8021-V2 was 0.12 at 10 ◦ C; 0.26 at 15 ◦ C; 0.17 at 20 ◦ C; and 0.05 at 30. Differences in WGI between HY344 and SC8021-V2 were 0.13 at 10 ◦ C; 0.37 at 15 ◦ C; 0.46 at 20 ◦ C; and 0.26 at 30 ◦ C.
ated at lower (10 and 15 ◦ C) and higher dormancy levels at higher temperature (30 ◦ C). It would appear that temperature has a marked influence on seed germination and is dependent on genotype. We conclude that germination temperature affects the relative dormancy level of a given wheat genotype and that for studying dormancy differences between wheat genotypes, 15 to 20 ◦ C appears ideal for the current set of genotypes.
Acknowledgements Discussion Based on the results of this study, temperature 10 and 15 ◦ C favored relatively high levels of germination i.e. reduced dormancy. This suggests that the genotypes tested lacked the ability to sustain high levels of seed dormancy at lower temperatures compared to higher temperatures such as 30 ◦ C. Thus factors controlling seed dormancy have an optimum temperature at which maximum effects are expressed. However, this may vary depending on genotype. Reddy et al. (1985) reported that the maximum potential seed dormancy a genotype contains can be determined by evaluating seeds in a range of germination temperature. In the present study of 10 genotypes, the dormancy differences are greatest among sprouting-resistant genotypes germinated at low temperature. At the more susceptible end of the temperature scale, differences in dormancy were observed at higher germination temperatures. The 30 ◦ C temperature only showed differences with the least dormant genotypes, but differences were also observed at lower temperatures. It might be expected that if the germination tests were carried out over a longer period, temperature effects would protract dormancy with increased temperature. However, there is good reason to germinate seeds as quickly as possible to avoid protracted germination tests. Protracted germination uses expensive growth facilities, may delay results, and place tests at a higher risk of contamination with microbial growth. Conversely too low a temperature also masks differences between genotypes. The 10 genotypes differed significantly in mature seed dormancy based on the average WGI values. At the four regimes of temperature used, HY344 showed the lowest degree of seed dormancy. The present report demonstrated relatively low dormancy levels when seeds of the 10 wheat genotypes were germin-
The authors appreciate the Western Grains Research Foundation and Matching Investment Initiative for their research grants. Many thanks to C. Wu, S. Stewart, I. Piche and B. Meyer for seed sampling, threshing and germination tests. The skill and help of the Cereals Technical Team, at the Semiarid Prairie Agricultural Research Center is much appreciated. We thank Dr Y. Gan and Mr M.P. Schellenberg for evaluating the manuscript and Carol Dyson for the valuable comments.
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