GERMINATION CHARACTERS AND SEED RESERVE MOBILIZATION DURING GERMINATION OF DIFFERENT WHEAT GENOTYPES UNDER VARIABLE TEMPERATURE REGIMES

A laboratory experiment was conducted in the Crop Botany Laboratory , a t Bangabandhu Sheikh Mujibur Rahman Agricultural University, Salna, Gazipur during 15 October to 15 November, 2001 to study the influence of three temperature regimes (15OC, 25OC and 35OC) on germination characters and seed reserve qobilization in six wheat genotypes. Three heat tolerant ( Aghrani, Kanchan and CB-30) and three heat sensitive (Sonora, CB-24 and CB-34) wheat genotypes were used as study material. Speed of germination expressed a s rate of germination, coefficient of germination and germination vigor index was found to be increased with increasing temperature from 15 OC to 35OC in all wheat genotypes, but heat tolerant ability based on speed of germination was not found. Generally shoot and root growth of HS genotype in relation to length and dry weight were affected more than those of HT genotypes a t high temperature. The amount of seed reserve loss a s respiration increased from 25OC to 35OC in all genotypes, but the increment was higher in Sonora (14.1%) and CB-24 (13.21%), moderate in Kanchan (6.36%), CB-30 (5.22%) and CB-34 (4.95%) and lowest in Aghrani (2.03%). At 15OC, the amount of seed reserve loss a s respiration was also increased from that a t 25°C in all genotypes except in Aghrani and CB-30. But again the increment was higher in Sonora, CB24 and CB-34 (2.79 to 8.84%) than that of Kanchan (1.67). In case of ~ ~ h r a n i a d CB-30, it even decreased. At 25OC temperature the seed reserve mobilization efficiency was significantly higher than that a t 15OC and 35OC in all wheat genotypes. But the heat sensitive genotypes had higher efficiency (2.58 to 3.48 glg) than that of the heat tolerant genotypes (1.99 to 2.09 gig) a t 25OC. Based on reduction of efficiency both a t 15OC and 35OC from that of 25OC, Sonora, CB-24 and CB-34 (HS) were found more affected than Aghrani, Kanchan and CB-30 (HT).


INTRODUCTION .
Late planting of wheat is one of the major reasons of yield reduction because of rice-wheat cropping system.l In Bangladesh, late planting wheat exposed to high temperature at reproductive stage causes reduced kernel number per and reduced kernel s i~e .~-~T h e net effect of these is the reduced grain yield. About 60% of the wheat crop is cultivated atYlate sowing condition after harvesting the transplanted' amah rice7 and this problem will be further increased due to .global warming because in Bangladesh, the annual mean temperature of 25.75OC will rise by about 0.21°C and 0.39"C by 2050 and 2100, re~pectively.~ In spite of low yield of wheat due to post anthesis heat stress, cultivation of wheat can not be avoided totally, because the irrigation dependent Boro rice cultivation may need to be replaced in future by partially irrigated or non-irrigated wheat cultivation to overcome arsenic problem. Therefore, effort ought to be made to minimize the late sown yield reduction of wheat by screening or developing high temperature tolerant genotypes1 varieties.
Membrane thermostability (MT) test, a widely used and acceptable method to evaluate heat tolerance and heat susceptibility index is used to evaluate yield parameters. Germination characters and seed reserve mobilization may vary in different temperature regimesg because temperature is a modifying factor in germination since it can influence the rate of water and other substrates supply necessary for growth.1° The magnitude of variation in seed reserve mobilization may also vary in different genotypes.ll But in wheat, the heat tolerance in relation to germination characters and seed reserve mobilization had not been evaluated not only in Bangladesh but also probably in other countries. In the present study, to elucidate heat tolerance of wheat we have investigated germination characters and seed reserve mobilization of wheat during germination in different temperature regimes.

METHODS AND MATERIALS
The experiment was conducted a t the Crop Botany Laboratory, Bangabandhu Sheikh Mujibur .Rahman Agricultural University, Salna, Gazipur during 15 October to 15 November, 2001. Three heat tolerant genotypes (Aghrani, Kanchan and CB-30,) having longer heat killing time and three heat sensitive wheat genotypes (Sonora, CB-24 and CB-34)) having shorter heat killing time in membrane thermostability test were germinated in three different (15,25 and 35OC) temperature regimes. Before placement of seed for germination, the seeds of a genotype were thoroughly mixed and moisture percentage was determined gravimetrically using a portion of the seeds and the remaining seeds were used for the experiment. Individual weight of 30 seeds for each genotype were measured and placed sequentially according to the marking on filter paper soaked with water in sterilized petridishes. Then the petridishes were kept in seed germinator (ATTEMPTER, Advantec, Japan) at 15,25 and 35OC. For each temperature, three batches of petridishes each containing 30 seeds were used. Water was added to the petridishes as necessary.
Rate ofgermination, co-efficient ofgermination andgermination vigor: Germination was counted a t 24-hour intervals and continued upto the Elth day (120 h). A seed was considered germinated if plumule and radicle came out and were >2 mm long.
The rate of germination was calculated following Krishnasamy and Seshu12 : No. of seeds germinated at 48 h Rate of germination (%) = X 100 No. of seeds germinated a t 120 h Co-efficient of germination and vigor index were calculated using the followidg formulae (Copeland 1976h6  Shoot and root length, dry matter partitioning: At Eith day after placement for germination, five seedlings from each petridish were sampled and the shoot and root length of individual seedlings were recorded manually. Then the shoot, root and remaining seeds were dried separately at 70°C for 72 h and weights were recorded. The mean length (cm) and dry weight were calculated for each treatment combination.
Seed reserve mobilization efficiency: Seed reserve mobilization efficiency in the present study is defined as the a,mount.of shoot and root dry matter (g) .produced from 1 unit (g) of seed .dry weight that was lost as respiration. Thus higher the value of seed reserve mobilization efficiency (SRME), the higher is the efficiency of seed as more seed reserves would be used for producing roots and shoots.
Amount of seed material lost as respiration (SMLR) was calculated as Where, SDW = Seed dry weight before germination SHW = Shoot dry weight RTW = Root dry weight RSW = Remaining seed dry weight Seed reserve mobilization efficiency (SRME) was calculated as-

Statistical analysis
The findings were analyzed by partitioning the total variance with the help of computer using MSTAT program. The treatment means were compared using Duncan's Multiple Range Test (DMRT) at 5% level of significance.

~ermination characteristics:
Rate of germination was significantly influenced by the interaction effect of temperature regimes and wheat genotypes (Table 1). Co-efficient of germination and germination vigor index of the genotypes were influenced significantly by temperature regimes without expressing any interaction (Table 1). At 15OC, the rate of germination was nil in all wheat genotypes i.e., none of the seeds were found to be germinated until 48 hours after placement for germination. At 25OC, all the wheat genotypes showed medium rate of germination (47.85 to 60.80%). The highest germination rate was found in CB-30 (98.89%) genotype at 35OC, which was statistically similar to all other genotypes except lower germination rate in Aghrani (75.14%).
Co-efficient of germination was increased with increasing temperature. The lowest value (24.53%) was found a t 15OC in CB-24 genotype at Eith day, which was similar.to all other wheat genotypes at 15OC. The highest value (28.54%) was found in Sonora, which was also statistically similar to all other genotypes at 35OC. At 25OC, all the genotypes showed a medium value (26.61 to 27.20%).
Germination vigor indices showed an increasing trend with increasing temperature. The lower indices (14.80 to 30.90) were found at 15OC, which increased to moderate level (28.83 to 30.90) a t 25OC and then to the highest level (33.33 to 37.48) a t 35OC in all wheat genotypes. However, the germination vigor indices increased to a greater extent as the temperature rises from 15OC to 25OC, while with further increase in temperature up to 35OC the increase was less.
Faster speed of germination a t higher temperature might be due to rapid hydrolysis and mobilization of seed reserves through higher alpha amylase activity at higher temperature. The close relation between germination of wheat seed and alpha amylase activity at various temperatures was reported by Sultana et a l l 5 In respect to speed of germination all wheat genotypes behaved almost similarly a t different temperatures, but genotypic differences in speed of germination due to increase in temperature did not appear clearly.

Height and dry weight of seedling:
The length and the dry weight of shoot and root of wheat seedlings were influenced significantly by the interaction effect of temperature and wheat genotypes ( Table  2). The shoot length was lowest in Kanchan (1.14 cm) which was statistically comparable to all other genotypes at 15OC. Shoot length increased (5.69 to 9.03 cm) a t 25OC which was considered to be the optimum temperature for wheat seedling growth. At 35OC, shoot length remained more or less the same (7.53 cm) in Aghrani as at 25OC increased in Kanchan (6.69 cm) and CB-30 (8.01 cm) but in HS wheat genotypes (Sonora, CB-24 and CB-34), it decreased (6.23 to 8.78cm) as compared to that a t 25%. Means followed by the same letter (s) did not differ significantly a t 5% level by DMRT In the case of root length, the lowest value was attained a t 15OC, increased at 25OC and thereafter decreased a t 35OC in all wheat genotypes. At 35OC, the reduction in root length from that a t 25OC was lower (2.84 to 4.74 cm) in Aghrani, Kanchan and CB-30 (HT) than that of heat sensitive Sonora, CB-24 and CB-30 (5.93 to 6.37 cm). The adverse effect of higher temperature (35OC) on root length was more clear than that on shoot length.
The shoot to root length, ratio increased with increasing temperature regimes in all wheat genotypes. It was low a t 15OC (0.33 to 0.42), moderate a t 25OC (0.59 to 1.05) and high at 35OC (1.26 to 1.84). The increasing shoot to root ratio length with increasing temperature regimes indicated that the root was more affected than the shoot a t high temperatures.
Generally, shoot dry weight increased significantly with increasing . temperature in all wheat genotypes, except in CB-34 a t 35OC. The increment of shoot dry weight from 25OC to 35OC was higher in Aghrani, Kanchan and CB-30 (1.27 to 1.48 mglseedling) than that of Sonora (0.31 mglseedling) and CB-24 (0.77 mglseedling), whereas in CB-34, it decreased. In the case of root dry weight, the highest value was found a t 25OC and was reduced significantly both at 15 and 35OC in all wheat genotypes.
The shoot to root dry weight ratio increased with increasing temperature in all wheat genotypes. Increment of ratio of shoot to root dry weight with increasing temperature indicated that root dry weight was reduced but shoot dry weight increased a t higher temperature. Based on increment of shoot to root dry weight ratio from 25OC to 35OC, HT genotypes showed higher increment in shoot dry weight (0.73 to 1.08mg) than HS genotypes (0.37 to 0.61mg).

Seed dry m a t t e r distribution:
The percent seed dry matter accumulated in shoot and root, the amount of dry matter loss during respiration and the dry matter remained in seed at 5 days after placement for germination a t 15 OC, 25OC and 35°C showed clear differences among the genotypes ( Table 3 and Figure 1). The amount of seed reserve lost due to respiration increased from 25OC to 35OC in all wheat genotypes, but the increment was higher in Sonora (14.1%) and CB-24 (13.19%), moderate in Kanchan (6.36%) and CB-34 (4.95%) and lowest in Aghrani (2.03%). At 15OC, the amount of respiratory loss of seed reserve was also increased a t 25OC in all genotypes, except in Aghrani and CB-30. The increment was higher in Sonora, CB-24 and CB-34 (2.79 to 8.84%) than in Kanchan (1.67%). In the case of Aghani and CB-30, it even decreased.
At low temperature (15OC), the proportion of dry matter distributed to shoot (2.37 to 3.28%) and root (4.7 to 5.81%) was lower in all genotypes compared to those at both higher temperatures (25OC & 35OC). Dry matter accumulation in shoot weight was increased significantly over a larger increase in temperature (15OC to 35OC). But dry matter accumulation in root was increased over a smaller range of increasing temperature (15OC to 25'0 and thereafter decreased at 35OC in all wheat Means followed by the same letter (s) do not differ significantly a t 5% level by DMRT genotypes. The results indicated that the optimum temperature for root growth was lower compared to that for shoot growth. merefore, when the root failed to continue tc increase dry matter at high temperature (35OC), the shoot was able to continue to gain dry matter showing a higher temperature optimum than root.

Effect of temperature on wheat germination
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Seed reserve mobilization efficiency (SRME):
Temperature had a profound effect on seed reserve mobilization efficiency (SRME) in all wheat genotypes (Figure 1). At 25OC, which is near to optimum temperature (22OC) for wheat seed germination, the SRME was significantly higher than that of at 15OC (Low temperature stress) and 35OC (High temperature stress) in all wheat genotypes. Sonora, CB-24 and CB-34 (HS) showed higher SRME (258 to 3.48 gig) than HT genotypes (1.99 to 2.09 glg) a t 25OC. Based on the amount of reduction in SRME both a t 15OC and 35OC compared to that a t 25OC, the HS genotypes (Sonora, CB-24 and CB-34) experienced larger reduction (1.04 to 2.69g/ g) than heat tolerant Aghrani, Kanchan and CB-30 (0.60 to 1.70glg).

Figure 1: Seed reserve mobilization efficiency in wheat genotype as influenced by temperature
The lowest SRME at 15OC temperature suggested that a t low temperature, respiration was not linked to a build-up of shoot and root biomass and could have led to thermal dissipation of the respiratory energy by way of alternate oxidase pathway or cyanide resistant pathway.13J4 Again a t high temperature (35OC) reduction in SRME under high temperature could be due to increased rate of respiration but the respiratory product somehow failed to accumulate in seedling, mainly in root.
Generally shoot and root growth of HS genotype in terms of length and dry weight were affected more than those of HT genotypes at high temperature. Seed reserve mobilization efficiency of HS genotypes was also affected to a greater extent than that of HT wheat genotypes under high temperature stress.