Pollen Quality, Seed Yield Components and Seed Quality Response to High Temperatures Stress During Reproductive Development in Soybean (Glycine Max L. Merr.)

Plant reproduction is highly vulnerable to temperature changes especially during reproductive growth stage. Although there are much research about the effect of daytime temperature during reproductive stage on soybean pollen and seed yield in temperature-controlled chambers, the effect in the field has been little explored. Long-term manipulative field experiments on temperature effect on plants are important tools to provide accurate information for revealing the impacts of climate change on crop yields and seed quality. A field study was conducted at Universiti Putra Malaysia (UPM), in 2008 and 2009, to determine the effects of temperature stress on pollen quality, seed yield components, and seed quality. Three soybean varieties; AGS 190, Dieng, and Willis, were exposed to three temperature levels (25, 30, 35°C) at R1-R2 or at R1-R5 reproductive growth stages. Temperature treatment imposed to the plants was done by placing transparent plastic cages above the canopy. The pollen quality determined were pollen production, pollen viability, pollen germination, pollen tube length and pollen load. Scanning Electron Microscopy (SEM) was used to evaluate the pollen morphology. Seed yield components were determined on the main stem and branches at harvest maturity growth stages (R8). Seed viability and vigour were based on standard germination (SG), 3d germination, and electrical conductivity (EC) tests. There was a strong linear trend of pollen production, viability, germination, tube length and pollen load decrease with increase in temperature. The SEM results showed that general morphology of pollen was affected when plants were subjected to higher temperature, which resulted in flattened and collapsed pollen grains. The apertures were missing in pollens from plants exposed to temperature >30ºC. The pollen morphological aberrations observed in this study may contribute to poor pollen germination and shorter tube length in sensitive varieties such as in Dieng variety. The crop growth stage during R1-R5 was shown to be sensitive to temperature increase as compared with that of crop growth stage during R1-R2. The high temperature (>30°C) during R1-R5 decreased the number of pod 35- 45% for all three varieties compared with the control. The decrease in the number of pods on the main stem was in the range of 39 - 43%, whereas the decrease of pods on the branches was 41- 46%. Willis variety produced as many pods per plant and more seeds per plant than the AGS190 and Dieng varieties under stressful conditions. Although the AGS190 variety had less individual seeds per plant, the resulting yield was greater than Dieng and Willis varieties. These results suggest that the AGS190 variety may have a better ability to produce high seed yield under high temperature stress. Seed viability based on standard germination test was always less sensitive to high temperature stress than vigor, which suggests that seed produced in high-temperature environments could have acceptable or high level of viability, but low vigor levels. Seed vigour based on electrical conductivity test, showed a greater decline from seed developed on the branches compared with the seeds from the main stem. The highest vigour declined was observed in AGS190. The data indicate that varieties with large seed size such as AGS190 are not suitable to be planted in high temperature field conditions. The three soybean varieties showed different response to high temperature with respect to pollen quality and seed quality. In the variety AGS190, pollen quality was good at high temperature but seed quality was low. On the other hand, for Dieng variety pollen quality was low under high temperature but seed quality was higher than AGS190. Our finding indicates that pollen quality has a significant influence on seed yield, but has no influence on seed quality in soybean. The results strongly suggest that pollen parameters would be good indicators in determining reproductive tolerance to high temperature and in relation to seed yield potential. However, the genetic base for these differences must be further examined.

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