Effects of Heat Stress During Flowering On Physiological and Ultra-Structural Changes in Soybean Leaves and Pollen Grains.

Soybean has a leading role in world vegetable oil production, since it accounts for about 35% of total oilseed area in world.  The crop’s share in global oilseed crop output is estimated at 44%. The remarkable success of this crop in temperate zones is well known, but there is also potential role for soybean in tropics and subtropics, where the daytime temperature is high compared to its optimum growing temperature. Soybean yield is limited by number of pods per plant which in turn depends on number of flowers produced per plant and pollen viability. Heat stress during flowering stage will eventually leads to loss of pollen viability and lower pod set ratio. The present study was formulated with a hypothesis that heat stress during flowering stage induces flower abortion by altering pollen viability and ultra structure along with reduced photosynthetic rate. The hypothesis was supported by the following objectives (i) to study the effect of heat stress during flowering stage on pollen viability and germination, (ii) to assess the impact of heat stress on leaf photosynthetic rate and (iii) to study heat stress the ultra-structural changes in pollen, vascular bundle and leaf induced by heat stress. The experiment was designed in completely randomized block design with five replications. The various physiological traits like leaf chlorophyll content, photosynthetic rate, stomatal conductance, PSII efficiency was recorded on the leaf at four days interval. The pollen viability, germination and ultra structure changes in pollen and leaf were also recorded. The result indicated that heat stress significantly reduced the chlorophyll content (SPAD unit), photosynthetic rate, stomatal conductance and PS II photochemistry of the leaf. Heat stress reduced the pollen viability and germination. The reduction in photosynthetic rate and stomatal conductance were correlated with ultra-structural changes in the leaf. The reduction in pollen viability was correlated with pollen morphology and ultra-structural changes.