Effect of low temperature stress on photosynthesis, total soluble sugars, grain filling rate and yield in rice (Oryza sativa L.)

A field experiment with 15 rice (Oryza sativa L.) genotypes was conducted during kharif (rainy) season of 2001 to understand the photosynthetic basis of productivity under low temperature conditions in hills. Exposure of rice genotypes to low temperature conditions (mean temperature <18°C and minimum temperature <12°C) during reproductive phase was made by altering the dates of planting i.e. normal planting (transplanting date 25^th June) and late planting (transplanting date 25th July). It was observed that the rates of photosynthesis and canopy photosynthesis showed significant positive association with grain yield during grain filling stage (15 days after anthesis) under normal planting and poor association under late planting. A significant reduction in rates of photosynthesis, canopy photosynthesis, and level of total chlorophyll was observed at grain filling stage under low temperature condition. Besides, a reduction in the grain-filling rate under late planting was also seen among rice genotypes. A significant enhancement in the level of total soluble sugars (TSS) in the flag leaf (L) while slight reduction in soluble sugars in panicle peduncle (P) was noted among rice genotypes exposed to low temperature condition. The total soluble sugars in the leaf showed significant negative association with photosynthesis rate. Moreover, highly significant positive association of harvest index (HI) with grain yield (r = 0.843**) under late planting indicated that the low temperature reduced the partitioning of photoassimilates resulting in poor yield. Three advance lines of rice viz. VL-94-3288, VL-98-3894 and VL-98-3895 proved their cold tolerance efficiency > 80%.