Growth and development of rice seedlings is severely affected by low temperature stress causing decreased germination and emergence, reduced growth and consequently lowering of yield (Cruz and Milach 2004, Jiang et al. 2008, Sharifi 2010). Temperatures below 200C decreases both the speed and percentage of germination (Yoshida 1981) and results in lower crop stands and higher production costs. Low temperature at vegetative stage can cause slow growth and reduced seedling vigor (Ali et al. 2006) and tillering (Shimono et al. 2002), increased plant mortality (Farrell et al. 2006, Baruah et al. 2009, Fujino et al. 2004) and growth period (Alvarado and Hernaiz 2007). Cool temperature (below 15-170C) causes severe injuries leading to poor establishment, decreased plant competitive ability against weeds, delayed crop maturation and subsequently decreased yields (Yoshida et al. 1996, Andaya and Makill 2003).
3.2 Reproductive stage
Low temperature often affects plant growth and crop productivity, which causes significant crop losses (Xin and Browse 2001). It is a major factor limiting rice production in temperate and high altitude areas (IRRI 1979; Sthapit and Witcombe 1998). The reproductive stage of rice is the most susceptible to cool weather (Nishiyama et al. 1995). Low temperature in the range of 15–19°C impairs microspore development and leads to the production of sterile pollen grains, resulting in poor grain filling and high spikelet sterility (Satake 1976). Analysis of mutants from the cultivar Taichung 65 treated with cold water at 19°C revealed that pollen development was inhibited, reducing spikelet fertility due to malformed embryo sac (Nagasawa et al. 1994). Low temperature stress in reproductive stage enhances panicle sterility and lower grain yield (Shimono et al. 2002). For example, in 1980 and 1993, low temperatures seriously damaged the Korean rice crop, with grain yields dropping by 26% and 9.2%, respectively, compared with the national average yield on either side of these years (MOAF, 1994). Low temperature stress is also a liming factor of Boro rice (November to May) production in Bangladesh affecting 10-15% of the total rice areas with varying degrees of cold damage at reproductive stage.