High - temperature stress in rice

This paper examines the occurrence and causes of high-temperature injuries at different stages of rice growth and in different varieties of rice and investigates genetic tolerance to heat-induced sterility. Exposure to temperatures higher than 35`C during the vegetative stage can reduce plant height, tiller number, and dry weight and can cause white leaf tip, chlorotic bands and blotches, and white bands and specks. The rice plant is most sensitive to high temperatures at flowering; the second most sensitive stage is 9 days before flowering. Exposure more than 15 days before or 3 days after flowering does not affect spikelet sterility. Exposure at the reproductive stage can cause white spikelets, white panicles, and reduced spikelet numbers. High temperature on the flowering day (anthesis) causes sterility, but high temperature (except 41`C or higher) for 5 consecutive days before or after flowering does not disturb fertilization of spikelets provided the temperature at anthesis is not too high. High temperatures during ripening can reduce grain filling. It appears that sterility under high temperature is caused by disturbed pollen shedding and impaired pollen germination, and not by an inactivated pistil. Early morning anthesis (when temperatures are lower) is one way to avoid sterility. The flowers of Oryza glaberrima open 3 hours earlier than those of IR36, a characteristic which can be incorporated into O. sativa. There are clear varietal differences in high temperature-induced spikelet sterility. Tolerant varieties (e.g., Agbede, Dular, N22) are 84-90% fertile following exposure to a temperature of 35`C, while susceptible varieties (e.g., Basmati 370, Pelita 1/1) are less than 10% fertile. Although rice varieties grown in heat-prone areas must be early maturing, it is not difficult to find varieties that are both heat-tolerant and early maturing. Heat tolerance has a fairly high heritability and most genetic variation is additive. Finally, a comparison of phytotron and field experiments confirmed that the phytotron can be used to screen heat tolerance. A 29-item list of references (1956-81) is provided.