When high temperature conditions occur during flowering, spikelet sterility can sometimes be seen on the windward side of Australian rice crops. Evaporative cooling can reduce canopy temperature by 7oC on hot windy days protecting spikelets from high temperatures at anthesis. Historically, high temperature induced sterility has not been a major problem in South East Asia because most rice is grown in the wet season. However, high temperatures and high humidity during flowering are becoming a constraint to rice production in the lowlands particularly in southern Laos. A recent report confirms that spikelet sterility under high temperature is increased with humidity (Matsui et al. 1997). The average maximum temperatures during March and April in the Champassak province, is 35.3°C and 35.8oC respectively (Figure 1-F). In the Sekong province the average maximum temperature was greater than 35oC from January through to May, 1998. The breeding program that produces cultivars for southern Laos should target improving the heat tolerance and attempting to induce earlier flowering.
The rice plant is most sensitive to high temperatures during flowering, causing impaired pollen germination and a reduced number of pollen on the stigma leading to spikelet sterility (Yoshida et al. 1981). IRRI (1979) confirmed genotypic variation existed by identifying 13 of 291 selections that were tolerant for high temperature damage at flowering. High temperatures disturb reproductive processes occurring within an hour after anthesis. This includes dehiscense of anther, pollen shedding, germination of pollen grains, and elongation of pollen tubes (Yoshida 1981). More than 10 germinated pollen grains on a stigma is needed for normal fertilisation (Togari and Kashiwakura 1958; cited in Yoshida 1981). Yoshida (1981) suggested that spikelet sterility was caused by high temperature on the day of flowering only. Anthesis usually occurs between 1000 hours and 1200 hours, with temperature rising in the morning and exceeding the critical temperature (35oC) by 1000 hours in high temperature areas. Therefore, early morning anthesis is highly desirable because high temperature is avoided and sterility is reduced (Yoshida 1981). The flowers of Oryza glaberrima, an African cultivated rice species, open early in the morning (IRRI, 1979) has been a source of earliness for Oryza sativa species of rice. The one-hour advance in the flower opening time may have a significant effect in decreasing sterility, since air temperature rises at a rate of 3-4oC/hr in many areas in the tropics. In screening tolerant materials, eight-hour treatments of 35oC and 38oC were effective in selecting heat susceptible and tolerant lines respectively (Yoshida 1981). Two methods are therefore possible to improve the heat tolerance cultivars in Laos. Increasing the true tolerance is possible through the use of cultivars that have improved pollen shedding and pollen germination, and avoidance of high temperatures is possible through flowering earlier in the day.
