C

Ziska and Manalo (1996)

Vegetative stage heat tolerance has been studied by using cellular membrane thermo-stability (CMT) to record the electrolyte leakage, as a measure of heat tolerance (Blum and Ebercon 1981; Reynolds et al. 1994; Fokar et al. 1998). In cowpea (Vigna unguiculata L.), a positive association between CMT and heat tolerance at flowering has been found (Ismail and Hall 1999). However, Prasad et al. (2006) found a poor correlation (r2 = 0.02) between heat tolerance during vegetative stage as measured by CMT and reproductive stage tolerance measured by spikelet fertility in 14 rice genotypes. A similar relation has also been found in peanuts (Kakani et al. 2002; Craufurd et al. 2003). Therefore, a measure of heat tolerance by CMT at the vegetative stage and response to heat at reproductive stage in rice could be different and hence cannot be extrapolated.

4.3 High Temperature and Reproductive Development of Rice

Temperature below or above optimum will slow down the developmental rate and increase the time to reach heading stage (Horie 1994). Reproductive stage in rice is more sensitive to heat than the vegetative stage (Yoshida et al. 1981). An independent extreme heat episode during vegetative stage has no influence on reproductive stage (Porter and Semenov 2005).

Rice genotypes can avoid high temperatures during anthesis, by heading during the cooler periods of the season (macro-escape) or by anthesing during cooler hours of early morning (micro-escape). An advancement of peak anthesis towards early hours of the morning (Prasad et al. 2006), is a genuine attempt to escape high temperatures during later hours of the day. Among the two cultivated rice spp. O. glaberrima has the ability to anthese during early morning (immediately after dawn) and escape high temperatures during the later hours of the day (Yoshida et al. 1981; Prasad et al. 2006). On the other hand, O. sativa spp. flower later during the day (Prasad et al. 2006) experiencing high day temperatures which results in the spikelet sterility. The early morning flowering advantage of O. glaberrima has been exploited in interspecific crosses between O. glaberrima and O. sativa to advance peak flowering towards early hours (Prasad et al. 2006). Alternatively, genotypes anthesing under high temperature and maintaining seed-set exhibit true heat tolerance (O. sativa sub-spp. indica cv. N22; Yoshida et al. 1981; Prasad et al. 2006). Anthesis/flowering is the most sensitive process during reproductive stage to high temperature (Nakagawa et al. 2002; Satake and Yoshida 1978), followed by microsporogenesis (Yoshida et al. 1981). High temperatures (35°C) during anthesis and microsporogenesis resulted in 71 and 34% decline in the spikelet fertility, respectively (Yoshida et al. 1981). Heat stress during anthesis leads to an irreversible effect, as there is no increase in panicle dry weight with subsequent improvement in the environment (Oh-e et al. 2007).

Anthesis is highly sensitive to heat as spikelet tissue temperature >33.7°C for < 1 h was sufficient to render a spikelet sterile (Jagadish et al. 2007). Spikelet opening even an hour before showed no effect of the subsequent heat exposure, while those spikelets anthesing within an hour after high temperature exposure were affected (Satake and Yoshida 1978), but not after one hour. The preceding high temperature affected either the anther or pollen in spikelet opening within an hour after high temperature (Matsui et al. 2000). Reciprocal studies with manual shedding of pollen from control plants on to the stigma exposed to high temperature and vice versa showed that the ability of the pistil to be fertilized remained unaffected even over a period of 5 days at 41 °C (Yoshida et al. 1981). Similarly, wheat spikelet fertility was increased from 30 to 80% by pollinating heat stressed pistil with unstressed pollen (Saini and Aspinall 1982). Hence, the male reproductive organ is mainly responsible for spikelet sterility under high temperature and hence, to be targeted for increasing tolerance to warmer climates. The male gametophyte was found highly sensitive to high temperature in majority of the cultivated crops (Table 4.2) and the mechanism leading to sterility could be similar to rice under high temperatures.

High temperatures during anthesis could result in spikelet sterility due to the sensitive physiological processes (anther dehiscence, pollination, pollen germination on the stigma, pollen tube growth or the early events of fertilization) being affected. A rice spikelet, on an average, open for 45 min, exposing the sensitive reproductive organs to ambient temperature and relative humidity. Anther dehiscence is the most susceptible process during anthesis under high temperature (Matsui et al. 1999). Earlier, anther dehiscence was perceived to be the result of a simple desicca-tory process, which if true, would be promoted by high ambient air temperature.

Table 4.2 Effect of high temperature on reproductive processes in various cultivated crops

Stage

Temperature

Summary of the effects

Crop - references

Microspor-ogeneis

30°C

Anthers with no pollen or

Triticum aestivum (Saini

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