Rice (O. sativa and O. glaberrima) is one of the world's most important crops, particularly in Asia, but increasingly so in Africa and Latin America as well. Rice is extensively grown in irrigated cropping systems, allowing production in the warmer, high radiation post-monsoon and summer months. Rice production has also intensified in rainfed-lowland and dryland (upland) cropping systems, many of which are prone to drought and high temperature (Coffman, 1977). Furthermore, global climate change is likely to exacerbate the current vulnerability of the crop to climate, with a projected global average surface temperature increase of 1.4–5.8 °C by 2100 and the possibility of increased variability about this mean (IPCC, 2001). Simulations by Horie et al. (1996), for example, have suggested that the yield of current varieties in southern Japan would be reduced by up to 40% in future climates.
Flowering (anthesis and fertilization), and to a lesser extent booting (microsporogenesis), are the most susceptible stages of development to temperature in rice (Satake and Yoshida, 1978; Farrell et al., 2006). Previous studies, summarized in Satake and Yoshida (1978), have shown that spikelets at anthesis that are exposed to temperatures >35°C for about 5 d during the flowering period are sterile and set no seed. Sterility is caused by poor anther dehiscence and low pollen production, and hence low numbers of germinating pollen grains on the stigma (Matsui et al., 2000, 2001; Prasad et al., 2006). There is genotypic variation in spikelet sterility at high temperature (Matsui et al., 2001; Satake and Yoshida, 1978; Prasad et al., 2006) that can be defined by different temperature thresholds (Matthews et al., 1995; Nakagawa et al., 2002). It has been suggested that indica spp are more tolerant to higher temperatures than japonica spp (Satake and Yoshida, 1978; Matsui et al., 2000), although heat-tolerant genotypes have been found in both subspecies (Prasad et al., 2006; Matsui et al., 2001). Genotypes N22 (Yoshida et al., 1981; Prasad et al., 2006) and Akitakomachi (Matsui et al., 2001) are the most tolerant genotypes found to date among indica and japonica spp, respectively.
The response to duration of exposure to temperature >35 °C appears to be quantitative, with shorter durations at higher temperatures having the same effect as longer durations at cooler temperatures (Satake, 1995). However, interactions between temperature and duration have not been quantified. Where responses to high temperature have been modelled, spikelet sterility increases in response to daily maximum temperature (Matthews et al., 1995; Horie et al., 1996; Nakagawa et al., 2002). If there is an interaction between temperature and duration, then the response of spikelet fertility to temperature may be better modelled by a cumulative temperature response above a threshold temperature (Vara Prasad et al., 1999, for peanut). Furthermore, if only a short period of high temperature causes sterility, then the timing of this episode in relation to peak flowering will be critical, both for phenotyping (i.e. to differentiate between escape and absolute tolerance) and modelling the impact of high temperature (Wheeler et al., 2000). It follows that effects of temperature on flowering pattern, which have not been studied, are also likely to be important with respect to escape and the total number of spikelets.
The overall objective of this work was to quantify the effects of duration of exposure and temperature on spikelet fertility in order to develop protocols to phenotype and map this important trait in rice. Accordingly, a widely grown lowland indica, IR64, and an upland japonica, Azucena, were subjected to a range of high temperature treatments during anthesis. Specifically, (i) the effect of temperature on flowering patterns; (ii) the effect of time of spikelet opening (anthesis) relative to a high temperature episode on spikelet fertility; and (iii) the interactions between duration of exposure and temperature on spikelet fertility were studied.
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