Increased biomass production of rice caused by elevated [CO2] has the potential to increase yield, provided flowering and grain-fill are not disrupted by some environmental stress such as drought or high temperature. Baker and Allen (1993) showed that equation 5.1 can also be applied to relative yield responses of rice to [CO2] for an indica-type rice (cv. IR30) subjected to long-term CO2 at constant day/night temperature of 31 °C under field-like conditions. The values of the parameters for the yield response (equation 5.1) were 284 |mmol mol-1 for Km, 2.24 for the asymptotic response parameter (Pmax) and -0.13 for the j-intercept (Pi). Using these parameters, they calculated that doubling [CO2] from 330 to 660 |mmol mol-1 increased yield by 44%. Ziska et al. (1997) obtained a yield increase of 27% with CO2 enrichment for the cultivar IR72 grown at ambient temperatures in the wet and dry seasons at Los Baños, the Philippines. The percentage yield increase due to a doubling of [CO2] for a japonica-type rice (cv. Akihikari) grown at ambient temperatures in 1991 and 1992 in Kyoto, Japan, ranged from 20 to 40% (Kim et al., 1996b). While there is considerable variation among these reports in the relative yield response to doubled [CO2] it appears that a 30% enhancement in seed yield may be a reasonable estimate for rice exposed to long-term doubled [CO2] under field conditions with moderate temperatures. This estimate for the relative yield increase also coincides with that for the relative biomass response under sufficient N application conditions.

In terms of predicting the effects of potential future global warming on rice yields, studies that examine the effects and interactions of both temperature and CO2 enrichment are far more relevant than studies that examine only the effects of CO2 enrichment. It has been well established that spikelet sterility in rice at mean daily temperatures below 20°C can be caused by a failure in pollen development at the microspore stage (Satake and Hayase, 1970) or injury sustained at flowering (Abe, 1969). Temperatures above 35°C at flowering can also cause spikelet sterility (Satake and Yoshida, 1976; Matsui et al., 1997a). Whether or not elevated [CO2] has any effect on ameliorating cool temperature damage on spikelets has not been studied. We consider spikelet sterility caused by high temperatures to be more important in the study of elevated [CO2] and global warming effects on rice. The optimum daily mean temperature for grain-fill in rice is 20—25°C (Yoshida, 1981) which is lower than that for many other growth and developmental processes. In general, temperatures above 25°C result in poor grain-fill, caused in part by reduced grain-filling duration which in turn results in reduced grain yield.

Summarizing data from several experiments on the cultivar IR30, an indica-type rice, Baker et al. (1995) reported that grain yields declined with increasing temperatures above 26°C to zero yield near 36°C for both ambient and elevated [CO2]. They estimated that rice yield declined by about 10% for each 1°C rise in daily mean temperature above 26°C. Ziska et al. (1997) reported significant yield reductions for the cultivar IR72 caused by a 4°C rise in air temperature above ambient temperatures in both the dry and wet seasons in the Philippines. They also reported a larger yield reduction caused by high temperature for elevated [CO2] compared with ambient [CO2] in the dry season.

Similar yield reductions at daily mean temperature above 26°C were reported for cultivar Akihikari a japonica-type rice under both doubled and ambient [CO2] (Kim et al., 1996b). In that experiment, rice grown at doubled [CO2] also suffered more severe yield reductions with increasing temperature than plants in the ambient [CO2]. They attributed the greater sensitivity of the [CO2] enriched plants to high temperatures to having both a shorter grain-fill duration (cf. section 5.2.4) and a lower spikelet fertility than plants grown in ambient [CO2] (Fig. 5.7). This reduction in spikelet fertility was mainly due to spikelet sterility induced by high temperatures during flowering. Since rice spikelets are most sensitive to high temperatures during flowering (Satake and Yoshida, 1976; Matsui et al., 1997a) and because flowering in rice usually occurs at midday, the daily maximum temperature is usually more indicative of

Fig. 5.7. Percentage fertility of rice spikelets vs. daily maximum air temperature (Tmax) averaged over the flowering period (7 days) for rice grown at 690 and 350 mmol mol-1 [CO2] in TGCs. (Adapted from Kim et al., 1996b.)

high-temperature-induced spikelet sterility than is daily mean temperature. As a result, the relative enhancement in yield from a doubling of [CO2] displays a sharp decline with increasing daily maximum air temperature averaged over the flowering period (usually about 7 days) and reaches negative values for air temperatures above 36.5°C (Fig. 5.8) (Kim et al., 1996b). These results indicate that the effects of elevated [CO2] on relative rice yield enhancement is strongly temperature dependent and may even become negative at extremely high temperatures during flowering.

This greater negative effect of elevated [CO2] on high temperature damage to spikelets was also reported by Matsui et al. (1997b). They counted the number of pollen grains both shed and germinated on stigma of the spikelets that opened at high temperatures for cultivar IR72 (indica type) grown at both doubled and ambient [CO2] at Los Baños, the Philippines. Shown in Fig. 5.9 are the percentages of spikelets having more than ten germinated pollen grains on the stigma as a function of temperature. This relationship was reduced sharply as temperature at flowering exceeded a specific threshold. This threshold temperature was 1-2°C lower for the CO2-enriched plants compared with the ambient controls (Fig. 5.9). Since spikelets having more than ten germinated pollen grains on the stigma is a good criterion for predicting successful spikelet fertilization (Satake and Yoshida, 1976; Matsui et al., 1997a), these results indicate that the elevated [CO2] increased spikelet susceptibility to high-temperature-induced sterility. This reduction at high temperatures was caused by reductions in both the number of pollen grains shed and subsequent pollen germination.

Although the exact mechanism through which the elevated [CO2] increased spikelet susceptibility to high-temperature-induced sterility is

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