The temperature optimum for maximum phenological developmental rate of rice, in terms of the number of days to reach heading, is generally 27-30°C. Above or below this temperature optimum, developmental rate slows and a longer time period is required for the crop to reach heading (Horie, 1994). Elevated [CO2] increases phenological developmental rate and hence reduces the number of days to heading (Baker et al., 1990b; Nakagawa et al., 1993; Kim et al., 1996a). This [CO2] effect on rice developmental rate has been shown to be temperature dependent. For the japonica-type rice cultivar Akihikari, CO2 enrichment reduced the number of days to heading by 6% and 11% at air temperatures of 28°C and 30°C, respectively (Kim et al., 1996a). There are two potential explanations for this accelerated crop developmental rate with [CO2] enrichment. First, elevated [CO2] could increase plant temperature by increasing rs, thus reducing transpirational cooling. Second, it has been shown that rice grown under elevated [CO2] generally has a higher carbon to nitrogen (C/N) ratio (Baker et al., 1992c; Kim 1996;

Ziska et al., 1996). A high C/N ratio has been shown to be associated with accelerated plant developmental rates (Zeevaart, 1976). However, CO2 enrichment effects on leaf temperature as the primary mechanism seems unlikely since leaf temperature differences between ambient and doubled [CO2] were very small at higher air temperatures (Homma et al., 1999). Furthermore, developmental rates are slowed as air temperature increases beyond the optimum for development.

Baker et al. (1990b) found that [CO2] did not significantly affect mainstem phyllochron intervals of rice. However, in that study, the final mainstem leaf number reduced with elevated [CO2] due to an acceleration in the phenological development. Reports on the effects of [CO2] on tiller production appear contradictory. Baker et al. (1990b, 1992b) found relatively minor effects of [CO2] on tiller production across the [CO2] range from 330 to 660 mmol mol-1. In contrast, Kim et al. (1996a) and Ziska et al. (1997) found a marked increase in tiller number caused by a doubling of [CO2]. This discrepancy is likely related to differences in plant densities among these studies. Kim et al. (1996a) and Ziska et al. (1997) utilized plant populations of 50 and 75 plants m-2, respectively, whereas Baker et al. (1990b, 1992b) used a density of 235 plants m-2, which is extraordinarily high. A high degree of mutual shading among plants at this high density would have suppressed the development of tiller primordia due to greater competition for light. Therefore, under planting densities usually practised in Asian rice cultures, elevated [CO2] may substantially promote tiller production.

Elevated [CO2] resulted in only minor effects on individual leaf size (Baker et al., 1990c) and also on total leaf area of rice except during the initial growth stage (Morrison and Gifford, 1984a; Imai et al., 1985; Baker et al., 1990c; Nakagawa et al., 1993; Kim et al., 1996a; Ziska et al., 1997). This appears to indicate that N uptake of rice was not influenced by elevated CO2 (Baker et al., 1992c; Ziska et al., 1996) except at the initial stage (Kim, 1996), since rice leaf area increases proportionally with plant N uptake (Murata, 1961; Miyasaka et al., 1975; Hasegawa and Horie, 1997).

Elevated [CO2] increased specific leaf weight of rice during early growth, but less so at later growth stages (Baker et al., 1990c). Also, the [CO2] effects on the root to shoot (R/S) ratio were significant during the early growth stages, with a higher R/S ratio at elevated [CO2] (Imai et al., 1985; Baker et al., 1990c; Ziska et al., 1996). However, for later developmental stages, the [CO2] effect on R/S ratio became negligibly small provided sufficient N fertilizer was applied (Ziska etal., 1996; Kim et al., 1996a).

Elevated [CO2] markedly increased rice spikelet number per unit area over a wide range of air temperatures, through both increases in the number of productive tillers per unit area and spikelets per tiller (Imai et al., 1985; Kim et al., 1996b). In rice, the spikelet number per unit area is generally proportional to plant N content (Shiga and Sekiya, 1976; Horie et al., 1997b) or to plant N concentration (Hasegawa et al., 1994) at the spikelet initiation stage. The fact that elevated [CO2] increased the spikelet number despite a reduction in plant N concentration suggests that elevated [CO2] promoted spikelet production efficiency per unit of plant N. Elevated [CO2] increases sink size by increasing spikelet number and this generally leads to increased yield, provided successful fertilization and grain-fill follows.

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