Biomass production

Crop growth rate (dw/dt; g m-2 day-1) can be generally defined as the product of canopy light interception and radiation utilization efficiency (RUE; g MJ-1) (Monteith, 1977; Horie and Sakuratani, 1985) and is represented by:

where S0 is incoming solar radiation (MJ m-2 day-1), k is the radiation extinction coefficient of the canopy, and LAI is leaf area index. As previously noted, the effects of [CO2] on LAI are small. Thus, differences in radiation interception among [CO2] are also small for rice except during early growth. Therefore, CO2 enrichment increases RUE and crop growth rate mainly by increasing photosynthesis and decreasing specific respiration rate. Rice biomass production is calculated by integrating equation 5.4 with respect to time for a given crop duration (d). Since elevated [CO2] increases RUE but decreases d, the overall effect on crop biomass production is a result of the differential effects of elevated [CO2] on these two parameters. Reported values of the relative enhancement of rice biomass caused by CO2 enrichment vary widely. This is probably due to the difference in cultivars, temperatures, solar radiation levels, N or other fertilizer applications, and various experimental methodologies (e.g. field vs. potted plant experiments). Determination of the interactive effects of [CO2] with these factors on rice biomass production is of primary importance.

With respect to temperature and [CO2] effects on rice biomass production, Horie (1993) analysed published data for long-term CO2 enrichment studies. Large differences in [CO2] enrichment effects on biomass production occurred for potted plant experiments compared with those under near field-like environments. He concluded that, under field-like conditions, [CO2] enrichment increases biomass production by 24% across a wide range of air temperature treatments. Nakagawa et al. (2000) conducted a similar analysis with more recently reported data (Fig. 5.6). This analysis revealed that [CO2] effects on biomass production for rice grown in pots showed a strong temperature dependency while rice grown under field-like conditions was little affected by temperature. A temperature coefficient (e.g. the slope of the regression line) of only 1.8% °C-1 was obtained for the field-like conditions (Fig. 5.6). Since the value of this temperature coefficient was small and significantly different from zero only at P < 0.05, the average value of a 24% relative enhancement of biomass production for a doubling of [CO2] for field-grown rice appears to be a reasonable estimate.

The reason pot-grown plants displayed a strong [CO2] by temperature interaction for biomass production may be that pot-grown rice continues to produce tillers for a longer time than does field-grown rice, especially at higher temperatures. This is likely due to reduced mutual shading in potted plants

Fig. 5.6. Effects of mean air temperature on the relative enhancement of rice crop biomass. Plants were grown season long in ambient and nearly doubled [CO2]. The open and closed symbols are for pot- and field-grown plants, respectively. The data are normalized to the values at ambient [CO2] as unity. (Adapted from Nakagawa et al, 2000.)

Fig. 5.6. Effects of mean air temperature on the relative enhancement of rice crop biomass. Plants were grown season long in ambient and nearly doubled [CO2]. The open and closed symbols are for pot- and field-grown plants, respectively. The data are normalized to the values at ambient [CO2] as unity. (Adapted from Nakagawa et al, 2000.)

compared with field-grown plants and is similar to the light interception effects on dry matter production and tiller development described in section 5.2.4. In contrast with the nearly isolated pot-grown rice, plants grown under field-like conditions develop a closed canopy, where mutual shading suppresses positive feedback effects of increased photosynthesis on growth. Also, the shortening of growth duration by elevated [CO2], especially at higher temperatures (Kim et al., 1996a), contributes to a weakening of [CO2] by temperature interactive effects on rice biomass production for field-grown plants, despite the appreciable interactive effects observed for single leaf photosynthesis (Nakagawa et al., 1993; Lin et al., 1997).

Since leaf photosynthesis depends strongly on leaf N concentration (Murata, 1961; Ishihara et al., 1979), RUE also depends on leaf N (Sinclair and Horie, 1989). These facts suggest that the relative enhancement of rice biomass production with [CO2] depends strongly on plant N content. In a fertilizer N by [CO2] experiment conducted on a japonica-type rice in TGCs, Nakagawa et al. (1994) found enhanced biomass production due to CO2 enrichment was 17, 26 and 30% at N applications rates of 40, 120 and 200 kg ha-1, respectively. This finding is similar to that reported for an indica-type rice by Ziska et al. (1996) where the increased biomass production due to CO2 enrichment was 0, 29 and 39% at N application rates of 0, 90 and 200 kg ha-1, respectively. These results clearly illustrate that the relative enhancement in biomass production due to CO2 enrichment depends strongly on N application rate. Furthermore, with adequate fertilizer N, the relative enhancement in rice biomass production due to CO2 enrichment can reach or exceed 30%.

Since global climate change could involve shifts in precipitation patterns, potential interactive effects of [CO2] and drought stress become another topic of interest. For pot-grown rice plants, CO2 enrichment under drought-stress conditions resulted in increased relative biomass production compared with drought-stressed ambient controls (Morrison and Gifford, 1984b; Rogers et al., 1984). This effect was attributed to higher leaf water potentials for the CO2-enriched plants under drought-stress conditions. Due to the anti-transpirant effect of CO2 enrichment on canopy evapotranspiration, Baker et al. (1997a) reported a modest reduction of about 10% in crop water use.

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