Photosynthesis and respiration

Rice, being a C3 plant, responds very well to increased [CO2]. Several long- and short-term CO2 enrichment studies show that doubling the current ambient [CO2] increased leaf-level photosynthetic rate by 30-70%. The magnitude of this increase depended on the particular rice cultivar, growth stage and environment (Imai and Murata, 1978; Akita, 1980; Morrison and Gifford, 1983;

Ambient C02 TGC

Ambient C02 TGC

Fig. 5.3. Schematic drawing of the temperature gradient chamber (TGC) for CO2 enrichment (bottom) and ambient [CO2] (top) developed at Kyoto, Japan. (Adapted from Horie et al., 1995c.) 1 = oil heater; 2 = stovepipe of oil heater; 3 = air exhaust window; 4 = variable speed exhaust fan; 5 = reversible exhaust fan; 6 = oscillating fan; 7 = CO2 controller; 8 = CO2 injection pipe; 9 = liquid CO2 tanks.

Fig. 5.3. Schematic drawing of the temperature gradient chamber (TGC) for CO2 enrichment (bottom) and ambient [CO2] (top) developed at Kyoto, Japan. (Adapted from Horie et al., 1995c.) 1 = oil heater; 2 = stovepipe of oil heater; 3 = air exhaust window; 4 = variable speed exhaust fan; 5 = reversible exhaust fan; 6 = oscillating fan; 7 = CO2 controller; 8 = CO2 injection pipe; 9 = liquid CO2 tanks.

Lin et al., 1997). The effects of a wide range of [CO2] on canopy photosynthesis of rice (cv. IR30) are shown in Fig. 5.4. The relative response of canopy net photosynthetic rate (P„) to [CO2], with 330 mmol mol-1 set to unity, was iteratively fitted to the following rectangular hyperbola (Baker and Allen, 1993):

where Pmax is the asymptotic response limit of (Pn - Pi) at high [CO2]; Pi is the intercept on the j-axis; and Km is the value of [CO2] at which (Pn - Pj) = 0.5Pmax. Values of parameter estimates were 70.83 mmol mol-1, 3.96 and -2.21 for Pmax, Km and Pi, respectively. Equation 5.1 indicates that the relative Pn reaches a ceiling value of 1.75 at infinite [CO2] and that doubling [CO2] from 330 to 660 mmol mol-1 increases rice canopy photosynthesis by 36%. Although this value of the relative response to doubled [CO2] would be influenced by environmental conditions, cultivars and developmental stages of rice, it can be accepted as a reasonable value in comparison with the relative response in rice biomass production as described in section 5.2.5.

Since it has been shown that the effects of CO2 enrichment on rice leaf-area development are small (Morrison and Gifford, 1984a; Imai et al., 1985; Baker et al., 1990c; Nakagawa et al., 1993; Kim et al., 1996a; Ziska et al., 1997), the canopy photosynthetic response to CO2 enrichment can be mainly

Fig. 5.4. Relative canopy net photosynthesis at a photon flux of 1500 |mmol m-2 : vs. season-long [CO2] treatment. (Adapted from Baker and Allen, 1993.)

Fig. 5.4. Relative canopy net photosynthesis at a photon flux of 1500 |mmol m-2 : vs. season-long [CO2] treatment. (Adapted from Baker and Allen, 1993.)

attributable to responses at the unit leaf-area level. This implies that responses to [CO2] in terms of net assimilation rate and radiation use efficiency are similar to those previously described for canopy net photosynthesis.

A major question in the study of global climate change effects on rice is whether or not [CO2] effects on rice photosynthesis are influenced by air temperature. Here, seemingly contradictory results have been reported. Lin et al. (1997) and Nakagawa et al. (1997) found that higher temperatures stimulated single-leaf photosynthesis of rice subjected to long-term [CO2] treatments during the vegetative stages. In contrast, Baker and Allen (1993) reported that rice canopy photosynthesis was relatively unaffected by a wide range of air temperatures. Since one of the major effects of elevated [CO2] on net photosynthesis is through the suppression of photorespiration, it could be expected that optimum temperature for photosynthesis shifts upward as [CO2] increases. Indeed, this type of interaction of temperature and [CO2] on leaf-level photosynthesis has been defined by Long (1991) for some C3 species. This leaf-level response has been confirmed for rice by Lin et al. (1997) and Nakagawa et al. (1997).

Conversely, very small interactive effects of temperature and [CO2] on canopy photosynthesis occur at the canopy level. This is consistent with the temperature effects on rice responses to [CO2] in terms of biomass production obtained under field-like conditions (Baker et al., 1992a; Horie, 1993; Kim et al., 1996a; Nakagawa et al., 2000; see also Fig. 5.7). Similarly, soybean (Glycine max, L.) canopy photosynthesis is also relatively insensitive to a rather wide range of air temperatures (Jones et al., 1985). Carbon dioxide enrichment causes partial stomatal closure, increased stomatal resistance, reduced leaf and whole canopy transpiration and warmer leaf and whole canopy temperatures (Baker and Allen, 1993). However, the lowered canopy surface temperature caused by increased transpirational cooling at higher air temperature (Baker and Allen, 1993) may be one of the reasons for this differential photosynthetic response to temperature between single leaves and whole canopies. Further studies are needed to arrive at a definitive conclusion on this topic.

Potential photosynthetic acclimation (or down-regulation of photosynthesis) in response to elevated [CO2] has been addressed in a few experiments conducted on rice. Baker et al. (1990a) grew rice (cv. IR-30) season-long at a wide range of [CO2]: subambient (160 and 250 ||mol mol-1), ambient (330 |mmol mol-1) and superambient (500, 660 and 900 |mmol mol-1). They tested for canopy photosynthetic acclimation to long-term [CO2] by comparing canopy photosynthesis at short-term [CO2] of 160, 330 and 660 |mmol mol-1. When all long-term [CO2] treatments were held at a common short-term [CO2] of 160 |mmol mol-1, canopy photosynthesis was decreased by 44% across the long-term [CO2] treatments from 160 to 900 |molmol-1. This decline in photosynthesis was accompanied by a 32% decrease in the amount of Rubisco protein relative to other soluble protein, and a 66% decrease in Rubisco activity (Rowland-Bamford et al., 1991). However, the majority of this down-regulation of photosynthesis, as determined by canopy gas-exchange measurements, occurred from the subambient (160 | mol mol-1) to ambient (330 |mol mol-1) long-term [CO2] treatments. The decline in canopy photosynthesis for the long-term ambient (330 |mol mol-1) and twice ambient (660 |mol mol-1) was only 3.6%.

In a subsequent experiment, Baker et al. (1997b) similarly tested for potential acclimation of rice (cv. IR-72) canopy photosynthesis to long-term [CO2] growth treatments of 350 and 700 |mol mol-1. They compared canopy photosynthesis across short-term [CO2] ranging from 160 to 1000 |mol mol-1 and found no photosynthetic down-regulation. Here, photosynthetic rate was a function of current short-term [CO2] rather than long-term [CO2] growth treatment. However, in the same experiment, Vu et al. (1998) found reductions in leaf Rubisco content ranging from 6 to 22% for the CO2-enriched treatments compared with ambient controls. Thus, while photosynthetic acclimation responses in terms of enzyme down-regulation may be detected at the single leaf biochemical level, these effects may or may not result in a detectable loss of canopy photosynthetic capacity when measured using gas-exchange techniques.

The response of leaf photosynthetic rate to intercellular [CO2] was similar for rice plants grown at different [CO2] and temperature regimes under field-like conditions during the vegetative phase of growth (Lin et al., 1997). Thus, photosynthetic acclimation to elevated [CO2] is not likely to occur for rice grown under field conditions within the [CO2] range up to twice the current ambient level. Conversely, photosynthetic acclimation to enriched [CO2] could result from plant exposures to much higher [CO2] (above 1000 |mol mol-1), as shown by Imai and Murata (1978), or from reduced sink size caused either by restricted root growth, in the case of potted plants (Arp, 1991), or by high-temperature-induced spikelet sterility (Lin et al., 1997). From the experimental evidence and analysis, we conclude that, across the range from current ambient [CO2] (near 360 |mol mol-1) to the approximate doubling of [CO2], projected for the mid to late 21st century, photosynthetic acclimation to elevated [CO2] may not be a large or even significant factor governing rice photosynthetic responses to [CO2].

Dark respiration rate of rice, expressed on a ground area basis, increased with increasing [CO2], due to increased biomass, but specific respiration rate per unit biomass decreased (Baker et al., 1992c). However, the differences in the specific respiration rate among rice crops subjected to different long-term [CO2] were shown to be derived from differences in the N concentration of above-ground biomass. Since elevated [CO2] can reduce plant tissue N concentration (Baker et al., 1992c; Nakagawa et al., 1993; Kim et al., 1996a; Ziska et al., 1996) and both the growth and maintenance components of respiration can be affected by tissue-N concentration (Penning de Vries et al., 1974; Amthor, 1994), it is possible that elevated [CO2] may reduce specific respiration.

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