Accumulated hourly temperature >33°C
Almost all previous studies on the response of rice or crops in general were conducted in temperature controlled chambers or temperature gradient tunnels until recently T-FACE (Temperature - Free Air Controlled Enhancement) was introduced. Initially heating systems were devised to heat ecosystems (e.g. soil) to increase soil temperature without increasing air temperature (Shaver et al. 2000; Shen and Harte 2000). To study the effects of global warming in warm open field trials, efficient infrared heater arrays have been tried (Kimball et al. 2008). Positioning the heaters at 45° from the initially tried horizontal position and arranging them in hexagonal array could uniformly warm an area of 3 m2. To begin with, the infrared heaters were maintained at constant power mode, resulting in excess heating during calm night time and less heating under variable day time condition (Harte and Shaw 1995; Harte et al. 1995). But Nijs et al. (1996) and Kimball (2005) devised controllers to maintain controlled warming system to simulate diurnal pattern of temperature increase during day and night. In future, a much more cost effective heating system could be available for in situ studies related to climate change.
4.4 Effect of Elevated CO2 on Vegetative Growth and Photosynthesis in Rice
CO2 could well be an evil in disguise, as elevated CO2, apart from enhancing photosynthesis in most crop plants (C3), has the ability to trap infra red radiations from earth surface and redirect them back to increase global mean temperature. Rice, being a C3 plant, responds positively to increase in atmospheric CO2. Several long and short-term CO2 enrichment studies show that doubling the current ambient CO2 increased leaf photosynthetic rate by 30-70% (Ghilidyal and Natu 2000). The magnitude of this increase depends on rice cultivar, growth stage and environment (Lin et al. 1997). Rice responses to CO2 are quite different for plants grown under isolated conditions (e.g. in pots) compared to those under field conditions (Nakagawa etal. 1994).
Horie (1993) reported large differences in CO2 enriched treatments on biomass production in pot experiments compared to field like environments. Horie et al. (2000) conducted a similar analysis and revealed that CO2 effects on biomass production for rice grown in pots showed a strong temperature dependency, while under field-like conditions, temperature had a minor effect. The pot-grown plants displayed a strong CO2 - temperature interaction because these plants continue to produce tillers for a longer time than the field-grown rice, especially at higher temperatures. This is attributed to reduced mutual shading in potted plants, while plants grown under field-like conditions develop a closed canopy and so mutual shading suppresses positive feedback effects of increased photosynthesis on growth.
Since one of the major effects of elevated CO2 on net photosynthesis is suppression of photorespiration, it could be expected that optimum temperature for photosynthesis could shift upwards. The effect of CO2 enrichment on rice leaf area developmentis small(Nakagawaet al. 1993;Kimetal. 1996a;Ziskaetal. 1997),but the canopy photosynthetic response is mainly attributed to responses at the unit leaf area level. 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 (Lin et al. 1997). These reports indicate that photosynthetic acclimation to elevated CO2 is not likely to occur in rice grown under field conditions within double the current CO2 concentration. However, photosynthetic acclimation to enriched CO2 could occur either at much higher CO2 concentration (above 1000 ^lmol-1) or from reduced sink size caused by restricted root growth (Arp 1991) or by high temperature induced spikelet sterility (Lin et al. 1997).
Rice cv. IR-30 was subjected to a wide range of CO2 [sub-ambient (160 and 250^lmol-1), ambient (330^lmol-1) and super-ambient (500, 660 and 900 ^lmol-1)] and tested for acclimation by comparing canopy photosynthesis at short and long-term with different levels of CO2 concentrations (Baker et al. 1990a). When all long-term CO2 treatments were compared to common short-term CO2 of 160 ^lmol-1, canopy photosynthesis was decreased by 44% across the long-term CO2 treatments from 160 to 900 ^lmol-1. This decline in photosynthesis was accompanied by a 32% decrease in Rubisco (Ribulose 1,5 Biphosphate Carboxylase) relative to other soluble protein with a 66% decrease in Rubisco activity (Rowland-Bamford et al. 1991). Similarly, Vu et al. (1998) found reductions in leaf Rubisco content ranging from 6 to 22% for the CO2 enriched treatments compared with ambient controls. Baker et al. (1990b) tested rice cv. IR-72 to short-term CO2 ranging from 160 to 1000 ^lmol-1 and found no photosynthetic down-regulation. 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. Respiration leads to loss of carbon fixed during photosynthesis. Increase in respiration with higher carbohydrates at elevated CO2 will be seen due to strong relationship between the two (Farrar 1985). The maintenance respiration increases with increase in growth and the positive relation between photosynthesis and respiration is nearly non-existent at senescence (Poorter et al. 1988). Therefore, the enrichment in canopy photosynthesis and respiration differs with plant growth and hence CO2 budgetting throughout crop cycle is essential (Sasaki et al. 2005).
Elevated CO2 increases phenological developmental rate and hence reduces the number of days to heading (Baker et al. 1990a; Nakagawa et al. 1993; Kim et al. 1996a). Baker et al. (1990a) found that elevated CO2 did not significantly affect the interval between similar growth stages of successive leaves (phyllochron) on the main-culm of rice, but the leaf number was reduced due to accelerated phe-nological development. Reports on the effects of CO2 on tiller production appear contradictory. Baker et al. (1990a, 1992b) found relatively minor effects of CO2 on tiller production across CO2 range of 330-660 ^lmol-1, with a density of 235 plants m-2. In contrast, Kim et al. (1996a) and Ziska et al. (1997) found a marked increase in tiller number caused by a doubling of CO2 at plant densities of 50 and 75 m-2, respectively. This discrepancy is possibly related to the differences in plant densities. A high degree of mutual shading among plants at higher density would have increased competition for light, suppressing the development of tiller primor-dia (Baker et al. 1990a, 1992b). Therefore, under planting densities usually practised in Asian rice cultures, elevated CO2 may substantially promote tiller production (Ziska et al. 1997).
To overcome limitations of controlled chamber studies, FACE (Free Air CO2 Enrichment) technology was developed (McLeod and Long 1999; Hendrey 1993; Long et al. 2006) for in situ study of elevated CO2 on crop plants. Rice grown under FACE (Ambient + 200 ^ mol of CO2 mol-1 of air) accumulated more biomass during the vegetative stage which increased partitioning of photosynthates to the ear. They concluded that with no significant increase in canopy photosynthesis during grain filling stage, the advantage obtained by CO2 enhancement during vegetative stage is lost during grain filling period (Sasaki et al. 2005). Long et al. (2006) questioned the possible effects of elevated CO2 in non FACE experiments and concluded that the advantage was roughly half compared to in situ FACE experiments and suggested a downward revisions of the estimated world food supply by the end of this century. However, another group (Tubiello et al. 2007) attributed the conclusions of Long et al. (2006) to technical inconsistencies and lack of statistical significance and concluded that the response of elevated CO2 was similar across FACE and non FACE experiments as revealed from previous comprehensive modelling and experimental analysis.
4.5 Effect of Elevated CO2 on Reproductive Development and Yield in Rice
Increased biomass production due to elevated CO2 has the potential to increase yield, provided flowering and grain-filling are not disrupted by environmental stresses, such as drought or high temperature. In rice, increase in grain yield can be associated with various components like tiller number per ground area, increased panicle weight at maturity, seed fill and individual grain weight. Two rice cultivars
(cv N22 and IR72) at elevated CO2 recorded an increase in grain yield at ambient temperature and the increase was associated to increase in tiller number and panicle weight, but cv NPT-2 had no effect on either tiller number or panicle weight. This shows that tiller formation may be a factor in optimizing the response of rice to increasing CO2 concentration (Moya et al. 1998). Elevated CO2 markedly increased rice spikelet number per unit area over a wide range of air temperatures, through increase 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 nitrogen content (Hasegawa et al. 1994) at the spikelet initiation stage. The fact that elevated CO2 increased the spikelet number despite a reduction in plant N content suggests that elevated CO2 promotes better nitrogen use efficiency.
Moya et al. (1998) reported significant enhancement in total plant biomass of rice (cv. IR 72) with increased CO2 concentration, but grain yield responded to a lesser extent than biomass. Ziska et al. (1997) obtained a yield increase of 27% with CO2 enrichment in cv. IR72 grown at ambient temperatures in both wet and dry seasons at the International Rice Research Institute, Philippines. The percentage yield increase due to a doubling of CO2 for a japonica cv. Akihikari, grown at ambient temperatures in Kyoto, Japan, ranged from 20 to 40% over two consecutive years (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 yield may be a reasonable estimate for rice exposed to long-term doubled CO2 concentration under field conditions with moderate temperatures.
4.6 Interaction Between Increasing Temperature and Elevated CO2 on Rice
Previously, an increase in atmospheric CO2 resulting in temperature increase by 2.5°C was recorded (Gutowski et al. 1998; Cohen 1990) and the same has been convincingly reported (increase by 2.0-4.5°C) in the present IPCC report (IPCC 2007). Hence, examining the interaction of CO2 and temperature is the need of the hour, as their concomitant occurrence is most likely in future climates. Several studies have examined the impact of increasing carbon dioxide concentration and ambient air temperature on rice growth and yield in controlled environment or field conditions (Baker et al. 1992b; Baker and Allen 1993; Ziska and Teramura 1992). Elevated CO2 effect on rice developmental rate leading to decrease in days to heading has been shown to be temperature dependent. Akihikari (japonica), headed earlier by 6 and 11% at ambient temperatures of 28 and 30°C, respectively (Kim et al. 1996b).
Response of rice with respect to biomass and yield under CO2 and/or temperature studies could vary considerably under field condition and in growth chamber studies (Ziska et al. 1996). Studies show much larger stimulation (70%) in growth chambers compared to field experiments (10-30%) (Baker et al. 1990a, 1992b; Horie 1993). Although cv. IAC 165 and N22 were recommended for elevated CO2 conditions, but none performed convincingly for sustained increase in production under concomitant increase in CO2 and temperature. Similarly, Moya et al. (1998), after studying long term responses, concluded that cv. N22 recorded highest biomass and yield under elevated CO2 concentration, but did not maintain the same under combined CO2 and temperature increase. Matsui et al. (1997a), studying the interaction of CO2 and temperature at reproductive stage, recorded an increase in canopy temperature due to closing of the stomata at high CO2 concentrations, resulting in low transpiration cooling.
Photosynthetic response of rice may vary under different regimes of air temperature. Nakagawa et al. (1997) found that higher temperature 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 range of air temperatures. Biochemically, increase in CO2 concentration stimulates an increase in Rubisco and photorespiration is partially inhibited reducing carbon loss. Hence, increasing temperature should result in higher net photosynthesis and CO2 uptake as seen in single leaf (Potvin 1994). The interaction of CO2 and temperature at both vegetative and reproductive stages has to be further explored to exploit the increasing CO2 for increasing yields.
Rice grains are a significant sink for the assimilates and removal or restriction of this carbon sink will fail to exploit the elevated CO2 due to photosynthetic insensi-tivity (Stitt 1991; Webber et al. 1994). Accordingly, Ziska et al. (1996) recorded a significant increase in root/shoot ratio with elevated CO2 with increasing temperature and hinted at alternative sinks becoming active recipients with reduced carbon sink capacity of the grains due to spikelet sterility from high temperature exposure. Increasing temperatures from 28/21 to 37/30°C decreased grain yield significantly even under 660 ^ mol of CO2 mol-1 of air (Baker et al. 1992a). Ziska et al. (1996) recorded 70 and 22% increase in biomass at elevated CO2 treatment under 29/21 and 37/29°C, respectively, while grain yield of 17 contrasting cultivars recorded <1% filled spikelets.
There is sufficient information available on the beneficial effects of elevated CO2 during the vegetative stage and the antagonistic effect of high temperature during reproductive stage. However, experimental data on the interactions between CO2 and temperature at critical growth and developmental stages is limited. Therefore, future studies should be concentrated on interaction studies to harness the beneficial effect of elevated CO2 and to minimize the deleterious effects of increasing temperature. This can be achieved by developing varieties which exhibit heat escape (flower early in the morning) or breeding varieties having heat tolerance during sensitive stages (panicle initiation, microsporogenesis and anthesis). With increasing purchasing power of people in the developing countries, nutritional aspects will be a priority in coming years and should be given importance in future breeding programme. Robust information collected and knowledge gained from environmentally controlled studies could be extended for in situ trials to study the crop response to elevated CO2 under the FACE system and increasing temperature under T-FACE system which, in future, could be used for interaction studies between temperature and CO2.
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