The interactive effects of elevated [CO2] and higher temperatures on plants are complex (Conroy et al., 1994) but can be simplified if one separates cases where heat stress limits the reproductive sink from cases in which heat stress limits the photosynthetic source. In many cases, reproductive development is more sensitive to high temperatures than overall plant biomass production. Several studies showed the detrimental effects of high temperatures on reproductive development are not ameliorated by elevated [CO2]. For soybean (Glycine max [L.] Merr.) grown under controlled-environment field conditions, HI progressively decreased with increasing temperature under either 660 or 330 |mmol mol-1 [CO2] (Baker et al., 1989). Reproductive development of Pima cotton is also sensitive to high temperatures, such that the plants may not produce either fruiting branches or bolls (Reddy et al., 1992). Studies in naturally sunlit, controlled-environment chambers demonstrated that elevated [CO2] of 700 |mmol mol-1 did not ameliorate this problem (Reddy et al., 1995, 1997). Controlled-environment field studies with rice demonstrated that grain yield decreased 10% °C-1 increase in average temperature above 26°C at [CO2]s of either 330 or 660 |mmol mol-1 (Baker and Allen, 1993). The decreases in rice grain yield were due mainly to fewer grains per panicle. High day and high night temperatures can cause decreases in viability of pollen grains at anthesis, increases in floret sterility and decreases in seed set in rice (Ziska and Manalo,
1996). Elevated [CO2] aggravated the effect on pollen, causing a 1°C decrease in the threshold maximum canopy surface temperature after which the percentage of spikelets having ten or more germinated pollen grains exhibited a precipitous decline (Matsui et al., 1997). Heat-induced increases in floral sterility may have been responsible for the down-regulation of photosynthesis observed in rice under high temperatures and elevated [CO2] through indirect effects associated with reductions in reproductive sink strength (Lin et al.,
With high night temperatures, many cowpea genotypes do not produce flowers, while others produce flowers but not pods (Ehlers and Hall, 1996). Growth chamber studies with contrasting cowpea genotypes grown in pots demonstrated that these heat-induced detrimental effects are present under either 350 or 700 |mmol mol-1 [CO2] (Ahmed et al, 1993). A heat-tolerant cowpea genotype had greater pod production under elevated [CO2] at both high and more optimal night temperatures than a genetically similar cultivar that does not have the heat-tolerance genes. This suggests that the heat-tolerance genes may confer responsiveness to elevated [CO2] in this species. The possibility that genes which enhance heat tolerance during reproductive development also enhance grain yield responses to elevated [CO2] under a range of temperatures should be evaluated with other species that exhibit sensitivity to heat during reproductive development.
For cases where the photosynthetic source is particularly sensitive to heat stress, the interactive effects of elevated [CO2] is less clear. The review of Allen (1994) indicates that for C3 plants, photosynthetic responses to elevated [CO2] of individual leaves often increased with increasing temperature, up to some maximum temperature, and in some cases biomass responded in the same way. Simulations based on a model of leaf photosynthesis, photorespiration and respiration predicted that the response of net canopy CO2 uptake to elevated [CO2] by C3 plants could be greater at higher temperatures (Long, 1991). However, studies of Ziska and Bunce (1997) with soybean demonstrated that even though photosynthetic responses to elevated [CO2] of individual leaves increased with increasing temperature, photosynthetic rates of whole plants did not. Ziska and Bunce (1995) also found that cultivars of soybean differed in their responsiveness to elevated [CO2] and higher temperature with respect to photosynthesis and biomass production. Criteria that breeders could use to select plants that have enhanced photosynthetic responses to elevated [CO2] have not been established but the study of Ziska and Bunce (1995) indicated that genetic variability for this trait may be present within species.
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