Substantial changes occurred in atmospheric [CO2] in the geological past and in the 20th century (Allen, 1994). Millions of years ago, atmospheric [CO2 ] was about 1200-4000 ||mol mol-1 but it decreased substantially over centuries (Sundquist, 1986). Analysis of air trapped in polar ice indicates that prior to 1800, atmospheric [CO2] fluctuated between 180-290 |mmol mol-1 for at least 220,000 years (Barnola et al., 1987; Jouzel et al., 1993). Since 1800, ice core data indicate accelerating increases in atmospheric [CO2] from 280 to 300 |mmol mol-1 by 1900 and 315 |mmol mol-1 by 1958. Direct measurements indicate substantial increases since 1958 from 315 to 360 |mmol mol-1 by the 1990s (Hall and Allen, 1993). Global reliance on fossil fuels could result in further increases in atmospheric [CO2]. The moderate scenarios in the report of the Intergovernmental Panel on Climate Change (IPCC; Houghton et al., 1996) predict that atmospheric [CO2] will exceed 600 |mol mol-1 by the end of the 21st century. Even with full implementation of the agreements to reduce the use of fossil fuels, made at the meeting in Kyoto, Japan, in December 1997, atmospheric [CO2] is expected to increase substantially in the 21st century (Bolin, 1998).
Plants with the C4 photosynthetic system evolved during an early period after the atmospheric [CO2] became low and this system represents a specific adaptation to the low [CO2] environments of the last 200,000 years (Bowes, 1993). The growth responses of C4 and crassulacean acid metabolism (CAM) plants to elevated [CO2 ] are smaller than those of C3 species (Poorter, 1993) and their responses to [CO2] will not be considered in this review.
The extent and nature of the evolution of plants with the C3 photo-synthetic system, with respect to low [CO2], are not known. Prior to 1900, these plants were subjected to low [CO2] for thousands of years. The characteristics of the enzyme involved in the initial fixation of CO2 in C3 plants, ribulose bisphosphate carboxylase (Rubisco), may not have changed very much (Morell et al., 1992). However, it is likely that low [CO2] resulted in evolutionary modifications to whole plant processes, such as increases in the ratio of photosynthetic source to carbohydrate sink tissues, and that some C3 plants may not be well adapted to either future or even present day levels of atmospheric [CO2].
Empirical selection for yield under field conditions may indirectly select plants that are responsive to the continually increasing levels of CO2 (Kimball, 1985). However, it is likely that indirect selection will be inefficient because the atmospheric [CO2] is increasing rapidly, and yield is dependent upon many abiotic and biotic factors. Direct selection in breeding nurseries that are subjected to elevated [CO2] would not appear feasible in commercial programmes due to the large numbers of plants that must be screened and the high costs of nursery environments that have elevated [CO2] compared with conventional field nurseries. A unique experiment was conducted by Maxon Smith (1977) over 5 years in special glasshouses. Segregating populations of lettuce (Lactuca sativa L.) were selected for yield and agronomic traits in two environments: day-time CO2 enrichment with higher temperatures, and ambient [CO2] with lower temperatures. In both environments, selection was effective in improving agronomic traits but did not enhance yield, and the selection in the elevated CO2 and high-temperature environment did not increase lettuce responsiveness to elevated CO2. As was pointed out by Maxon Smith (1977), the lack of success in increasing yield and responsiveness to elevated [CO2] could have been due to the small number of plants that were screened compared with the numbers that can be screened in field nurseries. However, if the biochemical, physiological and morphological traits that contribute to responsiveness to elevated [CO2] could be identified, they could be used to supplement empirical selection based on yield under field conditions and thereby enhance breeding programmes.
Possible beneficial modifications that could be exploited by plant breeders may be discovered by examining plant responses to elevated [CO2]. At intermediate temperatures, doubling atmospheric [CO2] increased grain yield of various small grain cereals by 32% and grain legumes by 54% (Kimball, 1983), but the increases often were less than the increases in photosynthesis that occur with short-term doubling of [CO2] at the same temperatures (Poorter, 1993; Allen, 1994). There are several possible explanations for the smaller yield responses to long-term CO2 enrichment compared with the short-term photo-synthetic responses. A major factor is the down-regulation of photosynthetic capacity under long-term exposure to elevated [CO2] that occurred in some experiments. This down-regulation could either be a consequence of artificial growth conditions or it may indicate that current cultivars of C3 plants are not well-adapted to elevated [CO2]. Down-regulation has been attributed to feedback mechanisms that operate when the supply of carbohydrates from photosynthesis exceeds sink demands for carbohydrates (Allen, 1994). Arp (1991) pointed out that strong down-regulation was observed when plants were grown in small pots that would have resulted in a much smaller root sink for carbohydrates than occurs in nature. In addition, CO2-induced growth enhancement can depend on nutrient supplies in the root zone (McConnaughay et al., 1993). Conroy (1992) proposed, based on a review of the literature, that the greatest absolute increases in productivity with CO2 enrichment will occur when soil N and P availabilities are high. More recently, Ziska et al. (1996b) showed for rice (Oryza sativa L.) that growth response to elevated [CO2] was negligible when soil N was very deficient and increased with greater supplies of soil N. Increases in growth responses to high [CO2] with increases in nitrogen supply also were observed for wheat (Triticum aestivum L.) and cotton (Gossypium hirsutum L.). These data were consistent with the hypothesis that increases in N supply increase the ratio of the carbohydrate sinks to the photosynthetic source which then result in increased growth responses to elevated [CO2] (Rogers et al., 1996a,b).
Large differences in the responses of growth or yield to elevated [CO2] have been observed among C3 species that could have been due to differences in either environmental conditions or genotypic effects. The genotypic effects are of particular importance to this review. In studies comparing C3 species, Poorter (1993) and Poorter et al. (1996) reported a positive correlation between responsiveness to elevated [CO2] (in terms of the absolute value of the increase in relative growth rate) and the relative growth rate in atmospheric [CO2]. More simply, faster-growing species were more responsive, and the authors commented that the more responsive plants may have had larger sink strengths. Comparisons of contrasting cultivars within species, however, would permit more rigorous tests of hypotheses concerning traits influencing responsiveness to elevated [CO2], because different species have substantial variation in genetic background that could obscure responses and hinder interpretations.
Progress during the 20th century in increasing the productivity of several annual C3 crops through plant breeding was estimated as mainly (77%) resulting from increases in harvest index (typically harvest index (HI) = grain yield/total shoot biomass) with only 23% due to increases in total shoot biomass (Gifford, 1986). A general explanation for the increases in grain yield associated with increases in HI is increased efficiency of carbohydrate partitioning, although evidence for traits that were consistently associated with the rise in HI is not available (Evans, 1993). An alternative explanation is that the increases in both grain yield and HI resulted from inadvertent selection of plant types that were better adapted to the higher [CO2]s of the last half of the 20th century compared with the low [CO2]s in earlier centuries. As hypothesized by Hall and Allen (1993), plants with higher HI would be expected to have greater sinks for carbohydrates in relation to their photosynthetic capacity and thus be more responsive to elevated [CO2].
Responses to elevated [CO2] of contrasting genetic lines of cowpea (Vigna unguiculata L. Walp.) support this hypothesis. In growth chamber studies, a genotype with enhanced pod set due to heat-tolerance genes (No. 518) produced more shoot biomass and had greater pod yield with elevated [CO2], under either high night temperatures or more optimal temperatures, than a genetically similar cultivar (CB5) that does not have the heat-tolerance genes (Ahmed et al., 1993). Subsequent field studies with six pairs of cowpea lines either having or not having the heat-tolerance genes demonstrated that the heat-tolerance genes cause dwarfing and enhance HI under both hot and more optimal temperatures (Ismail and Hall, 1998). Taken together, these studies indicate that selection for higher HI through enhancing pod set might produce cowpea cultivars whose grain yields are more responsive to elevated [CO2] under both hot and more optimal temperatures. This hypothesis has not been rigorously evaluated such as by testing the responses of these genetic lines to elevated [CO2] under field conditions.
Responses to elevated [CO2] of six contrasting cultivars of spring wheat were studied in open-top chambers in field conditions but with plants in pots (Manderscheid and Weigel, 1997). These cultivars had been introduced in Germany between 1890 and 1988. The more recent cultivars had less tillering, shorter stems, and higher HI, but they were less responsive to elevated [CO2] with respect to shoot biomass production, number of tillers, stem weight and stem height than the older cultivars. Grain yield responses varied inconsistently with the year of introduction. The authors argued that the greater shoot biomass responses of the older cultivars to elevated [CO2] were due to the increased sink strength resulting from their greater tendency to tiller. These results may not, however, be relevant to optimal field conditions in that the newest cultivars had similar grain yields as the oldest cultivars under ambient [CO2]. One would expect a cultivar released in 1988 to have higher grain yields under optimal present-day environmental conditions than a cultivar released in 1890. Generally, newer small grain cultivars with greater HI have greater grain yields under optimal field conditions than older cultivars. The results of Manderscheid and Weigel (1997) may be explained by their use of abnormally wide spacing that would have benefited the older cultivars due to their greater tendency for tillering and preventing the newer cultivars from achieving their potential grain yields per unit area. However, the number of spikes per plant (four to five) under ambient [CO2] were similar to the values expected to occur under the high plant densities of optimal field conditions and do not support this argument. If their results are relevant to optimal field conditions, they indicate the need to select wheat plants with a greater tillering tendency and smaller HI, which is the opposite of current breeding practice. The cultivar effects reported by Manderscheid and Weigel (1997) are consistent with the conclusion of Rogers et al. (1996a) that the greater responsiveness to elevated [CO2] of wheat under high soil N was due to stimulation of tillering by the high N. Neither of these studies gave a clear indication of whether cultivars of wheat with greater tillering tendency would be more responsive than modern cultivars to elevated [CO2] in terms of grain yield under optimal field conditions and this is a critical issue for plant breeding programmes.
Contrasting cultivars of rice were studied by Ziska et al. (1996a) with plants in pots at wide spacing in glasshouses. At intermediate temperatures (day/night, 29/21°C), there were substantial differences among 17 cultivars in responses of both total plant biomass and grain yield to [CO2] of 664 mmolmol-1. As with the studies of spring wheat by Manderscheid and Weigel (1997), responsiveness of rice to elevated [CO2] was associated with increased numbers of fertile tillers, there was no association between responsiveness and HI (measured as grain yield/total plant biomass), and the cultivars differed in many traits. Rice cultivar differences in response to elevated [CO2] also were observed in field conditions (Moya et al., 1998). Among three cultivars tested, one (N-22) exhibited a large increase in grain yield (+57%), another (IR72) had a 20% increase, and an experimental line had a non-significant 8% decrease in grain yield. Additional field studies with IR72 (Ziska et al., 1997) confirmed that grain yield responses to elevated [CO2] were associated with increases in numbers of fertile tillers; but the grain yield responses (+15% and +27%) were much smaller than the responses of total plant biomass (+31% and +40%). This indicates that grain yield may have been sink limited and that responsiveness to elevated [CO2] in rice might be enhanced by breeding to enhance reproductive sink strength.
To date, there have been few reports of studies aimed at determining the specific traits that enhance responsiveness of grain yield to elevated [CO2].
The few studies reported were constrained by the use of cultivars that differed in many traits and the use of growth chambers or greenhouses or potted plants, which makes it difficult to predict responses under optimal field conditions. More definitive tests of various hypotheses would be obtained by comparative studies of responsiveness to elevated [CO2] under optimal field conditions with genetic lines that differ mainly in HI (or some other trait such as tillering) and also exhibit positive correlations between grain yield and HI under ambient [CO2]. There is a critical need for field studies of this type, because current selection strategies may be appropriate or in the opposite direction from those needed for breeding cultivars for future environments that have elevated [CO2].
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