The debate about what limits plant growth oscillates between those who argue that carbon gain by photosynthesis sets the potential growth rate and those who argue that it is the sink's ability to grow and use photosynthate that regulates carbon gain. Therefore, if a C4 rice plant were engineered having a greater potential canopy rate of CO, assimilation, would the growth and yield increase? One way to address this question is to look at the situation where rice has been grown under elevated atmospheric CO:. While this does not eliminate photorespiration, it provides an analogy for what might be expected for a C4-like plant.
Numerous reports mention rice that has been grown under elevated atmospheric CO:, but the outcome can be illustrated with a select few. Imai et al (1985) grew rice under 350 and 700 |iL CO,. They showed that elevated CO; led to earlier flowering (another potential benefit of creating C4 photosynthesis that cannot be covered here) and increased biomass at tillering (94%), ripening (51%), and grain yield (39%). Baker et al (1990). using paddy culture in naturally lit controlled environment chambers with six CO, treatments ranging from 160 to 900 |iL L_1 CO,, found that, as atmospheric CO; concentration increased, tillering, leaf area, biomass, and number of panicles increased. The grain yield in the 660 versus the 330 (J.L L 1 C02 treatment increased by 32%. Ziska and Teramura (1992) grew two rice cultivars under 360 or 660 |iL L 1 C02 for 113d and also found that the increased CO, level increased biomass in the two cultivars by 27% and 34%, respectively. Gas exchange measurements revealed that CO, assimilation rate increased by 50% in the elevated CO, treatment. Although this was offset by a decrease in leaf area per unit leaf dry mass, it demonstrates that, if the rates of CO, assimilation per unit leaf area can be increased in rice plants, enhanced growth will follow.
One final note of caution. The harvested grain contains a significant amount of protein and hence nitrogen. When yields are increased, it will be necessary to ensure that the protein content is maintained. This is especially important for rice, which already has the lowest grain protein content among the cereals. If canopy nitrogen contents were not increased as yield increased, a larger fraction of plant nitrogen would need to be remobilized into the grain than at present just to maintain current grain protein content (Feil 1997). Alternatively, canopy nitrogen contents need to rise. This could happen by either increasing nitrogen contents per unit leaf area or increasing LAI. Because C4 leaves generally have less nitrogen per unit leaf area than C, leaves, the former seems unlikely. The prospect of having crops with an LAI much greater than 10 is also daunting, as this would require an even more erect habit than is currently achieved (Sinclair and Sheehy 1999). Given that CO, assimilation rate per unit nitrogen is greater in C4 leaves, this implies that a different balance would be expected for grain yield and grain nitrogen in C4 versus C, crops. However, as they tend not to be grown in the same environments, paired comparisons of nitrogen harvest index between the C, cereals (maize, sorghum, and millet) and rice are not made easily.
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