Influence of Rising Atmospheric CO₂ Concentrations and Temperature on Growth, Yield and Grain Quality of Cereal Crops

Abstract

A possible scenario for the end of the 21st century is that the atmospheric CO₂ concentration will be in the range of 510-760 μL L^-1 and that the mean global temperature will be 1.5-4.5ºC higher. Further, there may be greater incidences of extreme climatic events, which together with the CO₂ and temperature changes will influence development, growth and grain yield of cereals such as rice and wheat. For these C₃ plants, the driving force for the growth response to elevated CO₂ is higher leaf CO₂ assimilation rates (A). However, the response of A to CO₂ depends on temperature with maximum absolute increases occuring at temperatures which do not cause flower abortion, while negligible increases are observed at low temperatures. At high temperatures, where A is reduced because of partial inactivation of photosynthetic enzymes, the increase in A due to CO₂ enrichment is still observed. Other factors, such as changes in shoot water relations or hormone concentrations, may influence growth at elevated CO₂ concentrations. Wheat and rice development is accelerated by high temperature and consequently grain yield is reduced because there is less time for radiation to be intercepted during the vegetative phase. Although high CO₂ also accelerates development in rice and, to a lesser extent in wheat, the extra carbohydrate produced by increases in A results in at least a 40% increase in grain yield at temperatures which do not cause flower abortion. This is due mainly to increased tiller numbers rather than increases in the number or weight of individual grains. However, the yield enhancement due to high CO₂ will not necessarily compensate for decreases in yield caused by accelerated development at high temperatures. As predicted by the response of A to high CO₂, the relative increase in yield, due to rising CO₂ concentrations, is smaller at lower temperatures. Elevated atmospheric CO₂ may improve the tolerance of plants to heat-induced drought stress by facilitating the maintenance of cell volume and photosynthetic function in the leaves. Increased carbohydrate storage in the stems may also be an advantage during grain filling if the flag leaves senesce prematurely. However, it is unlikely that the effect of very high temperatures on flower abortion will be ameliorated by high CO₂. For bread making, the quality of flour produced from grain developed at high temperatures is poorer. High CO₂ may also have an effect through a reduction in the protein content of wheat grain. For rice, the amylose content of the grain, a major determinant of cooking quality is increased under elevated CO₂.