Effect of Radiation and Temperature on Efficiency of Cereal Leaves during Grain Growth

Grain: leaf ratio, G (the ratio of grain yield to leaf area duration between ear emergence and maturity), in 15 experiments on wheat and barley in different seasons (Group A experiments) was highly correlated with mean daily radiation, R, mean daily temperature, Tμ, and mean daily maximum temperature, Tmax, during the grain growth period. The regression of G on R accounted for 81 per cent of the variance of G, and introducing Tμ to the regression significantly increased this to 88 per cent. The regression of G on Tmax alone accounted for 87 per cent, perhaps because Tmax effectively integrates radiation and temperature. When R was varied artificially by shades in two experiments on wheat in different years (Group B experiments) the relationship between G and R was approximately linear in both, but the slope of the line was less in one year, when R and temperature were less, than in the other. For this second year, when R and temperatures were about the middle of the ranges found in Group A experiments, the calculated relationship agrees with the Group A results after correcting values of G for differences of Tu from its value in the shading experiment. A formula relating G and R derived from the results of both Group B experiments and the observed correlation of R and temperature in the field, assuming that the regression of G on R depends on temperature, agrees with the relationship between G and R in the Group A experiments. It is concluded that differences in radiation and temperature are about equally responsible for the differences in G found between seasons. The positive effect of temperature on G suggests that factors other than leaf photosynthesis, e.g. translocation rate or capacity of the grain to accumulate carbohydrate, are important in determining G.