Leaf area partitioning as an important factor in growth.

Despite continuing efforts to correlate unit area rates of photosynthesis of crop varieties with growth rates, there has been little or no success. It is reasonable to assume that partitioning of photosynthate into new leaf area is an important component of growth. Accordingly, an expression was developed to measure leaf area partitioning. Using growth analysis techniques, relative growth rates were compared to net assimilation rates, partitioning of daily weight gain into new leaf area, and partitioning of daily weight gain into new leaf weight of nine species grown in growth chambers under three temperature regimes. Day/night temperatures of 21/10, 32/21, and 38/27 C caused large differences in relative growth rates. Relative growth rates were closely correlated with leaf area partitioning in seven of the nine species, but were inversely correlated with leaf weight partitioning for six of the nine species. Relative growth rates were poorly correlated with net assimilation rates for five of the nine species. The product of net assimilation rate times leaf area partitioning is shown to be equal to the relative leaf area expansion rate.These results indicate that growth responses due to temperature shifts were more sensitive to changes in leaf area partitioning or relative leaf area expansion rates than to net assimilation rates. Because changes in leaf area partitioning or relative leaf area expansion rates can have an effect on relative growth rates that overshadow changes in net assimilation rates, and because net assimilation rates are largely a function of unit area rates of photosynthesis, the correlation of unit area rates of photosynthesis with growth should include consideration of leaf area partitioning or relative leaf area expansion rates.