The Relationship between the critical Leaf Area, Total Chlorophyll Content, and Maximum Growth-rate of some Pasture and Crop Planst

Abstract

The amounts of leaf and of chlorophyll per unit area of land exposed to light above 400–500 f.c. on sunny days at local noon were determined for pure stands of red clover, white clover, perennial ryegrass, short-rotation ryegrass, maize, kale, cocksfoot, and watercress, growing under near-optimum conditions. The values obtained for the grass species and maize were significantly higher than those of the dicotyledon species There was a highly significant correlation (r = + 0·912) between the maximum growth-rates¹ of the different species and the amounts of chlorophyll above the measurement level per unit area of land. However, the index $Growth-rateTotal chorophyll$ was higher for species in which the leaves were disposed horizontally or where flagging occurred (clovers, kale, and maize) than for the grass species, suggesting either that the chlorophyll of the first group of species was more active or efficient in photosynthesis than the chlorophyll in the species with erect leaves, or that a greater proportion of dry matter produced was accumulated in underground organs of these erect-leaved species. Growth-rate data were not available for cocksfoot or watercress. There was a significant correlation (r = + 0·815) between the leaf-area index absorbing 95 per cent. of the light and the maximum growth-rates of the different species, but this relationship did not take account of the photosynthetic activity of other parts of the plant or of species differences in chlorophyll content. The results are discussed in relation to the availability and utilization of light energy and carbon dioxide by the species examined.