Photosynthesis of tropical pasture plants III. Leaf age

Grass and legume plants were grown under near-optimum conditions in controlled-environment cabinets. Changes in net photosynthetic rate, dark respiration rate, and carbon dioxide transfer resistances during leaf ontogeny, and variability between leaves on grass tillers and legume runners were studied under controlled conditions in an open gas analysis system. As leaves aged dark respiration rate, light compensation point, and light saturation point declined progressively, whereas the initial slope of the light response curve and net photosynthetic rate measured at 10,000 f.c. increased to a maximum before declining. Immediately after unfolding, grass leaves photosynthesized at a rate more closely approximating the maximum than legume leaves which had a relatively low rate initially. For both grasses and legumes, the lower photosynthetic rate of young leaves was accompanied by high mesophyll resistances, but the decline from the maximum rate as leaves aged was associated with increases in both stomatal and mesophyll resistances. Stomatal resistance exerted a major control over the net photosynthetic rate of grasses, whereas mesophyll resistance predominated for legumes. The influence of chlorophyll concentration on the relationship between net photosynthesis and leaf age is discussed