Photosynthesis of tropical pasture plants II. Temperature and illuminance history

Abstract

Grasses and legumes were grown at two temperatures in controlled-environment rooms and at three illuminances (obtained by shading) in a glasshouse. Carbon dioxide and water vapour exchange of leaves were measured in an open gas analysis system. Net photosynthetic rates of grasses and legumes grown at 20°C and measured at 30°C were lower (and transfer resistances were concomitantly higher) than values for plants grown at 30°C, but almost complete acclimatization to the higher temperature occurred within 15 hr. Dark respiration rates varied with species and with illuminance prior to measurement, but were unaffected by growth temperature. Shading markedly affected the anatomy and the photosynthetic characteristics of both grass and legume leaves. Shaded leaves were thinner and contained fewer, smaller, and less densely packed cells than unshaded leaves. Light saturation point, light compensation point, and dark respiration rate declined as the level of shading increased, but the initial slope of the light response curve was unaffected. The lower net photosynthetic rate of shaded leaves was associated with increased stomatal and mesophyll resistances, and it is argued that the latter arose from higher carboxylation resistances. Net photosynthetic rate was positively related to leaf thickness, specific leaf weight, and the reciprocal of mesophyll resistance. These relationships and the relationship between net photosynthesis and chlorophyll content are discussed. © 1971 CSIRO.