Carbon dioxide and water vapour exchange of attached, young, fully expanded leaves of tropical pasture species were measured in an open gas analysis system. The influence of illuminance, carbon dioxide concentration, leaf temperature, and leaf-air vapour pressure difference on net photosynthetic rate and carbon dioxide transfer resistances was studied.

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.

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Two grasses and two legumes were grown at three illuminances: one grass and one legume in a growth cabinet (100, 50 and 34% relative illuminance) and one of each in a glasshouse (100, 30 and 10% relative illuminance). The response to shading was investigated by using classical growth analysis techniques, and the photosynthesis-respiration balance obtained by Watson and Hayashi's method. Shading caused a reduction in the proportion of root and a corresponding increase in the proportion of leaf, and an increase in the shoot/root ratio in all species except green panic. The relative growth rates of grasses (which were only clearly superior at the highest illuminance) were affected more by shading than were those of legumes, and the greater shading effect resulted from a greater decrease in net assimilation rate, and to a lesser extent, from a smaller compensatory increase in leaf area ratio in the grasses than in the legumes. The greater sensitivity of net assimilation rate of grasses to shading arose from a stronger dependence of the photosynthetic rate on illuminance. The significance of these findings for pasture production is discussed. Both tiller and runner production were dependent upon the total amount of radiant energy received by the plant, and in the growth cabinet, at least, were independent of its intensity, duration and sequence. ______________________ *Part IV, Aust. J. Agric. Res., 25: 415 (1974).