Studies on the productivity of tropical pasture plants. III.* Stand structure, light penetration, and photosynthesis in field swards of Setaria and green leaf Desmodium

Abstract

Inclined point quadrat analysis was applied to pure swards of two pasture species, Desmodium intortum and Setaria sphacelata. Canopy structures are described in terms of the denseness and inclination of foliage and stems. Light penetration patterns throughout the day were calculated. The two species differed greatly in these characteristics. Desmodium leaf was oriented at a uniform mean angle to the horizontal throughout the height of the canopy, but leaf occurred predominantly in the upper layers. In contrast, leaf angles in Setaria varied down the profile, with most of the leaf in the middle layers. There were diurnal variations between and within the species in the amounts and distribution of sunlit leaf area index.

Diurnal patterns of canopy photosynthesis were estimated from calculated leaf and light profiles, photosynthesis-illuminance data, and records of incident radiation. There was an overall 1.8 : 1.0 advantage in sward photosynthesis by Setaria, arising from a greater than two-fold superiority at high angles of the sun, falling to a 1.6 : 1 .0 advantage at low angles. This trend follows the diurnal patterns of light penetration into the two canopies.

From calculated sward photosynthesis, concurrently measured dry matter production, and other data it was concluded that the relative magnitude of productivity corresponded very closely to the relative photosynthetic capacities of the leaves. Therefore, canopy structures were comparatively unimportant as determinants of yield differences.

________________ *Part II, Aust. J. Agric. Res., 21: 183 (1970)