The Effect of Form and Concentration of Nitrogen Supply on Photosynthetic Temperature Optima and Light Saturation Levels in Selected C4 Photosynthetic Grasses

Abstract

The influence of increased nitrogen supply on the carbon dioxide gas exchange characteristics of indigenous C4 photosynthetic grasses and Zea mays L. were determined in this study. The grasses investigated were Themeda triandra Forsk, Hyparrhenia hirta (L) Stapf. and Eragrostis curvula (Schrad) Nees. Previous studies had established that the carbon dioxide compensation point increased beyond the levels usually associated with C4 photosynthetic plants, within 24h of increasing the nitrogen supply either as nitrate, or as ammonium nitrogen (Grossman and Cresswell, 1974). In this study the increased supply of nitrogen resulted in a shift of both the temperature optima as well as the optimum level of light intensity for photosynthetic activity. In both cases the shift was towards a lower optimum level, approximating the levels normally found associated with C3 photosynthetic plants. The changes in carbon dioxide gas exchange characteristics resulting from an increase in nitrogen supply are discussed