Activation Energy Analysis and Limiting Factors in Photosynthesis

Abstract

It has been proposed that activation energies of CO2 exchange obtained from Arrhenius plots of the temperature response of leaf CO2 exchange rates (or the equivalent QIO analysis) should elucidate the rate-limiting processes. Chmora and Oya (1967), for example, suggest that a QlO (15-25°0) of about 1 for maize photo-synthesis at low light and low CO2 concentration implies photochemical reactions are limiting, at high light and high CO2 a QIO of 1 ·6 implies enzyme reactions are limiting, whilst at high light and low CO2 a QlO of 1· 25 suggests diffusion is limiting. Bjorkman, Nobs, and Hiesey (1969) surmise that for Mimulus sp. at 0·07% CO2 the coincidence of QIO (15-30°0) for both CO2 exchange (at 1'5% oxygen and saturating light) and extracted carboxydismutase (QlO = 2·7-3·3) could reflect a causal relationship. Charles-Edwards and Charles-Edwards (1970) find that for clones of three grass species there is a clustering of the determinations of activation energy around certain values. It is suggested that each such value may be characteristic of a certain rate-limiting process.