Modification of the Induction of Photosynthesis in Wheat by Glyphosate, an Inhibitor of Amino Acid Metabolism

Abstract

Ireland, C. R., Percival, M, P. and Baker, N. R. 1986. Modification of the induction of photosynthesis in wheat by glyphosate, an inhibitor of amino acid metabolism.—J. exp. Bot. 37: 299-308. The effect of the herbicide glyphosate, an inhibitor of amino acid metabolism, upon the kinetics of induction of photosynthesis was investigated in wheat leaves. After treatment with a commercial preparation of glyphosate the induction period for both photosynthetic carbon assimilation and chlorophyll fluorescence emission was increased approximately by 50% after 2 d and by 100% after 5 d. The quenching of chlorophyll fluorescence became a slower process and the S-M transient was diminished. In the steady-state neither the rate of carbon fixation nor the level of fluorescence emission were altered. The changes in induction kinetics preceded the occurrence of visible leaf damage. The fluorescence emission was analysed by a technique which estimates the redox state of the PS2 primary electron acceptor, Q, and the photochemical and non-photochemical components of fluorescence quenching. Glyphosate produced no fundamental change in the relationship between the steady-state values of these parameters until the time when extensive leaf damage occurred. Thus there was no direct effect of glyphosate on the capacity for light capture or on the rate of photosynthetic electron transport. This was confirmed by the absence of any effect on the quantum efficiency of oxygen evolution. The slower rate of fluorescence quenching during induction is attributed to a slower generation of the trans-thylakoid A pH. It is proposed that glyphosate affects photosynthetic induction kinetics by an indirect modification of carbon metabolism which limits photosynthetic rate during this phase. The possibility of screening for rapid effects of non photosynthetic herbicides, such as glyphosate, by monitoring the kinetics of photosynthetic induction, is raised. Key words—Photosynthesis, induction, glyphosate (N-phosphonomethyl-glycine), wheat (Triticum aestivum L.). Correspondence to: Department of Biology, University of Essex, Wivenhoe Park, Colchester, Essex C04 3SQ, U.K.