ADENINE-NUCLEOTIDES AND THE XANTHOPHYLL CYCLE IN LEAVES .1. EFFECTS OF CO2- AND TEMPERATURE-LIMITED PHOTOSYNTHESIS ON ADENYLATE ENERGY CHARGE AND VIOLAXANTHIN DE-EPOXIDATION

The effects of varying the steady-state rate of non-cyclic photosynthe tic electron transport on the leaf adenylate energy charge and the epo xidation state of the xanthophyll-cycle pigments were determined in le aves of cotton (Gossypium hirsutum L.) and the mangrove (Aegialitis an nulata R.Br.). Different photosynthetic rates were obtained by varying the intercellular CO2 concentration and/or the leaf temperature, and in some cases, by changing the leaf conductance to CO2 diffusion. Also determined were the effects of these treatments on the changes in the adenylate energy charge and the epoxidation state of the xanthophyll- cycle pigments that occur after darkening of the leaves. The leaf aden ylate pool remained close to equilibrium with the adenylate kinase bot h in the light at steady state and during dark relaxation. The adenyla te energy charge increased as the photosynthetic rate decreased and ma ximal levels were obtained when CO2 assimilation and, therefore, non-c yclic electron flow were maximally inhibited. This implies that, in na ture, photophosphorylation may provide energy needed for ion-pumping a nd biosynthetic and repair processes, even under stress conditions tha t severely restrict or prevent photosynthetic gas exchange. High level s of de-epoxidized violaxanthin in the light did not necessarily indic ate or depend on a high adenylate energy charge. Dithiothreitol, an in hibitor of the violaxanthin de-epoxidase and ascorbate peroxidase, did not inhibit the adenylate energy charge in the light. Thus we conclud e that coupled electron transport during inhibited CO2 fixation was no t driven by a dithiothreitol-sensitive Mehler ascorbate-peroxidase rea ction. The changes in the adenylate energy charge and xanthophyll re-e poxidation that follow when leaves were darkened are strongly affected by the preceding photosynthetic rate. Postillumination fluctuations i n adenylate energy charge, both at 15 degrees and 27 degrees C, were m ost pronounced when the preceding photosynthetic rate was minimal and least pronounced when this rate was maximal. Temperature had a conside rably greater influence in the dark on xanthophyll re-epoxidation than on the pattern of adenylate relaxation.