Photoinhibition under Atmospheric O2, the Activation State of RuBP Carboxylase and the Content of Photosynthetic Intermediates in Soybean and Wheat

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Abstract
Ward, D. A. and Drake, B. G. 1987. Photoinhibition under atmospheric O2, the activation state of RuBP carboxylase and the content of photosynthetic intermediates in soybean and wheat.—J. exp. Bot. 38: 1937–1948. Associations between photosynthesis, the activation state of RuBP carboxylase and the contents of photosynthetic intermediates were compared in soybean and wheat leaves before and after exposure to photoinhibitory treatments in the presence of atmospheric O2. Exposing attached leaves to a supra-saturating irradiance (3 800 μmol quanta m– 2 s–1) for 2 h in CO2-free air decreased carboxylation efficiency and the light-saturated photosynthetic rate in air by approximately 50%. Exposure to the photoinhibitory treatment for periods in excess of 2 h did not cause a further decrease of photosynthesis in soybean. Although photosynthesis was reduced, the initial and total (fully-activated) activities of ribulose 1,5-bisphosphate carboxylase (RuBPCase) in leaf extracts were unaltered in each species by the photoinhibitory treatment. This was true for leaves sampled under both air and at a rate-limiting intercellular CO2 partial pressure (Ci) of 75 μPa Pa–1. The contents of ribulose l,5-bisphosphate (RuBP) and 3-phosphoglyceric acid (3-PGA) were reduced by the photoinhibitory treatment in soybean leaves sampled in air and at a rate-limiting Ci, although the RuBP/3-PGA ratio was unaffected. The relative reduction of RuBP content in soybean leaves at rate-limiting C1 was similar to the corresponding reduction of carboxylation efficiency. For wheat,the relative reduction of RuBP content at rate-limiting Ci (–19%) caused by the photoinhibitory treatment was considerably less than the corresponding decrease of carboxylation efficiency (–49%).The RuBP/3-PGA ratio of wheat was also increased significantly by the photoinhibitory treatment The significance of these observations to the regulation of CO2-limited photosynthesis in leaves experiencing photoinhibition under atmospheric oxygen is discussed. Consideration is also given to the previous contention that contemporary measurements of initial activity in crude extracts may provide a spurious indication of the amount of the enzyme-CO2-Mg2 + form of RuBP carboxylase present in the leaf.