Markedly low requirement of added CO₂ for photosynthesis by mesophyll protoplasts of pea (<i>Pisum sativum</i>): possible roles of photorespiratory CO₂ and carbonic anhydrase

Abstract

Mesophyll protoplasts of pea required only 74.1 μ M CO 2 for maximal photosynthesis, unlike chloroplasts, which required up to 588 μ M CO 2 . Such a markedly low requirement for CO 2 could be because of an internal carbon source and/or a CO 2 concentrating mechanism in mesophyll protoplasts. Ethoxyzolamide (EZA), an inhibitor of internal carbonic anhydrase (CA) suppressed photosynthesis by mesophyll protoplasts at low CO 2 (7.41 μ M ) but had no significant effect at high CO 2 (741 μ M ). However, acetazolamide, another inhibitor of CA, did not exert as much dramatic effect as EZA. Three photorespiratory inhibitors, aminoacetonitrile or glycine hydroxamate (GHA) or aminooxyacetate inhibited markedly photosynthesis at low CO 2 but not at high CO 2 . Inhibitors of glycolysis or tricarboxylic acid cycle (NaF, sodium malonate) or phosphoenolpyruvate carboxylase (3,3‐dichloro‐2‐dihydroxy phosphinoyl‐methyl‐2‐propenoate) had no significant effect on photosynthesis. The CO 2 requirement of protoplast photosynthesis and the sensitivity of photosynthesis to EZA were much higher at low oxygen (65 nmol ml −1 ) than that at normal oxygen (212 nmol ml −1 ). In contrast, the inhibitory effect of photorespiratory inhibitors on protoplast photosynthesis was similar in both normal and low oxygen medium. The marked elevation of glycine/serine ratio at low O 2 or in presence of GHA confirmed the suppression of photorespiratory decarboxylation by GHA. While demonstrating interesting difference between the response of protoplasts and chloroplasts to CO 2 , we suggest that photorespiration could be a significant source of CO 2 for photosynthesis in mesophyll protoplasts at limiting CO 2 and at atmospheric levels of oxygen. Obviously, carbonic anhydrase is essential to concentrate or retain CO 2 in mesophyll cells.