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

Abstract

Mesophyll protoplasts of pea required only 74.1 μM CO2 for maximal photosynthesis, unlike chloroplasts, which required up to 588 μM CO2. Such a markedly low requirement for CO2 could be because of an internal carbon source and/or a CO2 concentrating mechanism in mesophyll protoplasts. Ethoxyzolamide (EZA), an inhibitor of internal carbonic anhydrase (CA) suppressed photosynthesis by mesophyll protoplasts at low CO2 (7.41 μM) but had no significant effect at high CO2 (741 μM). 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 CO2 but not at high CO2. 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 CO2 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 O2 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 CO2, we suggest that photorespiration could be a significant source of CO2 for photosynthesis in mesophyll protoplasts at limiting CO2 and at atmospheric levels of oxygen. Obviously, carbonic anhydrase is essential to concentrate or retain CO2 in mesophyll cells.