Plant Science Letters, 5 (1975) 189--195 189 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands CARBONIC ANHYDRASE AND CARBOXYLASE ACTIVITIES FROM PLANTS (LOLIUM MULTIFLORUM) ADAPTED TO DIFFERENT LIGHT REGIMES A. REYSS and J.L. PRIOUL Laboratoire Structure et M~tabolisme des Plantes, associ~ au C.N.R.S. (L.A. 40), Universit~ de Paris-Sud, F. 91405 Orsay (France) (Received January 15th, 1975) (Revision received and accepted June 6th, 1975) SUMMARY As incident light received during growth increases, Lolium plants show an important adaptive enhancement of ribulosediphosphate (RuDP) carboxylase, phosphoenolpyruvate (PEP) carboxylase, and carbonic anhydrase as expressed on a leaf area basis. These results are paralleled with the associated increase in net maximum photosynthesis and related physiological parameters, notably the resistances to CO2 transfer. INTRODUCTION The dependence of light-saturated photosynthesis on the amount of incident light received during growth has been frequently reported, but the role and relative importance of internal intrinsic factors in determining the different photosynthetic characteristics are open to question. These internal factors may be roughly subdivided into two main components: firstly, transfer pro- cesses along the pathway from external air to carboxylation sites and second- ly, purely biochemical processes located in the Calvin cycle. By means of a convenient analogical model [ 1] photosynthetic light response curves can be analysed, thus permitting the determination of three resistances to CO2 transfer: rs (stomatal resistance), r m (mesophyll resistance) and rx (carboxyla- tion resistance). Previous results [2] based upon this method have shown a stepwise reduction of all the resistances as growth irradiance increases. What- ever the incident light may be, the CO2 transfer in the mesophyll (rm) appears to be the most important limiting factor. In contrast, several authors [ 3, 4] re- porting on correlations between RuDP carboxylase activity and light-saturated Abbreviations: diamox, 5-acetamido-l,3,4-thiadiazole-2-sulphonamide; PEP, phosphoenol- pyruvate; RuDP, ribulosediphosphate.