Limitations to photosynthesis at different temperatures in the leaves of Citrus limon

Abstract

The response of CO2 assimilation rate (A) to the intercellular partial pressure of CO2 (Ci) was measured on intact lemon leaves over a range of temperatures (10 to 40°C). The A/Ci response shows how change in the leaf temperature alters the activity of ribulose-1,5-bisphosphate (RuBP) carboxylase-oxygenase (Rubisco) and RuBP regeneration via electron transport. The rate of A reached a maximum of 7.9 to 8.9 μmol m-2 s-1 between 25 and 30°C, while dark respiration (Rd) increased with temperature from 0.4 μmol m -2 s-1 at 10°C to 1.4 μmol m-2 s -1 at 40°C. The maximum rates of carboxylation (V c,max) and the maximum rates of electron transport (Jmax) both increased over this temperature range from 7.5 to 142 μmol m -2 s-1 and from 23.5 to 152 μmol m-2 s -1, respectively. These temperature responses showed that A can be limited by either process depending on the leaf temperature, when Ci or stomatal conductance are not limiting. The decrease in A associated with higher temperatures is in part a response to the greater increase in the rate of oxygenation of RuBP compared with carboxylation and Rd at higher temperatures. Although A can in theory be limited at higher Ci by the rate of triose-phosphate utilization, this limitation was not evident in lemon leaves.