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 40ºC). 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 30ºC, while dark respiration (Rd) increased with temperature from 0.4 µmol m-2 s-1 at 10ºC to 1.4 µmol m-2 s-1 at 40ºC. The maximum rates of carboxylation (Vc,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.
A/Ci curves; gas exchange; lemon
