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

The response of CO₂ assimilation rate (A) to the intercellular partial pressure of CO₂ (C_i) was measured on intact lemon leaves over a range of temperatures (10 to 40°C). The A/C_i 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 (R_d) 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 (J_max) 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 C_i 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 R_d at higher temperatures. Although A can in theory be limited at higher C_i by the rate of triose-phosphate utilization, this limitation was not evident in lemon leaves.