Midday depression in photosynthesis: effect on sucrose-phosphate synthase and ribulose-1,5-bisphosphate carboxylase in leaves of prosopis juliflora (Swartz) DC

Abstract

The midday depression in net photosynthetic rate (P_N) and stomatal conductance (g_s) in Prosopis juliflora was studied in relation to two key enzymes of carbon metabolism. Diurnal gas exchange measurements carried out in autumn on P. juliflora showed a pronounced depression in P_N and g_s along with a decrease in apparent carboxylation efficiency (CE*) during midday. The activities of sucrose-phosphate synthase (SPS) and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) also showed large diurnal fluctuations. Initial RuBPCO activity (that present in vivo) and total activity (fully carbamylated activity) increased gradually with increase in irradiance, in the morning reaching a maximum by 08:00 h. The RuBPCO activity declined sharply during midday due to decrease in initial activity. The drop in the % activation of RuBPCO indicated that the deactivation of RuBPCO was achieved via a decarbamylation mechanism. There was a marked similarity in the diurnal patterns of SPS activity and the P_N. During a diurnal rhythm, SPS activity increased after irradiation, reaching a maximum at 08:00 h and then declined during midday. Diurnal fluctuations in SPS activity could be due to the changes in the amount of protein (changes in V_max) as well as to the changes in kinetic properties (changes in V_lim). Hence the midday decline in photosynthesis is closely associated with the regulation of RuBPCO and SPS activities.