Photosynthetic characteristics in leaves of ‘Valencia’ orange ( Citrus sinensis (L.) Osbeck) grown under high and low temperature regimes

Abstract

The photosynthetic activities of leaves of 1-year-old ‘Valencia’ orange ( Citrus sinensis (L.) Osbeck) were determined after trees were maintained for 30 consecutive days in controlled growth chambers under high temperature (HT, 32.2°C day/21.1°C night) and low temperature (LT, 15.6°C day/4.4°C night). Leaf CO 2 exchange rates ( CER), stomatal conductance ( C s), transpiration ( E), chlorophyll (Chl), soluble protein, and proline of the LT treatment were 48, 55, 26, 78, 113 and 265%, respectively, of the HT treatment. The water potentials (ψ) of leaves from HT treatment were about 0.5 MPa more negative than those from LT leaves. Activities of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and phosphoenolpyruvate carboxylase (PEPCase) of the HT treatment, expressed on a leaf fresh weight basis, were 91 and 49%, respectively, of the LT treatment. PEPCase of both temperature treatments, however, showed no differences in affinities for HCO 3 − and PEP, having a Km (HCO 3 −) of 0.61 mM and a Km (PEP) of 0.15 mM. The ratio of RuBPCase/PEPCase was 6.6 for the HT treatment, compared to 3.5 for the LT treatment. When the 30-day HT-treated trees were transferred to the LT chamber, CER, C s and ψ, determined 24 hr after transfer, were comparable to values of 30-day LT-treated trees; the RuBPCase activity decreased about 18% and PEPCase activity increased about 31% after 96 hr in the LT chamber. These relative changes decreased the RuBPCase/PEPCase ratio from 6.6 to 3.7. Switching trees from the 30-day LT treatment to the HT chamber resulted in increases in CER, C s and E and decreases in ψ; the RuBPCase activity decreased about 10%, but PEPCase activity was relatively unaffected after 96 hr following the transfer to the HT chamber. The LT-induced changes in the ratio of carboxylase activities would indicate that some alteration in the photosynthetic carbon metabolism might occur in ‘Valencia’ orange leaf tissues subjected to cold acclimation conditions.