Effects of continuous lighting with or without a diurnal temperature difference on photosynthetic characteristics of tomato leaves

Too long photoperiods such as continuous lighting (CL) lead to chlorosis-like physiological disorder (CL-induced injury) in leaves of tomato. On the other hand, CL-induced injury was not observed where there is a diurnal temperature difference (DIF) of 8-10°C or more. The objective of this study was to investigate the interactive effects of photoperiod and DIF on photosynthetic characteristics of tomato leaves. Four-week-old plants were treated with a photoperiod of 12 (P12) or 24 h (P24) and a DIF of 10 (25/15°C, D10) or 0°C (20/20°C, D0) for 12/12 h day/night cycle. After two-week treatment, the light-limited (A200) and light-saturated (A1,600) rates of photosynthesis at photosynthetic photo flux densities of 200 and 1,600 μmol m-2 s-1 and chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and N contents of leaves were examined. Severe and moderate CL-induced injury was observed on leaves in P24D0 and P24D10, respectively, but not at all in P12D10. In P12D0, the whole leaves became pale green. A200 was significantly lower in P24 treatments than in P12 treatments. There were no significant differences in A200 based on DIF. In contrast, A1,600 was significantly lower in D0 than in D10 irrespective of photoperiod. Thus, no interactive effect of photoperiod and DIF was found in the gas-exchange rates, suggesting that they affect different processes in leaf carbon metabolism independently. The Chl and Rubisco contents in P12D0 were significantly lower than those in the other treatments, associated with a lower leaf N content. Although the lower A1,600 in P12D0 was probably related to the lower Rubisco content, the other differences in gas-exchange rates among treatments cannot be accounted for by the differences in the amounts of photosynthetic components examined. More detailed physiological and biochemical characterization is necessary to understand the complex effects of photoperiod and DIF on leaf photosynthesis.