The aim of this study was to evaluate the effects of low air temperature during nocturnal (TN) and diurnal (TD) periods as well as the substrate temperature (TS) on photosynthesis of 'Valencia' orange tree grafted on Rangpur lime rootstock. The experiment was carried out in a growth chamber with seven-month-old plants. The plants were exposed to the following temperature regimes: low substrate temperature (LTS, with: TD = 28°C, TN = 20°C,
TS = 10°C); low air temperature during night (LTN, with: TD = 28°C, TN = 10°C, TS = 26°C); low temperature during nighttime and also low substrate temperature (LTSN, with: TD = 28°C, TN = 10°C,
TS = 10°C); low air temperature during both diurnal and nocturnal periods (LTND, with: TD = 17°C, TN = 10°C, TS = 26°C); and finally to low air temperature (night and day) and low substrate temperature (LTSND, with: TD = 17°C, TN = 10°C, TS = 10°C). As reference (control), plants were subjected to TD = 28°C, TN = 20°C, and TS = 26°C. Measurements of leaf gas exchange, photochemical activity and carbohydrate concentrations were performed after six days of exposure to each thermal treatment. Compared to the control, all thermal regimes caused reductions in photosynthesis due to diffusive and metabolic limitations. The photoinhibition was transient in plants exposed to night and substrate low temperatures, whereas it was severe and chronic in plants subjected to chilling during the diurnal period. However, the lowest photosynthesis was observed in plants with low substrate temperature of 10°C (in LTS, LTSND and LTSN treatments), regardless of air temperature. The occurrence of cold night and/or its combination with low substrate temperature caused accumulation of starch in leaves. When considering carbohydrate concentrations in stems and roots, it was not possible to establish a clear response pattern to chilling. In conclusion, the low substrate temperature causes a greater reduction of CO2 assimilation in citrus plants as compared to the occurrence of low air temperature, being such response a consequence of diffusive and biochemical limitations. and C. M. A. Santos ... [et al.].