Effect of low temperature and high humidity stress on physiology of cucumber at different leaf stages

Low temperature (LT) and high humidity (HH) are important environmental factors in greenhouses and plastic tunnels during the cold season, as they hamper plant growth and development. Here, we studied the effect of LT (day/night: 9/5 °C, 25/18 °C as control) and HH (95%, 80% as control) on young cucumber plants at the 2, 4 or 6 leaf stages. LT+HH stress resulted in a decline in shoot, root and total fresh and dry weights, and decreased P_n , g_s , T_r , F_v /F_m , qP, ETR and chlorophyll, and increased MDA, H₂ O₂ , O₂ ^- , NPQ and C_i as compared to the control at the 2 leaf stage. SOD, POD, CAT, APX and GR were upregulated under LT+HH stress as compared to the control at the 6 leaf stage. ABA and JA increased under LT+HH stress as compared to the control at the 6 leaf stage, while IAA and GA decreased under LT+HH stress as compared to the control at the 2 leaf stage. Our results show that LT+HH stress affects young cucumber plant photosynthetic efficiency, PSII activity, antioxidant defence system, ROS and hormone profile. Plants at the 6 leaf stage were more tolerant than at the 2 and 4 leaf stages under stress conditions.