Response and acclimatization of a CAM orchid, Dendrobium Sonia 'Earsakul' to drought, heat, and combined drought and heat stress

Response and the acclimation to abiotic stress, including drought, heat (40-45?), and combined drought and heat stress for 30 days, have been clarified in a crassulacean acid metabolism (CAM)-orchid, Dendrobium Sonia 'Earsakul'. The drought stress alone caused a more severe effect than the heat stress only. In the drought stress, at 30 days after stress treatments (DAT), the stomatal conductance (gs) and transpiration rate (TR) rapidly dropped after the peak time (6 pm) then the stomata were completely closed to maintain the leaf's relative water content (RWC) at 95.6%, resulting in a drop in the carbon exchange rate (CER). However, the phosphoenolpyruvate carboxylase (PEPC) activity, malondialdehyde (MDA), and antioxidant activity increased at 30 DAT.In heat stress, CER dropped significantly at 7 DAT and was retrieved to the same as the control's level during 14-30 DAT. Moreover, the gs and TR of the heat treatment were significantly increased during 14-30 DAT. In addition, we also found the presence of aerenchyma in leaf sheath in response to heat stress. The combined drought and heat stress caused the severest symptom. The significant loss of leaf RWC at the back pseudobulb was detected from 7 to 30 DAT, resulting in decreasing leaf CER, gs, and TR. However, an increase in PEPC, MDA, and antioxidant activity at 30 DAT was observed after facing both stress combinations.