Comparative effects of salt and alkali stress on photosynthesis and antioxidant system in tea plant (Camellia sinensis)

Salt and alkali stress are considered major abiotic stresses in agriculture. Tea plant (Camellia sinensis), an acidophilic economic crop, is seriously affected by salt and alkali stress, which severely restricts its widespread planting. However, the mechanisms underlying the response of tea plants to salt and alkali stress remain unclear. To understand the physiological and molecular responses of tea plants to salt and alkali stress, tea plants were treated with NaCl and NaHCO₃ to study the effects of salt and alkali stresses, respectively. Tea plants exhibited different symptoms under the NaCl and NaHCO₃ treatments. The leaves of tea plants suffered varying degrees of damage, and concentrations of the quality components epigallocatechin gallate, caffeine, and theanine in the leaves decreased significantly under the NaCl treatment. However, after NaHCO₃ treatment, epigallocatechin and theanine levels were significantly reduced. Photosynthesis and antioxidant enzymes changed significantly to varying degrees under both the NaCl and NaHCO₃ treatments. The stomata closed under both NaCl and NaHCO₃ treatments. After the transcriptome analysis of tea samples treated for one, three, and 7 days with NaCl or NaHCO₃, a large number of differentially expressed genes related to photosynthesis and the antioxidant system were identified. Analysis of the promoters of key differentially expressed genes revealed many light-responsive, hormone-responsive, and stress-responsive elements, and many corresponding upstream transcription factors were also differentially expressed. These results provide a basis for understanding the physiological and molecular responses of tea plants to salt and alkali stress.