Physiological and chlorophyll fluorescence characteristics of five afforestation cypress species under drought stress in the arid and semi-arid area

To investigate the effects of drought stress on the physiological and chlorophyll fluorescence characteristics of five typical afforestation cypress species in the arid and semi-arid regions of Lhasa, and comprehensive evaluates their drought resistance, and provide a theoretical basis for species selection and scientific configuration for afforestation in this region.Using the Cupressus gigantea、Juniperus tibetica、Juniperus pingii、Juniperus sabina、Platycladus orientalis as experimental materials, a pot water restriction experiment was conducted. Setted four water gradients: normal water supply (CK), light drought (L), mild drought (M), and high drought (H). After 28 days of continuous drought, a re-watering treatment (R) was applied. Systematically measured the osmotic adjustment substances, degree of membrane lipid peroxidation, antioxidant enzyme activities, and chlorophyll fluorescence parameters in plant leaves at each stage, and combined with principal component analysis to conduct a comprehensive evaluation of drought resistance. The results showed that : (1) The five cypress species exhibit different physiological strategies in response to drought. J. sabina displays “high basal defense” characteristics, maintaining consistently high levels of soluble sugars,PRO, and POD activity under all water conditions, with the smallest increase in membrane lipid peroxidation damage (MDA). C. gigantea and P. orientalis are classified as “stress-induced” types, showing a decline in osmotic adjustment substances under high drought while relying on a burst increase in POD activity to scavenge reactive oxygen species. J. tibetica exhibits the most sluggish defense mechanism, with extremely low POD activity, resulting in the most severe damage, as indicated by a 144.7% increase in MDA content. (2) High drought significantly inhibits photosystem II (PSII) in all species. Among them, C. gigantea and J. tibetica show the greatest reduction in maximum photochemical quantum yield (F_v/F_m), which drops to approximately 0.60, whereas J. pingii and J. sabina maintain relatively higher photosynthetic conversion efficiency. Following rewatering, J. sabina, J. pingii, and C. gigantea recover better in photosynthetic function, while recovery in P. orientalis and J. tibetica is markedly delayed, indicating irreversible photodamage. (3) Principal component analysis indicates that soluble sugar content, POD activity, and MDA content are the sensitive indicator combination for evaluating drought resistance among the five cypress species. (4) The comprehensive evaluation ranks the drought resistance of the five cypress species in descending order as follows: J. sabina > C. gigantea > J. pingii > P. orientalis > J. tibetica. To improve afforestation success in the arid and semi-arid region of Lhasa, it is recommended to use J. sabina on sites with severe drought or poor water retention. In areas with moderate drought or fluctuating water availability, C. gigantea and J. pingii can be combined to establish resilient mixed forests. P. orientalis should be limited to habitats with mild drought stress, while J. tibetica should only be planted in microsites with favorable water conditions.