Objective This paper aims to investigate the effects of drought stress on growth and photosynthetic physiological characteristics of Leucaena leucocephala, Pueraria montana var. lobata, and Cajanus cajan, and to evaluate the drought resistance of these three typical plants, providing theoretical basis for the selection of drought-tolerant plants and vegetation restoration in the dry and hot valley.

Method Using one-year-old seedlings of these three species as experimental materials, a pot experiment with controlled water supply was conducted. Four treatments were set as followings: control (CK, soil moisture content was 75%–80% of field capacity), mild drought (LD, 60%−65% of field capacity), moderate drought (MD, 45%−50% of field capacity), and severe drought (SD, 30%−35% of field capacity). The growth and photosynthetic physiological indicators of the three species under each treatment were measured, and the drought resistance was evaluated by the membership function method.

Result (1) With the intensification of drought, the plant height and ground diameter growth, relative water content of leaves, net photosynthetic rate, stomatal conductance, transpiration rate, carboxylation efficiency, and maximum photochemical efficiency of PSII decreased, while the water saturation deficit, malondialdehyde, and proline contents increased. (2) The intercellular CO₂ concentration decreased under MD stress, and increased under MD and SD stress. (3) The water use efficiency of Leucaena leucocephala and P. montana var. lobata increased under LD and MD stress, but decreased under SD stress; the water use efficiency of Cajanus cajan continuously decreased with the intensification of drought. (4) The initial fluorescence of the three species increased first and then decreased with the intensification of drought; the maximum fluorescence and variable fluorescence of L. leucocephala and C. cajan increased first and then decreased, while that of P. montana var. lobata continuously decreased. The potential activity of PSII varied significantly among varied species, with P. montana var. lobata increasing from MD to SD, and L. leucocephala and C. cajan continuously decreasing throughout the drought process. (5) Correlation analysis showed that plant height growth was significantly positively correlated with net photosynthetic rate, stomatal conductance, transpiration rate, and chlorophyll fluorescence parameters (P < 0.01); the relative water content was significantly positively correlated with plant height and ground diameter growth, net photosynthetic rate, stomatal conductance, and water saturation deficit, and negatively correlated with malondialdehyde (P < 0.01); the chlorophyll fluorescence parameters were strongly correlated with each other.

Conclusion Drought stress significantly inhibites the growth of three plant seedlings and reduces their physiological activities, causing damage to the cell membranes of leaves and slowing down the growth of plants. The seedlings enhance their drought adaptability through osmotic regulation and reducing stomatal conductance and transpiration rate, maintaining a relatively stable metabolic level. The comprehensive evaluation indicates that the drought resistance of the three species decreases from strong to weak is P. montana var. lobata, L. leucocephala and C. cajan. For vegetation restoration in the dry hot valley, it is recommended to choose P. montana var. lobata as the preferred species, with L. leucocephala as the companion species.