Responses of growth and photosynthetic physiological characteristics of typical plants in the dry-hot valley to drought stress

Objective To investigate the effects of drought stress on the 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 river valley.

Method Using the one-year-old seedlings of these three plants as experimental materials, a pot experiment with controlled water supply was conducted. Four treatments were set: control (CK, soil moisture content was 75%-80% of the field capacity), mild drought (LD, soil moisture content was 60%-65% of the field capacity), moderate drought (MD, soil moisture content was 45%-50% of the field capacity), and severe drought (SD, soil moisture content was 30%-35% of the field capacity). The growth and photosynthetic physiological indicators of the three plants 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 mild drought stress, and increased under moderate and severe drought stress. (3) The water use efficiency of Leucaena leucocephala and Pueraria montana var. lobata increased under mild and moderate drought, but decreased under severe drought; the water use efficiency of Cajanus cajan continuously decreased with the intensification of drought. (4) The initial fluorescence of the three plants increased first and then decreased with the intensification of drought; the maximum fluorescence and variable fluorescence of Leucaena leucocephala and Cajanus cajan increased first and then decreased, while that of Pueraria montana var. lobata continuously decreased. The potential activity of PSII varied significantly among species, with Pueraria montana var. lobata increasing from MD to SD, and Leucaena leucocephala and Cajanus cajan continuously decreasing throughout the drought process. (5) Correlation analysis showed that the 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 inhibited the growth of the three plant seedlings and reduced their physiological activities, causing damage to the cell membranes of leaves and slowing down the growth of plants. The seedlings enhanced their drought adaptability through osmotic regulation and reducing stomatal conductance and transpiration rate, maintaining a relatively stable metabolic level. The comprehensive evaluation indicated that the drought resistance of the three plants decreased from strong to weak: Pueraria montana var. lobata > Leucaena leucocephala > Cajanus cajan. For vegetation restoration in the dry and hot river valley, Pueraria montana var. lobata should be the preferred species and Leucaena leucocephala as a companion species.