Effect of drought stress on characteristics of photosynthesis and chlorophyll fluorescence of four species of Cassia

The effects of drought stress on the characteristics of photosynthesis and chlorophyll fluorescence of four species of Cassia were studied by conducting a water control experiment (mild drought, severe drought, and normal irrigation) using four Cassia species (C. obtusifolia, C. occidentalis, C. corymbosa, C. bicapsularis). The results showed that 1) drought stress resulted in a decline in the chlorophyll content in all four species of Cassia, and the stress degree of C. occidentalis was the lowest, meaning that C. occidentalis maintains the highest photosynthetic capacity under low moisture conditions. The chlorophyll content of C. corymbosa was the highest in all three treatments, indicating that the light energy absorption and transformation capacity of C. corymbosa are stronger than those of the other three species. 2) Drought stress resulted in all four species of Cassia displaying reduced stomatal conductance (Gs), while the stomatal limitation (Ls) value increased, which was accompanied by a decrease in intercellular CO2 concentration (Ci) and a decline in the rate of photosynthesis (Pn), indicating that drought stress affects stomatal factors. The exception to this was C. obtusifolia, which showed an increase in Ci and decrease in Ls, indicating that the results are because of non-stomatal factors. Under drought stress, C. occidentalis showed a higher photosynthetic rate, and C. corymbosa showed a relatively high degree of stomatal opening. 3) Drought stress led to a decrease in Fm and Fv/Fm, and an increase in Fo among the four species of Cassia, indicating that damage to photosystem Ⅱ(PSⅡ) or heat dissipation. The qP and ETR values decreased, while the NPQ value increased, indicating damage to PSⅡ by drought stress, but effectively avoiding or reducing photoinhibition and light oxidation, which is caused by PSⅡ absorbing excess light energy. C. occidentalis showed the most stable performance, which means that C. occidentalis has a higher drought resistance