Effects of water supply on photosynthesis and fluorescence characteristics of sweet potatoIpomoea batatas (L.) Lam. leaves and comparison of light response models

Water supply plays a vital role in the growth and yield of sweet potato. In this paper, the edible sweet potato 'Yanshu 25' was used to study the photosynthesis-light response process and fluorescence characteristics of sweet potato leaves under different water treatments. Different models were used to study the light response process, and the effects of different water treatments on photosynthesis and fluorescence characteristics of sweet potato were analyzed. The results showed that:drought and flooding treatment significantly reduced the net photosynthetic rate (P_n), stomatal conductance (G_s), and transpiration rate (T_r) of sweet potato leaves; at PAR ≤ 1 000 μmol·m^-2·s^-1, the decrease of P_n under drought and flooding treatment was induced by stomatal restrictions, and at PAR > 1 000 μmol·m^-2·s^-1, the decrease of P_n was induced by non-stomatal restrictions. The fluorescence parameters indicated that the light capture and absorption capacity of photosystem Ⅱ (PS Ⅱ) of sweet potato leaves decreased under drought and flooding treatment, while the heat dissipation increased. The modified rectangular hyperbola model demonstrated the best fit among all the light response models and matched the light saturation point. The model was, therefore, suitable for simulation of photoresponse simulation under different soil water environments. The model parameters showed that the initial quantum efficiency (α) of sweet potato leaves ranged from 0.039 to 0.055 under different water treatments, falling within the theoretical range of 0-0.125. The sweet potato leaves demonstrated significant photo-saturation and photo-inhibition under drought and flooding treatment, resulting in a reduced light utilization capacity. The light utilization capacity under flooding treatment was lower than that under drought treatment. It can be concluded that the modified rectangular hyperbola model is the optimal model to analyze light response changes of sweet potato under different water conditions. Both the drought and flooding treatment damage the photosynthetic system of sweet potato and reduce its photosynthetic capacity. Flooding is more likely to reduce the light utilization capacity of sweet potato leaves when compared with drought, and high light intensity increases the degree of water stress of sweet potato.