Abstract: Using artificial water supplementation and natural water control methods, we measured the light response process of photosynthetic physiological parameters of potato leaves under different water conditions and further analyzed the threshold effects for the responses to soil water. Soil moisture was set as 55%, 65% and 75% field water-holding capacity for drought stress treatments, 80% field water-holding capacity for control treatment and 85% field water-holding capacity for waterlogging treatment. The results showed that net photosynthetic rate (P_n), water use efficiency (WUE), and light response parameters of potato leaves had obvious water critical effects. The apparent quantum efficiency (AQY), maximum net photosynthetic efficiency (P_nmax), and light saturation point (LSP) increased first and then decreased with increasing soil water content, while light compensation point (LCP) and dark respiration rate (R_d) first decreased and then increased. Under waterlogging or drought stress, the ability of light absorption and conversion of potato leaves under weak light was reduced, the utilization of strong light and weak light was inhibited, and dry matter accumulation was maintained by reducing respiratory consumption. With increasing soil moisture, P_n and WUE increased first and then decreased, but the critical values of moisture were different. The relative soil water content (W_r) at 56.7% was the critical point of stomatal limitation in potato leaves. The W_r at 96.3% was water saturation point of P_n, and W_r at 70.1% was the high efficiency point of WUE. Photosynthetic capacity of potato leaves under waterlogging was higher than that under drought stress. The moisture threshold of potato in semi-arid area was determined by mathematical model. The W_r value between 70.1% and 96.3% was high-yield and high-efficiency water, in which the photosynthetic productivity and water use efficiency of potato were higher. Therefore, W_r between 70.1% and 96.3% is suitable for potato growth in semi-arid area. Photosynthetic capacity of potatoes under waterlogged conditions is significantly higher than that under drought stress in semi-arid area.

Key words: semi-arid region, soil moisture, water use efficiency, threshold effect, photosynthetic characteristics.