Abstract:Using pot experiment for 4 water-controlled treatments: control (80% soil water content), mild drought (65% soil water content), moderate drought (50% soil water content) and severe drought (35% soil water content), we explored the response of drought stress on photosynthetic characteristics of Polygonatum cyrtonema Hua. with 3-year-old seedlings. The results showed as following: (1) with the deepening of drought stress, the contents of chlorophyll (a+b) and chlorophyll b in the leaves showed a trend of increase first and then decrease, while the content of chlorophyll a decreased gradually. However, the chlorophyll a/b and carotenoids content showed an trend of “up-down-up”. (2) The diurnal variation of net photosynthetic rate (Pn) showed a “single-peak” curve under control and mild drought stress conditions, while under moderate and severe drought stress conditions, the diurnal variation of Pn showed a “double-peak” curve, and the main influencing factor of the “photosynthetic midday depression” phenomenon was non-stomatal limitation.(3) With the deepening of drought stress, the average daily net photosynthetic rate, average daily transpiration rate, average daily stomatal conductance and average daily light use efficiency were decreased, while the average water use efficiency was increased. (4) With the deepening of drought stress, the light compensation point decreased first and then increased sharply, while the light saturation point increased first and then decreased, and its apparent quantum efficiency was gradually decreased. Meanwhile, the CO2compensation point and CO2 saturation point both showed an trend of “increase-decrease-increase”, while their carboxylation rates gradually decreased, and they all were significantly different from the rest of the treatments under severe drought stress. (5) Under severe drought stress, the initial fluorescence (Fo), maximum fluorescence (Fm), potential activity of PSⅡ (Fv/Fo) and maximum photosynthetic efficiency of PSⅡ (Fv/Fm) were significantly lower than those of other treatments. It was found that P. cyrtonema has certain adaptability to mild drought stress, while the moderate drought stress may be the turning point of drought damage. However, the severe drought stress would seriously inhibit the photosynthetic capacity.