The diurnal variation of photosynthesis and effect of drought stress on photosynthetic characteristics of Lysimachia davurica

This study aimed to clarify the photosynthetic characteristics of Lysimachia davurica under natural conditions and drought stress conditions. The diurnal variation in photosynthesis under natural conditions, and the light response curve under natural drought conditions for 5, 10, and 15 days, were measured using a LI-6400 portable photosynthetic apparatus with young seedlings of Lysimachia davurica used as samples. The results showed that the net photosynthetic rate (P_n) of Lysimachia davurica appeared to have an " unimodal” pattern that lacked a " midday depression” on one day , which reached a peak of 9.18 µmol·(m²·s)^–1 at 14:00. Under drought stress conditions, P_n, stomatal conductance (G_s), and transpiration rate (T_r) decreased, while the internal CO₂ concentration (C_i) increased. The apparent quantum efficiency (AQE) decreased and the light compensation point (LCP) increased. The light saturation point (LSP) initially increased and then decreased. In addition, the net photosynthetic rate decreased to a negative value at 15 days after the application of stress. This indicated that Lysimachia davurica is only weakly able to adapt to dry environments. It can withstand drought stress for up to 10 days.