LAUSR

Chlorophyll a fluorescence transient can reveal the degree of thermal damage to plant photosynthetic organs and provide much physiological information about PSII fleetly. However, it is not clear how it reveals photosynthetic mechanisms of plants under decreasing soil moisture. In this paper, the photosynthesis characteristics and photosystem II (PSII) activities in leaves of Lonicera japonica under 11-level soil moisture were explored by gas-exchange analysis and JIP test. Both parameters of net photosynthetic rate (Pn) and water use efficiency (WUE) exhibited an increasing–decreasing trend with the decreasing relative soil water content (RSWC). At 29.7% ≤ RSWC≤ 79.6%, the main reason for Pn decrease was stomatal limitation; at 36.4% ≤ RSWC≤ 55.1%, both Pn and WUE were higher than the average. When RSWC was below 29.7%, Pn, Tr, and WUE significantly decreased. Meanwhile, a JIP-test analysis revealed a distinct K peak with decreases in the quantum yield and energy distribution ratio (ψo or φEo) and increases in the K phase (Wk) and J phase (Vj) at RSWC≤ 29.7%, indicating that severe drought (RSWC≤ 29.7%) damaged the leaf oxygen-evolving complex, caused the accumulation of QA− in PSII electron acceptors, and, therefore, hindered the progression of the PSII electron transport chain. In addition, at RSWC≤ 29.7%, the absorption flux per CS (ABS/CSo) increased, while the phenomenological energy fluxes for TRo/CSo, ETo/CSo, RC/CSo decreased, indicating that the PSII reaction center was damaged and excessive light energy was accumulated. Therefore, the main reason for the decrease of photosynthetic efficiency was non-stomatal limitation at RSWC≤ 29.7%, and the maximum moisture deficit for the growth of Lonicera japonica is RSWC= 29.7%.