Effects of soil drought stress on photosynthetic gas exchange traits and chlorophyll fluorescence in Forsythia suspensa

To clarify the changes in plant photosynthesis and mechanisms underlying those responses to gradually increasing soil drought stress and reveal quantitative relationships between photosynthesis and soil moisture, soil water conditions were controlled in greenhouse pot experiments using 2-year-old seedlings of Forsythia suspensa (Thunb.) Vahl. Photosynthetic gas exchange and chlorophyll fluorescence variables were measured and analyzed under 13 gradients of soil water content. Net photosynthetic rate (P (N)), stomatal conductance (g (s)), and water-use efficiency (W (UE)) in the seedlings exhibited a clear threshold response to the relative soil water content (R (SWC)). The highest P (N) and W (UE) occurred at R (SWC) of 51.84 and 64.10%, respectively. Both P (N) and W (UE) were higher than the average levels at 39.79% >= R (SWC) >= 73.04%. When R (SWC) decreased from 51.84 to 37.52%, P (N), g (s), and the intercellular CO2 concentration (C (i)) markedly decreased with increasing drought stress; the corresponding stomatal limitation (L (s)) substantially increased, and nonphotochemical quenching (N (PQ)) also tended to increase, indicating that within this range of soil water content, excessive excitation energy was dispersed from photosystem II (PSII) in the form of heat, and the reduction in P (N) was primarily due to stomatal limitation. While R (SWC) decreased below 37.52%, there were significant decreases in the maximal quantum yield of PSII photochemistry (F (v)/F (m)) and the effective quantum yield of PSII photochemistry (I broken vertical bar PSII), photochemical quenching (q (P)), and N (PQ); in contrast, minimal fluorescence yield of the dark-adapted state (F (0)) increased markedly. Thus, the major limiting factor for the P (N) reduction changed to a nonstomatal limitation due to PSII damage. Therefore, an R (SWC) of 37.52% is the maximum allowable water deficit for the normal growth of seedlings of F. suspensa, and a water content lower than this level should be avoided in field soil water management. Water contents should be maintained in the range of 39.79% >= R (SWC)>= 73.04% to ensure normal function of the photosynthetic apparatus and high levels of photosynthesis and efficiency in F. suspensa.