Abstract:

This research examined the effect of salt stress on photosynthesis responses in potted seedlings of the annual plant Gymnocarpos przewalskii. Salt stress was simulated using seven NaCl solutions of different concentrations （CK， 0.2%， 0.4%， 0.6%， 0.8%， 1.0% and 1.2%） and changes in leaf gas exchange parameters， light response curves and chlorophyll fluorescence parameters were determined and analyzed. It was found that with increasing salt concentration， leaf net photosynthetic rate （P_n）， stomatal conductance （G_s） and transpiration rate （T_r） all showed a downward trend. At NaCl concentrations ≥0.4%， P_n， G_s and T_r were all significantly lower than in CK plants. Stomatal limit value （L_s） and instantaneous water use efficiency （WUE） showed an initial upward with a maximum value in the 0.4% NaCl treatment， and then a downward trend at higher NaCl concentrations. Intercellular CO₂ concentration （C_i） showed the opposite trend， reaching its minimum value when the NaCl concentration was less than or equal to 0.4%. The decrease in P_n was mainly due to non-stomatal limitation when the NaCl concentration was equal to or greater than 0.6%， at lower NaCl levels， stomatal limitation was the main factor. As the degree of salt stress increased， the maximum net photosynthetic rate （P_nmax）， dark respiration rate （R_d）， light saturation point （L_SP） and the apparent quantum efficiency （A_QY） all gradually decreased， while the light compensation point （L_CP） gradually increased， indicating salt inhibits the ability of seedlings to absorb， utilize and convert light. Leaf PSII potential activity （F_v/F₀）， light energy conversion efficiency （F_v/F_m）， actual photochemical efficiency （Φ_PSII）， electron transfer rate （ETR） and photochemical quenching coefficient （q_p） all showed decreases with increasing salt concentration. The non-photochemical quenching coefficient （NPQ） was significantly increased compared with CK at NaCl concentrations ≤0.4%， but was significantly decreased when the salt concentration was ≥0.8%. At NaCl concentrations in the seedling substrate of 0.2% and 0.4%， the P_n， F_v/F₀， and F_v/F_m values of G. przewalskii leaves did not decrease significantly， relative to CK， and the WUE increased. The PSII system can protect the photosynthetic mechanism by dissipating excess light energy， thus conferring some salt tolerance. However when the salt concentration exceeded 0.6%， the light energy utilization range was narrowed， the photosynthetic apparatus was obviously damaged， and the photosynthetic capacity was significantly inhibited. Therefore， based on these results， 0.6% NaCl is the threshold concentration， above which salt stress injury for seedlings of G. przewalskii is likely to occur.

Key words: Gymnocarpos przewalskii, photosynthesis, light response curve, fluorescence parameters