Photosynthetic response of Apocynum venetum to salt stress under low K⁺ condition

Apocynum venetum can adapt to saline environments. Previous studies have found that A. venetum is a typical K⁺-efficient species, and the large absorption of Na⁺ and K⁺ to enhance the leaf osmotic adjustment ability is an important physiological strategy for this species to adapt to salt stress. However, the photosynthetic response of A. venetum to low-K⁺ and saline environments has not yet been investigated. In this study, the effects of 50 and 200 mmol·L⁻¹ NaCl treatment on the photosynthetic gas-exchange parameters, PSⅡ photochemical efficiency, and absorbed light allocation in A. venetum under low K⁺ (0.01 mmol·L⁻¹) and normal K⁺ (2.5 mmol·L⁻¹) conditions were investigated using a pot experiment. The results showed that low-K⁺ supplementation had no effect on the net photosynthetic rate (P_n), chlorophyll fluorescence kinetics, or absorbed light allocation when NaCl was not supplied. Under both low and normal K⁺ conditions, compared with the control, both 50 and 200 mmol·L⁻¹ NaCl treatments significantly decreased the P_n, stomatal conductance (G_s), transpiration rate (T_r), and intercellular CO₂ concentration (C_i), and increased stomatal limit values (L_s) (P < 0.05). Moreover, 200 mmol·L⁻¹ NaCl treatment also significantly reduced the maximal photochemical efficiency of PSⅡ (F_v/F_m), potential activities of PSⅡ (F_v/F_o), actual photochemical efficiency of PSⅡ (Φ_PSⅡ), excitation capture efficiency by PSⅡ (F_v'/F_m'), and photochemical quenching (qP) (P < 0.05). These results indicate that the photosynthesis of plants was inhibited by stomatal factors under slight salt stress but by both stomatal and non-stomatal factors under severe salt stress. Furthermore, A. venetum could cope with stomatal limitation under salt stress by improving its leaf water use efficiency. In addition, A. venetum was able to enhance leaf heat dissipation to relieve the damage to the photosynthetic system resulting from light redundancy under the 200 mmol·L⁻¹ NaCl treatment. These results provide a theoretical foundation for further clarifying the stress-resistance mechanism of A. venetum.