Photosynthetic characteristics of alfalfa seedlings under salt stress based on FvCB model

Understanding the photosynthetic physiological characteristics of alfalfa seedlings under salt stress is important for improving alfalfa growth, restoring the ecological environment, and promoting the development of the foraging industry in China. Alfalfa variety 'Adrenalin' seedlings were treated with different concentrations of NaCl (0 mmol·L^-1, CK; 40 mmol·L^-1, T1; 80 mmol·L^-1, T2; 120 mmol·L^-1, T3; and 160 mmol·L^-1, T4), the light response curves were measured with Li-6400XT, and the effects of NaCl stress on photosynthetic characteristics were analyzed by using FvCB model. The results showed that the leaf net photosynthetic rate (P_n) decreased significantly with increasing NaCl concentration by 1.44% (T1), 7.21% (T2), 7.90% (T3), and 3.85% (T4), respectively, compared with that of CK. The P_n in all treatments showed a rapid upward trend as photosynthetic effective radiation increased, and the P_n growth rate in the normal treatment (CK) was significantly higher than that in the other treatments. Compared with those in the CK, the T1 and T2 treatments increased the alfalfa seedling leaf maximum carboxylation rate (V_cmax; by 3.59% in T1, and 13.88% in T2) and maximum electron transfer rate (J_max; by 11.24% in T1, and 17.47% in T2), but the T3 and T4 treatments reduced V_cmax and J_max. Leaf conductance (g_m) and dark respiration rate (R_d) decreased with increasing NaCl concentration. Compared with those in the CK, the T1 and T2 treatments did not affect g_m, but significantly reduced R_d. The T3 and T4 treatments significantly reduced g_m and R_d compared with CK, T1, and T2 treatments. The FvCB model fitting results of alfalfa P_n under different stress treatments verified that the FvCB sub-model accurately estimated plant leaf photosynthesis, and the results showed that introducing g_m into the model had a high simulation accuracy and low average absolute error. The critical salt tolerance value for alfalfa seedlings was 80–120 mmol·L^-1. As the NaCl concentration increased, the photosynthetic limiting factors changed from mesophyll factors to damaged photosynthetic organs. These results may help formulate effective control measures in Northwest China saline land to improve plant salt tolerance.