Nitric oxide reduces freezing stress damage on leaf pigments, photosynthetic apparatus, and chlorophyll fluorescence of broad bean (Vicia faba L.)

Freezing stress is one of the abiotic stresses limiting the growth and development of fall crops, which are an important part of sustainable food production worldwide. To investigate the effect of foliar spraying of nitric oxide (NO) on reducing freezing stress damage in broad bean plants, an experiment was conducted under a controlled environment in 2022. NO at three levels (control [distilled water], 50, and 100 µM) and freezing stress at five levels (0, -4, -8, -12, and -16 °C; 2 h) were considered as the experimental treatments. The results showed that freezing stress reduced plant survival rate; however, 50 µM NO led to a 92% increase in survival rate compared with the control at -12 °C. 50 µM NO also increased the shoot and root dry weight of plants exposed to freezing stress. NO positively affected the net photosynthetic rate (A_N) of plants exposed to freezing temperatures. At -8 °C, leaf A_N increased by 33 and 32%, respectively, when 50 and 100 µM NO were applied, compared with the control. 100 µM NO at -4, -8, and -12 °C enhanced stomatal conductance by 34, 33, and 41%, respectively, compared with the control. The maximum efficiency of photosystem II (F_v´/F_m´) was recorded in 100 µM NO at the recovery time of 48 h. Generally, NO, especially at temperatures below -8 °C, was highly effective in improving the photosynthetic and physiological traits of broad beans. The results are promising for plants exposed to freezing stress, which are damaged by low temperatures during critical seasons, and can be helpful for better understanding the parameters affecting broad bean survival under freezing stress.