Effects of bioactive compounds on chlorophyll fluorescence parameters of Mexican lime seedlings (Citrus aurantifolia Swingle) under cold stress

Mexican lime is one of the commercial citrus cultivars in the world, which is commonly used as a citrus rootstock in the southern regions of Iran. This cultivar is one of the most sensitive citrus cultivars to cold stress and is used as a cold-sensitive plant in many citrus studies. This study aimed to investigate the effect of melatonin (0 and 500 μM), mannitol (0 and 50 μM), acetic acid (0 and 15 μM) and their combination on photosynthetic apparatus of Mexican lime seedlings under cold stress (0 and -6 °C). According to the results, the combined application of melatonin, mannitol, and acetic acid, by improving the photosynthetic parameters, reduced the destructive effects of cold stress (0 and -6 ºC) on photosynthetic machinery in Mexican lime seedlings. This treatment led to the highest performance index (PItotal) at both stress levels. The most suitable chlorophyll fluorescence parameters for identifying the best reducing treatment against cold stress were indicator of the activity of the oxygen evolving complex on the donor side of PSII (Fv/F0), variable fluorescence (Fv), maximum quantum yield of PSII photochemistry (Fv/Fm), thermal dissipation quantum yield (F0/Fm), and maximal fluorescence (Fm), respectively. The results of this experiment showed that the exogenous use of natural compounds (melatonin, mannitol) and a cost-effective agent (acetic acid) can protect Mexican lime photosynthetic apparatus under cold stress. Generally, the interactions of melatonin, mannitol, and acetic acid at both 0 and -6 °C provided the best results.