The effect of low temperature on fruit yield, quality and antioxidant properties of physalis under foliar spray of amino acids and selenium

Low temperature has a major influence on survival and geographical distribution of plants and as an important environmental factor, it affects plant productivity. Physalis (Physalis peruviana L.) belongs to the Solanaceae family and is a tropical and subtropical plant that is affected by low temperatures at beginning and end of the growing season. These fruits are highly beneficial to human health due to their nutritional and bioactive compounds (antioxidants, vitamins A, B, C, E and K1, flavonoids and carotenoids), so it has gained attention for cultivation worldwide. Amino acids (L-Phenylalanine and cysteine) or selenium play important roles in plant resistance to low temperature. However, the role of L-phenylalanine (Phe) and cysteine (Cys) or selenium (Se) in regulating chilling tolerance and improve fruit quality of physalis is still unknown. In this study, the effects of Phe, Cys and Se on quality and antioxidant capacity of physalis fruit under low-temperature stress were investigated. The experiment was conducted in a randomized complete block design with three replicates. The seeds of physalis (Physalis peruviana L.) were sown in seedling trays containing peat moss under optimal conditions of 25±2°C during the day and 20±2°C at night. Different concentrations of L-phenylalanine (0.75, 1.5, and 2.5 mM, Phe), L-cysteine (0.25, 0.5, and 0.75 mM, Cys), or sodium selenate (0.25, 0.5, and 1 mg L-1, Se) were sprayed on the seedlings at the 4–5th true leaf stage. Distilled water was used as the control treatment. To induce chilling stress, plants with the basically same growth were transferred to a 4 °C climate chamber for 48 hours. The control group was grown under optimal conditions. After applying the chilling stress, the plants were returned to optimal growth conditions for 24 hours. Plants transplanted to the field and foliar sprayed three times (growth stage, flowering and fruit set stages) with amino acids and Se. After exposed plants to autumn chilling, fruits harvested during calix and fruit color change from green to orange stage, and plant height, yield and the quality characteristics of the fruit including carotenoid content, membrane stability index, titratable acidity, phenolic compounds and antioxidant activity were evaluated. The results showed that low temperature stress significantly decreased fruit yield, carotenoids content and membrane stability index (MSI), and increased titratable acidity, phenols and flavonoids content and phenylalanine ammonia-lyase (PAL), superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities. Foliar spray of Physalis plants with Phe, Cys and Se under cold stress significantly increased plant height, fruit yield, antioxidant enzymes activity, MSI, carotenoids, phenolic compound and TA. The highest value of MSI (38.46 and 36.1) was obtained in fruit of plants treated with Se 0.25 and 0.5 mg L-1 under low temperature condition. The maximum phenol content (8.08 and 7.83 mg L-1) of fruit was observed with application of Phe 0.75 mM and Cys 0.25 mM in plants exposed to cold stress. Also, the highest antioxidant capacity was obtained in plant fruits treated with Phe 0.75 and 1.5 mM (60.8%), Cys 0.25 mM (60.26%) and Se 0.5 mg L-1, respectively under low temperature condition, and the lowest antioxidant capacity (50.5%) was observed in control plant fruits. The results of research showed that the foliar application of Phe, Cys or Se treatments, especially the levels of Phe 0.75 and 1.5 mM and Se 0.5 mg L-1 during seedling and field growing periods caused a significant increase in fruit yield, also increased the activity of antioxidant enzymes, phenolic compounds and the membrane stability index of fruits exposed to autumn cold and led to improvement of fruit quality.