The effect of foliar spray of phenylalanine, cysteine and selenium on yield and fruit quality of Physalis (Physalis peruviana L.) under cold stress condition

Introduction Physalis (Physalis peruviana L.) is a perennial plant belonging to the Solanaceae family, but it is grown commercially as an annual crop. Low temperature has been reported as one of the most restraining environmental factors for agricultural crops, particularly vegetables. Cold stress causes symptoms such as wilting, reduced growth and photosynthetic rate, chlorosis, necrosis, discoloration, abnormal ripening, increased susceptibility to diseases, ion leakage from cell membranes, and changes in respiration and ethylene production in plants. Amino acids are one of the possible approaches that induce cold stress tolerance in plants. Phenylalanine is one of the essential amino acids that is used as a nutritional enhancer, amino acid injection, and complex amino acid preparation. Foliar application of amino acids such as L-phenylalanine during plant growth increased anthocyanins and phenolic compounds contents in grape and strawberry fruits and cysteine also contains sulfur as an amino acid which is widely present in bacteria, yeast, plants, animals and certain single cells. Spraying amino acids on plants is one of the modern methods. Selenium (Se) and its salts protect plants against biotic (pathogens and herbivores) and abiotic (ultraviolet rays, heavy metals, arsenic) stresses. Also, the findings showed that selenium can effectively stimulate the phenylpropanoid metabolic pathway and it has been specifically determined that it protects plants against biological stresses. Materials and methods In order to investigate the effect of low temperature stress during seedling growth stage, and foliar application of amino acids L-phenylalanine (Phe), L-cysteine (Cys) and sodium selenite (Se) on yield and fruit quality of Physalis (Physalis peruviana L.), an experiment was conducted in greenhouse and field of University of Zanjan. The seeds of physalis were sown in seedling trays contain peat moss. The seedlings were grown under normal conditions (25±2 °C/day and 20±2 °C at night with 60-65% RH). The different concentrations of Phe (0.75, 1.5 and 2.5 mM), Cys (0.25, 0.5, and 0.75 mM) and Se (0.25, 0.5, and 1 mg.L-1) was sprayed on the seedling at 4–5th true leaf stage and distilled water was used for control treatment. For the chilling stress treatment of seedlings, plants were transferred to the 4 °C climate chamber for 24 h and two days. The control group (plants without chilling stress) was grown under normal conditions (25±2 °C/day and 20±2 °C at night with 60-65% RH). Plants transplanted to the field and foliar sprayed three times (growth stage, flowering and fruit set stages) with amino acids and Se. Fruits were harvested with the change of calyx and fruit color from green to orange and total fruit yield estimated as a kg.ha-1. Also, chlorophyll and carotenoid contents of leaf, fruit carotenoid, titratable acidity, soluble solids contents, vitamin C ant antioxidant activity were investigated. Results and discussion The results showed that low temperature significantly decreased total chlorophyll and carotenoid contents of leaves, titratable acidity (TA) and carotenoid contents of fruit, and caused increases in total soluble solid content of fruit, but had no significant difference on fruit yield, vitamin C content and antioxidant activity. Foliar application of Phe, Cys, and Se increased yield and fruit quality. The highest fruit yield (11419.1 kg.ha-1) was obtained in with application of Cys 0.5 mM in plants under low temperature stress. The maximum vitamin C content was obtained in plants sprayed with Phe 0.75 and 1.5 mM under normal condition. Also, the highest fruit antioxidant activity (56.9%) was observed with application of Phe 0.75 mM in plants without cold stress compared to other treatments and plants under stress. The beneficial role of Se and amino acids in increasing antioxidant capacity might be attributed to enhancing antioxidant enzymes activity, phenolic compounds and carotenoids content, which have antioxidant activity. Conclusion Therefore, the use of these compounds specially Phe 1.5 mM, Cys 0.5 mM and Se 1 mg.L-1 are suggested to improve the performance and quality of the fruit under low temperature stress or non-stress conditions.