The Effect of Foliar Application of Melatonin on Changes in Secondary Metabolite Contents in Two Citrus Species Under Drought Stress Conditions

Abstract

Plant secondary metabolites are compounds that play an important role in plant interactions and defense. Persian lime and Mexican lime as the two most important sour lime varieties with high levels of secondary metabolites, are widely cultivated in tropical and subtropical areas. Melatonin is a pleiotropic molecule that plays a key role in protecting plants against drought stress through regulating the secondary metabolite biosynthesis pathway. This study was performed as a factorial experiment consisting of three factors in a completely randomized design (CRD), including four concentrations of melatonin (0, 50, 100, and 150 μM), three levels of drought stress [100% (control), 75% (moderate stress), and 40% (severe stress) field capacity (FC)], and two Citrus cultivars. The experiment was conducted for 60 days in a greenhouse condition. Based on the results of this study under severe drought stress, melatonin-treated crops had higher total flavonoid and total phenolic contents than the untreated crops. The highest level of essential oils components was observed on 100 μM foliar application of melatonin under severe drought stress in both varieties. The main component of the essential oil was limonene in both Citrus species. Moreover, based on the analysis of the results, hesperidin was the main polyphenol in both varieties. Since the use of melatonin often increases the production of secondary metabolites, this study can be considered as a very effective method for controlling the adverse effects of drought stress in citrus for both industrial and horticultural aims.

Exogenous Melatonin Reprograms the Rhizosphere Microbial Community to Modulate the Responses of Barley to Drought Stress

“…It has been found that the exogenous application of melatonin regulates the soil microbiome under salt stress [ 63 ]. Also, exogenous melatonin affects plant root growth and metabolite levels under drought [ 28 , 64 , 65 ], hence altering plant–microbe interactions and potentially reprogramming the rhizosphere microbiome. For example, in this study, Patescibacteria and Proteobacteria were sensitive to melatonin application.…”

Crosstalk between Melatonin and Reactive Oxygen Species in Plant Abiotic Stress Responses: An Update

“…Moreover, to reestablish the redox homeostasis, melatonin also enhanced the expression of genes encoding antioxidant enzymes (such as APX1 , APX2 , CAT1 , FSD1 , CuZnSOD , and MnSOD ), and improved the activities of APX, SOD, CAT, POD, Δ1-pyrroline-5-carboxylate synthesis (P5CS), as well as the levels of antioxidants (ASC, GSH, proline, and total soluble carbohydrates) [ 12 , 35 , 36 , 40 , 42 , 47 , 54 , 55 , 56 , 57 , 58 , 59 ] ( Table 1 ). Similarly, it was well established that melatonin boosted the activities of many antioxidant enzymes (including SOD, POD, APX, CAT, DHAR, GST, GR, MDHAR, and PPO) and the levels of antioxidants (including ASC, DHA, GSH, proline, flavonoid, carotenoid, and phenolic compounds), thus reducing ROS levels and improving tolerance to drought stress in plants, such as maize, tomato, citrus, soybean, Malus , or kiwifruit plants [ 14 , 39 , 60 , 61 , 62 , 63 , 64 ] ( Table 1 ). Melatonin also acted as a priming agent to improve Medicago sativa tolerance to drought stress via the nitro-oxidative homeostasis [ 65 ].…”

Mechanism of calcium in melatonin enhancement of functional substance-phenolic acid in germinated hulless barley

“… 6 MT has been discovered in recent years to have a significant regulatory function in forming secondary metabolites in plants. Exogenous MT treatment could enhance levels of flavonoids, carotenoids, and isoflavones in citrus, 7 tomato, 8 and soybean, 9 respectively. Previously, we also found that MT considerably stimulated the biosynthesis of phenolic acids in barley under salt stress 10 and effectively improved plant stress resistance.…”