Melatonin Affects the Photosynthetic Performance of Pepper (Capsicum annuum L.) Seedlings under Cold Stress

Abstract

Photosynthesis is an important plant metabolic mechanism that improves carbon absorption and crop yield. Photosynthetic efficiency is greatly hampered by cold stress (CS). Melatonin (ME) is a new plant growth regulator that regulates a wide range of abiotic stress responses. However, the molecular mechanism of ME-mediated photosynthetic regulation in cold-stressed plants is not well understood. Our findings suggest that under low-temperature stress (15/5 °C for 7 days), spraying the plant with ME (200 µM) enhanced gas exchange characteristics and the photosynthetic pigment content of pepper seedlings, as well as upregulated their biosynthetic gene expression. Melatonin increased the activity of photosynthetic enzymes (Rubisco and fructose-1, 6-bisphosphatase) while also enhancing starch, sucrose, soluble sugar, and glucose content under CS conditions. Low-temperature stress significantly decreased the photochemical activity of photosystem II (PSII) and photosystem I (PSI), specifically their maximum quantum efficiency PSII (Fv/Fm) and PSI (Pm). In contrast, ME treatment improved the photochemical activity of PSII and PSI. Furthermore, CS dramatically reduced the actual PSII efficiency (ΦPSII), electron transport rate (ETR) and photochemical quenching coefficient (qP), while enhancing nonphotochemical quenching (NPQ); however, ME treatment substantially mitigated the effects of CS. Our results clearly show the probable function of ME treatment in mitigating the effects of CS by maintaining photosynthetic performance, which might be beneficial when screening genotypes for CS tolerance.

Impact of Exogenous Melatonin Application on Photosynthetic Machinery under Abiotic Stress Conditions

“…Photosynthesis acts as an important sensor of temperature stress by often being inhibited before other cellular processes are affected. To counteract the adverse impact of this stress, the application of melatonin emerges as a very promising strategy [ 87 , 88 ]. Melatonin has demonstrated significant effects on the production of total phenols and flavonoids, contributing to the enhancement of heat stress tolerance in grapes [ 89 ].…”

“…Furthermore, the exogenous supplementation of melatonin markedly improved photosynthetic pigment molecules, including Chlorophyll a , Chlorophyll b , and carotenoids. These improvement were attributed to the upregulation of pigments’ biosynthesis related-genes expression (Ca CB12 , Ca CAB4 , Ca CAB7 , Ca CAB8 , Ca CAB21 , and Ca CAB37 ) [ 88 ].…”

Melatonin Enhances the Photosynthesis and Antioxidant Enzyme Activities of Mung Bean under Drought and High-Temperature Stress Conditions

“…Melatonin is structurally related to tryptophan, and its biosynthesis is analogous to that of auxin, serotonin, and its isoforms [12]. It acts as an antioxidant and detoxifies ROS and RNS by enhancing the antioxidant enzyme activity under stress conditions [13]. Melatonin act as a promoter of seed germination [14] and lateral root formation [15], regulates flowering [16], and delays leaf senescence [17] to manage and control plant growth and development under challenging environments.…”

Melatonin Interaction with Other Phytohormones in the Regulation of Abiotic Stresses in Horticultural Plants

“…In addition, MT is a novel PGR that has strong antioxidant properties [24]. Furthermore, MT played a significant role in the regulation of photosynthetic machinery [25]; modification of root system architecture [26], antioxidant pool [27], mineral nutrient accumulation [28], flowering [29], fruit ripening [13], circadian rhythms [22]; and improvement of plant growth and development [30]. In addition, MT positively regulated the growth status of many plant species under salinity [31], drought [32], heat [33], cold [34], alkaline [35], and HM stress [20].…”