Mechanistic Approach on Melatonin-Induced Hormesis of Photosystem II Function in the Medicinal Plant Mentha spicata 

Abstract

Melatonin (MT) is considered a new plant hormone having a universal distribution from prokaryotic bacteria to higher plants. It has been characterized as an antistress molecule playing a positive role in the acclimation of plants to stress conditions, but its impact on plants under non-stressed conditions is not well understood. In the current research, we evaluated the impact of MT application (10 and 100 μM) on photosystem II (PSII) function, reactive oxygen species (ROS) generation, and chlorophyll content on mint (Mentha spicata L.) plants in order to elucidate the molecular mechanism of MT action on the photosynthetic electron transport process that under non-stressed conditions is still unclear. Seventy-two hours after the foliar spray of mint plants with 100 μM MT, the improved chlorophyll content imported a higher amount of light energy capture, which caused a 6% increase in the quantum yield of PSII photochemistry (ΦPSII) and electron transport rate (ETR). Nevertheless, the spray with 100 μM MT reduced the efficiency of the oxygen-evolving complex (OEC), causing donor-side photoinhibition, with a simultaneous slight increase in ROS. Even so, the application of 100 μM MT decreased the excess excitation energy at PSII implying superior PSII efficiency. The decreased excitation pressure at PSII, after 100 μM MT foliar spray, suggests that MT induced stomatal closure through ROS production. The response of ΦPSII to MT spray corresponds to a J-shaped hormetic curve, with ΦPSII enhancement by 100 μM MT. It is suggested that the hormetic stimulation of PSII functionality was triggered by the non-photochemical quenching (NPQ) mechanism that stimulated ROS production, which enhanced the photosynthetic function. It is concluded that MT molecules can be used under both stress and non-stressed conditions as photosynthetic biostimulants for enhancing crop yields.

Photosystem II Tolerance to Excess Zinc Exposure and High Light Stress in Salvia sclarea L.

“…Exposure of S. sclarea plants to Zn significantly increased leaf Fe levels (Figure 2a), accompanied by a rise in the ETR (Figure 8a). This increased ETR after 8 days of exposure to Zn (Figure 8a) was due to the decreased NPQ (Figure 6b) [103][104][105]. However, NPQ inhibits ROS formation, acting as a photoprotective mechanism [34,42,46,[106][107][108].…”

Physiological Characteristics and Transcriptomic Responses of Pinus yunnanensis Lateral Branching to Different Shading Environments

“…Photosynthesis represents a critical process underpinning plant growth. However, a plant’s ability to achieve high levels of photosynthesis is dependent on its environmental conditions [ 56 ]. Chlorophyll is a vital component of plant systems, as it absorbs, transports, and converts light energy [ 57 ].…”

Modulation of Photosystem II Function in Celery via Foliar-Applied Salicylic Acid during Gradual Water Deficit Stress

“…Singlet oxygen ( 1 O 2 ) is a reactive oxygen species (ROS) that can be developed in plants as a byproduct of photosynthesis [ 36 , 38 , 40 , 93 ]. ROS, such as superoxide anion radical (O 2 •− ), hydrogen peroxide (H 2 O 2 ), and singlet-excited oxygen ( 1 O 2 ), are constantly produced but are scavenged by the antioxidant enzymatic and non-enzymatic cellular mechanisms [ 40 , 94 , 95 , 96 , 97 , 98 , 99 ]. The function of chloroplast antioxidants is not to entirely remove ROS, but to accomplish a proper equilibrium between creation and removal so as to counterbalance the process of photosynthesis and document an efficient spread of signal wave [ 99 , 100 , 101 , 102 ].…”

“…ROS, such as superoxide anion radical (O 2 •− ), hydrogen peroxide (H 2 O 2 ), and singlet-excited oxygen ( 1 O 2 ), are constantly produced but are scavenged by the antioxidant enzymatic and non-enzymatic cellular mechanisms [ 40 , 94 , 95 , 96 , 97 , 98 , 99 ]. The function of chloroplast antioxidants is not to entirely remove ROS, but to accomplish a proper equilibrium between creation and removal so as to counterbalance the process of photosynthesis and document an efficient spread of signal wave [ 99 , 100 , 101 , 102 ]. ROS are fundamental signaling molecules that allow cells to respond speedily to diverse kinds of alterations to their homeostasis, contributing to the creation of defense mechanisms and plant resilience [ 103 , 104 ].…”