Along with cyclic electron flow and non-photochemical quenching, water-to-water cycle is involved uniquely in alleviating Zn stress-caused photodamage in Melia azedarach

Abstract Zinc (Zn) is a widespread industrial pollutant that has detrimental effects on plant growth and development. Photoprotective properties ensure plant survival during stress by protecting the photosynthetic apparatus. This occurs via numerous mechanisms, including non-photochemical quenching (NPQ), cyclic electron flow (CEF) and the water-to-water cycle (WWC). However, whether and how Zn stress affects the photoprotective properties of plants to enhance the tolerance of Zn toxicity remains unknown. In this study, we treated Melia azedarach plants with different Zn concentrations ranging from 200 to 1000 mg kg−1. We then analyzed the activities of two leaf photosynthetic pigment components—photosystems I and II (PSI and PSII)—and the relative expression levels of their subunit genes. As expected, we found that Zn treatment decreases photosynthesis and increases photodamage in M. azedarach leaves. The Zn treatments exacerbated a variety of photodamage phenotypes in photosystem activities and altered the expression levels of key photosystem complex genes and proteins. Furthermore, our results demonstrated that PSI was more seriously damaged than PSII under Zn stress. Subsequently, we compared differences in photodamage in the NPQ, CEF and WWC photoprotection pathways under Zn stress and found that each exerted a protective function again photodamage under 200 mg kg−1 Zn stress. The NPQ and CEF may also play major protective roles in the avoidance of irreversible photodamage and helping to ensure survival under higher (i.e., 500 and 1000 mg kg−1) levels of Zn stress. Thus, our study revealed that NPQ- and CEF-based photoprotection mechanisms are more effective than WWC in M. azedarach upon Zn stress.