Exogenously applied melatonin enhanced chromium tolerance in pepper by up-regulating the photosynthetic apparatus and antioxidant machinery

Abstract

Chromium (Cr) is widely recognized as a highly toxic metal pollutant that adversely affects the agricultural productivity and poses considerable risks to human health. The regulatory role of melatonin (MEL) in Cr stress is well recognized, however, the potential role of MEL in enhancing Cr stress tolerance in pepper is rarely investigated. This study examined the possible effect of MEL in alleviating the adverse effects of Cr toxicity in pepper seedlings. When pepper seedlings were exposed to Cr toxicity, they showed a significant reduction in growth traits, photosynthetic efficiency, root morphological characteristics, and biochemical attributes. Cr treatment greatly caused oxidative damage that was directly related to increased EL, H2O2, O2•−, and MDA production. Conversely, MEL supplementation significantly mitigated the phytotoxic effects of Cr on root morphology, gas exchange elements, and leaf photosynthetic capacity, and restored the growth of pepper seedlings. MEL application stimulated the plant's defense system by enhancing the modulation of antioxidant enzymes, AsA-GSH pools, maintaining the glyoxalase enzymes activity, and reducing oxidative damage. Notably, MEL supplementation significantly reduced Cr accumulation from root to shoot. Hence, MEL supplementation might be regarded as a successful approach for reducing Cr accumulation and its adverse effects in pepper seedlings cultivated in contaminated soils. The research may also provide novel perspectives on the potential application of MEL in mitigating heavy metal toxicity in pepper seedlings.