HEAVY METAL TOXICITY IN CUCUMBER: AN OVERVIEW ON USING GROWTH AND BIOCHEMICAL PARADIGMS

Abstract

Heavy metals (HMs) like cadmium (Cd), lead (Pb), copper (Cu) and zinc (Zn) are a major concern in agricultural soils, threatening global food security and ecosystem integrity. Fast-growing and physiologically well characterized cucumber (Cucumis sativus L.) is an excellent model species for assessing HM phytotoxicity because the plants are sensitive and exhibit a quantifiable stress response. This review aims to systematically compile existing information on toxicity due to HM exposure to cucumber for morphological, physiological and biochemical endpoints, providing a solid basis for HM ecotoxicology assessment. The results indicate that exposure to HM causes dose-dependent inhibition of seed germination, biomass and elongation of seedlings, root–shoot architecture impairments (stunted roots, necrotic lesions), water and nutrient imbalances, and a characteristic chlorosis or necrosis of leaves. The effects of HM stress at the subcellular level include a burst of reactive oxygen species (ROS), leading to lipid peroxidation, higher malondialdehyde (MDA) and electrolyte leakage, and substantial loss of photosynthetic pigments (chlorophyll a, b, and carotenoids). At the same time, affected plants regulate their antioxidant defense system, resulting in changes in the activity of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione S-transferase (GST). Along with relative water content and markers for membrane integrity, these biochemical changes indicate early and sensitive markers of oxidative injury. The review concludes that a combination of morpho-physiological growth traits with oxidation stress/antioxidant biomarkers is needed for a precise diagnosis of HM toxicity. These integrated solutions increase the diagnostic sensitivity and provide guidance for the screening of mitigation measures such as soil amendments, metal-tolerant breeding and application of bio stimulants that will make farming in contaminated environments safer.