Effects of 28-homobrassinoloid on key physiological attributes of Solanum lycopersicum seedlings under cadmium stress: Photosynthesis and nitrogen metabolism

Heavy metal accumulation due to environmental pollution, especially in agricultural ecosystem can cause serious deterioration of crop yield and quality. In present study we assessed the effect of exogenous 28-homobrassinoloid (HBL; 10−8 M) on growth, photosynthesis, indices of chlorophyll a fluorescence and nitrogen metabolism in Solanum lycopersicum seedlings grown under two doses (Cd1: 3 mg kg−1 sand and Cd2: 9 mg kg−1 sand) of cadmium. Accumulation of Cd in root tissues was considerably higher than shoot hence, Cd declined the growth, pigment contents, and photosynthetic O2 yield in its concentration dependent manner. Chlorophyll a fluorescence due to Cd stress was negatively affected as shown by decreased QA − reoxidation kinetics: φP0, ψ0, φE0 and PI_ABS and increased energy flux parameters: ABS/RC, TR0/RC, ET0/RC and DI0/RC. HBL application under Cd stress improved the photochemistry of photosystem II (PS II) by affecting these parameters positively. Treatment of Cd in test seedlings resulted into significant decrease in nitrate reductase, nitrite reductase, glutamine synthetase and glutamate synthase activities, and induced enhancing effect on ammonium content and glutamate dehydrogenase activity. Exogenous HBL treatment alleviated the negative effect of Cd on growth, photosynthesis, contents of protein, carbohydrate and inorganic nitrogen and nitrogen assimilating enzymes. The data indicate that exogenous HBL protects the test seedlings during the early growth phase against Cd phytotoxicity by regulating Cd accumulation in tissues and two key metabolic processes; photosynthesis and nitrogen metabolism.