Effects of Exogenous Proline Application on Redox Homeostasis and Photosynthetic Performance in Tomato Seedlings Under Copper Stress

Copper accumulation above the optimal range can rapidly become phytotoxic, suppressing photosynthesis and disturbing redox homeostasis in plants. This study evaluated whether foliar proline application mitigates copper-induced injury in tomato ( Solanum lycopersicum L.) seedlings and determined an effective proline concentration under copper stress. Seedlings grown under hydroponic conditions were first exposed to a range of CuSO₄ concentrations, and 500 µM CuSO₄ was selected as a sublethal stress level that markedly reduced photosynthetic performance while maintaining plant viability. Under this stress level, treatments included a water-sprayed control, proline-only treatments (0.5, 5, and 20 mM), CuSO₄-only treatment (500 µM), and combined CuSO₄ + proline treatments. Copper stress significantly decreased net photosynthetic rate and stomatal conductance and reduced photosystem II efficiency, accompanied by increased hydrogen peroxide and thiobarbituric acid reactive substances, indicating elevated oxidative load and membrane damage. Foliar proline improved gas exchange and chlorophyll fluorescence parameters and reduced oxidative damage markers, with the 5 mM dose providing the most consistent protection across physiological and biochemical endpoints. Elemental profiling by inductively coupled plasma mass spectrometry indicated that proline did not necessarily reduce copper accumulation, suggesting that its primary effect is enhancing tolerance under metal presence rather than restricting uptake. Overall, 5 mM foliar proline is identified as an effective dose to support photosynthesis and redox balance under copper stress in tomato seedlings.