Influence of nitrate levels on plant growth and carbon and nitrogen metabolism in cucumber (Cucumis sativus L.) under low light stress

Photosynthesis is critically dependent on both adequate light and nitrogen availability. In controlled agricultural environments with limited natural light, cucumber faces substantial challenges, including reduced photosynthetic efficiency and biomass accumulation. Nitrogen (N) is a vital component for the synthesis of various macromolecules, metabolites, and signaling molecules in plants. This study assessed the impact of different nitrate levels (LL-N, LL-1/2N, and LL-2N) on the physiological, biochemical, and metabolic responses of cucumber under low light conditions (80-100 mu mol m -2 s -1 photosynthetic photon flux density) in comparison to standard light (400-500 mu mol m -2 s -1 photosynthetic photon flux density) and unchanged nitrate. We observed that increasing nitrate levels to twice the standard concentration (LL-2N) significantly enhanced shoot morphology, including weight and size, and improved photosynthetic performance, stomatal conductance, and transpiration rates. In contrast, lower nitrate levels (LL-1/2N) exacerbated stress, leading to diminished photosynthesis and biomass. Biochemical analyses indicated that elevated nitrate concentrations activated key enzymes involved in the metabolism of carbon and nitrogen, including glutamine synthetase, sucrose phosphate synthase, nitrate reductase, and glutamate synthase. This led to improvement in the amounts of sucrose, fructose, soluble sugars, free amino acids, and soluble proteins. Additionally, the activities of antioxidant enzymes including superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase were considerably greater under conditions of increased nitrate, which reduced the levels of malondialdehyde and H2O2 as oxidative stress markers. Furthermore, enhanced nitrate levels upregulated critical photosynthetic genes (rbcL, rbc S, FBPase, and SBPase mRNA), supporting the RuBisCO contents and overall photosynthetic capacity. These findings underscore the effectiveness of strategic nitrate enrichment in mitigating low light stress, providing essential insights for optimizing cucumber cultivation in controlled environments and improving productivity.