Response of water balance and nitrogen assimilation in cucumber seedlings to CO₂ enrichment and salt stress

The effects of CO₂ enrichment on water balance and nitrogen (N) assimilation in cucumber (Cucumis sativus L. cv. Jinyou No.35) seedlings under salt stress were investigated. Two-way randomized block design was used: the main treatment consisted of two [CO₂] levels, ambient and enriched (400 and 800 ± 40 μmol mol^-1, respectively), and the minor treatment consisted on two salinity treatment levels, 0 and 80 mmol L^-1 NaCl. The results showed that, under the experimental conditions, enriched [CO₂] and salt stress significantly inhibited the N assimilation process in cucumber leaves; however, enriched [CO₂] had no effect on the nitrate (NO₃^-) reduction or ammonium (NH₄⁺) assimilation of leaves under salt stress, inhibiting only the transamination. Moreover, enriched [CO₂] increased the plasma membrane H⁺-ATPase activity, vacuolar membrane H⁺-ATPase activity and root hydraulic conductivity under salt stress, thereby increasing the ion selective absorption and water absorption capacity. To a certain extent, enriched [CO₂] promoted the accumulation of K⁺ in plants, which significantly reduced the Na⁺/K⁺ ratio; moreover, the enrichment ultimately improved the water state conditions and helped to maintain the ion balance in plants under stress, ensuring normal enzymatic activity.