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

Abstract The effects of CO 2 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 [CO 2 ] 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 [CO 2 ] and salt stress significantly inhibited the N assimilation process in cucumber leaves; however, enriched [CO 2 ] had no effect on the nitrate (NO 3 − ) reduction or ammonium (NH 4 + ) assimilation of leaves under salt stress, inhibiting only the transamination. Moreover, enriched [CO 2 ] 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 [CO 2 ] 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.