Change of Physiological Properties and Ion Distribution by Synergistic Effect of Ca²⁺ and Grafting under Salt Stress on Cucumber Seedlings

Salinization is an important soil environmental problem, which severely restricts the sustainable production of cucumbers. Therefore, how to improve the salt tolerance of cucumbers is a global problem. Grafting improves the resistance of crops, and calcium ion (Ca²⁺) weakens the permeability of the plasma membrane. In this paper, grafting cucumber with NaCl-free treatment was the control treatment (CK). Under salt stress, grafting combined different concentrations of CaCl₂ and non-grafted (NG) were considered as treatments. The synergistic effect of grafting and Ca²⁺ to relieve salt stress on cucumber seedlings was investigated. The results revealed that grafting (G), Ca²⁺, and their interaction significantly influenced plant growth, osmotic adjustment substances, enzyme activities, and iron distribution. Under salt stress, grafting increased the absorption of potassium ion (K⁺) and Ca²⁺ in cucumber stems and leaves, but compared with NG, it significantly reduced the accumulation of Na⁺ in those parts by 61.58–89.40%. Moreover, supplication suitable Ca²⁺ content had a similar effect. Supplemental Ca²⁺ promoted the shoot and root biomass. The 10 mM L⁻¹ Ca²⁺ had the highest biomass, compared with CK and NG, an increase of 49.95% and 20.47%, respectively; the lowest sodium ion (Na⁺). The highest Ca²⁺ accumulation in cucumber stem and leaves was found in 10 mM L⁻¹ Ca²⁺ treatment. Supplemental Ca²⁺ increased free proline (Pro) and decreased malondialdehyde (MDA) content during the entire salt stress period. At 11 days, compared with 0 mM L⁻¹ Ca²⁺ treatment, pro content was increased by 4.70–25.31, and MDA content was decreased by 1.08–4.90 times, respectively. Superoxide dismutase (SOD) activity, relative growth rate of plant height (PH), and stem volume (SV), and K⁺/Na⁺ and K⁺/Ca²⁺ in cucumber leaves had significantly negative correlations with a salt damage score. The combination of grafting and supplemental 5–20 mM L⁻¹ Ca²⁺ relieved salt damage to cucumber seedlings. The best synergistic effect was obtained with grafting and 10 mM L⁻¹ Ca²⁺ treatment.