ObjectivesIn northern China, cucumber seedlings are often subjected to suboptimal temperature (15℃/8℃) stress during spring or winter season. Under such environment, the hydroponic cucumber seedlings using standard nutrient solution are weak and their leaves become yellow in color. A study was conducted aiming to relieve the adverse effects of suboptimal temperature stress by adjusting magnesium (Mg) and potassium (K) concentrations in the nutrient solution.

MethodsHydroponic experiment was conducted inside a climatic chamber with cucumber ‘Bo Nai 3000’ as materials. The day/night culture temperature was 25℃/18℃ for control and 15℃/8℃ for treatment. On the base of a standard Yamazaki cucumber nutrient solution in the control, two levels of K⁺ (6 and 12 mmol/L) and four Mg²⁺ (2, 4, 6 and 8 mmol/L) were set for the treatments under suboptimal temperature. The root morphology, dry matter accumulation and distribution as well as K and Mg absorption in cucumber seedlings were investigated every 7 days since the cultivation, and 4 investigations were did in total.

Results1) Appropriate increasing of the Mg²⁺ concentration in the culture solution significantly improved healthy seedling index under suboptimal temperature. When the Mg²⁺ and K⁺ concentrations were all 6 mmol/L, the seedling index reached the maximum value of 0.17. 2) The effects of Mg²⁺ and K⁺ on root morphology were different. K⁺ concentration was positively correlated with the total root length and root surface area, the effect was mainly on roots of 0–0.5 mm in diameter; Mg²⁺ was positively correlated with the average diameter of the root system, the effect was mainly on roots of >1.0 mm in diameter; higher K/Mg ratio was beneficial to the increase of total length and volume of roots of 0–0.5 mm in diameter, but not for those of roots of 0.5–1.0 mm and >1.0 mm in diameter. 3) Compared to normal temperature, the dry biomass of stems and leaves were decreased while the root dry biomass increased under suboptimal temperature. Under K⁺ concentration of 6 mmol/L, root and stem dry biomass were increased with enhanced Mg²⁺ concentration. The high Mg²⁺concentration of nutrient solution contributed to the distribution of dry matter to leaf. 4) When Mg²⁺ was less than 6 mmol/L, interaction effect existed between Mg²⁺ and K⁺, while significant antagonism effect existed when Mg²⁺ was higher than 6 mmol/L. The absorptive amount of Mg under suboptimal temperature decreased by 25%, 72% and 58% in root, stem and leaf respectively, compared to CK.

ConclusionsUnder suboptimal temperature, 6 mmol/L Mg²⁺ and K⁺ in Yamasaki cucumber nutrient solution improved the seedling index of cucumber seedlings, root morphology and dry matter accumulation. It can effectively resist the adverse effects of suboptimal temperature on cucumber seedlings.