Alginate Oligosaccharides Promoted the Nutrient Uptake and Growth of Cucumber Seedlings Under Suboptimal Tempera-Ture Conditions

Suboptimal temperatures severely repress the nutrient absorption, growth, and yield formation of vegetables in solar greenhouse during winter and early spring in China. Alginate oligosaccharides (AOS) are anionic acidic polysaccharides derived from brown algae, known for promoting plant growth and alleviating abiotic stress. In this study, we aimed to investigate the effects of different nutrient solution concentration combined with AOS on the growth and nutrient uptake of cucumber seedling under suboptimal temperatures (15/8 °C, day/night). Potted ‘Jinchun No. 4’ cucumber seedlings grown in coconut coir were treated with 0.5×, 1.0×, or 1.5× strength of Hogland solution alone (N0.5, N1, N1.5), or with 30 mg·L⁻¹ AOS (A0.5, A1, A1.5). The results showed that growth attributes, and nitrogen accumulation of cucumber plants of N1 and N1.5 were significantly higher than those of N0.5. And plants of A0.5 exhibited significantly higher plant height, chlorophyll a content, root surface area, root volume, root vitality, nitrogen metabolism enzyme (NR, GDH, GS) activities and N accumulation, than those under N0.5, N1, or N1.5. Moreover, compared to A0.5, the net photosynthetic rate, total root length, root surface area, root N content, leaf nitrate reductase activity, root glutamate dehydrogenase activity and N accumulation of A1 and A1.5 were significantly higher than those of A0.5. Correlation analysis indicated strong positive relationships between root surface area, root volume, root vitality, and N content in plant organs. In summary, AOS enhanced the nutrient uptake and growth of cucumber seedlings through promoting root surface area, root vitality, and nitrogen metabolism enzyme activities under suboptimal temperatures conditions, therefor it has a good application prospect and great ecological value for increasing nutrient use efficiency and reducing the use of chemical fertilizers.