Mechanisms Of Root-zone Temperature Affected Acclimation To Sub-optimal Temperature Stress In Cucumber Seedlings

During winter and spring vegetable production, sub-optimal temperature stress is a major limiting factor for crop growth. How to realise efficient use of energy in greenhouse temperature control in order to achieve the purpose of improving crop growth and development, is an important issue needs to be addressed for sustainable greenhouse vegetable production. In this experiment, cucumber (Cucumis sativus L.) was taken as a test material to study the relevant mechanisms of improving root-zone temperature to relieve growth and physiological activity of cucumber seedlings under sub-optimal temperature stress, and by comparison of different treatment temperatures on the shoot-root environment for crop growth balance interaction, as well as by the addition of polyethylene glycol (PEG) to analyze the contribution of root-sourced water supply. The main results are as follows:1) Under sub-optimal temperature stress, developmental processes and biomass accumulation rate in cucumber seedlings are decreased, root-zone warming can significantly ease the rate of biomass accumulation, but can not promote the new leaves senescent process.2) Under sub-optimal temperature stress, cucumber seedlings root growth and absorption are severely suppressed, root-zone warming can significantly increase the length of cucumber seedling roots, length of the main root, number and length of first and secondary lateral roots were significantly promoted. At the same time, root-zone warming can effectively promote root absorption of various mineral elements, including phosphorus, potassium, manganese, zinc and copper by directly enhancing the absorption rate of the unit root length, or nitrogen, calcium, magnesium and iron by increasing the total root length.3) Under sub-optimal temperature stress, photosynthetic capacity of cucumber seedling leaves are subject to different degrees of inhibition. For the first true leaf, root-zone warming energy reduces its stomatal and non-stomatal limitation by increasing stomatal conductance, easing Rubisco carboxylation enzyme expression and vitality, restorating photo-reactive quinone electron transfer process. For newly unfolded second true leaf, root-zone warming reduces the extent of stomatal limitations, and facilitate its expansion by improving root-sourced water supply.4) Among the changes during root-zone warming, of the increase of carbon assimilation in cucumber seedlings,12% is from the promoted net photosynthetic rate and 88% is from the increased leaf area, and leaf area growth is almost entirely due to the increased specific leaf area; of the increase of nitrogen assimilation,16% is from the increased absorption rate and 84% is from the lengtherned total root length, and total root length is increased 70% by specific root length and 30% by root mass fraction. For relative growth rates, the contributions are assimilation vitality> morphological features> biomass allocation.5) According to the experiment results, when confronted with sub-optimal temperature stress during greenhouse production or nursery, if there is no limit of calendar age of seedlings, it is recommended that priorly enhance the rhizosphere temperature by 3-6℃ to ensure the robustness of seedlings, if there is time limit or the duration of sub-optimal temperature is too long, it is recommended to enhance the air temperature in order to promote the development of seedlings.