Study On Physiological Mechanism Of Different Rootstocks Relieving Heat Stress Of Grafted Cucumber

Temperature control in vegetable cultivation is facing a great challenge because the increasing global warming. Cucumber (Cucumis sativus L.), as one of the main kind of vegetables in greenhouse production, its year-round production is always the pursuit of goal for horticulturists. Improving the ability of resistance to high temperature stress and the adaption to the tread of global warming of cucumber to provide reasonable guidance for the cucumber cultivation in greenhouse has become the focus of scholars paying attention to. The physiological response of Chunqiuwang No.2 cucumber under 35℃ high root-zone temperature with hydroponic cultivation is studied and arootstock of towel gourd named as Wuyexiang is selected to resist high root-zone temperature stress from five cucurbit crops by chlorophyll fluorescence kinetics method and the change of gas exchange parameters. Then high root and air temperature treatment, growth and fruit quality of Wuyexiang towel gourd grafted cucumber plants and self-root grafted cucumber plants are further studied. The major results are as follows:1. Cucumber seedlings were treated by four root-zone temperature 20℃、25℃、 30℃、35℃. Five days later, autioxidant enzyme activities (G-POD, CAT, SOD, APX) of leaf and root varied as the root-zone temperature changes. Seedlings grew well in 20℃、 25℃, especially in 25℃. Root under 35℃ treatment turned brown and had local necrosis symptoms, root activity, leaf relative chlorophyll content (SPAD) and net photosynthetic rate (Pn) were reduced obviously, malondialdehyde. (MDA) was accumulated in leaf and root, proline and soluble protein contents were increased in the leaf and decreased in the root, causing the growth of the whole seedlings to be restrained seriously.2. Photosynthetic fluorescence characteristics of five cucurbit crops, namely, fig-leaf gourd, cucumber cultivar Chunqiuwang No.2, Xingshu and Wuyexiang towel gourd, Aomei bitter gourd, treating with 35℃ high root-zone temperature after 0 days,3 days,5 days, and recovering 5 days later were studied. The two towel gourd, especially Wuyexiang, showed well in this trial. There were insignificant in Pn, excitation energy capture efficiency of photosystem (PS II) reaction center (Fv’/Fm’), actual photochemical efficiency of PSⅡ(ΦPS II), actual photochemical yield (Yield), apparent photosynthetic electron transport rate (ETR), photochemical fluorescence quenching coefficient (qP) and the energy ratio of photochemistry reaction (P) between control and heat treatments in the studied time, while the above indexes of the other three crops were reduced after treating 3 days, and showed more seriously reduction 5 days later. P value deduced and the energy ratio of antenna heat dissipation (D) increased, showing that the three crops protected themselves from high root-zone temperature stress by dissipating most of light energy absorbed by chlorophyll, while the high resistance to heat stress of the towel gourd could be associated with its higher photosynthetic electron transmissi capacity and photorespiratory protection pathway during long-term evolution. This experiment provided certain theoretical basis for selecting rootstock resistance to high root-zone temperature stress of cucurbit crops.3. Wuyexiang towel gourd rootstock and cucumber self-root grafted seedlings were treated in 35°C high root-zone temperature,45/35±1℃ high air temperature (day/night) and the above temperature interaction (overall heat stress). Towel gourd rootstock grafted seedlings showed higher SPAD, Fv’/Fm’,ΦPSII, Pn, transpiration rate (Tr) and qP than cucumber self-root grafted seedlings under all treatments. And the former ones in root had lower MDA content and higher proline content than the latter ones. The results showed that grafting could relieve the heat stress of cucumber in some extent.4. Wuyexiang towel gourd rootstock and cucumber self-root grafted plants grew in plastic greenhouse from August to December. The former ones had higer stem diameter, Pn and early yield, lower virus disease incidence rates and earlier first flowering date than the latter ones. The difference of vatimin C (Vc) and soluble protein content between them were not significant. The free amino acid contents of the former ones were higher than the latter ones, but the soluble suger contents had a reverse trend.Above all, the ability of resistance to high air and root-zone temperature, disease resistance of cucumber plants and the contents of free amino acid in the fruits were improved significantly using Wuyexiang towel gourd as rootstock. The aboved results can provide some reference for cucumber cultivation in summer and fall.