Effects Of Rootstock With Different Thermophibicity And Cytokinin On The Chilling Tolerance Of Photosynthetic Apparatus In Cucumis Sativus

Plant growth and development is an intergrated process with a series of physiological metabolisms.The leaves are the place for photosynthesis in which carbohydrates are synthesized for the fruits and the roots.In contrast,root absorb water and nutrients in order to meet the shoot requirement at the same time. Consequently,all of these constituents work as a whole.Meanwhile both shoot and root can perceive the changes in environment,and produce communication substances subsequently which can be transported between root and shoot by xylem sap and phloem sap separately,resulting in changed gene expression,physiological metabolization and growth in other organs.The present study was taken to investigate the different responses of both shoot and root to low temperature.The cucurbit species with different sensitivities to temperature were used as rootstocks to investigate the effects of grafting and spraying cytokinin(CTK)on the resistance of cucumber photosynthesis apparatus to low temperature.The main results are as follows:1.The effects of shoot or root chilling(12℃)on the biomass,gas exchange and chlorophyll fluorescence parameter,antioxidant enzyme system and the metabolism of carbohydrate were examined in cucumber seedlings which were grafted on cucumber themselves or figleaf gourd.Chilling resulted in a significant reduction in the seedlings biomass,the light saturated rate of the CO₂ assimilation(A_sat),stomatal conductance(G_s),the quantum efficiency of PSII(Φ_PSII),photochemical quenching coefficient(qP)and maximum photochemical efficiency(F_v/F_m).In contrast,chilling significantly increased the activities of antioxidant enzymes,the content of malonaldehyde(MDA)and carbohydrate.In comparison with cucumber shoots grafting on cucumber roots,shoots grafting on figleaf gourd showed less change,and root-chilled plants showed less change compared to shoot-chilled plants.2.Three cucurbit crops different in chill-tolerance:chill-tolerant figleaf gourd, warm-adapted cucumber and heat-tolerant bitter melon were used as rootstock. Roots chilled for 10 days resulted in an obvious decrease in shoot fresh weight, especially in those grafted onto bitter melon.These results were well in agreement with the changes in leaf chlorophyll content and the different response of photosynthesis system and respiration system.Our results showed that C/Kt had a most obviously reduction in the chlorophyll content and photosynthesis,and had the highest increase in respiration.C/Ct took the second place,and the changes in C/Pt were the least.It seems likely that bitter melon need to produce more energy and intermediate through respiration in order to repair leaf photosynthetic system, meanwhile alleviate the injury of root by low temperature stress through phloem sap flux.3.Experiment was carded out to investigate the effects of exogenous CTK on the chill-induced reduction in photosynthesis.Results showed that grafting on figleaf gourd and spraying 6-benzyladenine(6-BA)could alleviate low-root temperature-induced reduction in shoot and root growth mass,chlorophyll content of leaf and the xylem sap exudation rate,light saturated rate of the CO₂ assimilation (A_sat),stomatal conductance(G_s)and transpiration rate(Tr).Besides,spraying 6-BA could obviously increase chlorophyll content,and restrain the decreases of chla/b both in old and new leaves when seedling suffering low temperature stress.Using 6-BA improved the xylem sap exudation rate in root stress,maybe this had some connection with the increase of leaves transpiration.