Ameliorative Effects Of 24-epibrassinolide On Grape Seedlings Under Water Stress

Drought stress leads to various changes in grapes. These changes include the membrane structure damage, growth inhibition, impairment of photosynthesis and changes in endogenous phytohormones levels. BRs, a natural plant hormone, which play an essential role in plant growth and development, have been implicated in plant responses to abiotic stress.The Riesling grape seedlings(Vitis vinifera L.) were subjected to 10%(w/v) polyethylene glycol(PEG-6000) and treated with 0.05, 0.10 or 0.20 mg/L 24-epibrassinolide(EBR), respectively. The effects of EBR on cellular ultrastructure, photosynthesis characters, reactive oxygen metabolism, antioxidant system and endogenous hormone levels under water stress investigated. The main results demonstrated as follows:(1)Compared with normal control, water stress induced the degradation of chlorophyll and change of chlorophyll fluorescence parameters. EBR application increased chlorophyll contents, maximum photochemical efficiency of PSII(Fv/Fm), the effective photochemical quantum yield of PSII(ФPSⅡ) and non-photochemical quenching coefficient(NPQ) under water stress in each concentration.(2)Water stress induced inhibiting photosynthesis through decreasing in stomatal opening degree and CO2 assimilation capacity, resulted in restraining electron transfer and photophosphorylation through collapsing organelle structure and decreased of photosynthetic rate. However, EBR application increased stomatal opening degree and stomatal density, counteracted the detrimental effects of water stress on the structure of the photosynthetic apparatus. These results suggested that exogenous EBR could alleviate water stress-induced inhibition of photosynthesis on grape possibly through lessening the stomatal and non-stomatal limitation of photosynthesis performance.(3)Water stress caused an increase in superoxide anion(O2·-), hydrogen peroxide(H2O2) and malonaldehyde(MDA) contents in grape seedlings that disturbed balance of reactive oxygen species production and aggravated the membrane lipid peroxide level. Application of EBR significantly increased the contents of soluble protein and proline, activities of antioxidant enzymes(SOD, CAT, POD and APX) and levels of antioxidants(As A and GSH) which decreased the levels of O2·-,H2O2 and MDA under water stress. These results suggested that EBR alleviated water stress-induced oxidative stress through regulating antioxidant and osmotic compounds of grape seedlings.(4)Water stress could alter hormonal balance of grape seedlings. EBR application increased the contents of abscisic acid, cytokinin and salicylic acid, while decreased the contents of auxin and gibberellin, which regulated the levels of phytohormones to maintain homeostasis. On the other hand, in the middle and later periods of water stress, it has been showed that co-application of both EBR and water stress to grape seedlings resulted in increased endogenous castasterone, brassinolide and 6-deoxocastasterone which enhanced grape seedlings tolerance to water stress.In conclusion, EBR application preserved integrity of cellular structures which increased photosynthesis, enhanced the contents of osmoregulation substance, the activities of antioxidant enzymes and levels of antioxidants which decreased the levels of ROS, regulated the cross-talk of phyohormones which maintain homeostasis. These results suggested that EBR alleviated the adverse effect of water stress in grape seedlings and enhanced drought tolerance. Compared to other treatments, treatment of 0.10mg/L EBR had best ameliorative effect against water stress.