| BRs, one of the six natural plant hormones, play an essential role in the growth and adverse environment adjustment among high plants and thus were widely studied. ABA is also a highly recognised and studied drought stress plant hormone, whose content increases significantly during stress conditions. However, little is known about their biological functions during the drought tolerance of Carbernet Sauvignon grape variety. In addition, the role ABA acts during BRs’ amelioration effects on water-stressed grape seedlings remains to be further researched.The Carbernet Sauvignon grape seedlings（Vitis vinifera L.） were subjected to 10%（w/v） polyethylene glycol（PEG-6000） and treated with 0.1 mg/l 24-epibrassinolide（EBR）, 20 mg/l Abscisic acid（ABA）, or both 0.1 mg/l EBR and 2.95 mg/l NDGA before water stress, respectively. Another two treatment groups were CS seedlings under normal conditions（CK1） or water-stressed conditions without any plant hormones application（CK2）. The effects of exogenous plant growth regulators（PGRs） on photosynthesis characters, reactive oxygen metabolism, antioxidant system and As A-GSH cycle under water stress were investigated. The main results are demonstrated as follows:（1） Compared to CK1, water stress resulted in the degradation of chlorophyll and chlorophyll fluorescence parameters alterations, while PGRs application ameliorated chlorophyll degradation and increased chlorophyll fluorescence parameters such as Fv/Fm, ФPSⅡand NPQ, i.e. maximum photochemical efficiency of PSⅡ, the effective photochemical quantum yield of PSⅡ, and non-photochemical quenching coefficient, respectively. EBR and EBR+NDGA treatment both effectively ameliorated chlorophyll degradation and increased chlorophyll fluorescence parameters yet they showed differences means that ABA is involved in EBR’s ameliorating photosynthesis inhibition of water-stressed grape seedlings.（2） Water stress caused an increase in superoxide anion（O2·-） and malonaldehyde（MDA） contents in Cabernet Sauvignon seedlings which disturbed the balance of reactive oxygen species production and aggravated the membrane lipid peroxide level. Application of PGRs significantly increased the contents of soluble protein and proline, activities of antioxidant enzymes（SOD, CAT, POD） and decreased the levels of O2·- and MDA under water stress. Such results suggested that EBR alleviated water stress-induced oxidative stress through regulating antioxidant and osmotic compounds and ABA was only partly involved during the process, so EBR is able to ameliorate water-stressed grape seedlings through ABA-independent signaling pathways.（3） PGRs application promoted the operation rate of As A-GSH cycle by enhancing the level of antioxidants（As A and GSH）, decreasing GSSG content, and increasing the activity of APX and GR in order to efficiently eliminate ROS and maintain a high ratio of GSH/GSSG As A/（As A+DHA）. The fact that EBR and EBR+NDGA treatment effectively promoted As A-GSH circulation, suggested EBR is able to ameliorate As A-GSH cycle of water-stressed grape seedlings through ABA-dependent signaling pathways.（4） Water stress altered hormonal balance of grape seedlings. PGRs application increased the contents of abscisic acid, Jamonic acid,gibberellin and salicylic acid, which regulated the levels of phytohormones to maintain homeostasis, suggesting that EBR is not fully able to ameliorate the phytohormones homeostasis of water-stressed CS seedlings through ABA-independent signaling pathways.In conclusion, compared to CK2, PGRs application increased photosynthesis, enhanced the contents of osmoregulation substance and activities of antioxidant enzymes as well as the levels of antioxidants which decreased the levels of ROS, regulated the As A-GSH cycle which maintains a high ability of reduction, and regulated the cross-talk of phyohormones which maintain homeostasis. These results suggested that ABA was involved in EBR’s alleviating the adverse effect of water stress in grape seedlings, however, it was not the most significant limiting factor. |
