Effects Of Exogenous Strigolactones On Growth And Physiology Of Grapevines Under Drought Stress

The Northwest China is one of the major areas producing grape and wine,but its arid and semi-arid climate conditions and the lack of water resources have greatly restricted the development of grape and wine industry.Strigolactones（SLs）,as a new class of plant hormone,has many functions such as stimulating the germination parasitic weeds,inhibiting shoot branching,inducing hyphal branching of the symbiotic arbuscular mycorrhizal fungi,regulating root architecture,etc.In recent years,more and more studies have found that they could also act as positive regulators in resisting abiotic stress.In this experiment,the‘Cabernet Sauvignon’cuttings（V.vinifera L.）were used,and pretreated with three concentrations（1μM,3μM,5μM）of strigolactone analogues（GR24）,and the distilled water was sprayed as control.After the pretreatment,the cuttings were subjected to 7%（w/v）polyethylene glycol（PEG-6000）,and leaves were sprayed with GR24treatment at the same time.In this study,by comparing the plant morphology,photosynthetic characteristics,microscopic structure,antioxidant system,osmotic substances,and endogenous hormone contents of‘Cabernet Sauvignon’,the effects of GR24 treatment on cuttings under drought stress were investigated.The main results were as follows:（1）GR24 treatment could effectively reduce the wilting of grape leaves response to drought stress.GR24 treatment led to an increase of root/shoot ratio during the process of drought stress,which contributed to absorb water and nutrient elements.Moreover,GR24treatment strengthened the leaf moisture content rising and the increasing trend of electrical conductivity was effectively alleviated.And the effect was strengthened with increasing of GR24 concentration.（2）Drought stress caused stomatal closure of the grapevine leaves,and decreased stomatal conductance（Gs）and transpiration rate（Tr）,while GR24 treatment resulted in lower stomatal conductance at the early stage of drought stress.After 96 hours,the leaf net photosynthetic rate（Pn）gradually increased with the increase of hormone concentration.In addition,GR24 treatment could effectively reduce the decrease of chlorophyll content,and significantly increase the maximum photochemical quantum yield（Fv/Fm）and non-photochemical quenching coefficient q N of grapevine PSII.（3）With increased drought stress,microstructure of grape leaf and root changed:the thickness ratio of palisade to spongy rised,and leaf tissue structure were closely arranged;the ratio of stockade tissue thickness to sponge tissue thickness was higher with GR24 treatment.Under drought stress,the root tissues were shrunk,root diameter decreased,the thickness of endothelium and pericycle increased and the area of vessel decreased.GR24 treatment could alleviate the decrease of stomatal area and vessel area caused by drought stress,which contributed to moisture absorption and improved water transportation.（4）Drought stress improved H₂O₂ and O₂^·-accumulation of grape leaves but the enrichment of H₂O₂ and O₂^·-alleviated after GR24 treatment.GR24 treatment could also significantly promote the activities of peroxidase（POD）,catalase（CAT）,ascorbate peroxidase（APX）,glutathione reductase（GR）and glutathione（GSH）contents,decrease the content of membrane peroxidation product malondialdehyde（MDA）,preventing over-accumulations of active oxygen and enhancing the ability of drought-resistance of grape cuttings.（5）Drought stress significantly increased the content of osmotic substances,while GR24treatment triggered different changes in the content of soluble sugar content,soluble protein content and proline content in grape root.Besides,GR24 spraying treatment significantly decreased malondialdehyde and superoxide anion content.Strigolactones could take protection on root system under drought stress.（6）GR24 treatment could alter the level of hormones in leaves and roots of grapevine during drought stress.After 72 hours of stress,5μM GR24 spraying treatment increased the contents of ZR and IAA in grape leaves,and the levels of IAA/ABA and ZR/ABA ratios was increased significantly.On the other hand,in the root system,3μM GR24 application increased significantly the ratio of IAA/ABA and ZR/ABA.Through Orobanche seeds germination experiment,it was found that drought stress induced down-regulation of SLs contents in grape roots,while the opposite trend was showed in seedling leaves.In summary,under the conditions of drought,GR24 treatment can effectively promote the stomatal closure of the grape leaves,reduce transpiration,alleviate water loss,enhance the antioxidant defense ability,and at the same time have a good maintenance of the integrity of the leaf and root tissue structure.The effect is to relieve the damage caused by drought stress to the root system and promote the absorption of water by the grape root system,thereby enhancing the drought resistance of the grape.In this study,treatment of 5μM GR24had b est ameliorative effect among three concentrations against drought stress.