Effects Of Exogenous Abscisic Acid On The Physiology And Growth Of Grafted Watermelon Seedlings

Grafting can control soil-borne diseases of watermelon,but grafting operation can cause water stress of watermelon scions during the healing period,affecting survival and growth of grafted seedlings.Abscisic acid（ABA）is known as a"stress hormone"and can alleviate plant damage caused by many abiotic stresses.In this experiment,six ABA concentrations(0,0.05,0.1,0.5,1 and 5μmol·L^-1)and two application methods（spraying and irrigating）were used to study their effects on the physiology and growth of grafted watermelon seedlings with pumpkin’Ribenxuesong’as rootstock and watermelon’Zaojia（84-24）’as scion.We aimed to reveal the physiological mechanism of exogenous ABA to alleviate grafting stress,clarify the regulatory effect on graft formation in watermelon,and provide some references for the application of ABA in watermelon grafting seedlings.The main results are as follows:（1）The treatment with appropriate concentration of exogenous ABA was beneficial to the rapid rise after the reduction of the maximum photosynthetic efficiency,actual photosynthetic efficiency（Y（II））,relative electron transfer rate（r ETR）and photochemical quenching（q L）and the rapid fall after the rise in non-photochemical quenching（NPQ）of grafted watermelon seedlings during the healing period;it could increase the chlorophyll content of grafted watermelon seedlings and enhanced superoxide dismutase,catalase,peroxidase and ascorbate peroxidase activities,reduced malondialdehyde（MDA）content,alleviated water stress damage to the scion caused by grafting,and significantly increased plant height,scion height,fresh weight and dry weight of grafted watermelon seedlings.Most of the indicators showed a trend of increasing and then decreasing with the increase of exogenous ABA treatment concentrations,and the best treatment effect was achieved at the application concentration of 0.5μmol·L^-1 in a comprehensive comparison.（2）When treated with different application methods,exogenous ABA spraying and root irrigation significantly increased Y（II）,r ETR,q L values,chlorophyll content and antioxidant enzyme activities of grafted watermelon seedlings,decreased NPQ value and MDA content,increased fresh weight and dry weight of scions;It also increased the potential maximum relative electron transfer rate（r ETRmax）,half-saturating light intensity（lk）,Y（II）and r ETR values under light,indicating that exogenous ABA treatment promoted the effective absorption and utilization of light energy by grafted watermelon seedlings;at the same time,exogenous ABA alleviated the level of water loss in the scion leaves and brought them back to normal quickly.ABA spraying resulted in lower MDA content and higher biomass,r ETRmax and lk in treated grafted seedlings compared to root irrigation,yielding the best results when the treatment was spraying.（3）Exogenous ABA treatment of scions increased the content of endogenous ABA,indoleacetic acid and trans-zeatin,reduced the content of gibberellin 3,regulated the partial closure of stomata of watermelon seedling leaves in response to adversity stress,accelerated the formation of vascular bridges between scion and rootstock of grafted watermelon and improved survival rate,indicating that exogenous ABA treatment promoted the formation and accumulation of endogenous ABA,which was conducive to activating drought-tolerant physiological responses,combined with the regulation of endogenous hormone levels promoting the survival and growth of grafted watermelon seedlings.