Effects Of ALA On Physiological Characteristics Of Hydrangea Under High Temperature And Drought Stress

Hydrangea macrophylla,a genus of Saxifragaceae(Hydrangea),is a deciduous ornamental shrub with flowering period from May to August,when high temperature and drought can easily lead to flower and leaf scorching and affect the ornamental effect.As a new plant growth regulator,ALA can effectively alleviate plant adversity injury.In this study,in order to find an effective way to improve the tolerance of H.macrophylla to high temperature and drought,exogenous application of different mass concentrations(0,25,50,75,100 mg·L-1)of ALA at two temperature gradients(35 and 42℃)and two moisture gradients(40%FC and 20%FC)were applied to second-year potted seedlings of H.macrophylla variety ’Endless Summer’.The effects of exogenous ALA on H.macrophylla under high temperature and drought stress were investigated in terms of physiological and biochemical characteristics,leaf anatomy and photosynthetic characteristics.The main results of the study are as follows:(1)High temperature and drought stress led to the increase of SOD,POD,CAT and APX activities,soluble protein,soluble sugar and free proline contents in H.macrophylla leaves,and the trend of continuous increase with the increase of stress temperature and drought degree;O2·-production rate decreased and then increased with the increase of stress temperature and drought degree;MDA content increased continuously with the increase of stress temperature,and decreased and then increased with the increase of drought degree The MDA content decreased and then increased with the increase of drought.Exogenous application of ALA solution increased SOD,POD,CAT and APX activities in leaves to different degrees,inhibited the production of MDA and O2·-induced by high temperature and drought stress,reduced the degree of membrane lipid peroxidation and alleviated reactive oxygen species injury;exogenous ALA induced H.macrophylla to accumulate more soluble proteins,soluble sugars,free proline and other osmoregulatory substances,which effectively alleviated osmotic stress The exogenous ALA induced H.macrophylla to accumulate more soluble protein,soluble sugar,free proline and other osmoregulatory substances,which effectively alleviated osmotic stress and mitigated the damage to H.macrophylla by high temperature and drought stress.Among them,50 mg·L-1 of ALA at 35℃ and 75 mg·L-1 of ALA at 42℃ were the most effective in alleviating reactive oxygen species and osmotic stress damage;50 mg·L-1 of ALA at moderate and severe drought were the most effective in alleviating reactive oxygen species and osmotic stress damage.(2)Exogenous ALA induced an increase in leaf thickness and upper and lower epidermal thickness of H.macrophylla under high temperature and drought,which enhanced the water storage capacity of H.macrophylla leaves,induced an increase in the fenestra/sea ratio and compactness of leaf flesh structure,and increased the efficiency of light energy utilization of leaves,which played a positive role in the repair and regulation of leaves under high temperature and drought,and effectively alleviated the anatomical structural damage of leaves caused by abiotic stresses.Among them,75 mg·L-1 of ALA at 42℃ had the best effect on the repair and regulation of leaf structural damage,and 50 mg·L-1 of ALA at severe drought had the best effect on the repair and regulation of leaf structural damage.(3)The accumulation of chlorophyll a,chlorophyll b and total chlorophyll content in H.macrophylla leaves was significantly inhibited by high temperature and drought stress,and Pn,Tr,and Gs decreased and Ci increased,leading to the decrease of Pn in H.macrophylla mainly by non-stomatal factors.Exogenous ALA induced chlorophyll accumulation in H.macrophylla under high temperature and drought stress,improved photosynthetic pigment depletion in H.macrophylla,and induced lower Ci and higher Tr and Gs to enhance the net photosynthetic rate and improve the photosynthetic capacity of H.macrophylla.Among them,the best effect was achieved with 75 mg·L-1 of ALA to enhance photosynthetic capacity under high temperature at 42℃,and 50 mg·L-1 of ALA to enhance photosynthetic capacity under severe drought.