Xylem Embolism Repair Of Platycladus Orientalis And Quercus Variabilis And Its Response To Foliar Water Uptake

Climate change has altered the spatiotemporal distribution pattern of rainfall,and drought and temperature rise have led to a wide range of forest recession and death.Hydraulic failure was likely the main factor to cause tree death.Xylem embolism,affecting the long-distance water transport of woody plants has been recognized as the key hydraulic trait for understanding of the plant-water relation and the mechanism regulating drought death of plants.Recent studies found that more plants can use leaves to absorb water and lead water redistribution in plants,which might provide potential water sources for repairing xylem embolism in veins and branches.However,the xylem embolism repair mechanisms in plants were still not clear and few studies have been conducted to examine the relationships between foliar water uptake and xylem embolism repairing.In this study,the temporal and spatial variation of leaf water content,sap flow,branch water conductivity and xylem embolism were investigated in Platycladus orientalis and Quercus variabilis tree species in Beijing mountainous area,where seasonal drought often occurred during a growing season.The purpose of the project was to reveal the response of xylem embolism repairing to foliar water uptake under drought stress.The study is significance to further understand the plant-water relationship,the adaptation mechanisms of plants to drought stress,and the ecological effect of foliar water uptake.The results showed:(1)The study area exhibited a distinguish dry-wet season during the growing season of 2021.The leaf water status in P.orientalis and Q.variabilis were significantly improved(p<0.05)in the wet months(July-September).There was no significant difference in leaf water status between the rainfall exclusion plots and the control plots,indicating foliar water uptake may occur.The reverse flow phenomenon was observed in Q.variabilis in the control plots and the reverse flow rate increased monthly,which implied foliar water uptake and water redistribution from leaf to stem occurred.The reverse flow rate was strongly negatively correlated with relative humidity and rainfall,and was strongly correlated with temperature,solar radiation,VPD and wind speed(p<0.001).(2)During the growing season,the xylem water conductivity and embolism repair of P.orientalis and Q.variabilis changed dynamically at both daily and monthly scales.Under drought stress,the water conductivity of branches was limited by soil water conditions,and water was mainly used to leaf transpiration was mainly when soil water was sufficient.At daily scale,P.orientalis and Q.variabilis would adjust the period of xylem embolism repair to deal with drought stress.When soil water was limited,they would actively repair embolism in the morning.When soil water was sufficient,they would tend to withstand higher degrees of embolism during the daytime,and the embolism repairing occurred more at night.At the monthly scale,the occurrence and repair of the xylem embolism of P.orientalis and Q.variabilis were very frequently.At the end of the growing season,the xylem embolism of Q.variabilis was alleviated in the control plots,but the reverse flow rates remained similar in other studied plots.The two species had different xylem embolism repair strategies in response to drought stress and seasonal dry-wet changes:P.orientalis had a low degree of xylem embolism and was more sensitive to drought stress.The xylem embolism of P.orientalis was below 20%in the drought months(May-June)and partial drought month(August).The species was more likely to withstand high levels of xylem embolism in the wet months of July and September(Percentage loss of hydraulic conductivity(PLC)was above 30%in the control plots,and about 25%in the rainfall exclusion plots).The xylem embolism degree of Q.variabilis was high(the average embolism value was close to 50%)and its anti-embolization ability was weak under drought stress.The xylem embolism repairing of Q.variabilis was more dependent on precipitation.The embolism value of Q.variabilis was more than 60%in the rainfall exclusion plots in July.After the water supplement of foliar water uptake and reverse flow,the embolism value significantly decreased in August-September(42.67%,p<0.05).The degree of xylem embolism of Q.variabilis was significantly negatively correlated with the reverse flow rate in rainfall exclusion plots.(3)Both P.orientalis and Q.variabilis could use instantaneous rainfall to improve leaf water and to repair xylem embolism.These tree species could actively use rainfall to repair their xylem embolism in the control plots where sufficient soil moisture were available.The mitigation rates of P.orientalis and Q.variabilis were 61.15%and 67.72%in the control plots,respectively.In contrast,the mitigation rates of P.orientalis and Q.variabilis were 44.64%and 8.38%in the rainfall exclusion plots,respectively.The low rate of xylem embolism in Q.variabilis might be related to the preferential supplement of trunk water supplied by reverse flow.The xylem embolism repair of Q.variabilis can respond to the continuous reverse flow in the wet months.The remission rates of xylem embolism Q.variabilis were 13.62%and 8.86%before dawn(05:00 am)and at sunset(19:30 pm)of the reverse flow,respectively.