The Responses Of Sap Flow Density And Physiological Characteristics In The Canopy Of Moso Bamboo (Phyllostachys Edulis) Culms To Throughfall Reduction And Nitrogen Addition

Climate-change-induced drought and increasingly serious atmospheric nitrogen deposition,the main characteristics of global change,influence plant growth and physiology directly or indirectly.Importantly,plant growth and physiology are closely related to sap flow and physiological characteristics in the canopy,such as leaf water potential,photosynthetic rate,as well as nonstructural carbohydrate content of leaves and branches.The responses of plant growth and physiology to drought or atmospheric nitrogen deposition have been widely concerned,but how the plant sap flow and physiological characteristics in the canopy respond to the interaction effects of drought and nitrogen deposition remain poorly understood.As one of the most widespread bamboo species with highly economic value,Moso bamboo?Phyllostachys edulis?is widely planted in southwest China,where was likely to occur exacerbated drought events and atmospheric nitrogen deposition.Therefore,there is a need to study the response of sap flow and physiological characteristics in the canopy of Moso bamboo to drought and nitrogen deposition in order to improve models,parameterization,bamboo forest operation and management as well as develop strategies for sustainable management of bamboo forest.In this study,we manipulated drought by excluding throughfall and simulated atmospheric nitrogen deposition by nitrogen addition in a stand scale of a Moso bamboo forest,with a randomized complete block design,4 replicates,to elucidate how the Moso bamboo responds to manipulated drought,nitrogen addition and their interaction effect in sap flow and physiological characteristics in the canopy during growing seasons?from May to September?in 2017-2018.Three age groups of bamboo culms?i.e.,young,mature and old?were investigated for the dynamic variations of sap flow density?Js?,and its relationship with leaf water potential(?_leaf),leaf water potential at turgor loss(?_TLP),stomatal size and density,and the maximum stomatal conductance?gw?max??,and analyzed the responses of the light saturated maximum photosynthesis rate?Pmax?,leaf area index?LAI?and nonstructural carbohydrate content to throughfall reduction?TR?and nitrogen deposition?N?,relationships between Pn and nocturnal sap flow and intrinsic water use efficiency?WUEi?.The main results show that,?1?Throughfall reduction treatment?TR?decreased soil water content?SWC?by 31.4%?p<0.01?and 35.6%?p<0.01?in 2017 and 2018,respectively,while nitrogen addition treatment?N?didn't change SWC.Additionally,the interaction effect of TR and N was nonsignificant on SWC.TR didn't affect soil total nitrogen content?TN?,but N increased TN by 17.5%?p=0.02?.TR decreased the amount of newly developing bamboos in 2018 by 29.8%?p=0.03?,while N didn't change that amount.?2?TR decreased Js and Pmax by 28.3%?p=0.01?and 14.4%?p=0.01?,respectively,but didn't influence WUE_i,which probably led to the decease of LAI?p=0.03?.N didn't change J_s,while increased Pmax and WUEi,which probably caused the increase of LAI?all p=0.01?.In addition,N alleviated the drought stress on Js and Pmax,accompanying with the increase of LAI?all p=0.01?.?3?TR decreased?_pre and?_mid,which was probably attributed to the unchanged g_s and decreased?_TLP?all p=0.01?,while N increased?_pre and?_mid,which was probably due to the decreased g_sand increased?_TLP?p=0.01?.The interaction effect of TR and N was nonsignificant on?_pre and?_mid.?4?The effects of TR and N were different on the sap flow density and photosynthesis of Moso bamboo of different age culms.TR decreased Js and Pmax of the mature culms by 42.9%?p<0.05?and 11.1%?p<0.01?,respectively,but didn't changed that of the young and old culms.N decreased Js of the mature culms?p=0.04?,but increased Pmax?all p<0.01?of all three age groups.Additionally,N alleviated the decrease of Js and Pmax of the mature culms under TR?p<0.01?.We concluded that throughfall reduction treatment decreased sap flow density and photosynthesis of Moso bamboo in some degree,while nitrogen addition didn't affect sap flow density but raised photosynthesis,and nitrogen addition alleviated the water stress induced by throughfall reduction on sap flow density and photosynthesis.Moreover,throughfall reduction decreased leaf water potential,turgor loss point and the light saturated maximum photosynthesis rate,but nitrogen addition increased the value of these characteristics,thus moderating the effect of drought on water use and photosynthesis.Besides,the responses of Moso bamboo culms in different age to throughfall reduction and nitrogen addition were different.The old culms tended to more conservative strategy comparing to the young culms under drought.