The Correlations Of Leaf Hydraulic Conductance And Xylem Embolism To Drought

Under extreme drought stress,the maintenance of leaf hydraulic conductance(K_leaf)is the basis for its normal photosynthesis and drought tolerance.It has been suggested that the leaf water transport pathway is composed of the xylem pathway(K_x)and the out-side of xylem pathway(K_ox).There has been much debate that the decline of K_leafeaf under drought stress depends on whether the decline of K_x or K_ox.In this paper,the optical vulnerability technique that directly reflects the loss of function in the xylem of the leaf is used as the means.And the vitro drought experiment was performed using the angiosperm:Populus euphratica Oliv.,Fraxinus chinensis Roxb,Caragana Korshinskii,Caragana boisi,Pharbitis purpurea(L.)Voisgt,Cynanchum chinense R.Br and the fern Athyrium filix-femina,Coniogramme japonica(Thunb.)Diels.The dynamics of K_leaf were analyzed to confirm the relationship between K_leaf decline and K_x and K_ox decline.The main results are as follows:1.For the sprouting angiosperm,with the increase of growth years,the water potential(?_pd)of Caragana korshinskii decreased gradually,which range was-1.0^-2.6 MPa.The maximum leaf hydraulic conductance(K_leaf-max)gradually decreased,which range was 10.3⁵.0 mmol m^-2 s^-1MPa^-1.The leaf water potential at which K_leaf declined by 50%(K_leaf P₅₀)continued to decrease from-1.8 MPa for one-year-old sprouts to-3.4 MPa for fourteen-year-old sprouts.However,when the leaf water potential was lower than-4 MPa,the xylem started to embolize,and the water potential at which cause 50%embolism of the xylem(K_x P₅₀)was between-4.3 and-7.3 MPa.It was confirmed that the K_leafeaf decline was driven by K_ox and was decoupled with the development ages.It is considered that the adaptation of plant hydraulic structure to drought stress had plasticity,but the factors causing the decline of K_leaf didn't change.2.For the angiosperm,K_leaf-max of Fraxinus chinensis Roxb and Caragana boisi was 3.0 and6.2 mmol m^-2 s^-1 MPa^-1,respectively,and K_leaf P₅₀ was-0.6 and-1.7 MPa,respectively,and K_x P₅₀was-3.3 and-5.3 MPa,respectively.The K_leaf-max Pharbitis purpurea(L.)Voisgt was 6.28 mmol m^-2 s^-1 MPa^-1,K_leaf P50 was-0.5 MPa,and no embolism occurred in the xylem of the leaves.The results showed that the K_leaf decline was caused by the decline of K_ox.The K_leaf-max of Populus euphratica Oliv was 2.9 mmol m^-2 s^-1 MPa^-1,K_leafeaf P₅₀ was-2.3 MPa,and K_x P₅₀ was-2.8 MPa.As the xylem embolism increased,K_leafeaf decreased linearly,confirming that the K_leaf decline was driven by K_x.It is believed that in angiosperms,K_leaf decline is driven by K_x or K_ox,which was species-specific.3.For the fern,K_leaf-max of Athyrium filix-femina was 2.31 mmol m^-2 s^-1 MPa^-1,and the K_leaf P₅₀was-0.8 MPa.There was no embolism in the xylem of the pinna,which confirmed that the decrease of K_leaf was driven by the K_ox.The K_leaf-max Coniogramme japonica(Thunb.)Diels was 14.6 mmol m^-2 s^-1 MPa^-1,K_leaf P₅₀ was-1.0 MPa,and K_x P₅₀ was-1.3 MPa.When the xylem embolism occurred,the K_leaf declined by 51.6%,indicating that the loss of K_leaf is caused by K_x and K_ox.It is believed that in ferns,factors affecting the decline of K_leafeaf are species-specific.The above results indicate that whether it was angiosperm or fern,K_leaf decline is driven by the xylem embolism,which is species-specific.