Effects Of Drought Stress On Nonstructural Carbohydrates In Robinia Pseudoacacia And Pinus Tabuliformis Saplings

Drought stress has been threating the forests throughout the world and its frequency,severity and duration has been projected to be more acute under climate change.Carbon starvation has been put forward as one of the hypothesis explaining drought induced tree mortality.However,research on the effect of drought stress on tree nonstructural carbohydrate(NSC)has not come to a conclusion.In this study,Robinia pseudoacacia and Pinus tabuliformis,two species that were widely used for afforestation in the Loess Plateau of China,were selected as model species.Two continuous controlled drought pot experiments were performed in 2013 and 2014.The effects of drought characteristics(intensity,duration and timing),tissues,species on the NSC concentration dynamics in different tissues and on the whole sapling scale and NSC distribution in different sapling tissues were investigated.This study aims to enrich the carbon starvation hypothesis,provide theory basis for elucidating the mechanism underling the undergrowth and dieback of R.pseudoacacia and P.tabuliformis resulted from drought stress,forecasting the forest ecosystem dynamics of this region under climate change in the future,tending management of plantation,vegetation restoration and ecological reconstruction in the Loess Plateau.The main results are as follows:1 Drought intensity and duration both were the factors influencing the response of NSC of R.pseudoacacia and P.tabuliformis saplings to drought stress.Soluble sugar concentrations and the ratios of soluble sugar to starch in each tissue of R.pseudoacacia and P.tabuliformis saplings all increased significantly with drought intensity.Exept that there were no significant differences beteen starch and NSC concentrations in branches,stems,coarse roots,and fine roots under moderate drought stress and under the absence of drought stress,starch and NSC concentrations in each tissue of R.pseudoacacia all decreased with drought intensity.Starch and NSC concentrations in P.tabuliformis increased with drought intensity in leaves.Those inceased firstly and then decreased in branches,stems and fine roots,and their peak values appeared under the severe drought stress,the severe drought stress and the moderate drought stress,respectively.Those decreased in coarse roots.The significant decrease of starch and NSC concentrations in branches and stems and on the whole sapling scale of R.pseudoacacia only appearenced at the end of growing season.The effects of drought stress on NSC concentrations on the whole sapling scale of R.pseudoacacia and P.tabuliformis saplings were not significant,while those on starch and soluble sugar and those on NSC ratios of each tissue were significant.It indicates that R.pseudoacacia and P.tabuliformis saplings may tend to matain the NSC on the whole sapling scale stable,but increase the ratios of soluble sugar to starch and change the NSC distribution in different tissues to cope with drought stress.2 The effect of drought stress on NSC in R.pseudoacacia and P.tabuliformis saplings was specific to tissue type.The significant decrease of starch and NSC concentration in saplings under the continuous drought stress on the growing season only appeared in branches and stems.For R.pseudoacacia saplings,drought stress induced that NSC ratios decreased in leaves,coarse roots and fine roots,incteased in stems.For P.tabuliformis saplings,drought stress induced that NSC ratios decreased in stems and coarse roots,increased in leaves and fine roots.Drought stress at the end of growing season decreased the NSC concentration in branches,stems,and coarse roots,while increased those in leaves and fine roots.3 The effect of drought stress on NSC of R.pseudoacacia and P.tabuliformis saplings was different.The major composition of NSC was starch in R.pseudoacacia saplings,while soluble sugar in P.tabuliformis saplings.NSC concentration in each tissue of R.pseudoacacia saplings under drought stress decreased significantly,while that of P.tabuliformis saplings increased significantly except in coarse roots.NSC concentration in each tissue of R.pseudoacacia saplings was less sensitive than that of P.tabuliformis saplings.The time scale of NSC stability of R.pseudoacacia saplings under drought stress was shorter than that of P.tabuliformis saplings.For R.pseudoacacia saplings under drought stress,they allocated more NSC to stems,while for P.tabuliformis saplings,they allocate more NSC to fine roots.As NSC play important roles in drought stress and leaves and fine roots are the most important tissues under drought stress,the differences between of R.pseudoacacia and P.tabuliformis saplings above may the reason why the drought sesistanse of R.pseudoacacia is weaker than that of P.tabuliformis.4 The effects of drought stress at the end of growing season on the NSC dynamics were different from that at the other seasons.Drought stress at the end of growing season resulted in the significant decrease of leaf biomass,stomatal conductance and net photosynthetic rates,as a result of the starch and the NSC concentration on the whole sapling scale.Our results suggested that the premature defoliation and decrease of photosynthetic rate of R.pseudoacacia saplings resulted from drought stress at the end of the growing season could facilitate the transformation of starch to soluble sugars and then decrease the accumulation of starch,as a result,would induce the premature consuming of NSC.The premature consuming of NSC may induced R.pseudoacacia saplings to be vulnerable to frost and other stresses during the dormant season or may be unable to sprout in the following spring.