Effects Of Drought Stress On The Allocation Of Carbohydrates In Organs Of Forsythia Suspensa Saplings

Global climate extreme events such like severe drought are predicted to take place more frequently,leading to the imbalance of carbon which may cause severe damage to the forest all around the world.Water deficiency caused by drought can influence plant growth and alter the location of structural（SC）and nonstructural carbohydrates（NSC）,and then influencing the structure and function of forest ecosystems.Recently,the mechanism by which drought stress affects the NSC and SC in plants still remains a weak link and restricts our understanding of the question how drought affects plant growth or even kills them.Forsythia suspensa,which is native to the Loess Plateaus,is taken as the experimental material to study the effect of drought（of different intensity and time scale）on the content and distribution pattern of biomass,NSC,SC of each organ（foliage,stem,root（0-1mm、1-2mm and>2mm））and the entire plants saplings,and the influence of drought stress at the end of growing season on the concentration and stoichiometry of C,N,P in different organs by means of pot experiment and isotope labeling.Three moisture levels we set in the experiment were as follows:75%±5%FC（as control）,55%±5%FC（moderate drought stress）and 35%±5%FC（severe drought stress）.The main results are as follows:（1）Drought stress restricted the growth of whole plant and various organs of Forsythia suspensa saplings.Compared with the normal control group,the saplings under severe or moderate drought stress significantly had less biomass.The stem biomass of saplings under moderate and severe drought stress was 38.5%or 46.7%higher than those in control group,respectively.The intensity of drought could also influence the distribution of biomass between aboveground（foliage and stem）and underground part（0-1mm、1-2mm and>2mm roots）of Forsythia suspensa.Drought stress increased the proportion of the roots（mainly 0-1mm ones）and decreased that of underground part（mainly stem）.Especially,moderate drought treatment significantly increased the root:shoot ratio of Forsythia suspensa saplings by 12.8%.（2）Drought stress influenced the NSC dynamic change.Firstly,the soluble carbohydrate concentration in foliage,stem and root（0-1mm、1-2mm and>2mm）increased significantly under drought stress.And for foliage,the soluble carbohydrate content of saplings under moderate or severe drought stress was 44.6%or 33.2%higher than that in control group.Secondly,the starch concentration in organs of Forsythia suspensa under severe stress was significantly lower than that under moderate stress and normal condition.The starch content in leaves and 0-1mm roots was significantly promoted by moderate stress,especially during the early stage of stress.The starch concentration in foliage of saplings under moderate stress was 40%and 36%higher than that of the other two groups,respectively,while in 0-1mm parts was 56.5%and 50.1%.As for the concentration of starch in stems,roots 1-2mm or>2mm,there was no significant difference among groups.With increasing stress intensity,the concentration of soluble sugar and soluble sugar:starch ratio in different organs of Forsythia suspensa also increased significantly,and so did the soluble sugar level in the entire sapling.In comparison with the control group,moderate drought stress increased the starch concentration,however,the severe drought stress decreased it.Drought stress also increased the soluble sugar:starch ratio in the whole Forsythia suspensa saplings.（3）SC and its components（lignin and cellulose）in each organ showed differences in response to drought stress of different intensities.In comparison to saplings supplied with adequate water,a significant rise of lignin of those under stress was observed in coarse roots（>2mm）,and a decrease in foliage and 0-1mm roots.Especially,the lignin concentration in 0-1mm roots decreased by 27.1%（severe stress group）and 30%（moderate stress group）,but there was no significant change in stems.As the drought stress went on,the cellulose concentration increased in fine roots（0-1mm,1-2mm）and decreased significantly in the coarse roots（>2mm）.As for the whole plant,drought stress（severe or moderate）decreased the lignin concentration,and a higher cellulose level was observed in saplings under mild（moderate）drought stress.The SC level did not change with the treatment of Moderate drought,but became kind of lower in the severe-drought-stress group.What’s more,drought treatment showed no significant effect on the lignin,cellulose,and SC concentration of the whole plant on different time scales.（4）The concentration ofδ¹³C changed significantly in every organ of Forsythia suspensa after isotope labeling,especially in foliage,where it increased from-29.15‰（one day after labeling）to 74.41‰（the ninetieth day after labeling）.Theδ¹³C decreased by 27.3%and 45.5%respectively in severe and moderate drought group in comparison with that in control group,indicating that drought stress had negative influence on the carbon sequestration capacity of Forsythia suspensa.As time went by,the content of ¹³C decrease gradually in aboveground part（foliage and stem）,especially in foliage,which showed a 89.7%loss of the ¹³C assimilated in the growth period.In contrast,the ¹³C content underground increased obviously,especial for>2mm roots,which increased by80.6%when compared with the first day after labeling.And the drought stress made the increment of ¹³C in>2mm roots 30.9%-39.4%lower than that in the control group.（5）As for the research on effects of drought stress on the content and stoichiometry of C,N,P,we found that the C content in each organ was not influenced by drought with significance（P>0.05）.It is showed that drought stress increased N content in leaves,stems and fine roots（0-1mm、1-2mm）.However,P content decreased significantly in foliage and increased in stems and roots with the diameter between 1-2 mm.The C:N,C:P ratio in each organ decreased as the drought intensity increased（except for the C:P ratio in leaves）,indicating that drought reduced the utilization efficiency of N and P,which restricted the growth rate of plant.The N:P ratio was between 1.28-9.61,lower than 14,indicating that the N content of soil might be the major restriction.Moreover,drought stress increased the N:P ratio significantly,which tended to alleviate this restriction.（6）The correlation analysis conducted among C,N,P,soluble carbohydrate and NSC in each organ of Forsythia suspensa indicated that the content of N in leaves,1-2mm and>2mm roots and C in leaves correlated positively and significantly with soluble carbohydrate（P<0.05）.There was also a correlation between C and NSC in 0-1mm roots.The findings indicate that C,N and P may play an important role in plant growth and metabolism,including the synthesis and allocation of NSC and its components.In conclusion,drought stress restricted the growth of Forsythia suspensa saplings biomass and increased the proportion of soluble sugar and NSC in various organs and whole plants.In addition,severe drought stress reduced the content of SC and its components（mainly lignin）.Drought stress affects the allometric growth of various organs by affecting the distribution of structural and non-structural carbohydrates in different organs of Forsythia suspensa saplings.