Moisture Migration Mechanism Of Dwarfing Apple Tree Under Water Storage Pit Irrigation Based On Stable Isotope Technology

How to efficiently utilize water resources under water shortage to achieve the objectives of costs saving,quality improvement,and efficiency enhancement is an issue to be solved of sustainable development of modern orchard.With the application of water storage pit irrigation?WSPI?in northern orchard of China,to study the moisture migration mechanism of apple tree under WSPI in depth and further reveal water saving mechanism of WSPI is of great significance to guide the rational irrigation of orchards under WSPI.This paper combined indoor laboratory with field experiments to investigate the applicability of stable isotope technology in exploring the moisture migration of dwarfing apple trees.Five-year-old dwarfing apple trees are study objectives.Three treatments were set:surface irrigation?SI?,and WSPI with same irrigation amount as well as the WSPI with half amount.When using stable isotope technology,we systematically explained the interaction mechanism of moisture among the all processes of apple tree soil-root-shoot under WSPI.Finally,the results of study were shown as follows:?1?The feasibility of studying root water uptake of dwarfing apple trees using stable isotope was verified.In order to obtain accurate soil water and shoot water stable isotopes information,via describing the potential fractionation effect of extraction and testing,it was proposed that the optimal extraction strategy of soil water was carried out under the heating temperature of 90?,the filler of sponge,and the extraction time of 90 min.The sample sequencing optimization scheme based on improving efficiency was established and the fractionation effect of branch water was found during the non-growth stage of apple trees.It is concluded that the contribution rate of soil water was positively correlated with both the proportion of root length and the proportion of soil water consumption.?2?The transfer mechanism of moisture in the root zone under WSPI was explained.Through the physical model simuation of horizontal infilitation in soil colum,the fractionation mechanism of soil water during horizontral migration was grasped.It is shown the spatial and temporal distribution characteristics of soil water stable isotopes based on field experiments under WSPI.The radial heterogeneity of isotope could be ignored under single canopy.The cluster analysis method was adopted to classify the soil water layers in orchard,which could provide a basis for quantifying the root water uptake layers.It is concluded that both the slope and intercept of the soil water evaporation curve under WSPI treatment were smaller than that under SI,indicating that the soil evaporation intensity underWSPI was less.?3?Studing on moisture migration in the soil-root interface,the water-saving and drought-resisitant mechanism for WSPI was revelaed based on the variation of root growth and soil microenvironment in root zone,and the main layer of root water uptake under WSPI was quantified using stable isotope technology.Compared with SI,WSPI promoted the content of soil moisture below 40cm,improved the surface soil temperature,and increased the surface fungal content,so that the root systems of apple were distributed in the range of 0-100cm,induced deep rooting and the peak growth zone of root system was moved from20-40cm to 40-60cm.The optimal analysis method,namely?¹⁸O combined with Romero-Saltos and IsoSource models,was obtained via evaluating the difference and accuracy of the output results of five isotope models.Combine the results of model calculation with deuterium stable isotope?D₂O?injection,the main uptake depth of apple root under SI was quantified as ranging from 0 to 60cm,with the main soil water contribution area at 0-40cm soil layers.Under WSPI,the main uptake depth of apple root ranged from 20 to 100 cm,and significantly increased the water contribution rate of 40-100 cm soil layers.?4?The mechanism of moisture migration in the shoot of dwarfing apple under WSPI was investigated.Using D₂O injection and thermal diffusion techniques to study the moisture transfer of trunks,it is indicated that WSPI increased the sap flow rate of apple trees.A new method for predicting the water content of apple trunks via the maximum temperature difference?dTm?was proposed.The water status of apple trees was indicated by stem diamerter variations.Compared with SI,WSPI with the same irrigation amount increased the daily growth rate of the trunk and reduced the maximum daily shrinkage?MDS?of trunk diameter,while WSPI with the half irrigation amount could maintain similar variation of trunk diameter with SI treatment.The MDS under WSPI was positively correlated with the sap flow rate,and negatively correlated with soil moisture content of 40-80cm.MDS prediction models of multiple regression model and radial basis function?RBF?neural network based on daily average atmospheric temperature and daily average reference crop evapotranspiration under WSPI were constructed,and the average relative error for RBF was 4.53%.The effects of different irrigation methods on water use and consumption of leaves were analyzed using the variations of leaf water potential??_p?and stomatal conductance?g_s?.Compared with SI,WSPI could significantly increase?_p and g_s,and gs model under WSPI was established based on the leaf meteorological factors,which the average relative error was 1.89-8.93%.The transpiration line of the leaf water under WSPI was fitted,and the feasibility of characterization of stomatal conductance using leaf oxygen isotope discrimination was analyzed.?5?The mechanism of improving yield under WSPI based on leaf photosynthetic methanism was clarified,and the water use efficiency at different scales was quantified.Via improving leaf water status,promoting leaf stomatal opening,increasing chlorophyll content,enhancing leaf light energy utilization efficiency,and delaying leaf senescence,WSPI could enhance the ability of leaf photosynthesis,then increase the single fruit weight and soluble solid content of apple trees,and ultimately improve the fruit yield and quality of apple trees.Under the same irrigation amount,WSPI increased the yield by 25%compared with SI,but the yield had no significant difference when the irrigation amount of WSPI was reduced by half.The instantaneous water use efficiency of apple trees increased by 13-19%,the water use efficiency of yield level increased by 26.5-28.6%,and the leaf carbon stable isotope ratio increased significantly under different treatments of WSPI.The results of this paper are theoretically significant and can be utilized as guidance for appropriate irrigation management and eventually contribute to improve water use efficiency and increase yield in apple orchards under WSPI.