Research On Sunflower Growth Mechanism And Water Transport Simulation By Considering Vertical Non-uniform Water-salt Stress

Improving the efficiency of saline soil using is significant to solve the contradictions between water-soil resource scarcity and food security in China.While clarifying the mechanisms of crop growth and water consumption under water and salt stress is the prerequisite for the agricultural production in saline soils.Affected by the complex natural and human factors such as soil evaporation,root water uptake,rainfall,irrigation leaching,and groundwater fluctuation,the vertical distributions of soil water and salt inevitably exhibit a non-uniform status in saline soils,which will regulate the crop growth and change the water transport of the whole soil-crop system.However,the relevant experimental,theoretical and simulation studies are still insufficient.Based on the outdoor sunflower growth experiments at the pot,microplot,and field scales carried out in the Hetao Irrigation District in Inner Mongolia and combined multidisciplinary interdisciplinary theories such as soil physics,meteorology,agronomy,plant physiology,and a variety of mathematical statistics and numerical calculation methods,the morpho-physiological responses of sunflower shoot and root at different growth stages under non-uniform soil water and salt conditions were studied in this thesis.Meanwhile,through coupled an improved macroscopic physical root water uptake model,the stomatal conductance control model,and several hydraulic resistance variation functions,a hydrodynamic model（SR-HRV）were proposed,which can simulate water transport by considering the non-uniform stresses of soil water and salt and the hydraulic properties responses of the soil-crop system.The processes of water transport and the sunflower growth were simulated in saline farmland.The obtained main conclusions as follow:（1）Three types of quantitative indexes for the vertical non-uniform characteristics of soil water and salt,including positional,average and extreme indexes,were proposed based on the soil salt and water status analysis in the pots,microplots and fields experiments,and the quantitative methods of non-uniform indexes in different experiments were also introduced.（2）Morpho-physiological characteristics of potted sunflowers were rarely affected by root distribution,while which were mainly related to the minimum salinity status of three soil layers(S_Min),the good linear relationships between morpho-physiological characteristics and S_Min were established（R²～0.72-0.85）;however,the root distribution had a greater effect on the relative increases of height and LAI（REΔH and REΔLAI）of microplots sunflower in each growth stage,and REΔH and REΔLAI were most relevant with the average water and salt stress coefficient weighted by root length density（ω）,the quantitative relationships ofω-REΔLAI andω-REΔH in different growth stage were also established（R²～0.17-0.57）.（3）Soil salinity degree and its vertical non-uniform distribution both affected sunflower growth.Non-uniform soil salt distribution could effectively alleviate salt stress under moderate salt stress conditions(S_Min≈7.5 d S m^-1),and the low-salt soil located in the lower root zone（V-type）was more beneficial for sunflower growth;however,under the high salt stress(S_Min≥9 d S m^-1),the mitigating effect of the non-uniform salt distribution was weakened,and the low-salt soil located in the upper root zone（A-type）was more contribute to sunflower growth.（4）Based on the principle of water balance,Penman-Montes formula and Feddes model,the cumulative transpiration of microplots sunflower at each growth stage was calculated.The environmental stress factorδthat reflected the non-uniform characteristics of soil water and salt was proposed,and the critical compensatory factorω_c of Feddes Jarvis model at different sunflower growth stages was inversed and quantified.The quantitative relationships ofδ-ω_c showed the minimum values ofω_c(ω_cmin)were 0.50,0.28,0.48 and0.63 at the seedling,bud,flowering,and maturity stage of sunflower,and the corresponding criticalδvalues（δ_c）were 16.62,87.48,63.50 and 32.41,respectively.（5）For sunflower,the effects of leaf water potential on stomatal conductance were weak,which resulted in the relatively high stomatal conductance and transpiration during the initial SR-HRV model simulation stage,while also accelerated the leaf water potential decreasing during the whole simulation;meanwhile,it also caused the hydraulics properties,transpiration,and root water uptake patterns of sunflower more sensitive to hydraulic resistance variations:the xylem water potential gradients and the hydraulic resistance variabilities among leaf,root collar,and xylem bottom were enlarged under the stem（SXC）and root xylem（RXC）cavitation situations;the root surface water potential further decreased under root-soil contact loosening（RSCL）situation;the root water uptake capacity significantly increased when the soil was wetted under root ABA aquaporin response（RAAR）situation.（6）SXC exacerbated the reduction of crop leaf water potential,but has little effect on transpiration and root water absorption patterns;RXC increased the xylem hydraulic resistance of deep root water absorption,and lead to early attenuation of transpiration,but the impact was relatively limited.Specifically,RAAR could effectively improve the water absorption capacity of the root system in the humid soil.Meanwhile,it would also accelerate the transpiration and reduce the leaf water potential.In low transpiration requirements,root anatomical structure adjustment（RASA）and RSCL had similar effects on transpiration.However,in the case of RSCL,the transpiration reduction was delayed while the reduction rate was faster,and the root water absorption also disappeared later.（7）SR-HRV model obtained the best calibration performances for soil volumetric water content（SWC）in all salt fields,RRMSE was smaller than others soil-plant-atmospheric continuum（SPAC）water transport models,followed by Feddes Jarvis model,but de Jong van Lier model and Feddes model could not accurately simulate SWC in the high-salt field.The accuracy of soil salt content（SSC）calibration of each SPAC model was slightly lower,especially in the high-salt field（R²<0.25,RRMSE>0.3）,while the performances of SR-HRV model were better than other SPAC models in the low-and high-salt fields.Based on the SR-HRV model,the calibration accuracy of LAI,cumulative dry matter（CWDM）,and grain yield（Yields）were generally better than other SPAC models.For the de Jong van Lier model,the calibrated sunflower growth statuses were almost higher than actual growth in all salt field;as for the empirical model,the similar and accurate calibrated sunflower growth was simulated in mid-and low-salt fields,but underestimated the sunflower growth in the high-salt field,especially based on the Feddes model.（8）The verification accuracy of SWC was less affected by the crop root water uptake methods,but the SR-HRV model had better performances than other tested models（R² is the largest）.Affected by the rise of groundwater level,lateral salt replenishment,and uncertainty of soil water and salt transport parameters,the SSC model had poor SSC verification accuracy,especially in the mid-and high-salt fields.The simulated value of 0-50 cm SSC was lower than the measured value.Except for the SR-HRV model,the SPAC models had relatively large verified results of sunflower growth in all fields,especially in high-salt fields.based on the SR-HRV model,the verification accuracy of sunflower’s CWDM and Yields in low-salt fields were lower,while the verification accuracy of sunflower’s growth characteristics in the mid-high salt fields was better than other SPAC models.