Physiological Mechanism Of Continuous Drought On Sweet Potato Growth And Sucrose-starch Conversion In Storage Root

Under the guidance of national macro-control,a variety of staple crops,such as cotton and rice,has been constantly shifting to arid and saline-alkali areas,and the climate characteristics of these areas determine that crop production will often suffer drought stress and reduce production.How to regulate the growth of crop roots under drought stress is an important problem.At present,more than one-third of the world’s arable land is facing water shortages.Morover,due to the double intensification of greenhouse effect and climate change,drought stress has become one of the major environmental factors limiting crop yields.Due to high yield,stable yield,strong resistance to adversity and wide adaptability,sweet potato（Ipomoea batatas（L.）Lam.）is a very important food,vegetable,industrial raw material and energy crop in China,among which the output accounts for more than80%of the world’s total output.However,frequent drought significantly reduced the number of storage root per plant,then resulting in a serious decrease in the yield of sweet potato.In this study,under the condition of pot experiment,the response of photosynthetic characteristics and yield formation of sweet potato to soil drought stress was studied,aiming to clarify the influence of different degrees of drought on the growth and development of sweet potato.In addition,the response of sucrose-starch metabolism in sweet potato storage root to drought stress and the relationship with starch accumulation were studied.These studies were to clarify the physiological and ecological mechanism of starch accumulation in sweet potato storage root in response to drought stress.The results of the study were as follows:1)With the decrease of soil relative water content,leaf chlorophyll content of sweet potato,plant biomass,storage root biomass,the proportion of storage root biomass,and storage root yield all showed a decreasing trend.In addition,the plant biomass had significant positive relationships with chlorophyll content,light saturation point(I_sat),apparent quantum efficiency（AQE）,maximum net photosynthetic rate(Pn_max)and dark respiration rate（R_d）,and significant negative correlations with light compensation point（I_c）,so as storage root yield.In this study,under drought stress,the main reasons for the decrease in plant biomass and storage root yield were:（1）Lower I_sat,AQE,Pn_max and R_dreduced photosynthetic capacity;Moreover,Jishu 26 was more adaptable to light and drought than Xushu 32,which is beneficial to high yield;（2）Lower fresh weight per storage root decreased sink capacity,mainly due to the decrease in average accumulation rate（V_T）.These results indicated that under drought stress,the source-sink ratio of sweet potato should be balanced,such as increasing the above-ground biomass of sweet potato in the early growth period,and increasing V_T in the storage root biomass rapid accumulation period.2)With the decrease of soil relative water content,leaf net photosynthetic rate（Pn）,stomatal conductance（gs）,transpiration rate（Tr）and intercellular CO₂ concentration（Ci）,were decreased.In addition,in the storage root,the starch content,alkaline invertase（AKI）,acid invertase（AI）,ADPG pyrophosphorylase（AGPase）,granular-bound starch synthase（GBSS）,soluble starch synthase（SSS）,as well as the expression of most their coding genes,were reduced,while soluble sugar,sucrose content,sucrose synthase（Su Sy）activity,starch branching enzyme（SBE）and starch debranching enzyme（DBE）increased.Forthermore,there were significant positive correlations between starch accumulation with V_T and maximum growth rate(V_max).Additionly,starch accumulation had significant positive relationships with AI,AKI,AGPase,GBSS and SBE.Therefore,the decrease of starch accumulation under drought was due to lower V_T and V_max,and the decrease in sucrose decomposition of storage root was mainly affected by AKI,and the decrease in starch synthesis was mainly due to the reduced activities of AGPase,GBSS and SBE along with the decreased expression of their coding genes,and GBSS was the key enzyme.These results provided theoretical support for clarifying the effects of different degrees of drought on the growth and development of sweet potato,and elucidate the physiological and ecological mechanism of starch accumulation in sweet potato storage root in response to drought stress.It could provide a reference for understanding the drought tolerance and dry land cultivation of sweet potato.