Studies On The Effects Of Drought And Different Nitrogen Levels On Nitrogen Uptake And Metabolism In Malus

Drought is one of the most serious problems for sustainable agriculture,which influence plant growth and crop yield in agricultural production in many regions of the world.In China,Northwest China is one of the largest regions for apple production.However,annual precipitation in this region is less and extreme imbalance.Arid become the main factor of apple development in the region.Excessive nitrogen（N）fertilization in apple production is common in the Loess Plateau area of China,which directly lead to a decrease in nitrogen fertilizer rates,and also result in greater production costs and very significant levels of environmental risks,including nitrogen deposition,greenhouse effect and fruit diseases.Therefore,to control the amount of nitrogen fertilizers and to improve the nitrogen use efficiency is the way leading to sustainable development of apple industry.NH₄⁺and NO₃^-are the two major inorganic N forms in the soil,and the uptake of both ions by roots is closely related to water uptake and soil water content.In order to improve the efficient management of nitrogen fertilizers in arid regions,it is important to investigate the uptake properties of different nitrogen forms by apple roots under drought stress.In this study,apple rootstocks and cultivars were used to study the uptake properties of NH₄⁺and NO₃^-by apple roots under drought treatment.And the effects of drought and different nitrogen levels on plant growth,physiology,morphology,nitrogen and water uptake,and relevant gene expressions were also investigated.The results provided a theoretical basis for N uptake mechanism and N fertilization improvement in arid regions.The main results are documented as follows.1.We investigated the net fluxes of NO₃^-and NH₄⁺at the root surface along the roots of Malus hupehensis Rehd.and Malus prunifolia seedlings by non-invasive micro-test technique for the first time.And the effects of PEG-induced drought and N levels on the net fluxes of NO₃^-and NH₄⁺were also studied.The net fluxes of NH₄⁺and NO₃^-were different in roots of M.hupehensis and M.prunifolia.Under control conditions,net NO₃^-influx was much higher than that of NH₄⁺in roots of M.hupehensis,while the net influx of both ions were high in M.prunifolia roots.Under drought treatment,the net influxes of NH₄⁺were much higher than that of NO₃^-in the roots of both apple rootstocks.In response to drought stress,the net influx of NH₄⁺increased dramatically,while that of NO₃^-became much lower in M.prunifolia and changed little in M.hupehensis.In addition,as the N levels decreased,the net influx of NO₃^-declined while that of NH₄⁺enhanced.2.The effects of drought and N levels on the growth,physiology,morphology,anatomy and transcript regulation in M.hupehensis seedlings were investigated.Irrespective of N levels,drought decreased photosynthesis and biomass accumulation,inhibited root growth and uptake ability of water,and slowed down the process of N assimilation.The concentrations of NO₃^-and NH₄⁺and activity of NR were declined significantly in roots,and the expression levels of most NRTs and relevant genes encoding enzymes involved in N assimilation were also inhibited.Drought stress resulted in relatively increased uptake of NH₄⁺,and the transcript levels of AMT4;2 and AMT4;3 enhanced notably.Besides,the stable¹⁵N isotope analysis showed that the absorption of ¹⁵NH₄⁺increased significantly while¹⁵NO₃^-absorption remained unchanged or even decreased in response to drought stress.And¹⁵NH₄⁺absorption was significant higher than ¹⁵NO₃^-absorption by roots of M.hupehensis seedlings.Additionally,as the N levels decreased,the negative effect of drought on M.hupehensis seedlings was obviously alleviated.3.The apple cultivars Qinguan and Honeycrisp were used to study the effect of drought and N levels on plant growth and N metabolism.Drought inhibited the plant growth and N metabolism of both cultivars,and resulted in more serious negative effect in Honeycrisp seedlings.Additionally,Honeycrisp seedlings were more sensitive to reduced N levels than Qinguan.Asthe N levels decreased,the photosynthesis,biomass production and N accumulation were inhibted in both Qinguan and Honeycrisp seedlings.However,low N availability enhanced the root growth of Qinguan while inhibited that of Honeycrisp.Reducing N levels alleviated the negative effect of drought in Qinguan but aggravated the negative effect in Honeycrisp.The reason may be related to the distict N uptake and use efficiency between the two apple cultivars.4.We indentified 15 ammonium transporter（AMT）family members in apple,and investigated the phylogenetic analysis,conserved domain,gene structure,chromosome location and cis-acting elements existed in the promoters of apple AMT genes.The expression patterns of apple AMTs in different tissues were also studied,as well as the transcript response to N regime and diurnal variations.Functional analysis of apple AMTs in an ammonium uptake-deficient yeast mutant showed that apple AMT1;1/1;2/1;6/4;3 could facilitate NH₄⁺uptake.