Mechanisms Of Root Responses To Low Phosphorus Stress In Different Crop Species/genotypes With Contrasting Root Systems

Phosphorus(P)is the second largest mineral nutrient element required for plant growth and development.It participates in various physiological and biochemical processes,such as plant metabolism and regulation,and is an indispensable component of plant.Phosphorus nutrition required by plants mainly depends on the root system to absorb soluble P from the soil.However,the P element in the soil is easily fixed,forming compounds that are difficult to be directly intercepted and absorbed by plant roots.As a result,the benefit of phosphate fertilizer is low,and the waste of P resources also causes the corresponding environmental pollution.Therefore,improving the plant's absorption and utilization efficiency of the existing P in the soil has become an urgent need to be fulfilled in the sustainable development of agriculture.The root system is an organ that directly interacts with the soil.The different plant root systems have their advantages.When encountering low P stress,roots show the plasticity reflection with typical characteristics of the species,which provides the possibility to improve the availability of soil P for plants.In this study,five representative crops with different types of root systems(wheat,maize,soybean,white lupin and narrow-leafed lupin)were selected,and the high-throughput semi-hydroponic root phenotyping platform(Chapter2)and rhizobox cultivation technique(Chapter 3 and Chapter 4)were used to examine root responses to low-P stress at the seedling stage.The aim of this research was to reveal the mechanisms of crop species or genotypes with contrasting root systems in response to lowP stress and their adaptability to low-P environment.The main research findings obtained in the thesis are as follows:(1)Low P stress significantly reduced the total root length,root surface area,vertical root depth,root average diameter and basal root number of wheat and maize,but promoted the length of fine lateral roots(significant level at P < 0.05,the same in this thesis).Wheat and maize plants increased the P acquisition by increasing the length of fine roots,which reduced the cost of root exploration,and increased the distribution ratio of roots in the 0–30cm surface layer to improve the P utilization efficiency,and the adaptability to low-P environments.No significant changes of acid phosphatase activity(APase)in rhizosphere were found(rhizoboxes experiment).Therefore,the mechanism of these two fibrous root crops,wheat and maize,in response to low-P stress was through alteration of root phenotypic traits.The growth of the aboveground part of maize acquired a large amount of P and the root system was well developed,making the P absorption efficiency of maize was 1.7(rhizoboxes experiment)to 2.2(semi-hydroponic phenotyping experiment)times higher than that of wheat.(2)Low P stress inhibited the total root length,root surface area,vertical root length,basal root number and angle between the basal root and the soil surface in soybean,whilst improved the growth and distribution of roots in the topsoil layer(0–20 cm)(accounting for70% of the total root length).Low P supply had no significant effect on root diameter and rhizosphere APase activity(rhizoboxes experiment).The mechanism of soybean in response to low P stress was to increase the P absorption efficiency by expanding the distribution of roots in the topsoil layer.Compared with maize,the P absorption efficiency of soybean was significantly higher than that of maize(up to 2.3 times more).(3)Low P conditions promoted the formation of cluster roots in white lupin.Under the semi-hydroponic conditions,the total root length and root surface area increased significantly,but under the soil-filled rhizoboxconditions,the total root length,root surface area and vertical root length were significantly reduced,and the rhizosphere APase activity increased by 2.1 times.Therefore,the mechanism of white lupin responded to low P stress is by changing the root morphology and rhizosphere APase activity;however,the response mechanism varied with the cultivation environments: under hydroponic conditions,alteration in root morphology was the main response,and under soil conditions,increase in the rhizosphere APase activity was the main response.The P accumulation of white lupins was lower than soybean(0.81 times of soybeans),but the P absorption per root length was1.5 times higher than that of soybean(semi-hydroponic phenotyping experiment).(4)Low P stress had litter impact on the root morphology of narrow-leafed lupin,but it significantly promoted the rhizosphere APase activity and the secretion of a variety of organic acids,especially malic acid and lactic acid.Therefore,the mechanism of narrowleafed lupin in response to low P stress was by adjusting the physical and chemical properties of the rhizosphere.The root system of narrow-leafed lupin was relatively stronger with low root length and less P accumulation than other four crops.But narrow-leafed lupin had high efficiency of P absorption per unit root length 15.2,1.9,1.1 and 1.1 times higher than that of wheat,maize,soybean and white lupin,respectively.(5)According to the two standards for evaluating P efficiency,i.e.the biomass accumulation,and P accumulation under low P conditions,among the five crop species,soybean had the highest P efficiency,followed by white lupin,maize,and narrow-leaf lupin,and wheat had the lowest P efficiency.Between the two genotypes of the same crop species,wheat genotype Changwu 4,maize genotype Shaandan 618,soybean genotype K117–3,white lupin genotypes Kiev Mutant,and narrow-leafed lupin genotypes Jenabillup and #24showed higher P efficiency.(6)In the studies of crop root response to low P stress,the semi-hydroponic nondesctructive root illustration system is more suitable for evaluating crops that respond to low P mainly through root morphology,such as wheat,maize,and soybean;and soil-filled rhizoboxes could be used for crops that respond to low P through root physiology such as white lupin and narrow-leafed lupin.This thesis explored the mechanism of root responses to low P stress,revealed the different strategies of crop species/genotypes with contrasting root systems in response to low P stress;evaluated the P efficiency of the studied crop species/genotypes;and compared the different cultivation and root research methods.The outcomes of this study provide certain guiding significance in theory and practice for agricultural production and crop breeding.