Effects Of Exogenous Phosphous And Hydrogen Peroxide Supply On Wheat Root Growth

Plants acquire phosphorus (P) in the form of phosphate (Pi) in soils, the concentration of which is often limited for plant uptake. Modification of plant root growth and architecture is a well-documented response to Pi starvation. However, plant root growth and development under the conditions of high Pi supply in the intensive agricultural production system has receiced so little attention. A rhizobox experiment with soil and whole root culture experiment with nutrient solution were carried out to study effects of various concentrations of Pi supply on root morphology, rhizosphere processes, and plant P uptake of wheat. Two wheat varieties including Shimai 15, and Hengguan 35 were selected as the tested crops. Hydrogen peroxide (H₂O₂), an active oxygen species, is widely generated in many biological systems and regulates various physiological and biochemical processes in plants, but the roles of H₂O₂ in mediating root development of plants with different concentrations of Pi supply are unclear. The nutrient solution experiment was conducted to examine effects of exogenous H₂O₂ treatments on root growth of wheat under different levels of Pi supply. Ascorbic acid (AsA) and diphenylene iodonium (DPI) were added to nutrient solution. AsA is an H₂O₂ scavenger and one of the most important reducing substrates for H₂O₂ removal in cells. DPI is a specific inhibitor of NADPH oxidase activity and strongly inhibits the activity of NADPH oxidase, which is one of the main sources of H₂O₂ formation in plant cells. The main results and conclusions are as follows:(1) Two wheat varieties showed significantly different responses to soil Pi conditions. With more Pi supply, Shimai 15 appeared greater increases of shoot biomass and P content than Hengguan 35.Under limited Pi supply, Hengguan35 had more shoot biomass and P content than Simai15. Moreover, the decreases of shoot biomass and P accumulation due to Pi starvation were much lower in Hengguan35 than Shimai 15, indicating that Hengguan35 has higher tolerance to low Pi stress than Simai15.There were different morphological changes of roots in two wheat genotypes to cope with different Pi rates. Under high Pi supply, the fine root length within≤0.16 mm diameter and its percentage in total root length were greater in Shimai 15 than Hengguan35. Contrarily, without Pi fertilization, the total root length and the proportion of fine roots with diameter≤0.16 mm in total roots were larger in Hengguan 35 than Shimai 15. The roots of Hengguan 35 become finer. Phosphorus content in wheat shoots was significantly positively correlated with total root length and fine root length with≤0.16 mm diameter, respectively.With high Pi supply, the higher soil organic P content and the lower soil microbial biomass P was observed in rhizosphere. While without Pi supply, the activity of soil alkaline phosphatase was higher and soil organic P content and soil pH were lower in rhizosphere than in non-rhizosphere. Compared with optimum Pi supply, Pi over supply increased soil pH and decreased the activity of alkaline phosphatase in rhizosphere. However, low Pi stress decreased soil pH in rhizosphere.It is suggested that the adaptations of plants on two fronts are occurred when wheat plants subjected to low Pi stress. One is better root characteristics, such as longer roots, especially with more fine roots, is important for plant to efficiently utilize soil Pi. The other is the rhizosphere processes, such as soil acidification and the transformation of soil organic P to Pi, help plant to acquire more Pi from rhizosphere soils and benefit plant's tolerance to low Pi stress. However, the only change of root morphology occurs under high Pi supply, which possibly promote P accumulation of wheat plants(2) Low P application (0.005 and 0.01 mmol/L) to nutrient solution reduced lateral root number, but increased average lateral root length. The changes of root dry weight and total root length were not obvious under low P supply. The treatments at Pi concentration ranges of 2.50⁵.00 mmol/L did not significantly affect the root morphology of two wheat genotypes.The lateral root density of wheat plants was significantly increased by the treatment of 1.0 mmol/L H₂O₂ and decreased by the treatment of 10.0 mmol/L H₂O₂. More increase of lateral root density caused by 1.0 mmol/L H₂O₂ treatment was observed under the condition of low Pi (0.005 mmol/L) supply. Exogenous addition of 2.5 mmol/L AsA could eliminate the stimulatory effect of 1.0 mmol/L H₂O₂ on lateral root development. The application of 5.0μmol/L DPI reduced lateral root density, but such effect was relieved by exogenous H₂O₂ supply.The formation of nodal roots was obviously induced and the number of nodal roots was increased by the treatment of 10.0 mmol/L H₂O₂. The exogenous treatment of 2.5 mmol/L AsA could alleviate the inductive effect of exogenous H₂O₂ on nodal root development. The addition of 5.0μmol/L DPI reduced the number of nodal roots. Exogenous H₂O₂ treatments reduced the inhibitory effect of DPI treatment on nodal root formation.It is seemed that the morphological changes in wheat roots upon exogenous H₂O₂ are associated with H₂O₂ concentration and Pi supply conditions. The high level of H₂O₂ treatment would result in an increase of the nodal roots number and a reduction of lateral root density. While the low concentration of H₂O₂ treatment would enhance lateral root density, and such the stimulatory effect would be more significant in Pi deficient plants.