Genotypic Differences In Response Of Wheat Root Morphology To Nitrate Supply

Nitrate (NO3-) is the main source of inorganic nitrogen for plants in aerobic soil conditions. Adequate supply of nitrate is necessary for good plant growth. In barley, Arabidopsis thaliana and maize, the effects of the nitrate supply on root growth, and development have been identified. These are a localized stimulatory effect of external nitrate on lateral root elongation and a systemic inhibitory effect of high tissue nitrate concentrations on lateral root growth. However, little information is available on root morphological characteristics of different wheat genotypes influenced by different concentrations of nitrate. Whole root culture experiment with nutrient solution, split-root experiment, and soil column experiment with splitting layer were carried out to study effects of various concentrations of nitrate supply on root morphology and plant nitrogen uptake of wheat. Four wheat varieties, H10, L14, Shimai 15, and Hengguan 35 were selected as the tested crops. The main results and conclusions are as follows:(1)When whole roots of wheat plants were grown in the nutrient solution with 0.00~25.0 mmol/L of nitrate supply for 13 days, shoot biomass, shoot nitrogen contents and root development did not changed. After 22 days of nitrate application, the biomass and the nitrogen content in shoots increased significantly. Shimai 15, H10 and Hengguan 35 increased more than L14. Nitrate application resulted in a decrease in the ratio of root to shoot. With nitrate treatments at concentration range of 0.25~20.0 mmol/L did not significantly affected the root morphology of four genotypes of wheat. However, when supplied with low nitrate (0.05 mmol/L), four genotypes of wheat plants exhibited a notable increase in the average length of lateral roots, H10 and L14 had a larger proportion of fine root length with 0.15~0.45 mm diameter in total root length. There was a quadratic relationship between lateral root length, average lateral root length and wheat shoot N concentration. A positive correlative relationship was found between lateral root density and shoot N concentration.Without nitrate supply or under 0.05 mmol/L nitrate supply, Shimai 15 showed increases in primary root length, lateral root total length and total root length, a reduction in main root number, and no remarkable change in lateral root density; H10 appeared an increase in total length of lateral roots, a decrease in lateral root density, and no marked variation in primary root length, Hengguan 35 presented increases in main root number and lateral root density, a decrease in total length of lateral roots, and no obvious alteration in total root length; L14 had no significant changes in primary root length, total root length, lateral root length, total length, and lateral root density. It was suggested genotypic differences in wheat plants of root development response to various concentration of nitrate supply.(2) Under the addition of 50.0 mmol/L nitrate in both compartments of split root culture, dry matter weight and nitrogen concentration of wheat shoots were higher in comparison to those without nitrate supply. More root biomass and lateral root density of Hengguan 35 were in the compartment with 2.50 mmol/L nitrate treatment than those without added nitrate, while for the wheat cultivar L14, the 50.0 mmol/L nitrate increased root biomass and lateral root density in the nitrogen applied side. In the side with 2.50 mmol/L nitrate application, total lateral root length and average lateral root length of three wheat genotypes H10, Hengguan 35 and Simai 15 were higher than no N side. The stimulatory effect of 2.50 mmol/L nitrate on lateral root elongation was most in Shimai15. Compared with the side without nitrate, total lateral root length and average lateral root length of four wheat genotypes were longer in the side with 50.0 mmol/L nitrate application. The most increase of lateral root elongation under 50.0 mmol/L nitrate treatment was found in H10. In the root split treatments, N concentration in wheat shoots was positively correlated with the increase rates of the average lateral roots length induced by nitrate supply, suggesting that localized nitrate supply stimulates later root elongation of wheat, which would benefit plant nitrogen uptake.(3) Wheat roots displayed considerable developmental plasticity in response to variations in N supply in soil layers. Under the conditions of low nitrate rates in topsoils, more roots distributed to in subsoils where nitrate application rates were high as the ratio of total root length and root surface area in upper to lower layers of soil increased. Contrarily, less root length, surface area, and biomass allocated in the topsoil where nitrate rates were high as the ratio of total root length and root surface area in upper to lower layers of soil decreased. Nitrogen concentration and N accumulation in shoots had significant positive correlations with total root length, root surface area, and fine root length within 0~0.15 mm diameter respectively, while the soil nitrate contents showed negative correlations with those parameters of wheat roots. Some root phenotypes such as fine root length density, root surface area, and root developmental plasticity were important for improvements to nitrate acquisition and uptake of wheat plants from soils.