Growth, Physiological and Molecular Mechanisms of Low Nitrogen Stress Tolerance in Perennial Ryegrass

Perennial ryegrass is a widely used cool-season grass for forage and turfgrass in the temperate regions. Perennial ryegrass needs high fertility levels to maintain its turf quality and production. Nitrogen (N) is a major macronutrient for plant growth and development. The deficiency of N can cause chlorosis, loss of shoot density and decrease in growth. However, the overuse of N can cause pollution in atmosphere, water and soil. The objectives of this study were to: (i) investigate growth and physiological mechanisms of perennial ryegrass accessions exposed to low N stress; (ii) examine the effects of low N stress on antioxidant enzyme activities, malondialdehyde (MDA) and carbohydrate content in two genotypes of perennial ryegrass differing in low N tolerance; and (iii) analyze gene expression involved in N transporter and assimilation in young seedlings of perennial ryegrass subject to different time of low and high N treatments. Two low N tolerant (PI 231578 and BrightStar SLT) and two sensitive (PI 306292 and PI 547390) accessions of perennial ryegrass were exposed to low (0.75 mM) N and normal N (7.5 mM, control) nitrogen for 20 d. Plants grown under low N had reductions in chlorophyll content, and shoot and roots dry weight, and lower N content in the shoots and roots as well as higher shoot and root N use efficiency. The tolerant accessions maintained shoot dry weight and shoot carbon content, less reductions in shoot fresh and leaf chlorophyll content. The shoot total soluble protein significantly decreased in the sensitive accession but remained unchanged in the tolerant accession. The tolerant accession had greater increase in activities of shoot ascorbate peroxidase, root catalase, root peroxidase, and root superoxide dismutases. The amount of MDA generally increased in plants grown under low N. BrightStar SLT was grown in sands supplied with high (7.5 mM) and low (0.75mM) N solution for 14 d. Plant height, shoot dry weight, shoot carbon content, and chlorophyll content decreased at 14 d under low N. The expressions of nitrate transporter genes LpNRT1.1 and LpNRT2.1a and LpNR, LpNiR, LpGS1b and LpGOGAT involved in N assimilation were highly upregulated at 1-, 3-, or/and 7-d, respectively, compared to the high N control. The long-term goal of our research is to elucidate physiological, molecular and genetic mechanisms of low N tolerance in perennial ryegrass. The results from this thesis have laid a solid foundation for the future research on low N tolerance in more diverse perennial ryegrass accessions and cultivars. The results would also be valuable for studying on low N tolerance mechanisms in other perennial grass species