The effect of nitrogen on novel endophyte concentration in standard and high sugar ryegrasses: A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Previous studies have found that endophyte concentration can be reduced in high nitrogen environments and within high sugar grasses (Rasmussen et al., 2006; Stewart 1986). The interactions between endophyte and nitrogen are not well understood, but the implications to pasture persistence under pest attack are very important. A two part study was conducted at Lincoln University, Canterbury, New Zealand to test some of the mechanisms leading to the reduction in endophyte under high N. The first part was a field trial to examine impact of N and endophyte on aphid feeding. This involved two levels of nitrogen (0 and 160 kg N/ha) and three cultivars with two endophyte strains (Impact AR1 and nil, Samson AR1 and nil and Aberdart AR37, AR1 and nil). Data for root aphid numbers, tiller weights and root and shoot dry matter were collected. It was found that Aberdart had a tendency to have a higher level of root aphid (318.3 root aphids/m² ) than Impact which would suggest that it had a lower level of endophyte present and therefore a reduced capacity to defend against insect attack. It was also found that AR37 significantly reduced the root aphid numbers (144.3 root aphids/m² ) compared to ARl (355.2 root aphids/m² ) and nil (414.3 root aphids/m²) endophyte which were not significantly different. Nitrogen had no effect on the number of root aphids present. The second study, a pot trial, was designed to test some of the mechanisms involved in endophyte loss. Aberdart and Impact both with AR37 endophyte at three nitrogen levels, high, low and variable (low for ten weeks, high for four weeks) was run for a total of 14 weeks. At the end a destructive harvest was carried out and a Tissue Print Immunoassay of endophyte presence and qPCR for endophyte quantity was done. For the final four weeks after the variable nitrogen treatment increased from low to high, measurements of leaf elongation and tiller emergence were recorded. The key finding of the pot trial was a reduction in endophyte concentration under the high nitrogen treatment (709 ng⁻¹/gDNA) compared to low nitrogen treatment (1236 ng⁻¹/gDNA). The variable nitrogen treatment also had a tendency to have a lower endophyte content than the low nitrogen treatment (752 ng⁻¹/gDNA). There was also tendency for the standard cultivar Impact to have greater reduction in endophyte than Aberdart (a high sugar grass). These findings could have serious implications for farmers who rely on endophyte for protection from insect attack and have high sugar grass cultivars or use large amounts of nitrogen for increased production as it may reduce a pastures ability to defend against insect attack and reduce the persistence. In the future it may also have serious implications for seed production containing endophyte as it has been shown that successive generations do not recover the endophyte (Stewart, 1986). However, more work is needed before quality measures are implemented.