Understanding the physiology and agronomic performance of four perennial grass species under different nitrogen and water conditions in New Zealand drylands : a thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy

The main aim of this thesis was to quantify the effects of nitrogen (N) and moisture on monocultures of brome, cocksfoot, perennial ryegrass and tall fescue over time and understand any species differences in the physiology responses. According to previous studies available in the literature, those species perfom agronomically differently on water and nitrogen lacking environments. Our hypothesis is that, physiliogically, the species will show a difference that will justify the different agronomic performance. The pastures were established, at Ladbrooks and Ashley Dene, Canterbury, New Zealand, in the agronomic year of 2014/2015 and this thesis reports assessments for the years 2018/2019 (Year 5) and 2019/2020 (Year 6). The were selected because they have contrasting soil types that leads differences in plant available water content (PAWC). The Wakanui soil at Ladbrooks (LB) had a PAWC, of ~194 mm compared with ~131 mm for the stony Lismore silt at Ashley Dene (AD). The experimental design was a strip-plot design with the four grass species, fertilised (N+) or not (N-) with urea, with four replicates. This research focused on Year 6 and Year 6 after establishment. Total dry matter production at LB was 18100 kg ha-1 in Year 5 and Year 6 for the N+ pastures but 5360 and 6000 kg ha-1 for those unfertilised (N-). At AD the pastures yielded 9340 and 6000 kg ha-1 for N+ and N- in Year 5 but both yielded ~7395 kg ha-1 in Year 6. Cocksfoot yielded the highest (2630 kg ha-1) vegetative dry matter (leaves and swards, VDM) after six years under the driest conditions at Ashley Dene. The effects of water and nitrogen availability were quantified using a thermal time dry matter accumulation model, with a base temperature of 3 °C for all species. The temperature growth rates ranged from 0.3 kg DM °Cd-1 ha-1 during the water restricted summer dry periods, for N- plants, at Ashley Dene, up to 13.1 kg DM °Cd-1 ha-1 for N+ plants during the Spring 2019 at Ladbrooks when water is not restricted. The highest N+ pasture yields at Ladbrooks were explained by the higher photosynthetically active radiation (PAR) interceptance, and higher radiation use efficiency (RUE) coupled with greater water extraction, water use (WU) and water use efficiency (WUE) than at Ashley Dene. Plant physiology traits showed that under nitrogen or water deficit, the species had different response strategies. Cocksfoot and brome maintain their leaf area by keeping the cells turgid, with a higher relative water content and osmotic potential than p. ryegrass and tall fescue. These species protected their photosynthetic apparatus and chlorophyll concentration by accumulating higher levels of proline, particulary when nitrogen was provided to enable its synthesis. The strategy of preserving cell turgor and leaf area allowed greater vegetative material for animal production systems at Ashley Dene, which meant cocksfoot provided higher quality forage than the other three species in this dry environment six years after the establishment.