Response to water deficit stress in the native New Zealand tussock, Festuca novae-zealandiae : submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Department of Plant Biology and Biotechnology, Massey University, Palmerston North, New Zealand
Responses to water-deficit stress by a xerophytic tussock native to sub-alpine grasslands of New Zealand have been investigated. Festuca novae-zealandiae possesses some structural adaptions such as curled laminae, limited stomatal distribution, and dense trichomes characteristic of dry-land adapted species. These features were compared with those of a mesophyte, F. arundinacea. In addition to structural adaptions, evidence that metabolic changes contribute to the drought tolerance of F. novae-zealandiae was sought. Plants of F. arundinacea and F. novae-zealandiae were grown in glasshouse pots and were subjected to water-deficit. Concentrations of the phytohormone ABA and the osmoprotectants proline and glycine-betaine increased in mature laminae of F. arundinacea as SWC declined to 8%. In F. novac-zealandiae, ABA concentration increased from 25-50 ng/gDW in two steps. At the first step (at 9-10% SWC), the increase was greatest in leaf bases (to 150 ng/gDW), and the second increase (at 6% SWC) was greatest in laminae (to 320 ng/gDW) and non-existent in leaf bases. In water-sufficient tillers, proline concentration was elevated in leaf bases (5-8 mg/ g DW) compared to laminae (2 mg/ g DW) and increased to 28 mg/ g DW in leaf bases, and to 14 mg/ g DW in laminae as SWC declined to 8-9%. Glycine-betaine concentration (40-100 µmol/ gDW) was unchanged by water-deficit stress. Leaf and tiller counts, and estimates of leaf tip scorching indicated that leaf death progressed with increasing water-deficit. Although leaf elongation declined, it did not cease until the last tillers had died. SDS-PAGE and Western analysis of soluble proteins using antisera to ubiquitin indicated that protein turnover increased in lamina, but not in leaf bases in response to water-deficit. It was concluded that in response to water-deficit stress, leaf bases of F. novae-zealandiae retained turgor and remained viable at the expense of existing leaf laminae, which died back. Southern analysis suggested that the F. novae-zealandiae genome contains sequence(s) with homology to group 2 LEA proteins. Western analysis using antisera raised against the dehydrin consensus sequence KIKEKLPG revealed several related proteins in seeds of F. novae-zealandiae, but these were not induced by water-deficit stress in leaves.