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Drought resistance mechanisms in "Mediterranean" perennial ryegrass (Lolium perenne L.) and potential for introgression of "Mediterranean" germplasm into New Zealand commercial cultivars : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science, Massey University, Institute of Agriculture and Environment, College of Sciences, Palmerston North, New Zealand
The unique topography of New Zealand creates a wide variation in rainfall and temperature between and within the two islands of the country. As a result, successful use of perennial ryegrass (Lolium perenne L.), the backbone of New Zealand’s agricultural economy, has been restricted to only the higher rainfall and cooler areas of the country. However, there has been only limited analysis of drought resistance in forage grasses at the trait level. This PhD study was conducted on a perennial ryegrass cultivar “Medea” developed in Adelaide in the 1960’s from reportedly drought resistant and summer dormant germplasm of North African origin. The main objectives of the study were to compare Medea with a high yielding but drought susceptible current New Zealand cultivar, Grasslands Samson for their drought resistance potential and to evaluate Medea for its suitability for introgression with Grasslands Samson, in a plant improvement programme. Drought resistance strategies of Tolosa, Matrix and Ceres One50 were also evaluated.
In total six glasshouse experiments were conducted. Experiment 1 (April – September 2008) compared winter vegetative growth of potted plants of Grasslands Samson and Medea. Yield of Medea was <50% that of Grasslands Samson, but glasshouse temperature at times exceeded 25°C, so it is possible that this temperature was high enough to partially trigger summer dormancy in Medea.
In Experiment 2 (summer 2008 – 2009) techniques for assessing drought resistance were developed, and in Experiment 2 and Experiment 5 (summer 2009 – 2010) drought resistance strategies exhibited by individual cultivars were evaluated. Experiment 2 included Medea, Grasslands Samson, an unreleased tetraploid breeding line developed from Grasslands Samson and Tolosa. Experiment 5 evaluated Matrix and Ceres One50, in addition to Grasslands Samson and Medea. Drought resistance strategies observed in Medea included deep rootedness and high leaf proline contents, but there was some evidence for lack of transpiration reduction in water deficit stress. Medea had prolific flowering. Grasslands Samson and its tetraploid were more productive than Medea in these experiments. However, Tolosa produced the same shoot DW as Grasslands Samson with greater retention of soil moisture, indicating higher water use efficiency.
Experiment 3 (March 2009 – February 2010) compared five family groups, each comprising a Grasslands Samson and a Medea parent, and three of their F1 progeny. In this experiment plants were 11 months old when root traits were evaluated and for these older plants, Grasslands Samson had a higher root to shoot ratio and deep rootedness than Medea. Medea plants had similar shoot DW to Grasslands Samson plants during winter, but 46% lower shoot DW in summer. The F1 progeny showed positive mid-parent heterosis for deep rootedness, but negative mid-parent heterosis for shoot DW, and tended to reflect the prolific flowering of the Medea parent.
Experiment 4 (December 2009 – June 2010) compared six family groups of F2 progeny for traits related to drought resistance. Although plant numbers were small compared with a commercial breeding programme, it was evident some family groups combined both drought resistance and productivity traits.
Experiment 6 (September 2011 – February 2012) evaluated Grasslands Samson, Medea, and F1 and F2 progeny for drought resistance traits. Some useful traits expressed strongly in the F1 generation reverted to mid-parent values in the F2 generation. Some genotypes of Grasslands Samson exhibited higher water use efficiency (reduced soil moisture extraction with high shoot DW) and this warrants further research.
It is concluded that some desirable genes for traits contributing to drought resistance, such as deep rootedness and osmotic adjustment might be obtained from Medea. However, the drought resistance strategy of Medea involving reduction in plant size in summer, deep rooting and comparatively high transpiration would have pros and cons for New Zealand farmers as a trait combination. Reduced depletion of soil moisture under water deficit might assist survival of companion plants such as white clover; but high transpiration would decrease water use efficiency. Therefore, improving the water use efficiency of Grasslands Samson or use of material such as Tolosa, which has a comparatively low soil water use per unit of dry matter produced among the cultivars tested, would appear to be a preferred breeding strategy for future breeding programmes in New Zealand.