El profesorado investigador

3.1.1 Introduction

Plant domestication and modern breeding practices have been very successful in optimising plant performance to suit the needs of farmers and consumers (Moose & Mumm, 2008). This strong selective pressure exerted by humans on plant genetic diversity has a history of about ten thousand years for crops like wheat, and a few centuries for European forage grasses. However, selection pressure has resulted in genetic bottlenecks and therefore reduced the capability of the improved cultivars to adapt to the changing environment of many crops (Tanksley & McCouch, 1997). Perennial ryegrass is one of the most widely distributed grass species throughout the temperate regions of the world. Perennial ryegrass in Australasia was imported from Europe, and in New Zealand two superior ecotypes referred to as ‘Hawkes Bay’ and ‘Mangere’ ecotypes and Spanish germplasm have contributed strongly to the genetic structure of the cultivars currently sold to farmers (Stewart, 2006). However, germplasm from various other sources such as the European high sugar ryegrasses has also been used e.g. cultivar Aberdart (Stewart, 2006), and in Australia the possible use of Mediterranean germplasm has been explored among other initiatives (Silsbury, 1961).

The rainfall requirement for unrestricted perennial ryegrass growth depends on site factors such as temperature, evaporation potential, and soil water holding capacity. A recent New Zealand assessment of perennial ryegrass water requirements, using models developed at Wageningen, indicated that a potential perennial ryegrass yield of 15.8 t DM/ha/year in cooler conditions of Southland would require 1208 mm annual rainfall while a potential yield of 19.9 t DM/ha/year in warmer conditions of Waikato would require 1332 mm annual rainfall (Matthew et al., 2012). Against this, average annual rainfall (2001–2010) was 1134, and 1121 mm for Southland and Waikato, respectively, indicating a moisture deficit of over 200 mm per year in Waikato. Moreover, perennial ryegrass is commonly grown in drier regions of South Australia, Victoria, New South Wales and New Zealand regions with annual rainfall of 600–1000 mm. This implicit water deficit and associated production limitation is

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compounded by the occurrence of drier years and a trend towards warming temperatures and decreasing rainfall in some regions (Smith, 2012). To develop perennial ryegrass genetics that will ensure productivity from grazed pasture under water limitation, there is a need to identify genetic variation in plant performance of existing perennial ryegrass cultivars under summer drought.

Members of the fungal family Clavicipitaceae are found throughout the tropical and temperate regions of the world with many forming associations with various fungal, plant and invertebrate hosts. These relationships span the continuum from pathogenic, as with the entomopathogenic genus Metarrhizium, to mutualistic as with endophytic asexual species of Epichloë that colonise certain members of the grass family Poaceae (Leuchtmann et al., 2014). Epichloë festucae var. lolii naturally colonises perennial ryegrass (Leuchtmann et al., 2014). The wild type endophyte (common toxic endophyte) produces toxic metabolites which protect hosts from insects while also causing health issues for the grazing animals (Johnson et al., 2013; Thom et al., 2012). Selected endophytes, usually identified by screening a range of collected wild type strains for absence or reduced levels of toxic alkaloids harmful to mammals while retaining those alkaloids responsible for insect deterrence (Thom et al., 2014; Thom et al., 2013), are now widely used in the Australasian pastoral industries with the intention of enhancing ryegrass and tall fescue persistence.

Current research focuses on the use of selected symbiotic endophytes as a means to improve the tolerance of grasses, including cereals such as wheat (Hubbard et al., 2014; Simpson et al., 2014), to biotic and abiotic stressors. Published results on the effects of endophyte status on drought tolerance of perennial ryegrass are quite inconsistent, with some showing enhanced drought tolerance in endophyte infected plants, e.g. compared to endophyte-free plants, endophyte-infected plants showed more live tiller number (Amalric et al., 1999; Kane, 2011; Ravel et al., 1997), higher leaf water status (Amalric et al., 1999; Hahn et al., 2008) and more osmotic adjustment (Ravel et al., 1997) under drought conditions. In response to endophyte infection, perennial ryegrass has been found to show increased root dry matter (Hesse et al., 2003; Latch et al., 1985) or modified root distribution (Crush et al., 2004), however, no direct relationship between these root characteristics and drought tolerance was documented for endophyte-infected plants. Some other research

43 showed no effect of endophyte on drought tolerance of perennial ryegrass (Barker et al., 1997; Briggs et al., 2013; Cheplick et al., 2000; Marks & Clay, 2007), and sometimes even detrimental (Cheplick, 2004). However, a majority of this published research was done with the wild type endophyte. So far, there is limited information about the role of selected endophyte strains in drought tolerance of their associated perennial ryegrass cultivars.

Therefore, based on the information needs identified above, an experiment was set up at Palmerston North, New Zealand in 2012, aiming to i) evaluate variability in adaptation to moisture deficit in modern perennial or long-rotation ryegrass cultivars; and ii) identify whether presence of a commercial endophyte symbiont in the respective cultivar would improve plant performance during and after moisture deficit stress. Associated with these aims, research hypotheses were: (1) evaluated ryegrass cultivars would differ in agronomic drought tolerance, as assessed by herbage mass reduction under water deficit, compared to adequate watering, (Chapter 4); and (2) since perennial ryegrass is an out-crossing plant species, genotypes within each cultivar would show different drought tolerance levels (Chapter 5); (3) herbage mass responses to drought would likely be accompanied by differences in leaf water relations, and study of these responses might enhance understanding of defence against water deficit in ryegrass (Chapter 6); (4) leaf water relations might be modified by the presence of fungal endophyte, which in turn might vary depending on the host cultivar and endophyte strain (Chapter 6); (5) nutrient uptake is usually limited under drought conditions, although this response is not a criterion for drought tolerance evaluation, the author was interested to test whether cultivars show different nitrogen uptake abilities in response to drought; also, whether endophyte presence influences nitrogen uptake of the host under different water regimes (Chapter 7).

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3.1.2 Structure

This chapter introduces the plant material and methods of the rainout shelter experiment. Results of this experiment are presented in Chapters 4 to 7. The morphological data, including shoot dry matter, score of tiller survival rate and reproductive development and total length of new roots are given in Chapter 4. Chapter 5 explores genotype variations within each cultivar and interactions between plant genotype and irrigation treatment or endophyte status for herbage yield. The physiological data, including plant water relations, chlorophyll florescence, proline concentration and carbon isotope discrimination are described in Chapter 6. Data for nitrogen uptake and nitrogen concentration of shoots are reported in Chapter 7.