Questions relevant to further component research

Present understanding of developmental morphology of a grass tiller is the synthesis of sixty years of extensive research studies in different areas, including mass flow or tissue flow in terms of growth and death of leaves and roots, segmental morphology of the above-ground and below-ground components of tillers, physiological processes and anatomical organisation of the grass shoot and root systems. Section 2.3 illustrates that even though the presence of segmental units in the grass shoot system was understood in the 1950’s and 1960’s, the understanding that the same organisational patterns apply to root development took another 40 years to emerge. Some authors have reviewed the ecophysiological basis for different levels of plant-plant and plant-environment interactions. For example, Robson et al. (1988) described the form and function of grass plants from seed germination, vegetative development, reproductive development and related physiological events; Chapman and Lemaire (1993) discussed morphogenetic factors that can change the course of plant growth and development, Nelson (2000)

Chapter 2 Literature review

44 discussed the reciprocal interactions between leaf growth and tillering, Schnyder et al. (2000) and Thornton et al. (2000) reviewed the C and N use in growth zones and Dawson et al. (2000) described ecophysiological aspects of root form and function. Nevertheless the detail surrounding segmental organisation in the grass root system and how this impinges on root-shoot relations and interactions between tillers in a hierarchical relationship remains imperfectly described. This point is highlighted by the review article ‘Understanding shoot and root development’ (Matthew et al., 2001) which has traversed these points and indicated ways in which improved knowledge of the segmental architecture of in the grass shoot can potentially contribute to future improvement of plant performance.

As introduced in section 2.4.2.7, some earlier reports in the New Zealand and United Kingdom describe the annual pattern of root replacement from the grass swards (e.g., Stuckey, 1941; Troughton, 1951; Jacques and Schwass, 1956; Baker and Garwood, 1959; Caradus and Evans, 1977). Those results showed inconsistency with each other and failed to clearly explain the root growth pattern at the grass sward. Conversely, from another series of studies which emphasized the evidence of presence of segmental organisation in the grass tiller suggested that root formation and development in the grass tiller is a continuous process (Matthew et al., 1991; Matthew and Kemball, 1997; Yang et al., 1998; Matthew et al., 2001; Lattanzi et al., 2005b). Further study is therefore needed to confirm whether root initiation and root development in the tiller axis has any seasonal pattern during continuous root production at the tiller axis (Durand et al., 2000; Fournier et al., 2005; Verdenal et al., 2008).

Section 2.4.2.7 on ‘Root formation and development’ reviewed the existing knowledge of root development in L. perenne. As indicated in that section, information on perennial ryegrass root developmental processes, and seasonal variation in nodal root formation and branching pattern are also scarce in the literature. The present study will therefore try to fill the gaps in the literature to some extent. To better understand root development and turnover, a possible approach is to dissect roots from the tiller axis in order of root age, so that root development can be documented in a Alf time scale, similar to Sharman’s (1942)

leaf turnover cycle in a plastochron time scale. A study of this kind will also describe the co-ordination between successive phytomers during root development in the same way

Chapter 2 Literature review

45 that progressive leaf development at successive phytomers as described by Fournier et al. (2005).

A recently developed viewpoint is root appearance on the tiller axis is governed by seasonal variation in day length and temperature that primarily modifies the leaf appearance interval over time and so timing of root appearance for that position on the tiller axis. Day length and temperature determine the time interval between the emergence of two successive leaves and thus the time interval between the events of appearing roots at the two successive root-bearing phytomers due to delay in root appearance at the same phytomer (e.g., Hunt and Thomas, 1985; Matthew et al., 1998). Seasonal variations in day length and temperature coupled with a delay of several phyllochrons between leaf and root appearance events at a phytomer mean that there is a possibility of ‘desynchronisation’ between shoot and root formation cycles, and hence of a morphogenetic variation in root to shoot allocation (Matthew et al., 1998).

As illustrated in Section 2.4.4.2 the work of Carvalho et al. (2006) and some other previous studies such as Clifford et al. (1973) and Colvill and Marshall (1981) revealed that DTs feed the roots of MT once they are established and they probably have a significant role in feeding older roots of MT which suffer C starvation due to reduced supply from the tiller of origin (Matthew and Kemball, 1997). The interrelations between MTs and DTs for dry matter partitioning, and C and N exchange are much less studied in

L. perenne swards.

Considering the above information, the following broad aims were determined for the three year Ph D research programme:

1. To provide a description of root development for L. perenne in phyllochron time units, similar to that of Sharman (1942) for leaves and an analysis of implications for root ecophysiology;

2. To conduct a preliminary investigation of whether or not seasonal ‘decoupling’ (desynchronisation) of phyllochron and rhizochron influences root:shoot relations. 3. To investigate aspects of root-shoot and tiller-tiller translocation with

consideration of root age and position when sampling roots for measurement. 4. To provide information on how daughter tillers might modify the pattern of root