Determination of tiller and root appearance in perennial ryegrass (Lolium perenne) swards by observation of the tiller axis, and potential application in mechanistic modelling

Abstract

Recording of bud status at successive phytomers on the tiller axis in perennial ryegrass was shown to provide information similar to that obtained from conventional recording of tiller appearance by monitoring marked tillers. The technique can be extended to monitor the number of roots formed. These observations lead to the establishment of a notation which describes tiller and root appearance, respectively, on the basis of probability or frequency per phytomer. These statistics are analogous to but less ambiguous in their interpretation than site filling statistics currently used to define tiller, and sometimes root, appearance. Potential modelling applications of quantifying sward dynamics at the phytomer level are discussed. In particular, nodal frequency statistics allow development of mass flow equations to describe leaf, tiller, and root production, and seasonal variation in allocation of photosynthate to developing tillers and roots.

Forage Plant Ecophysiology: A Discipline Come of Age

“…Here it is established from data on root ingrowth to refilled cores that root mass deposition is typically about 15% of the aboveground herbage dry matter accumulation, with seasonal periods of root formation activity typically preceding shoot activity by a few weeks [15]. The field data presented support a previously published hypothesis [18] that plant architecture, through the delay between leaf and root formation at a given phytomer, does provide a mechanism to increase root growth in early summer and decrease it in early winter [15]. Furthermore, a novel functional ecology insight emerges that the cessation of new root production in summer dry conditions [19] allows for the supply of photosynthetic substrate, which would have been captured by the newly formed young roots, to continue to reach older roots and so allow those roots to penetrate deeper in summer dry conditions than at times when surface soil layers have available water [15].…”

Do Phytomer Turnover Models of Plant Morphology Describe Perennial Ryegrass Root Data from Field Swards?

“…We selected Equation (5) of Matthew et al [26] for use in this study, which, when reorganized to suit the data collected in this experiment can be expressed (with reconciliation of units below):…”

“…where Fr denotes mass flux of root formation (kg DM/ha/day); T denotes tiller population density per m 2 ; P is the number of phyllochrons per day (units: days´1); AS (architectural signal) is the ratio between the phyllochron at the time of leaf formation and the phyllochron at the time of root formation, denoted by [26] as I t-d /I t ; R n denotes the average number of roots produced by each phytomer on the tiller axis (assuming steady state turnover); W r denotes the average weight of a single root (mg root´1);…”

“…The development of a theoretical framework for phytomer-based turnover of the root system represents a step change from previous thinking which has generally assumed that roots are replaced annually [31,32]. One feature that needs inclusion in the calculations is a representation of the proposed 'architectural signal' (AS) [26,33], whereby root:shoot ratio potentially changes during the year, reflecting variation in the number of leaves available to 'feed' developing roots at different times of the year. The model should also be capable of providing insight into the effects of lax and hard grazing on root development.…”

“…Here we add to the previously published data for 0-250 mm soil depth, data for the 250-600 mm soil layer and seasonal root and shoot production data. We draw on emerging awareness of phytomer-level organisation of the grass root system [21][22][23][24][26][27][28][29] to develop a phytomer-oriented mechanistic model; and attempt to reconcile the annual cycle of field data collected from December 1986 to January 1988 with the output from the phytomer based model. The aim is to show the quantitative link between root appearance rates measured in units of mass flow typically used for herbage accumulation (kg DM/ha/day) in the field swards of perennial ryegrass described above on the one hand, and root formation events at each phytomer on individual tillers on the other hand.…”