Comparative growth forms of dryland forage legumes

Abstract

Basic forage legume morphology and development are described with reference to white, red, subterranean and caucasian clovers, lotus and lucerne, with emphasis on characteristics that relate to persistence in dryland environments, including crown formation, contractile tap roots and rhizomes. The range of structural variation is considered in detail by comparing development of plants with two extreme growth forms, i.e. white clover, which has branching horizontal growth and nodal roots, and lucerne and caucasian clover, which have vertical growth, permanent crowns and deep tap roots. Emphasis is placed on carbon allocation strategies during establishment and the inevitable compromise between shoot and root growth, which results in slow seedling establishment of the tap-rooted species, and on the effect of nodal roots as competitive sinks for carbon, which affect tap root development and longevity in white clover. The influence of storage tissues and the number of viable buds on regrowth following drought, overwintering and grazing is discussed together with the impact of variations in flowering shoot formation. All existing growth forms are seen as having evolved as compromise solutions to the problem of combining maximum growth rates with optimum drought tolerance. The possibility, or otherwise, of being able to combine rapid establishment, involving maximised leaf production, with development of a persistent deeply penetrating tap root is considered in light of present understanding. Key words: carbon allocation, dryland environment, forage legume, morphology, nodal roots, persistence, root:shoot ratio, seedling establishment, tap roots

A generic individual-based model to simulate morphogenesis, C–N acquisition and population dynamics in contrasting forage legumes

“…The greater number of root apices produced by multiple sub-root systems each receive a lower proportion of the amount of C allocated to roots, resulting in reduced root elongation rates. This behaviour produced plant phenotypes globally consistent with root mutants reported for white clover: larger and deeper seminal taproot in nodal mutants as compared to wild types (White et al, 1998;Thomas, 2003), and preferential development of nodal roots from phytomers connected with vigorous axillary branches in wild types (Thomas et al, 2002).…”

“…Concerning roots, a wide range of morphologies can also be simulated in terms of root depth, prospected root volumes, root length density and relative investment in a perennial taproot. These general features cover the range of architectures usually reported in forage legumes (Forde et al, 1989;Matches, 1989;Thomas, 2003) and a wide range of adaptations to grassland management and pedo-climatic conditions (Beuselinck et al, 1994). Based on a literature survey (Table S2), the main parameters discriminating between the tested plant morphologies principally involved differences in branching ability (Phyllo 1 , Tilr, D min ), maximal organ growth (L max , D max ), shoot bearing (g stem ) and rooting ability (p rhizo , Del Nodal ).…”

Identification of pasture mixtures that maximise dry matter yield

“…White clover also seemed to interact positively with red clover. This was unexpected, but possible mechanisms that may combine to produce such an effect include differences in shoots and light capture (Black et al 2009) and in roots and access to water and nutrients (Thomas 2003) between the two clovers.…”

Phenological development of subterranean clover cultivars under contrasting environments

“…This stage corresponds approximately with Stage 30 on the BBHC scale. Important to note that in the BBHC scale (Enriquez‐Hidalgo et al, 2020) the term shoot elongation is used to indicate the structure commonly known as a runner (Moot et al, 2003; Thomas, 2003). Figure 5 illustrates the phenophases of subterranean clover ‘Antas’.…”

“…The extension of the runner was recorded as when the secondary stem (runner) was formed and had grown more than 20 mm away from the centre of the plant (Moot et al, 2003; Thomas, 2003). This criterion was used based on the observations from Moot et al (2003) who recorded the appearance of the first runner when one leaf had emerged on the axillary bud under controlled environment conditions, but they stated that runners remained short.…”