Comparative growth forms of dryland forage legumes

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