Control of tiller recruitment in bunchgrasses: Uniting physiology and ecology

Abstract

1. Bunchgrasses are clonal plants whose dominance of moist grasslands worldwide is maintained largely through tiller recruitment. Tiller recruitment in clonal plants is a subset of the problem of lateral bud outgrowth in higher plants. This paper proposes that three currently competing hypotheses of lateral bud outgrowth - apical dominance; the nutrition hypothesis; and photosensitivity to the red: far-red light ratio - all operate in a manner dependent on environment and plant form. 2. The evidence for each hypothesis is reviewed, following which an integrated model is provided that links the three hypotheses into a cohesive strategy. Consequently we assess tiller recruitment by bunchgrasses in terms of the constraints of their functional growth form and their environment. Of the mineral nutrients, only nitrogen is considered because it is the only nutrient whose relationship with tiller recruitment is well established. 3. The integrated model maintains the accepted paradigm that actual bud release is hormonally controlled by the auxin: cytokinin ratio, although local nutrient concentrations may also be inhibitory. Importantly, each hormone is controlled by local signals in the shoots and roots, respectively, facilitating appropriate responses to environmental conditions. Auxin production and export from the shoots is moderated by phytochrome responses to red: far-red light ratios. Cytokinin production is mediated by root N concentration which, in turn, is a function of N absorption from the soil and seasonal reallocation of tissue N. 4. The growth form of bunchgrasses and the environment in which they are found emphasize that N has a strong mediatory role over tiller production which allows the grass plant to respond appropriately to shifts in this limiting resource. This suggests that control of lateral bud outgrowth may have an evolutionary basis in resource competition for N.