Regulation of shoot branching patterns by the basal root system: towards a predictive model

This study aimed to underpin the development of a generic predictive model of the regulation of shoot branching by roots in nodally rooting perennial prostrate-stemmed species using knowledge gained from physiological studies of Trifolium repens. Experiment 1 demonstrated that the net stimulatory influence from the basal rooted region of the plant on growth of newly emerging axillary buds on the primary stem decreased as their phytomeric distance from the basal root system increased. Experiment 2 found that at any one time the distribution of net root stimulus (NRS) to the apical bud on the primary stem and all lateral branches was fairly uniform within a single plant. Thus, although NRS availability was uniform throughout the shoot system at any point in time, it progressively decreased as shoot apical buds grew away from the basal root system. Based on these findings, a preliminary predictive model of the physiological regulation of branching pattern was developed. This model can explain the decline in growth rate of buds on a primary stem as it grows away from its basal root system but not the rapid progressive decline in secondary branch development on successive lateral branches. Thus knowledge of NRS availability to emerging buds is not, by itself, a sufficient basis from which to construct a predictive model. In addition, it seems that the ability of an emerging bud to become activated in response to its local NRS availability is, at least in part, directly influenced by the activation level of its parent apical bud. The experimental testing of this hypothesis, required for continued development of the model, is proceeding.