Shoot branching in nutrient-limited Trifolium repens is primarily restricted by shortage of root-derived promoter signals

Two experiments were used to test the hypothesis that regulation of axillary bud outgrowth in nutrient-limited Trifolium repens L. (white clover) is primarily via variation in the net supply of root-derived promoter signal rather than via direct nutrient effects or inhibitory influences from apical or basal tissues. In the first experiment, foliar nutrient applications to a non-rooted portion of a nutrient-limited stem increased nutrient content, size of organs and rate of growth in the treated region but branch development remained suppressed, indicating that nutrient supply does not directly regulate branching. The second experiment, using decapitation and basal branch excision treatments, showed that excision of basal branches had a major stimulatory effect on bud outgrowth whereas decapitation of the primary stem had only a minor effect. This indicates dominant and minor roles in branching regulation for, respectively, root-derived promoter signal(s) and inhibitory apical influences (apical dominance), and that any possible influence of the inhibitory strigolactone pathway on bud outgrowth is captured within the net root-derived promoter influence. Thus, the proposed hypothesis was supported by our results. These findings may be relevant for all species within the group of prostrate nodally-rooting clonal herbs.