Regulation of correlative inhibition of axillary bud outgrowth by basal branches varies with growth stage in Trifolium repens

Abstract

In Trifolium repens the decline in bud outgrowth that occurs with distance from basal root systems is due to correlative inhibition by the first-formed basal branches. The apical and axillary buds on these basal branches are the source of the inhibitory effect, but their mode of action is unclear. Inhibition might occur via basal branches being a sink for xylem-transported branching stimulants or alternatively by providing a source of inhibitory signals, or by both mechanisms. To distinguish between these mechanisms, four experiments were conducted on plants varying in initial growth stage from 10 to 19 nodes along their main stems to determine any variation in the relative importance of the operative mechanisms of correlative inhibition. Inhibitory signal exported into the main stem, detected as a branching response to girdling of basal branches, was relatively more significant in smaller (initially with 10-15 nodes on the main stem) than in larger (>19 nodes on main stem) plants. This signal was shown not to involve auxin fluxes, and is unidentified. However, across all stages of growth, the predominant mechanism driving correlative inhibition was the action of axillary and apical buds of basal branches as sinks for the stimulatory signal. This study indicates that the relative importance of the mechanisms regulating bud outgrowth in T. repens varies with growth stage and that, during intermediate stages, regulation has some similarity to that in Pisum.

Keywords: Apical dominance; auxin; branching regulation; correlative inhibition; cytokinin; prostrate clonal herbs; white clover (Trifolium repens)..

Fig. 1.

The core design and experimental procedures for Experiments 1–3: (A) growth stages of the plants at the time of treatment application for plants designated as small, medium, and large. DB and UE are distal branches and unemerged buds in the distal response zone, respectively; BB, basal branches in the treatment zone; ODB, the oldest branch in the distal response zone; Y indicates the position of the node (N) bearing the youngest fully unfolded leaf on the main stem. Nodes on the main stem are numbered acropetally from its base. (B) For each treatment, a diagram of one of the identically treated basal branches in the treatment zone for control, girdled, disbudded, and basal branch excision treatments. Open circles depict the nodes from which axillary buds and leaves were removed; arrows indicate intact active axillary or apical buds; and short double lines indicate points of stem excision or axillary bud removal. The open rectangle shows the position of the wax girdle.

Fig. 2.

For Experiment 1, the increase in (A) the number of primary branches and (B) their total length in the distal response zone during the 7-day period following treatment application for the control, girdled, and disbudded treatments in small-, medium-, and large-sized plants, n = 6. Asterisks indicate significantly (P < 0.05) different values from the control treatment.

Fig. 3.

Growth of axillary buds at nodes emerged successively from the apical buds of the main stem and branch 5 (the oldest branch in the distal response zone) during the 14-day period following treatment application in Experiment 2. Size of buds at the end of that period was measured as (A) the length of their stems and (B) the number of leaves that had emerged on them. Y is the node bearing the youngest unfolded leaf at the start of the experiment; +1, +2, and +3 are successively later-emerged nodes. Plants either remained intact (control) or their basal branches were girdled or totally excised; n = 6. Error bars indicate SE of means and asterisks indicate values significantly (P < 0.05) different from the control value.

Fig. 4.

Growth of axillary buds at nodes that emerged successively from the apical buds of the main stem and branch 7 (the oldest branch in the distal response zone) during the 14-day period following treatment application in Experiment 3A. Bud sizes at the end of that period are presented as (A) the lengths of their stems and (B) the number of leaves that had emerged on them. Y is the node bearing the youngest unfolded leaf at the start of the experiment; +1, +2, and +3 are successively later-emerged nodes. Plants either remained intact (control) or their basal six branches were girdled, disbudded, disbudded and girdled, or totally excised; n = 8. Error bars indicate SE of means and asterisks indicate values significantly (P < 0.05) different from the control value.

Fig. 5.

Axillary bud growth on branch 7 (the oldest intact branch in the distal response zone) in response to girdling of basal branches in Experiment 3A. Basal branches on control plants remained intact. Measurements of the length of each bud’s second oldest leaf (A) and stem (B) were made with the aid of microscopic examination at the end of the 14-day response period. Node 1 was the oldest node at the base of the branch and nodes 2, 3, etc. were successively younger. At the time of treatment, node 5 bore the youngest unfolded leaf (Y1); after 14 days the youngest unfolded leaf (Y2) was at node 8 on the branch and the axillary buds at nodes 9 to 12 were still within the branch’s apical bud; n = 8. Error bars indicate SE of means.

Fig. 6.

Axillary bud stem lengths at newly emerged distal nodes on the main stem in response to treatment in Experiment 3B. Basal branches either remained untreated (C) or were girdled (G), disbudded (D), or totally excised (X); n = 6. At the time of treatment application the youngest unfolded leaf (Y) was at node 15 on the main stem. Nodes that emerged successively from the main stem apical bud over a 19-day period are numbered 16–23. Error bars indicate SE of means.

Fig. 7.

For Experiment 2, the increase in number of primary (A) and secondary (B) branches in the distal response zone over the 14-day period following treatment application; n = 6. Treatments: application (+) or not (–) of NPA to basal branches of intact plants; application (+) or not (–) of NAA to stumps of excised basal branches; or girdling of each of the intact basal branches. Asterisks indicate significantly (P < 0.05) different values from the control treatment.