Branch and root formation in Trifolium repens is influenced by the light environment of unfolded leaves

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Abstract
In plagiotropic plants, axillary buds on the stolon can be exposed to low red:far-red (R:FR) ratios, while the leaves may be positioned in the uppermost layer of the sward where they are exposed to a high R:FR ratio. We tested whether the light environment of unfolded leaves influences outgrowth of the axillary buds and the formation of nodal roots of Trifolium repens. Single plants were grown in a growth cabinet with high photosynthetic photon flux rate (PPFR) and a high R:FR ratio (FHRH, control), low PPFR and high R:FR (FLRH) or low PPFR and low R:FR (FLRL). In an additional treatment (SS), only stolons were shaded so that developing leaves grew into light conditions similar to the control treatment. Neutral shading (FLRH) had a minor effect on branching and did not influence root formation. A reduction in the R:FR ratio (FLRL) significantly delayed the outgrowth of axillary buds so that, compared to the control plants, the percentage of branched phytomers was reduced by 43% on the parent axis and by 75% on primary branches. Furthermore, the number of nodal roots per plant was reduced by about 30%. When only the stolons were shaded (SS), the percentage of branched and rooted phytomers was similar to that of the control plants. Extension of petioles and leaves was very variable, increasing the values in the FLRL treatment at least 2.5-fold compared with the control plants. It was concluded that the light environment of the unfolded leaves had a significant influence on the regulation of the outgrowth of axillary buds and that the high plasticity in petiole growth allows the positioning of the leaves in a light environment conducive to the stimulation of branch outgrowth.

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