Branching responses of a plagiotropic clonal herb to localised incidence of light simulating that reflected from vegetation

Plants sense the presence of neighbouring vegetation through phytochrome ph otoreceptors perceiving a lowered red to far-red ratio (R:FR) of light refl ected from such vegetation. We hypothesised that it would be advantageous f or the grassland clonal herb, Trifolium repens, to have an inhibitory branc hing response to perception by leaves of light reflected from neighbouring vegetation (i.e. light with lowered R:FR ratio) but have no response to int erception of such light by the plagiotropic stem. We tested whether photore ception of reflected light by plagiotropic stems resulted in a different br anching response to photoreception by leaves and whether leaf ontogeny infl uenced the response. To simulate light reflected from vegetation, FR light- emitting-diodes were used to supplement controlled environment room Light s o that the R:FR ratio, but not the photosynthetic photon fluence rate, of l ight incident at the stem or leaf of a phytomer of T. repens was lowered fr om 1.20 to 0.25. The plagiotropic stems were unresponsive to light simulati ng that reflected from vegetation. This response differs from that of stems of orthotropic species, indicating that plagiotropic stems have evolved an organ-specific photobiology. Treatment of the mature leaf with light of lo wered FR ratio reduced phytomer production only of the branch in the axil o f the treated leaf. Similar treatment of the immature leaf retarded, in add ition, branching at basal phytomers on the same side of the primary stem ax is. Thus the response to light simulating that reflected from neighbouring vegetation depended upon whether the light was incident at the stem or the leaf and on the stage of leaf development. We argue that such responses imp rove the performance and fitness of T. repens within grassland habitats by allowing axillary buds on plagiotropic stems to branch freely when stems ar e in receipt of Light reflected from vegetation while leaves are in full li ght.