THE EFFECT OF LOCAL SHADING AND DARKENING ON BRANCH GROWTH, DEVELOPMENT AND SURVIVAL IN TRIFOLIUM-REPENS AND GALIUM-APARINE

Various measures of growth and development together with C-14-assimilate distribution patterns were used to investigate the response of shading or darkening of a single well established basal branch in two contrasting species, Galium aparine and Trifolium repens. Shading a basal branch had similar effects in both species in that its production of axillary branches and biomass were reduced; G. aparine also showed reduced leaf whorl production. In G. aparine darkening an established basal branch resulted in its rapid death, but in T. repens stolon elongation continued as normal but axillary branching ceased. With the exception of the root system, neither shading nor darkening had any significant effect upon the growth and development of the rest of the plant. The supply of C-14-assimilate from the main shoot or main stolon to the basal branch was not affected by either shading or darkening treatments. In G. aparine there was negligible movement of assimilate to an established basal branch, whilst in T. repens a significant proportion of assimilate (10-20%) was exported to such a branch, and within the shaded branch this was associated with a reorganisation of partitioning. Branch survival and mortality are discussed in relation to the differing growth strategies of the two species. Three potential physiological responses are proposed in response to localised shading and darkening.