Influence of low light intensity on growth and biomass allocation, leaf photosynthesis and canopy radiation interception and use in two forage species of Centrosema (DC.) Benth.

Sustainable silvopastoral systems rely on the capacity of forage species to withstand shade, competition for soil resources and grazing. On a field-plot experiment, we studied the effects of artificial shade on the performance of two tropical forage legumes: Centrosema molle (CIAT 15160) and C. macrocarpum (CIAT 5713). Acclimation to shade in these two species was investigated at individual leaf and whole canopies, to explain differences in the efficiency of radiation interception and use, and thus forage yield. Under 30% sunlight, midday leaf net photosynthesis (A) was reduced by 32%-44%, although instantaneous light use efficiency increased up to 44%-140%. Almost 50% increase in leaf specific area was accompanied by 61% in total chlorophyll content. From analysis of the light-response curves, net photosynthesis at light saturation was only reduced (by 36%) in C. macrocarpum. Efficiency of chloroplast photosystem II (PSII) was unaffected by shade. Canopy structure for light interception was not modified by shade, although conversion into biomass and final yield were significantly reduced (40%-43%) in C. macrocarpum. Results are discussed in terms of factors affecting potential acclimation to shade and the agronomic implications of using both forage legumes in mixed pastures under tree canopies.