Effect of shade on the carbon and nitrogen allocation in a perennial tropical grass, Dichanthium aristatum

Changes in sward growth above ground by shade is a controversial subject for field experimentation because the differential allocation of carbon (C) to shoots and roots is difficult to measure. In this experiment the effect of three levels of irradiance (100%, 56% and 33% of full sunlight) on C and nitrogen (N) allocation in Dichanthium aristatum were studied under well-watered and well-fertilized conditions. Dry matter accumulation, weight ratio per organ and N allocation indices were studied during two cycles of growth, after planting and after cutting. Shade had no effect on the C and N influxes into the whole plant, showing that the N absorption is regulated by the C assimilation. However, C, and principally N, were preferentially allocated to the laminae under reduced irradiance. Under 100% and 56% of full sunlight, more N was allocated to the stubble component. This situation was reversed in the lowest radiation level, indicating that N reserves, and not exclusively C reserves, may limit the regrowth of this perennial grass when growing under high levels of shade. The higher shoot:root ratio under shade shows the disadvantage in the use of radiation use efficiency calculated from aerial biomass data when comparing differents levels of shade.