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 controversialsubject for field experimentation because the differential allocationof carbon (C) to shoots and roots is difficult to measure. Inthis experiment the effect of three levels of irradiance (100%,56% and 33% of full sunlight) on C and nitrogen (N) allocationin Dichanthium aristatum were studied under well-watered andwell-fertilized conditions. Dry matter accumulation, weightratio per organ and N allocation indices were studied duringtwo cycles of growth, after planting and after cutting. Shadehad 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 allocatedto 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, indicatingthat N reserves, and not exclusively C reserves, may limit theregrowth of this perennial grass when growing under high levelsof shade. The higher shoot:root ratio under shade shows thedisadvantage in the use of radiation use efficiency calculatedfrom aerial biomass data when comparing differents levels ofshade. Key words: Growth, shading, reserves, C₄ species