Abstract
The components of radiative transfer (incident, reflected and transmitted photosynthetic active radiation (PAR)) were continuously measured with amorphous silicon cells in a stand of pearl millet ( Pennisetum glaucum (L.) R.Br.) grown under optimum conditions in Niger. These measurements were combined with biomass and leaf area index ( L) measurements to calculate the radiation use efficiency of the crop (dry matter production per unit of incoming solar energy): ϵ=ϵ s ϵ a ϵ c . The climatic efficiency, ϵ s , may be considered as a constant during a season and is 0.47 for the rainy season. The absorption efficiency, ϵ a , is larger than the interception efficiency, and can be expressed as a linear function of L. The maximum conversion efficiency, ϵ c , is equal to 2.9 g MJ −1 of absorbed PAR and decreases throughout the cropping cycle. When estimated under non-optimum conditions, assuming that ϵ s and the L-ϵ a relationship were stable, ϵ c decreases to a minimum value of 1.8 g MJ −1. It is hypothesized that the main limiting factor is drought, and we suggest the use of a water stress index for biomass estimation. Grain yield is evaluated from the ratio of grain yield to aboveground biomass, as established in Sahelian conditions.
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