RADIATION USE EFFICIENCY OF PEARL-MILLET IN THE SAHELIAN ZONE

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): epsilon = epsilon-s-epsilon-a-epsilon-c. The climatic efficiency, epsilon-s, may be considered as a constant during a season and is 0.47 for the rainy season. The absorption efficiency, epsilon-a, is larger than the interception efficiency, and can be expressed as a linear function of L. The maximum conversion efficiency, epsilon-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 epsilon-s and the L-epsilon-a relationship were stable, epsilon-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.