CLIMATIC DETERMINANTS OF IRRIGATED RICE PERFORMANCE IN THE SAHEL .1. PHOTOTHERMAL AND MICRO-CLIMATIC RESPONSES OF FLOWERING

In the Sahel, variable crop duration of irrigated rice poses serious timing problems for intensification of production. Photothermal effects on phenology have been studied to develop simulation tools for breeding and cropping systems research. Forty-nine genotypes were planted at monthly intervals in various rice-garden trials. Environment variability among seasons, sites and within the crop canopy was' characterized to develop a field-based photot-thermal model for flowering. Basic concepts were summation of heat units and a linear thermal response of development rate having upper (Topt) and lower (Tbase) response limits. Photoperiodism was modeled by a slope constant (CPP) and a basic vegetative phase (BVP). Photoperiodism and transplanting shock acted as modifiers of heat requirements (Tsum), thereby having greater effects on duration at low than at high temperatures. Tbase, Topt, Tsum, BVP and CPP were considered genotypic constants and calibrated by optimization. Daily input temperature for the model was the physiolagically relevant temperature Tphys at the shoot apex. Tphys depended on apex submergence, water temperature and diurnal temperature patterns Diurnal temperature segments exceeding the Tbase-Topt range were disregarded. Mean water temperature was below air temperature, particularly at high leaf area index and on dry days. Mean air temperature was closer to the minimum than to the maximum when amplitudes were high or days short. Minimum temperatures below 18 degrees C at booting stage resulted in near total spikelet sterility and a specific delay in heading. The model was validated for a site thermally different from the site of calibration.