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 t iming problems for intensification of production. Photothermal effects on phenology have been studied to develop simulation tools for breedi ng and cropping systems research. Forty-nine genotypes were planted at monthly intervals in various rice-garden trials. Environment variabil ity among seasons, sites and within the crop canopy was' characterized to develop a field-based photot-thermal model for flowering. Basic co ncepts were summation of heat units and a linear thermal response of d evelopment rate having upper (Topt) and lower (Tbase) response limits. Photoperiodism was modeled by a slope constant (CPP) and a basic vege tative phase (BVP). Photoperiodism and transplanting shock acted as mo difiers 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 optimizatio n. Daily input temperature for the model was the physiolagically relev ant temperature Tphys at the shoot apex. Tphys depended on apex submer gence, water temperature and diurnal temperature patterns Diurnal temp erature segments exceeding the Tbase-Topt range were disregarded. Mean water temperature was below air temperature, particularly at high lea f area index and on dry days. Mean air temperature was closer to the m inimum than to the maximum when amplitudes were high or days short. Mi nimum temperatures below 18 degrees C at booting stage resulted in nea r total spikelet sterility and a specific delay in heading. The model was validated for a site thermally different from the site of calibrat ion.