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Soil optical properties affect spectral response of crop canopies and induce noise onto the relationships between reflectance data and crop characteristics such as leaf area index (LAI) or absorbed PAR (APAR). Different combinations of red and near infrared bands have been proposed but still suffered from a high sensitivity to soil brightness. As ideal vegetation index does not exist, we describe an improved vegetation index, the transformed soil adjusted vegetation index (TSAVI). It is based on similar principles as those stated by Huete (1988) and Major et al. (1988). We have used the SAIL model to simulate in different conditions the relationships between TSAVI and LAI or APAR. Experimental data recorded on wheat crops during the growing season are in good agreement with previous theoretical results.The W -TSAVI relationship can be set in the form oE TSAVI=TSAVI,(l-exp( -KTSAVI. LAI)) TSAVI, depends slightly on leaf angle distribution. The extinction coefficient, KTSAVI, depends on solar zenith angle and leaf angle distribution. The relationship between TSAVI measured at solar noon and MAR can be approximated to the linear simple form: MAR= c.TSAVIThis relationship is also in good agreement with experimental results with c = 1.205.

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