Many studies suggest that measuring spectral reflectance is useful to evaluate above-ground biomasses and qualities of standing plants. Few, if any, attempts to use spectral reflectance to estimate the growth conditions of creeping plants, however, have been done. In this study, therefore, the practical use of spectral reflectance data was investigated in the field of agriculture especially for sweet potato (Ipomoea batatas Lam. cv. kogane-sengan), which is one of the economically important crops in Kagoshima, Japan. It is well known that the total leaf area of a plant reflects its growth condition since the area affects its photosynthetic rate. In this study, the total leaf areas of sweet potato plants and their root yields were observed periodically during the cultivation period. But before digging up and destroying the samples, the spectral properties of their canopies were measured with a handheld spectroradiometer (400nm to 1000nm at 3 nm interval, IFOV with 3 degrees), and their numbers of leaves were counted nondestructively. These spectral curves were differentiated with respect to wavelength on the first and the second orders so that inflection points representing peaks and bottoms were found on the curves. Then the relationships among the wavelength corresponding to each peak, total leaf area, and root yields were analyzed. Normalized differential index with the reflectances of 520 nm and 710 nm, ND520,710 = (R520|R710) /(R520+R710) showed the close relation with total leaf area, the amount of yield, and nitrogen content of leaves, too. This result demonstrates the potentiality of the use of spectral reflectance signatures for estimating the yield of sweet potato before harvest.
