Fundamental study on the estimation of weight of tuberous root of sweet potato with spectral reflectance

Remote sensing has been studied for evaluation and/or estimation of growth and yields of many types of crops. Most of these were reported on the crops with leaves distribution vertically. On the other hand, for the crops with leaves of horizontal distribution, few remote sensing studies have been reported. Especially, for sweet potato (Ipomoea batatas Lam. cv. kogane-sengan), development of the monitoring technique during cultivating period has been expected. This study was conducted to clarify the relationships between a tuberous root fresh weight per plant and leaf area, experimentally. The spectral characteristics from the leaves of sweet potato were measured by a handheld spectroradiometer during cultivating period. And the relationships among the tuberous root fresh weight, leaf area (LA) and the spectral characteristics were analyzed. The growth estimation index (ND_710,510) was defined as following formula used spectral reflectance of 710 nm and 510 nm; ND₇₁₀ = (R₇₁₀-R₅₁₀)/(R₇₁₀+.R₅₁₀). It was confirmed that there was a negative correlation between LA and the growth index with negative correlation coefficient. It is thought that this originates in the change of the spectral reflectance immediately after planting. Moreover nitrogen content in leaves per plant in growing period was estimated NDVI, RVI and leaf contents indices shown by the following expressions; ND_770,600 = (R₇₇₀-R₆₆₀)/(R₇₇₀-R₆₆₀), ND_530,510 = (R₅₃₀-R₅₁₀)/(R₅₃₀-R₅₁₀). These results showed the possibility of estimating the mass tuberous root during growth with nondestructive technique using spectral reflectance of sweet potato leaves.