Seasonal Changes in Canopy Spectra of Sweet Potato

Abstract

In recent years use of multispectral satellite data to analyze plant cover and for developing phenological models has been prosperous. However, a ground-based continued and detailed study of spectral measurements throughout the growing season would be useful, by which not only spectral differences within and among seasons may be clarified but also the characterized wavelengths of crop species and variety may be identified. In the present study the reflectance spectra from 350 to 1100 nm were measured and the characterized wavelengths were recognized for the canopy cover of sweet potato (Ipomoea batatas cv. Tainung 57) grown in the field in 1995 and 1996. It was found the reflectance spectra of fresh canopy grown in different years were roughly similar, varied with environment and growth status. The ratio of reflected radiance in the UV region (＜400 nm) was far below 1%. The visible wavelengths (400-740 nm) were ranged from 20 to 27%, while near IR band (＞740 nm) were ranged from 20 to 27%, while near IR band (＞740 nm) were fluctuated in 70-80%. Patterns of spectral irradiance and reflectance look similar to other green crops. The same 28 wavelengths were identified from the spectral radiance curves while only 6 characterized wavelengths were selected from reflectance spectra of sweet potato in both 1995 and 1996. Three wavelengths from spectral irradiance, 554, 688, and 754 nm, were further chosen for the calculations of vegetation indices. The vegetation index regression curves were curvilinear after planting, the increase of the curves stands for the building up of plant canopy and the curves fall of as plant aged.