SEMIEMPIRICAL INDEXES TO ASSESS CAROTENOIDS CHLOROPHYLL-A RATIO FROM LEAF SPECTRAL REFLECTANCE

The ratio between carotenoid and chlorophyll a concentrations (Car/Chla) is indicative of the physiology and phenology of plants. With the aim of assessing this Car/Chla pigment ratio from reflectance (R), a wide range of leaves from several species and conditions were measured with high spectral resolution spectroradiometers for lambda between 400 and 800 nn. The performances of three pigment reflectance indices: (1) simple ratio pigment index (SRPI = R(lambda 1)/R(lambda 2)), (2) normalized difference pigment index [NDPI = (R(lambda 1) - R(lambda 2))/(R(lambda 1) + R(lambda 2))], and (3) the structure insensitive pigment index [SIPI = (R(800) - R(lambda 1))/(R(800) - R(lambda 2))] were tested. For each pigment index, every set of wavebands [lambda(1), lambda(2)] was systematically tested. High correlations with Car/Chla were found for all these pigment indices in the blue-red domain [400 nm<lambda 1<530 nm, 600 nm<lambda(2)<700 nm] as expected since both Chl and Car absorb in the blue; while only Chl absorbs in the red. The best semi-empirical estimation of the Car/Chla ratio was provided by SIPI for the wavelengths 445 and 680 nm: Car/Chla = 4.44 - 6.77 exp[-0.48 (R(800) - R(445))/(R(800) - R(680))] This index minimizes the confounding effects of leaf surface and mesophyll structure. These reflectance pigment indices provide new insight in the use of remote sensing for the assessment of physiology and phenology of vegetation.