Response of green reflectance continuum removal index to the xanthophyll de-epoxidation cycle in Norway spruce needles

A dedicated field experiment was conducted to investigate the response of a green reflectance continuum removalbased optical index, called area under the curve normalized to maximal band depth between 511 nm and 557 nm (ANMB_511–557), to light-induced transformations in xanthophyll cycle pigments of Norway spruce [Picea abies (L.) Karst] needles. The performance of ANMB_511–557 was compared with the photochemical reflectance index (PRI) computed from the same leaf reflectance measurements. Needles of four crown whorls (fifth, eighth, 10th, and 15th counted from the top) were sampled from a 27-year-old spruce tree throughout a cloudy and a sunny day. Needle optical properties were measured together with the composition of the photosynthetic pigments to investigate their influence on both optical indices. Analyses of pigments showed that the needles of the examined whorls varied significantly in chlorophyll content and also in related pigment characteristics, such as the chlorophyll/carotenoid ratio. The investigation of the ANMB_511–557 diurnal behaviour revealed that the index is able to follow the dynamic changes in the xanthophyll cycle independently of the actual content of foliar pigments. Nevertheless, ANMB_511–557 lost the ability to predict the xanthophyll cycle behaviour during noon on the sunny day, when the needles were exposed to irradiance exceeding 1000 μmol m^–2 s^–1. Despite this, ANMB_511–557 rendered a better performance for tracking xanthophyll cycle reactions than PRI. Although declining PRI values generally responded to excessive solar irradiance, they were not able to predict the actual de-epoxidation state in the needles examined.