Assessing the xanthophyll cycle in natural beech leaves with hyperspectral reflectance.

The xanthophyll cycle is critical for protecting the photosynthetic apparatus from light-induced oxidative stress. A clear view of the xanthophyll cycle is thus important for understanding abiotic stresses that are closely related to plant growth and reproduction. The epoxidation state (EPS) is well correlated with the photosynthetic radiation use efficiency, and is widely used for assessing the xanthophyll cycle. The hyperspectral index, photochemical reflectance index (PRI), has been claimed to be closely related with the EPS, and offers instantaneous information of photosynthetic activity: its applications are, however, largely limited to herbaceous and coniferous species, and few studies have ever focussed on both sunlit and shaded leaves of deciduous plants. In the present study, we examined the possibility of applying PRI for tracing the xanthophyll cycle in a typical deciduous species (Fagus crenata Blume) as well as other species in a cold-temperate mountainous area. This is based on a series of experiments with only light stress and other inhibited treatments. Furthermore, searching for new hyperspectral indices has also been attempted based on both original and first derivative spectra. Results revealed that PRI had low correlations with the EPS of deciduous leaves, especially for sunlit leaves. As a comparison, the newly identified dD677, 803, a differential type of index using reflectance derivatives at 677 and 803nm, had a much better performance. The robustness of the newly identified index has been confirmed from both inhibitor-treatments and an additional dataset from other deciduous species. The proposed index is hence applicable for tracing the xanthophyll cycle in deciduous species.