Facultative and constitutive pigment effects on the Photochemical Reflectance Index (PRI) in sun and shade conifer needles

Abstract

Leaf pigment content and spectral reflectance were examined in four conifer species from the Pacific Northwest and Canadian boreal forest. Our goal was to evaluate the causes of within- and between-stand variation in the Photochemical Reflectance Index (PRI), an indicator of xanthophyll cycle activity and carotenoid pigment content that often scales with photosynthetic light-use efficiency. Both the dark-state PRI values and the change in PRI upon dark-light transition (ΔPRI) were measured in situ in leaves from different canopy positions (top vs. bottom) having contrasting light histories (sun vs. shade). PRI varied with species, canopy position, and with the pool sizes of several photoprotective carotenoid pigments (relative to chlorophyll). Upper-canopy leaves had a greater Δ PRI than their shaded counterparts lower in the canopy, reflecting a higher investment of the photoprotective xanthophyll cycle pigments for sun-exposed top-canopy leaves. These results indicate that the relative concentration of ...