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

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 different pigment groups and associated PRI responses varied with canopy position and light history over more than one time scale, and included rapidly changing (facultative) and slowly changing (constitutive) components. Most of the PRI variability among the forest trees sampled was due to constitutive pigment pool size variation associated with species and canopy position. We conclude that both facultative and constitutive pigment components should be considered when applying PRI to photosynthetic studies of forest stands with remote sensing. Leaf-level measurements of PRI and ΔPRI provide non-destructive probes of both facultative and constitutive pigment changes within plant canopies that could help interpret variation in PRI signal viewed from remote sensing platforms.