Multiple drivers of seasonal change in PRI: Implications for photosynthesis 1. Leaf level

The goal of this study was to explore the relationships between the photochemical reflectance index (PRI) at the leaf level and pigment pools, focusing on the constitutive role of pigments in influencing PRI over seasonal or ontogenetic time frames. The purpose was to re-evaluate the role of PRI as an indicator of seasonally shifting pigment (chlorophyll, carotenoid and anthocyanin) contents, and hence photosynthetic activity, across a range of tree and crop species. We studied natural vegetation - three tree species (maple, chestnut and beech) and two managed irrigated and rainfed crop species (maize and soybean), contrasting in photosynthetic pathway and leaf structure, and having wide variation of pigment content and composition. In anthocyanin-free leaves, PRI related to both total chlorophyll (Chl) and carotenoid (Car) contents, however, much closer relationships were found between PRI and Car to Chl ratio (Car/Chl). The sensitivity of PRI to Car/Chl varied widely in tree species with the degree of secondary carotenoids. In crop leaves where the Car vs. Chl relationship was very close, the slopes of PRI vs. Car/Chl relationships for maize and soybean were almost identical. PRI vs. Car/Chl relationships for leaves of different tree species formed a significant, uniform relationship with PRI. Two crops also formed a significant, uniform PRI vs. Car/Chl relationship with a slope half the value found for trees. In anthocyanin-containing leaves, PRI did not clearly relate to any pigment content because reflectance values at both PRI wavebands are affected by anthocyanin content. The findings of a strong link between leaf level PRI and Car/Chl over seasonal and ontogenetic time spans supports recent findings calling for a more careful evaluation of the relationship between PRI and either LUE or photosynthetic activity. In particular, studies that contrast short-term (e.g. diurnal) vs. long-term (e.g. seasonal) pigment, PRI, and photosynthetic responses in contrasting vegetation types are needed.