Abstract
The spectral properties of plant leaves relate to the state of their photosynthetic apparatus and the surrounding environment. An example is the well known photosynthetic downregulation, active on the time scale from minutes to hours, caused by reversible changes in the xanthophyll cycle pigments. These changes affect leaf spectral absorption and are frequently quantified using the photochemical reflectance index (PRI). This index can be used to remotely monitor the photosynthetic status of vegetation, and allows for a global satellite-based measurement of photosynthesis. Such earth observation satellites in near-polar orbits usually cover the same geographical location at the same local solar time at regular intervals. To facilitate the interpretation of these instantaneous remote PRI measurements and upscale them temporally, we measured the daily course of leaf PRI in two evergreen biomes—a European boreal forest and an Amazon rainforest. The daily course of PRI was different for the two locations: At the Amazonian forest, the PRI of Manilkara elata leaves was correlated with the average photosynthetic photon flux density (PPFD) ( $R^{2}=0.59$ , $p ) of the 40 minutes preceding the leaf measurement. In the boreal location, the variations in Pinus sylvestris needle PRI were only weakly ( $R^{2}=0.27$ , $p ) correlated with mean PPFD of the preceding two hours; for Betula pendula , the correlation was insignificant ( $p>0.5)$ regardless of the averaging period. The measured daily PRI curves were specific to species and/or environmental conditions. Hence, for a proper interpretation of satellite-measured instantaneous photosynthesis, the scaling of PRI measurements should be supported with information on its correlation with PPFD.
Highlights
T HE spectral absorption characteristics of leaves undergo subtle changes when the solar irradiance they experience exceeds the photosynthetic capacity of the leaf under existing environmental conditions
In the boreal forest, leaf photochemical reflectance index (PRI) remained nearly constant before noon with a dip in the afternoon [see Fig. 3(a) and (b)] and full recovery visible in the Pinus sylvestris data
The leaves started the day with a high PRI value with the PRI decreasing during the day
Summary
T HE spectral absorption characteristics of leaves undergo subtle changes when the solar irradiance they experience exceeds the photosynthetic capacity of the leaf under existing environmental conditions These changes are commonly quantified using the photochemical reflectance index (PRI) [1] calculated from leaf optical properties as PRI = R(531) R(531) − + R(570) R(570) (1). If the excitation energy arriving as photons to the antenna system in a green leaf becomes too large, de-epoxidation of the xanthophyll cycle redirects it to pathways which dissipate it safely as heat. This downregulation of photosynthesis is evident as a decrease in leaf PRI. The dissipated energy is not available for photosynthesis, and lowers the light use efficiency (LUE) of the vegetation canopy defined as the ratio of gross primary production to the photosynthetic photon flux density (PPFD) absorbed by (the green fraction of) the vegetation [3]
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