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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&#x2014;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) (  $R^{2}=0.59$  ,   $p&lt; 0.01$  ) of the 40 minutes preceding the leaf measurement. In the boreal location, the variations in  Pinus sylvestris  needle PRI were only weakly (  $R^{2}=0.27$  ,   $p&lt; 0.05$  ) correlated with mean PPFD of the preceding two hours; for  Betula pendula , the correlation was insignificant (  $p&gt;0.5)$   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.

Diurnal Changes in Leaf Photochemical Reflectance Index in Two Evergreen Forest Canopies