Deconvolution of pigment and physiologically related photochemical reflectance index variability at the canopy scale over an entire growing season

Abstract

The sensitivity of the photochemical reflectance index (PRI) to leaf pigmentation and its impacts on its potential as a proxy for light-use efficiency (LUE) have recently been shown to be problematic at the leaf scale. Most leaf-to-leaf and seasonal variability can be explained by such a confounding effect. This study relies on the analysis of PRI light curves that were generated at the canopy scale under natural conditions to derive a precise deconvolution of pigment-related and physiologically related variability in the PRI. These sources of variability were explained by measured or estimated physiologically relevant variables, such as soil water content, that can be used as indicators of water availability and canopy chlorophyll content. The PRI mainly reflected the variability in the pigment content of the canopy. However, the corrected PRI, which was obtained by subtracting the pigment-related seasonal variability from the PRI measurement, was highly correlated with the upscaled LUE measurements. Moreover, the sensitivity of the PRI to the leaf pigment content may mask the PRI versus LUE relationship or result in an artificial relationship that reflects the relationship of chlorophyll versus LUE, depending on the species phenology.

Affecting Factors and Recent Improvements of the Photochemical Reflectance Index (PRI) for Remotely Sensing Foliar, Canopy and Ecosystemic Radiation-Use Efficiencies

“…Median R 2 (0.79 and 0.65, respectively; Figure 7d) was 7.0% and 4.0% higher for broadleaved and coniferous trees, respectively, than those (R 2 =0 .72 and 0.61, respectively) in Figure 5 in Garbulsky et al [20]. PRI accounted for 66% of the variability of RUE over an entire growing season for a mixed forest [107]. Four articles published after 2010 illustrated that PRI was strongly correlated with changes in net CO2 uptake for broadleaved (R 2 = 0.59 and 0.75) and herbaceous/crop (R 2 = 0.54 and 0.81) plants (Figure 7e).…”

“…Recent studies have indicated that the changes in the patterns of seasonal and inter-annual PRI were correlated with chlorophyllic and carotenoid pool sizes and structural properties [76,107,123,124]. Structural changes of the canopy caused by sustained water stress [120,123,125,126] or a varying leaf area index (LAI) [127] over the season led to PRI variability and loss of the seasonal relationship between PRI and RUE.…”

“…The status of epoxidation of the xanthophyllic cycle accounted for most of the diurnal PRI variability [76,106], especially in the early morning and afternoon [107], and short-term adaptations to varying levels of solar irradiance increased PRI variability [106]. Damm et al [108] demonstrated the sensitivity of canopy reflectance and vegetation indices derived from spatial and spectral high-resolution data to varying irradiances, and reported that unknown direct/diffuse irradiance caused by complex interactions of surface irradiance and reflectance anisotropy accounted for up to 32% of the uncertainty of PRI for crops.…”

Potential of Photochemical Reflectance Index for Indicating Photochemistry and Light Use Efficiency in Leaves of European Beech and Norway Spruce Trees

“…Separate modelling between sunlit and shaded portions of canopy yielding different Chla+b/Carx+c and PRI was the most efficient way to simulate daily photosynthesis from multi-angular PRI measurements [81]. Contrary to the direct use of PRI, recent pioneering studies employing procedures to estimate ∆PRI from PRI 0 and PRI are leading to attempts to use ∆PRI for the purpose of estimating various stresses [25,75,78]. We also suggest that ability to discern between the two effects of Chla+b/Carx+c and daily dynamics in xanthophylls in PRI 0 should play a crucial role in evaluating ∆PRI.…”

“…Similarly, Rippullone et al [77] tracked the photosynthetic responses of various tree species using ∆PRI under varying conditions of water stress. Hmimina et al [25] used ∆PRI to detect changes in LUE induced by water limitation in oak, beech, and pine saplings. Zhang et al [78] used ∆PRI to track photosynthesis responses to experimental drought and warming in a Mediterranean shrubland.…”