Do Daily and Seasonal Trends in Leaf Solar Induced Fluorescence Reflect Changes in Photosynthesis, Growth or Light Exposure?

Abstract

Solar induced chlorophyll fluorescence (SIF) emissions of photosynthetically active plants retrieved from space-borne observations have been used to improve models of global primary productivity. However, the relationship between SIF and photosynthesis in diurnal and seasonal cycles is still not fully understood, especially at large spatial scales, where direct measurements of photosynthesis are unfeasible. Motivated by up-scaling potential, this study examined the diurnal and seasonal relationship between SIF and photosynthetic parameters measured at the level of individual leaves. We monitored SIF in two plant species, avocado (Persea Americana) and orange jasmine (Murraya paniculatta), throughout 18 diurnal cycles during the Southern Hemisphere spring, summer and autumn, and compared them with simultaneous measurements of photosynthetic yields, and leaf and global irradiances. Results showed that at seasonal time scales SIF is principally correlated with changes in leaf irradiance, electron transport rates (ETR) and constitutive heat dissipation (YNO; p < 0.001). Multiple regression models of correlations between photosynthetic parameters and SIF at diurnal time scales identified leaf irradiance as the principle predictor of SIF (p < 0.001). Previous studies have identified correlations between photosynthetic yields, ETR and SIF at larger spatial scales, where heterogeneous canopy architecture and landscape spatial patterns influence the spectral and photosynthetic measurements. Although this study found a significant correlation between leaf-measured YNO and SIF, future dedicated up-scaling experiments are required to elucidate if these observations are also found at larger spatial scales.

Diurnal and Seasonal Variations in Chlorophyll Fluorescence Associated with Photosynthesis at Leaf and Canopy Scales

“…Our results report linear relationships between the SIF metrics (i.e., SIF A , SIF B , and SIF A+B ) and YII, and between SIF and ETR, primarily during the mature stage ( Figure 6), when canopy chlorophyll levels and Cl red were highest. Prior investigations [42,44] have reported that YII correlates well with SIF Ay for C3 canopies, which our findings did not support for corn, a C4 crop.…”

“…Recently, similar diurnal patterns and positive linear relationships between GPP and SIF A were reported using field and satellite observations for C3 and C4 canopies [36,37,[42][43][44][45]50]. Based on the established correlations between diurnal observations (i.e., 8:45-16:45), we also report strong positive correlations to GPP for SIF A and SIF B .…”

“…To our knowledge, very few studies have evaluated the correlation between the diurnal (i.e., 8:45-16:15 timeframe) dynamics in photosynthetic function in SIF A and SIF B across seasonality or multiple crop growth stages [42,[49][50][51][52]66]. Most investigations have focused on the relationship of SIF A to GPP, since SIF A can be obtained globally from satellites, and evaluated either the SIF diurnal patterns or phenological changes.…”

“…Further research, investigating the capability of SIF metrics for scaling and spatially extrapolating leaf YII and ETR under varying PAR conditions, is warranted for both C3 and C4 canopies. Recent studies [42,44,47] measuring C3 vegetation used supplemental dark-adapted measurements to augment PAM light-adapted observations and successfully estimate non-photochemical quenching during the day (NPQ) in a modeling framework, and thus enabled the partitioning of leaf PAR into YII, NPQ, and heat dissipation. Future efforts enhancing the current dataset with estimates of NPQ would yield a long term dataset for partitioning of APAR for C4 vegetation, which is not currently available.…”

“…Recent technology developments have enabled the continuous diurnal and seasonal measurements of SIF at the leaf and canopy level [26,28,[37][38][39][40][41][42][43][44][45][46] for extended periods, using automated very high resolution (~0.2 nm) spectral systems [47][48][49][50]. Systems that are able to dynamically adjust the optimization settings with the changing light levels provide a consistent stream of comparable data capturing the SIF diurnal cycles throughout the growing season.…”