Abstract
There is a critical need for sensitive remote sensing approaches to monitor the parameters governing photosynthesis, at the temporal scales relevant to their natural dynamics. The photochemical reflectance index (PRI) and chlorophyll fluorescence (F) offer a strong potential for monitoring photosynthesis at local, regional, and global scales, however the relationships between photosynthesis and solar induced F (SIF) on diurnal and seasonal scales are not fully understood. This study examines how the fine spatial and temporal scale SIF observations relate to leaf level chlorophyll fluorescence metrics (i.e., PSII yield, YII and electron transport rate, ETR), canopy gross primary productivity (GPP), and PRI. The results contribute to enhancing the understanding of how SIF can be used to monitor canopy photosynthesis. This effort captured the seasonal and diurnal variation in GPP, reflectance, F, and SIF in the O2A (SIFA) and O2B (SIFB) atmospheric bands for corn (Zea mays L.) at a study site in Greenbelt, MD. Positive linear relationships of SIF to canopy GPP and to leaf ETR were documented, corroborating published reports. Our findings demonstrate that canopy SIF metrics are able to capture the dynamics in photosynthesis at both leaf and canopy levels, and show that the relationship between GPP and SIF metrics differs depending on the light conditions (i.e., above or below saturation level for photosynthesis). The sum of SIFA and SIFB (SIFA+B), as well as the SIFA+B yield, captured the dynamics in GPP and light use efficiency, suggesting the importance of including SIFB in monitoring photosynthetic function. Further efforts are required to determine if these findings will scale successfully to airborne and satellite levels, and to document the effects of data uncertainties on the scaling.
Highlights
Climate change and anthropogenic activities have significantly affected agricultural and forest production by imposing novel combinations of multiple stresses, which act to alter plant function and productivity
This research effort assembled a set of corresponding leaf and canopy chlorophyll fluorescence and reflectance measurements, representative of the diurnal dynamics in the major growth stages for corn, which is supported by canopy gross primary productivity (GPP) collected by an eddy co-variance tower
(2) As compared to absorbed photosynthetically active radiation (APAR) and photochemical reflectance index (PRI), solar induced fluorescence (SIF) metrics provided stronger relationships to GPP and light use efficiency (LUE), which differed depending on Photosynthetically active radiation (PAR) levels: strongest correlations to GPP under low PAR were provided by SIF in the O2A (SIFA)+B and SIFA, under high PAR by SIFA+By and SIFAy; PRI had the strongest correlation to LUE under low PAR, while under high PAR, SIFA+By and SIFBy provided stronger correlations
Summary
Climate change and anthropogenic activities have significantly affected agricultural and forest production by imposing novel combinations of multiple stresses, which act to alter plant function and productivity. Plant photosynthesis is a fundamental biochemical process for carbon fixation that occurs within chloroplasts, primarily in the cells of the leaf mesophyll tissue [2]. Chl is arguably the key component enabling vegetation photosynthesis, and is an important indicator of plant function and productivity [3,4,5]. Plants continuously regulate their photosynthetic processes in response to the changing environmental conditions on diurnal and seasonal timescales. To capture the diurnal responses and seasonal changes in plant photosynthesis, there is a critical need for temporally dense remote sensing time series to accurately monitor the key parameters governing vegetation function. Observations at relevant time scales would enable the detection of diurnal and seasonal changes in plant condition that may otherwise be overlooked
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