Retrieval of the photochemical reflectance index for assessing xanthophyll cycle activity: a comparison of near-surface optical sensors

[摘要] Unattended optical sensors are increasingly being deployed on eddy covarianceflux towers and are often used to complement existing vegetation andmicrometeorological measurements to enable assessment of biophysical statesand biogeochemical processes over a range of spatial scales. Of particularinterest are sensors that can measure the photochemical reflectance index(PRI), which can provide information pertaining to leaf pigments andphotosynthetic activity. This interest has facilitated the production of anew range of lower-cost multispectral sensors specifically designed tomeasure temporal changes in the PRI signal. However, little is known aboutthe characteristics (spectral, radiometric and temporal) of many of these PRIsensors, making it difficult to compare data obtained from these sensorsacross time, geographical locations and instruments. Furthermore, directtesting of the capability of these sensors to actually detect the conversionof the xanthophyll cycle, which is the original biological basis of the PRIdiurnal signal, is largely absent, often resulting in an unclearinterpretation of the signal, particularly given the wide range of factorsnow known to influence PRI. Through a series of experiments, we assess thesensitivity of one of the leading brands of PRI sensor (Skye SKR 1800) tochanges in vegetation photosynthetic activity in response to changingirradiance. We compare the results with those obtained using a more expensiveindustry-standard visible to near-infrared hyperspectral spectrometer(PP Systems UniSpec) and determine the radiometric compatibility ofmeasurements made by the different instruments. Results suggest that the SKR1800 instrument is able to track rapid (seconds to minutes) and more gradualdiurnal changes in photosynthetic activity associated with xanthophyll cyclepigment conversion. Measurements obtained from both the high and lower costinstrument were significantly linearly correlated but were subject to a largesystematic bias, illustrating that differences in instrument configuration(e.g. spectral response functions and band positions) can have a large impacton the PRI measurement values obtained. Despite differences in absolute PRIvalues, significant correlations were observed between the canopy PRI derivedfrom both the SKR 1800 and the UniSpec instruments, and the epoxidation stateof the xanthophyll cycle (r² = 0.46 p < 0.05 and r² = 0.76 p <0.01, respectively). However, the dynamic range of the SKR 1800 PRI signalwas often lower than more expensive instruments and thus the lower costmultispectral instrument may be less sensitive to pigment dynamics related tophotosynthetic activity. Based on our findings, we make a series ofrecommendations for the effective use of such sensors under field conditionsand advocate that sensors should be fully characterized prior to their fielddeployment.

[学科分类] 地球化学与岩石