Abstract
Hyperspectral reflectance is becoming more frequently used for measuring the functions and productivity of ecosystems. The purpose of this study was to re-evaluate the potential of the photochemical reflectance index (PRI) for evaluating physiological status of plants. This is needed because the reasons for variation in PRI and its relationships to physiological traits remain poorly understood. We examined the relationships between PRI and photosynthetic parameters in evergreen Norway spruce and deciduous European beech grown in controlled conditions during several consecutive periods of 10–12 days between which the irradiance and air temperature were changed stepwise. These regime changes induced significant changes in foliar biochemistry and physiology. The responses of PRI corresponded particularly to alterations in the actual quantum yield of photosystem II photochemistry (ΦPSII). Acclimation responses of both species led to loss of PRI sensitivity to light use efficiency (LUE). The procedure of measuring PRI at multiple irradiance-temperature conditions has been designed also for testing accuracy of ΔPRI in estimating LUE. A correction mechanism of subtracting daily measured PRI from early morning PRI has been performed to account for differences in photosynthetic pigments between irradiance-temperature regimes. Introducing ΔPRI, which provided a better estimate of non-photochemical quenching (NPQ) compared to PRI, also improved the accuracy of LUE estimation. Furthermore, ΔPRI was able to detect the effect of drought, which is poorly observable from PRI.
Highlights
Photosynthetic CO2 uptake by the leaves is a key process determining plant productivity
Inasmuch as photochemical reflectance index (PRI) of natural canopies is sensitive to directional irradiance [64], our results suggest that shifts in carotenoid contents are responsible for the variation in the PRI–light use efficiency (LUE)
CO2 uptake result in decoupling in the ΦPSII –LUE and PRI–LUE relationships that may occur at the daily timescale
Summary
Photosynthetic CO2 uptake by the leaves is a key process determining plant productivity. A major part of the light absorbed by leaves is used in the photosynthetic light and associated electron transport reactions to form energy carriers ATP and NADPH, which are required to assimilate CO2 in the carbon reduction cycle [1]. The absorbed light energy is used to convert CO2 into carbohydrates, which are used as building blocks for plant growth. Processes involved in photosynthetic CO2 uptake are affected by environmental factors such as light intensity, water availability, and temperature [2]. Accurate measurement of photosynthetic CO2 uptake is possible at leaf level using gas-exchange technique or at ecosystem level using eddy-covariance systems. There is increasing potential to use measurements of hyperspectral reflectance in monitoring the functioning and productivity of terrestrial ecosystems and their spatial and temporal variability. While net productivity and amount of ground biomass can be assessed using the normalized difference vegetation index (NDVI)
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