Response of Mangrove Carbon Fluxes to Drought Stress Detected by Photochemical Reflectance Index

The photochemical reflectance index (PRI) has been often used as a physiology-based remote sensing indicator of ecosystem carbon fluxes. However, the assessments of PRI in tracking long-term carbon fluxes with climatic anomalies in mangroves are very limited. In this study, four-year (2017–2020) continuous time series measurements from tower-based eddy covariance and spectral systems in a subtropical mangrove were used to explore the ability of PRI in tracking the response of mangrove carbon fluxes to climate fluctuations and drought stress. The results showed that the temporal dynamics of daily PRI and carbon fluxes shared similar variation patterns over the study period, experiencing simultaneously decreasing trends under drought stress. Compared with the first three years, annual mean values of NEE in 2020 decreased by 10.7% and PRI decreased by 29.0%, correspondingly. PRI and carbon fluxes were significantly correlated across diurnal, seasonal, and annual time scales with better fitness under drought stress. Dark-state PRI (PRI0), the constitutive component of PRI variation due to seasonally changing pigment pool size, showed similar temporal variation as PRI in response to drought stress, while delta PRI (ΔPRI), the facultative component of PRI variation due to diurnal xanthophyll cycle, showed no response to drought stress. This study confirms the ability of PRI to track temporal dynamics of mangrove carbon fluxes on both short-term and long-term scales, with the temporal variation of PRI largely affected by the long-term constitutive pigment pool size. This study highlights the potential of PRI to serve as an early and readily detectable indicator to track the response of the mangrove carbon cycle to climatic anomalies such as drought stress.

Proximal Imaging of Changes in Photochemical Reflectance Index in Leaves Based on Using Pulses of Green-Yellow Light

Plants are affected by numerous environmental factors that influence their physiological processes and productivity. Early revealing of their action based on measuring spectra of reflected light and calculating reflectance indices is an important stage in the protection of agricultural plants. Photochemical reflectance index (PRI) is a widely used parameter related to photosynthetic changes in plants under action of stressors. We developed a new system for proximal imaging of PRI based on using short pulses of measuring light detected simultaneously in green (530 nm) and yellow (570 nm) spectral bands. The system has several advances compared to those reported in literature. Active light illumination and subtraction of the ambient light allow for PRI measurements without periodic calibrations. Short duration of measuring pulses (18 ms) minimizes their influence on plants. Measurements in two spectral bands operated by separate cameras with aligned fields of visualization allow one to exclude mechanically switchable parts like filter wheels thus minimizing acquisition time and increasing durability of the setup. Absolute values of PRI and light-induced changes in PRI (ΔPRI) in pea leaves and changes of these parameters under action of light with different intensities, water shortage, and heating have been investigated using the developed setup. Changes in ΔPRI are shown to be more robust than the changes in the absolute value of PRI which is in a good agreement with our previous studies. Values of PRI and, especially, ΔPRI are strongly linearly related to the energy-dependent component of the non-photochemical quenching and can be potentially used for estimation of this component. Additionally, we demonstrate that the developed system can also measure fast changes in PRI (hundreds of milliseconds and seconds) under leaf illumination by the pulsed green-yellow measuring light. Thus, the developed system of proximal PRI imaging can be used for PRI measurements (including fast changes in PRI) and estimation of stressors-induced photosynthetic changes.

Winter leaf reddening phenomenon: the long-term track of PRI and phenological changes in a temperate Japanese cypress forest at Kiryu Japan.

<p>Winter leaf reddening is a phenomenon that evergreen species’ leaf color changes into red resulting from the accumulation of red pigments before or during winter, which persists for several months before dissipating with springtime warming. Among the many hypotheses about the winter leaf reddening, photoprotection is currently the favored hypothesis. Several studies focused on leaf reddening in angiosperms species. Yet, little researches concerned about leaf reddening in gymnosperms species. In gymnosperms, a kind of xanthophyll pigment rhodoxanthin was reported to play an important role. However, the xanthophyll cycle is the main protection mechanism of plants to deal with excessive light energy.</p><p>To track the winter leaf reddening phenomenon, we utilized the carotenoid-based vegetation index, the photochemical reflectance index (PRI), which is sensitive to changes in carotenoid pigments (e. g. xanthophyll pigments) in live foliage, as a tool to reflect the invisible phenology of photosynthesis by assessing carotenoid pigment dynamics. We used the CO2 flux data and the micrometeorological data collected from the temperate Japanese cypress forest from 2014 to 2019. We also made use of the digital camera to monitor the canopy phenology changes from 2016 to 2019. The digital camera took photos in 3 hours intervals with 3 different ROI (region of interest), the RGB channels of image data were extracted to calculate the RGB chromatic coordinates and the Red-Green vegetation index (RGVI).</p><p>Our findings demonstrated that air temperature reached the lowest point had a one-month lag in the time than that of PAR. The imbalance between light energy absorption and light energy utilization might activate the photoprotection mechanism. The change in light use efficiency (LUE) might confirm this conjecture. LUE reached its peak at the end of December and then dropped sharply. It suggested the photoprotection mechanism was activated. The RGVI fluctuation showed the seasonal changes with that of PRI almost in contrast. PRI was highly correlated with RGVI (<em>R=-0.806928034317071 in Pearson’s correlation test</em>). It suggested that the winter leaf reddening phenomenon caused the decline of PRI. Further, the PRI and RGVI both were highly correlated with air temperature and PAR. Based on current observations, there are still many unclear mechanisms. In the future, we will try to better explain the mechanism of winter reddening with a new set of experiments.</p><p> </p><p>Keyword: winter leaf reddening, Japanese cypress, photochemical reflectance index (PRI), Red-Green vegetation index (RGVI), phenological analysis, digital camera</p>