Relationships between chlorophyll fluorescence parameters and photochemical reflectance index of tree species adapted to different temperature regimes

Establishing a Gross Primary Productivity Model by SIF and PRI on the Rice Canopy

Solar-induced chlorophyll fluorescence (SIF) has shown remarkable results in estimating vegetation carbon cycles, and combining it with the photochemical reflectance index (PRI) has great potential for estimating gross primary productivity (GPP). However, few studies have used SIF combined with PRI to estimate crop canopy GPP. Large temporal and spatial variability between SIF, PRI, and GPP has also been found in remote sensing observations, and the observed PRI and SIF are influenced by the ratio of different observed information (e.g., background, direct sunlit, and shaded leaves) and the physiological state of the vegetation. In this study, the PRI and SIF from a multi-angle spectrometer and the GPP from an eddy covariance system were used to assess the ability of the PRI to enhance the SIF-GPP estimation model. A semi-empirical kernel-driven Bidirectional Reflectance Distribution Function (BRDF) model was used to describe the hotspot PRI/SIF (PRIhs/SIFhs), and a modified two-leaf model was used to calculate the total canopy PRI/SIF (PRItot/SIFtot). We compared the accuracies of PRIhs/SIFhs and PRItot/SIFtot in estimating GPP. The results indicated that the PRItot+SIFtot-GPP model performed the best, with a correlation coefficient (R2) of the validation dataset of 0.88, a root mean square error (RMSE) of 3.74, and relative prediction deviation (RPD) of 2.71. The leaf area index (LAI) had a linear effect on the PRI/SIF estimation of GPP, but the temperature and vapor pressure differences had nonlinear effects. Compared with hotspot PRIhs/SIFhs, PRItot/SIFtot exhibited better consistency with GPP across different time series. Our research demonstrates that PRI is effective in enhancing SIF and PRI for estimating GPP on the rice canopy and also suggests that the two-leaf model would contribute to the vegetation index tracking the real-time crop productivity.

Effectiveness of the photochemical reflectance index to track photosynthetic activity over a range of forest tree species and plant water statuses

In this study, we investigated the potential of the photochemical resistance index (PRI) to track photosynthetic activity under water stress conditions by measuring PRI, leaf fluorescence, the xanthophyll cycle and photosynthetic activity in different forest tree species subjected to progressive drought. The PRI declined with pre-dawn water potential and a significant relationship between PRI and the xanthophyll de-epoxidation state (DEPS) was observed, although with large interspecific variability in the sensitivity of PRI to changes in DEPS. For single tree species, a strong relationship was observed on either PRI light saturated photosynthesis or PRI maximum photochemical efficiency of PSII (ΔF/Fm'); a larger variability in both relationships was apparent when data from different species were pooled together. However, an improved correlation was shown only in the former relationship by plotting the ΔPRI (dawn PRI minus the midday PRI values). Thus, we conclude that PRI is able to provide a good estimate of maximum CO2 assimilation at saturating light and ΔF/Fm' for single tree species, despite the severe drought conditions applied. PRI should be applied more cautiously when dealing with multispecific forests because of confounding factors such as the strong interspecific differences in the initial value of PRI and in the sensitivity of PRI to changes in DEPS in response to drought.

Relationship between photochemical efficiency of photosystem II and the photochemical reflectance index of mango tree: merging data from different illuminations, seasons and leaf colors

In order to elucidate the effects of chlorophyll concentration and seasonal temperature on the relationship between photosystem II (PSII) efficiency and the photochemical reflectance index (PRI) of leaves under different light intensity, mango (Mangifera indica), a low-temperature-sensitive species, was used for the study. From early winter to summer, we selected several days to measure chlorophyll fluorescence and leaf spectral reflectance of mango leaves with dark green to yellow green colors, under natural sunlight from predawn to sunset and under six levels (0, 200, 400, 800, 1200 and 2000 mumol m(-2) s(-1)) of artificial illumination. When leaves were exposed to light, both PRI and PSII efficiency decreased with the increase in illumination, yet the PSII efficiency-PRI relationship varied with temperature and leaf color. Both predawn PRI and the X-intercept of the PSII efficiency-PRI regression equations were higher in dark green leaves and on the day with higher minimum air temperature, and lower on opposite conditions. These were due to the influence of chlorophyll on the reflection of wavebands for detecting PRI, and leaves retained a higher degree of epoxidation state of xanthophyll cycle pigments in cold predawn. Therefore, when data obtained at different seasons and with different leaf colors were pooled for analysis, PRI was not closely related to PSII efficiency. Yet, either in the darkness of predawn or under a given level of illumination, PSII efficiency always showed a significant positive correlation with PRI, with data from different leaf colors and seasons merged for statistics analysis. Because both the intercept and slope of the PSII efficiency-PRI equation showed a negative regression with photosynthetic photon flux (PPF), an empirical regression model, i.e., PSII efficiency = c + d . PPF + e . (PPF)(2) + f . PRI + g . PPF . PRI, could be fitted for multiple regression analysis. Based on the close correlation between the estimated and measured PSII efficiency (r(2) = 0.844-0.907, P < 0.001), using dynamic data obtained from leaves with yellow green to dark green colors, measurement was taken at predawn (F(v)/F(m)) and under any given strength of sunlight and artificial illumination (DeltaF/F(m)') through different seasons. We, thus, concluded that this empirical regression model could simulate both the seasonal and diurnal variations of PSII efficiency.

Biochemical constrains limit the potential of the photochemical reflectance index as a predictor of effective quantum efficiency of photosynthesis during the winter spring transition in Jack pine seedlings

Leaf reflectance spectral measurements are an emerging non-invasive technique that can be used to derive the photochemical reflectance index (PRI) to assess the physiological state of plants from leaf to ecosystem level. Changes in PRI are associated with changes in the xanthophyll cycle activity and provide an estimate of changes in the effective photochemical quantum efficiency (Φ_II) during the growing season. However, we hypothesised that the correlation between PRI and Φ_II might be poor when the xanthophyll cycle is primed for sustained thermal dissipation of the light energy absorbed. To test our hypothesis, we studied the recovery of winter acclimated Jack pine (Pinus banksiana Lamb.) seedlings that were exposed to different simulated spring recovery treatments in controlled environments. Different growth temperatures and light intensities were used to dissect the effect of these two factors on chlorophyll fluorescence, pigment composition and leaf reflectance. Φ_II showed a clear response to temperature whereas PRI was mostly affected by light intensity. In contrast, the de-epoxidation state of the xanthophyll cycle pigments was both temperature and light dependent. Our data suggest that zeaxanthin-independent non-photochemical quenching is employed to various degrees in the different treatments. As a result, within the limits of our experimental setup, PRI could not explain the variation in Φ_II. This indicates that an improved understanding of the different energy quenching mechanisms is critical to accurately interpret the PRI signal under environmental conditions where the predominant mode of excess energy dissipation does not involve a dynamic operation of the xanthophyll cycle, but a sustained mechanism of energy dissipation.

Near-distance imaging spectroscopy investigating chlorophyll fluorescence and photosynthetic activity of grassland in the daily course

Detection of grassland canopy chlorophyll fluorescence (Chl-F) conducted with an imaging spectroradiometer provided evidence of potential remote sensing estimation of steady-state Chl-F (Chl-F_s). Daily near-nadir views of extremely high spatial resolution hyperspectral images were acquired from a distance of 4 m for temperate montane grassland in the Czech Republic. Simultaneously, measurements of Chl-F and total chlorophyll content (Chl_a + b) were made on a single leaf at ground level were collected. A specifically designed 'shade removal' experiment revealed the influence of dynamic physiological plant processes on hyperspectral reflectance of three wavelengths: 532, 686 and 740 nm. Based on this information, the vegetation indexes R₆₈₆/R₆₃₀, R₇₄₀/R₈₀₀ and PRI calculated as (R₅₃₂-R₅₇₀)/(R₅₃₂+R₅₇₀) were tested for statistical significance with directly measured Chl-F parameters (maximum fluorescence yield, F_v/F_m; steady-state chlorophyll fluorescence, Chl-F_s and actual quantum yield, Ф_II). The grassland species under investigation were: Festuca rubra agg. (L.), Hieracium sp., Plantago sp., Nardus stricta (L.) and Jacea pseudophrygia (C.A. Meyer). The coefficients of determination (R²) for best-fit relationships between PRI-Ф_II and PRI-Chl-F_s, measured in the daily course, show a high variability of 0.23-0.78 and 0.20-0.65, respectively. Similarly, R² for the R₆₈₆/R₆₃₀-Ф_II and R₆₈₆/R₆₃₀-Chl-F_s relationships varied between 0.20-0.73 and 0.41-0.70, respectively. The highest average R² values were found between PRI and Chl_a + b (0.63) and R₆₈₆/R₆₃₀ and Chl_a + b (0.72). The ratio R₇₄₀/R₈₀₀ did not yield a statistically significant relation with Chl-F parameters.

Photochemistry, remotely sensed physiological reflectance index and de-epoxidation state of the xanthophyll cycle in Quercus coccifera under intense drought

Quercus coccifera L. is a Mediterranean sclerophyllous shrub with a high capacity to resist intense drought stress. Therefore, it could be used in the study of physiological changes suffered by plants at very low water potentials. A remote sensing sensor was used to measure continuously the physiological reflectance index (PRI; defined as the changes in reflectance at 531 nm with respect to those at 570 nm; PRI = [(R531 - R570)/(R531 + R570)] at canopy level and under field conditions in an artificial carpet of seedlings of Q. coccifera during a drought cycle. Correlations between leaf level-measured chlorophyll fluorescence parameters as well as the de-epoxidation state of the xanthophyll cycle [(A + Z)/(V + A + Z)] and canopy level-measured PRI were reasonably good (R (2) = 0.57-0.63, P < 0.01), and quite interesting for water stress remote sensing purposes. The instrument's temporal resolution allowed us to follow the rapid response of PRI to changing photosynthetic active radiation, and to resolve, in response to cloud-induced changes in light intensity, a fast and a slow PRI component. We report the disappearance of the rapid one under conditions of intense drought in response to a sudden increase in light intensity. The underlying photoprotection mechanisms that Q. coccifera shows in response to intense drought stress periods seem to be related to the existence of a low intrathylakoid lumenal pH at the end of the drought cycle. Under intense drought, these mechanisms allow this species to avoid oxidative damage, which was evidenced by the maintenance of an unaltered photosynthetic pigment composition and constant photosystem II efficiency in the mornings. It is concluded that, contrary to early reports, PRI is a sensible, indirect, non-destructive water stress indicator, even in plants experiencing intense drought.