Photochemical Reflectance Index Use Research Articles

Estimation of photosynthetic dynamics in forests from daily measured fluorescence and PRI data with adjustment for canopy shadow fraction

We conducted year-long measurements of the photochemical reflectance index (PRI) and solar-induced fluorescence in the O2A oxygen band (SIFA) at a Norway spruce forest and a European beech forest to study relationships of these remote sensing variables to photosynthesis by trees in grown forest stands. Measured PRI and SIFA values were linked to changes in forest gross primary productivity (GPP) and light-use efficiency (LUE). Changes in the shadow fraction (αS) within tree crowns influenced PRI and fluorescence signals. In the spruce forest, the quantum yield of SIFA (FYSIFA) decreased around midday together with photosynthesis and GPP. Such decreases in FYSIFA were accompanied by an increase in the αS. In the beech forest, we detected an increase in FYSIFA together with a decrease in αS in the afternoon hours. The overall sensitivity of PRI to LUE was variable according to the season, presumably influenced by complex changes in photosynthetic pigments. PRI and FYSIFA showed weak correlations with canopy LUE; however, when considered together, the correlation was strengthened (R2 was 0.63 and 0.34 in spruce and beech forest, respectively). Our model predicting LUE dynamics includes a diurnal minimum of PRI and canopy αS to make allowances for seasonal changes in photosynthetic pigments and for diurnal variability of the shadow fraction in forests. The incorporation of these correcting factors allowed us to estimate LUE at R2 = 0.68 (spruce) and 0.53 (beech). The modeling equations appeared sensitive to the absorbed photosynthetically active radiation (APAR), but less sensitive to the GPP of these forests. Substituting pigments correction with introducing differential PRI (ΔPRI) into the model did not significantly improve the LUE estimation across the season. Our results show that the joint use of PRI and fluorescence improves LUE and GPP estimation accuracy in both daily and seasonal observations.

Using Hyperspectral Imagery to Detect an Invasive Fungal Pathogen and Symptom Severity in Pinus strobiformis Seedlings of Different Genotypes

Finding trees that are resistant to pathogens is key in preparing for current and future disease threats such as the invasive white pine blister rust. In this study, we analyzed the potential of using hyperspectral imaging to find and diagnose the degree of infection of the non-native white pine blister rust in southwestern white pine seedlings from different seed-source families. A support vector machine was able to automatically detect infection with a classification accuracy of 87% (κ = 0.75) over 16 image collection dates. Hyperspectral imaging only missed 4% of infected seedlings that were impacted in terms of vigor according to expert’s assessments. Classification accuracy per family was highly correlated with mortality rate within a family. Moreover, classifying seedlings into a ‘growth vigor’ grouping used to identify the degree of impact of the disease was possible with 79.7% (κ = 0.69) accuracy. We ranked hyperspectral features for their importance in both classification tasks using the following features: 84 vegetation indices, simple ratios, normalized difference indices, and first derivatives. The most informative features were identified using a ‘new search algorithm’ that combines both the p-value of a 2-sample t-test and the Bhattacharyya distance. We ranked the normalized photochemical reflectance index (PRIn) first for infection detection. This index also had the highest classification accuracy (83.6%). Indices such as PRIn use only a small subset of the reflectance bands. This could be used for future developments of less expensive and more data-parsimonious multispectral cameras.

Time course of the photochemical reflectance index during photosynthetic induction: its relationship with the photochemical yield of photosystem II.

Time courses of photochemical reflectance index (PRI) of an attached cucumber leaf during a dark-light transition were compared with those of photochemical yields of photosystem II (YII ) to discuss the feasibility of PRI imaging for estimating the efficiency of photosynthetic light use. YII and PRI were simultaneously evaluated with a pulse-amplitude modulation chlorophyll fluorometer and a low-cost imaging system consisting of digital cameras and band-pass filters, respectively. YII decreased immediately after the transition and then increased under various photon flux densities. Although PRI exhibited delayed time courses with respect to YII under low light conditions, PRI decreased monotonically under high light conditions. There was no correlation between YII and the changes in PRI (ΔPRI) immediately after the transition but YII was correlated with ΔPRI under the steady-state photosynthesis. These results indicate that the use of PRI to estimate YII under fluctuating light based on the regression obtained at steady state can overestimate YII . The imaging system was also applied to evaluate the spatial PRI distribution within a leaf. While PRI of leaf areas that remained untreated, or had been treated with H2 O again, first dropped and then rose under low light and monotonically decreased under high light conditions, leaf areas treated with inhibitor (dichlorophenyl dimethylurea) did not exhibit any changes. It is likely that the inhibitor suppressed lumen acidification, which triggers a decrease in PRI. It was suggested that YII of leaves with malfunctions in the photosynthetic electron transport can be overestimated by the PRI-based estimation.

Remote sensing of assimilating branch light use efficiency using the photochemical reflectance index inHaloxylon ammodendronforest

Five healthy Haloxylon ammodendron trees were randomly selected to monitor the photochemical reflectance index (PRI) and its relation to light use efficiency (LUE) in shaded and sunlit assimilating branches. Linear regression was performed where the results revealed that LUE decreased with seasonal courses with vivid midday depression regardless of light variation within canopies. PRI and LUE decoupled in early spring when the foliage was deeply downregulated, with high correlation ( R 2 = 0.8 , p 0.001 ) during most parts of the year. Measured and estimated LUE were highly correlated for the July to September dataset ( R 2 = 0.73 , p 0.001 ) compared to May to June datasets ( R 2 = 0.35 , p 0.001 ). The estimated LUE for young assimilating branches could be obtained with a high RMSE of 0.01, compared to mature branches with an RMSE of 0.003. The measured LUE values were ∼ 20% higher than estimated LUE during the period from May to June, and values were slightly lower at the end of the growing season. This was attributed to differences in the physiological and biochemical mechanisms controlling the seasonal dynamics of PRI. Therefore, PRI is the best estimator of LUE when age-related changes and canopy light partitioning are considered in assimilating organs. Combined use of PRI with pigments could improve the remote evaluation of LUE.

Photochemical reflectance index as a mean of monitoring early water stress

Water stress in plants affects a number of physiological processes such as photosynthetic rate, stomatal conductance as well as the operating efficiency of photosystem II (PSII) and non‐photochemical quenching (NPQ). Photochemical reflectance index (PRI) is reported to be sensitive to changes in xanthophyll cycle which occur during stress and could possibly be used to monitor changes in the parameters mentioned before. Therefore, the aim of this study was to evaluate the use of PRI as an early water stress indicator. Water stress treatment was imposed in a greenhouse tomato crop. CO2 assimilation, stomatal conductance, light‐adapted and dark‐adapted fluorescence as well as PRI and relative water content (RWCs%) of the rooting medium were repeatedly measured. The same measurements were also performed on well‐irrigated plants that acted as a reference. The experiment was repeated in four consecutive weeks. Results showed a strong correlation between RWCs% and photosynthetic rate, stomatal conductance, NPQ and operating efficiency of PSII but not with PRI when the whole dataset was considered. Nevertheless, more detailed analysis revealed that PRI gave a good correlation when light levels were above 700 µmol m−2 s−1. Therefore, the use of PRI as a water stress indicator cannot be independent of the ambient light conditions.

Seasonal changes in the relationship between photochemical reflectance index and photosynthetic light use efficiency of Japanese larch needles

We investigated seasonal changes in the photochemical reflectance index (PRI) and its relation to the diurnal profile of photosynthetic light use efficiency (LUE) in mature Japanese larch (Larix kaempferi Sarg.) forest throughout the growing season from June to October 2003. The daily mean value of needle PRI showed seasonal variation, strongly correlated with the chlorophyll concentration and carotenoid/chlorophyll ratio of the needles. During the green period from early June to late September, the hourly values of both PRI and LUE showed significant midday depression, and were positively correlated. In late October, however, because the PRI of yellowing needles tended to increase slightly at midday in contrast to the LUE, this correlation became negative. Even before autumn senescence, the sensitivity of PRI to LUE changed with the season. Correlation analysis indicated that the slope and intercept of the regression line of the PRI–LUE relationship increased during summer, with peaks in July and August, respectively. The seasonal change in slope was strongly correlated with the foliar photosynthetic pigment concentration, nitrogen concentration, air temperature and the daily mean value of the normalized difference vegetation index (NDVI). The value of the intercept was positively correlated with the daily mean PRI. These results suggest that although diurnal change in LUE cannot be estimated quantitatively from PRI on its own throughout the growing season, the combined use of PRI and other variables such as foliar pigments or NDVI could improve the remote evaluation of seasonal changes in LUE of deciduous tree leaves.

The Photochemical Reflectance Index (PRI) as a water-stress index

Measurements of leaf reflectance in two narrow wavelength bands centred around 531 and 570 nm were carried out during withering of two contrasting functional and structural plant types: Chenopodium quinoa (Willd.) and Arbutus unedo (L.). The Photochemical Reflectance Index (PRI)=( R 531 - R 570 )/ ( R 531 + R 570 ) was calculated. The results showed that PRI could be used as a reliable water-stress index. However, some limits to the use of PRI have to be emphasized, e.g. the wilting of leaves during dry periods. If such limits are under control, PRI could be a non-destructive and low cost method for assessing plant water status.

The photochemical reflectance index: an optical indicator of photosynthetic radiation use efficiency across species, functional types, and nutrient levels.

The photochemical reflectance index (PRI), derived from narrow-band reflectance at 531 and 570 nm, was explored as an indicator of photosynthetic radiation use efficiency for 20 species representing three functional types: annual, deciduous perennial, and evergreen perennial. Across species, top-canopy leaves in full sun at midday exhibited a strong correlation between PRI and ΔF/Fm', a fluorescence-based index of photosystem II (PSII) photochemical efficiency. PRI was also significantly correlated with both net CO2 uptake and radiation use efficiency measured by gas exchange. When species were examined by functional type, evergreens exhibited significantly reduced midday photosynthetic rates relative to annual and deciduous species. This midday reduction was associated with reduced radiation use efficiency, detectable as reduced net CO2 uptake, PRI, and ΔF/Fm' values, and increased levels of the photoprotective xanthophyll cycle pigment zeaxanthin. For each functional type, nutrient deficiency led to reductions in both PRI and ΔF/Fm' relative to fertilized controls. Laboratory experiments exposing leaves to diurnal courses of radiation and simulated midday stomatal closure demonstrated that PRI changed rapidly with both irradiance and leaf physiological state. In these studies, PRI was closely correlated with both ΔF/Fm' and radiation use efficiency determined from gas exchange at all but the lowest light levels. Examination of the difference spectra upon exposure to increasing light levels revealed that the 531 nm Δ reflectance signal was composed of two spectral components. At low irradiance, this signal was dominated by a 545-nm component, which was not closely related to radiation use efficiency. At progressively higher light levels above 100 μmol m-2 s-1, the 531-nm signal was increasingly dominated by a 526-nm component, which was correlated with light use efficiency and with the conversion of the xanthophyll pigment violaxanthin to antheraxanthin and zeaxanthin. Further consideration of the two components composing the 531-nm signal could lead to an index of photosynthetic function applicable over a wide range of illumination. The results of this study support the use of PRI as an interspecific index of photosynthetic radiation use efficiency for leaves and canopies in full sun, but not across wide ranges in illumination from deep shade to full sun. The discovery of a consistent relationship between PRI and photosynthetic radiation use efficiency for top-canopy leaves across species, functional types, and nutrient treatments suggests that relative photosynthetic rates could be derived with the "view from above" provided by remote reflectance measurements if issues of canopy and stand structure can be resolved.