In situ measurement of Scots pine needle PRI

Abstract

BackgroundThe Photochemical Reflectance Index (PRI) calculated from narrow-band spectral reflectance data is a vegetation index which is increasingly used as an indicator of photosynthetic activity. The leaf-level link between the status of photosynthetic apparatus and PRI has been robustly established under controlled light conditions. However, when a whole canopy is measured instantaneously, the PRI signal is heavily modified by vegetation structure and local variations in incident light conditions. To apply PRI for monitoring the photosynthesis of whole canopies under natural conditions, these large-scale measurements need to be validated against simultaneous leaf PRI. Unfortunately, PRI changes dynamically with incident light and has a large natural variation. No generally accepted procedure exists today for determining the PRI of canopy elements in situ.ResultsWe present a successful procedure for in situ measurements of needle PRI. We describe, characterize and test an optical measurement protocol and demonstrate its applicability in field conditions. The measurement apparatus consisted of a light source, needle clip, spectroradiometer and a controlling computer. The light level inside the clip was approximately two-thirds of that on sunlit needle surfaces at midday. During each measurement the needle was inserted into the clip for approximately 5 s. We found no near-instantaneous changes (sub-second scale jumps) in PRI during the measurements. The time constants for PRI variation in light to full shade acclimations were approximately 10 s. The procedure was successfully applied to monitor the greening-up of Scots pine trees. We detected both facultative (diurnal) PRI changes of 0.02 (unitless) and constitutive (seasonal) variations of 0.1. In order to reliably detect the facultative PRI change of 0.02, 20 needles need to be sampled from both sunlit and shaded locations.ConclusionsWe established a robust procedure for irradiance-dependent leaf (needle) PRI measurements, facilitating empirical scaling of PRI from leaf (needle) to full canopy level and the application of PRI to monitoring the changes in highly structured vegetation. The measured time constants, and facultative and constitutive PRI variations support the use of an artificial light for in situ PRI measurements at leaf (needle) level.

Evaluation of Accuracy and Practical Applicability of Methods for Measuring Leaf Reflectance and Transmittance Spectra

“…Because leaf spectral BRDFs may vary between species [30], a contact probe is probably not the best choice for comparing spectral differences between species. However, it is well suitable for monitoring relative changes in reflectance and thus physiological status of plants in field conditions, which is where it is usually applied [8][9][10].…”

“…In the case of LC, the influence of BRDF is obvious since the measurement is performed in hotspot geometry, but also the integrating spheres may not integrate over the entire hemisphere, because there are walls around the sample port. To further complicate the matter, the optical properties of green leaves depend on incident irradiation and the duration of exposure to measurement light (e.g., [9,10]) which vary between methods. These changes are difficult to quantify and depend on the seasonally varying biochemical composition of the leaf as well as leaf's growing environment.…”

“…The other leg of the bifurcated cable is then used for transporting the signal reflected from the sample to the spectrometer. The system was described in detail in [10]. The only differences to our study are that we used the Ocean Optics HL-2000-HP-FHSA 20 W halogen light source and a different spectrometer.…”

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

“…On the other hand, the subsequent increase in PRI was not observed in several studies (Peñuelas et al , Rahimzadeh‐Bajgiran et al ). Several recent studies also reported monotonic decreases in PRI values after the transitions although Y II was not evaluated simultaneously (Peguero‐Pina et al , Mõttus et al ). Considering these contrasting results, it appears that the tracking performance of PRI during photosynthetic induction has not yet been well characterized.…”

Spectral Properties of Coniferous Forests: A Review of In Situ and Laboratory Measurements

“…Contact probes and bare spectrometer detectors have also been used (e.g., [17][18][19][20]). By using a contact probe together with a needle clip, it is possible to measure only one needle at a time [21][22][23]. This approach is best suited for measuring living needles in the field.…”