Physiology of the seasonal relationship between the photochemical reflectance index and photosynthetic light use efficiency

Abstract

The photochemical reflectance index (PRI) is regarded as a promising proxy to track the dynamics of photosynthetic light use efficiency (LUE) via remote sensing. The implementation of this approach requires the relationship between PRI and LUE to scale not only in space but also in time. The short-term relationship between PRI and LUE is well known and is based on the regulative process of non-photochemical quenching (NPQ), but at the seasonal timescale the mechanisms behind the relationship remain unclear. We examined to what extent sustained forms of NPQ, photoinhibition of reaction centres, seasonal changes in leaf pigment concentrations, or adjustments in the capacity of alternative energy sinks affect the seasonal relationship between PRI and LUE during the year in needles of boreal Scots pine. PRI and NPQ were highly correlated during most of the year but decoupled in early spring when the foliage was deeply downregulated. This phenomenon was attributed to differences in the physiological mechanisms controlling the seasonal dynamics of PRI and NPQ. Seasonal adjustments in the pool size of the xanthophyll cycle pigments, on a chlorophyll basis, controlled the dynamics of PRI, whereas the xanthophyll de-epoxidation status and other xanthophyll-independent mechanisms controlled the dynamics of NPQ at the seasonal timescale. We conclude that the PRI leads to an underestimation of NPQ, and consequently overestimation of LUE, under conditions of severe stress in overwintering Scots pine, and most likely also in species experiencing severe drought. This severe stress-induced decoupling may challenge the implementation of the PRI approach.

Photochemical Reflectance Index (PRI) for Detecting Responses of Diurnal and Seasonal Photosynthetic Activity to Experimental Drought and Warming in a Mediterranean Shrubland

“…NDCI was also strongly seasonally correlated with A ( Figure S4a), and SIPI was weakly and negatively correlated with A ( Figure S4b), but neither was as strongly correlated as PRI. PRI was not only sensitive to long-term carotenoid/chlorophyll changes, but also to short-term xanthophyll pigment conversion [46,50,66]. The strong correlations of PRI with NDCI and SIPI ( Figure S5) supported the importance of pigments in using PRI to monitor the photosynthetic apparatus at seasonal timescales [46], because NDCI is an index of chlorophyll changes [51,61] and SIPI is associated with carotenoid and chlorophyll ratios [62].…”

Connection of the Photochemical Reflectance Index (PRI) with the Photosystem II Quantum Yield and Nonphotochemical Quenching Can Be Dependent on Variations of Photosynthetic Parameters among Investigated Plants: A Meta-Analysis

“…In contrast, very long-term stress can change the content of chlorophyll and the pool size of the xanthophyll cycle pigments. It is known that similar changes can also influence PRI [48,122].…”

“…Connections between PRI and other photosynthetic parameters, including a quantum yield of photosystem II (∆F/F m ') [38,41,42,[45][46][47][48][49], photosynthetic light use efficiency (LUE) [3,48,[50][51][52][53][54][55], and net CO 2 uptake [47,[56][57][58][59], are actively being investigated. However, the results of these different works vary considerably, e.g., the linear correlation coefficients between PRI and NPQ can range from −0.90 [38,49,60] to +0.86 [41] in different investigations.…”

“…However, the results of these different works vary considerably, e.g., the linear correlation coefficients between PRI and NPQ can range from −0.90 [38,49,60] to +0.86 [41] in different investigations. It is probable that differences are mostly connected to the various conditions of the investigations, e.g., PRI seems to be more responsive to chlorophyll content then to the xanthophyll cycle over long time periods [48]. As a result, an analysis of factors influencing the connection between PRI and photosynthetic parameters is very important for the practical application of the photochemical reflectance index.…”

Detecting Inter-Annual Variations in the Phenology of Evergreen Conifers Using Long-Term MODIS Vegetation Index Time Series

“…Xanthophyll cycle pigments provide a means of dissipating excess energy when light absorbed by plants exceeds the photosynthetic capacity to avoid damage to the photosynthetic system, which influences the PRI signal on a diurnal timescale. It was later demonstrated by Porcar-Castell et al [19] that on seasonal scales the main physiological mechanism controlling the relationship between PRI and LUE is not the xanthophyll cycle, but rather dynamics in the relative pigments pools of chlorophyll and carotenoids. This was further confirmed by Gamon et al [20] and Wong and Gamon [14], who observed high correlations between PRI responses and adjustments in chlorophyll:carotenoid ratios.…”