## 1 Introduction
The rapid changes in carbon uptake for evergreen vegetation in densely vegetated areas have been captured by PRI, but not by widely used greenness indices such as NDVI which can be saturated due to stabilized canopy greenness [20; 28; 39; 61; 62; 63]. PRI, a narrow-band spectral index as a proxy of ecophysiological parameters based on the changes in energetic status thus provides a quick, simple, nondestructive, labor-saving and cost-efficient means of optical sampling for exploring interactions of plant-ecosystem carbon fluxes and for promisingly monitoring and mapping RUE/GPP at larger spatiotemporal scales [11; 20; 64; 65; 66]. Several studies, however, have shown that PRI can be affected by solar angle, illumination, canopy structure, atmosphere, pigments and soil background [10; 11; 20; 23; 54; 67; 68; 69; 70; 71; 72; 73; 74; 75]. Additionally, the interpretation of PRI becomes more difficult for mixed species or landscapes with varying canopy fractions [76; 77], although several studies have reported correlations between PRI and photosynthetic performance [34; 35; 78; 79]. The variation in PRI also depended on the site difference caused by vegetation types and canopy structure [62; 77], which generally affected the PRI-RUE relationships. Much of the long-term variation in PRI and its tracking of carbon assimilation has recently been attributed to changes in the sizes of constitutive (slow) pigment pools that also impacted long-term tracking of carbon assimilation by PRI [23; 54; 55; 67; 76]. However, such studies have not been expanded to a wide range of species and sites, so the physiological mechanism of long-term PRI variation still remains to be clearly understood. Furthermore, the not yet resolved physiological mechanism of PRI and its high sensitivity to various extraneous effects might impede the evaluation of photosynthetic performance, when scaling up from foliar to canopy, and ultimately, landscape and global scales [24]. Even so, recent findings of links between RUE/GPP and PRI and the integration of PRI with other parameters such as foliar pigments [80], solar-induced fluorescence (SIF) [81] or vapor-pressure deficit (VPD) [77] may provide new opportunities for the continuous assessment of carbon uptake. In a review of the literature, [PERSON] et al. [20] found that PRI was significantly correlated with RUE and other relevant ecophysiological variables across vegetation types and spatiotemporal scales. The consistent relationships with RUE-PRI at different spatiotemporal scales were summarized by synthetic analyses of publications from 1992 to 2009. The kinds of factors that control PRI variation in different plant functional types, organizational levels and temporal scales, how and the extent to which these factors control changes in PRI and how these effects can be decreased or avoided, however, remain to be clarified. An integrated and robust model for remotely sensing carbon uptake using PRI at different scales will also need to be developed for using PRI as an estimator of carbon uptake. In this review, we analyze the main factors that drive changes in PRI and its assessment of RUE at foliar, canopy and ecosystem levels based on the scientific literature reviewed by [PERSON] et al. [20] and on an additional review of the literature published between 2010 and 2015, and we summarize the applications of PRI for interpreting ecophysiological variables. We also describe the principal suggestions for improving the estimation of RUE and carbon uptake using PRI. The primary purpose of this study is thus to determine if PRI, a simple, optical remote-sensing index, is a good proxy of RUE at both short and long timescales from foliar to global levels.