The increasing amount of continuous time series of solar-induced fluorescence (SIF) and vegetation indices (e.g. Photochemical Reflectance Index, PRI) acquired with high temporal (sub-minute) frequencies is foreseen to allow tracking of the structural and physiological changes of vegetation in a variety of ecosystems. Coupled with observations of CO₂, water, and energy fluxes from eddy covariance flux towers, these measurements can bring new insights into the remote monitoring of ecosystem functioning. However, continuously changing solar-view geometry imposes directional effects on diurnal cycles of the fluorescence radiance in the observation direction (F) and PRI, controlled by structural and biochemical vegetation properties. An improved understanding of these variations can potentially help to disentangle directional responses of vegetation from physiological ones in the continuous long-term optical measurements and, therefore, allow to deconvolve the physiological information relevant to ecosystem functioning. Moreover, this will also be useful for better interpreting and validating F and PRI satellite products (e.g., from the upcoming ESA FLEX mission).

Many previous studies focused on the characterization of reflectance directionality, but only a handful of studies investigated directional effects on F and vegetation indices related to plant physiology. The aim of this study is to contribute to the understanding of red (F₆₈₇) and far-red (F₇₆₀) fluorescence and PRI anisotropy based on field spectroscopy data and simulations with the Soil-Canopy Observation of Photochemistry and Energy fluxes (SCOPE) model. We present an extensive dataset of multi-angular measurements of F and PRI collected at canopy level with a high-resolution instrument (FloX, JB Hyperspectral Devices UG, Germany) over different ecosystems: Mediterranean grassland, alfalfa, chickpea and rice.

We found, that F₇₆₀ and F₆₈₇ directional responses of horizontally homogeneous canopies are characterized by higher values in the backscatter direction with a maximum in the hotspot and lower values in the forward scatter direction. The PRI exhibited similar response due to its sensitivity to sunlit-shaded canopy fractions.

As confirmed by radiative transfer forward simulations, we show that in the field measurements leaf inclination distribution function controls the shape of F and PRI anisotropic response (bowl-like/dome-like shapes), while leaf area index and the ratio of leaf width to canopy height affect the magnitude and the width of the hotspot. Finally, we discuss the implications of off-nadir viewing geometry for continuous ground measurements. F observations under oblique viewing angles showed up to 67 % difference compared to nadir observations, therefore, we suggest maintaining nadir viewing geometry for continuous measurements of F and vegetation indices. Alternatively, a correction scheme should be developed and tested against multi-angular measurements to properly account for anisotropy of canopy F and PRI observations. The quantitative characterization of these effects in varying illumination geometries for different canopies that was performed in this study will also be useful for the validation of remote sensing F and PRI products at different spatial and temporal scales.

可以预见到，以高时间（亚分钟）频率获取的太阳诱导荧光（SIF）和植被指数（例如，光化学反射指数，PRI）的连续时间序列的数量将不断增加，从而可以追踪植被的结构和生理变化。各种生态系统。结合观测到的涡旋协方差流量塔中的CO ₂，水和能量通量，这些测量结果可以为生态系统功能的远程监测带来新的见解。但是，不断变化的太阳视几何形状会在观察方向（F）和PRI上对荧光辐射的日循环施加方向性影响，受结构和生化植被特性控制。对这些变化的更好理解可以潜在地帮助在连续的长期光学测量中使植被的方向响应与生理响应分开，因此可以解卷积与生态系统功能相关的生理信息。此外，这也将有助于更好地解释和验证F和PRI卫星产品（例如，来自即将到来的ESA FLEX任务）。

先前的许多研究都集中在反射率方向性的表征上，但是只有少数研究研究了对F和与植物生理相关的植被指数的方向性影响。这项研究的目的是基于野外光谱数据和光化学和能量通量土壤盖层观测（SCOPE）的模拟，有助于理解红色（F ₆₈₇）和远红色（F ₇₆₀）荧光以及PRI各向异性。模型。我们展示了F和PRI的多角度测量的广泛数据集 使用高分辨率仪器（FloX，JB高光谱设备UG，德国）在冠层水平上采集不同生态系统：地中海草原，苜蓿，鹰嘴豆和水稻。

我们发现，水平均质冠层的F ₇₆₀和F ₆₈₇定向响应的特征是反向散射方向的值较高，热点最大，而向前散射方向的值较低。该PRI由于其阳光照射-凉棚馏分灵敏度表现出类似的响应。

正如辐射转移正演模拟所证实的那样，我们表明，在野外测量中，叶片倾斜度分布函数控制着F和PRI各向异性响应的形状（碗状/圆顶状），而叶面积指数和叶宽与冠层高度会影响热点的大小和宽度。最后，我们讨论了从天底观看几何图形对于连续地面测量的含义。斜视角下的F观测值与天底观测值相比，差异高达67％，因此，我们建议保持天底观测几何形状以连续测量F和植被指数。或者，应制定校正方案并针对多角度测量进行测试，以正确考虑冠层F和PRI观测值的各向异性。在这项研究中对不同冠层的不同照明几何形状中的这些效应进行定量表征，也将有助于验证不同时空尺度上的遥感F和PRI产品。