Abstract. Estimates of the gross terrestrial carbon uptake exhibit large uncertainties. Sun-induced chlorophyll fluorescence (SIF) has an apparent near-linear relationship with gross primary production (GPP). This relationship will potentially facilitate the monitoring of photosynthesis from space. However, the exact mechanistic connection between SIF and GPP is still not clear. To explore the physical and physiological basis for their relationship, we used a unique dataset comprising continuous field measurements of leaf and canopy fluorescence and photosynthesis of corn over a growing season. We found that, at canopy scale, the positive relationship between SIF and GPP was dominated by absorbed photosynthetically active radiation (APAR), which was equally affected by variations in incoming radiation and changes in canopy structure. After statistically controlling these underlying physical effects, the remaining correlation between far-red SIF and GPP due solely to the functional link between fluorescence and photosynthesis at the photochemical level was much weaker. Active leaf-level fluorescence measurements revealed a moderate correlation between the efficiencies of fluorescence emission and photochemistry for sunlit leaves but a weak correlation for shaded leaves. Differentiating sunlit and shaded leaves in the light use efficiency (LUE) models for SIF and GPP facilitates a better understanding of the SIF-GPP relationship at different environmental and canopy conditions. Leaf-level fluorescence measurements also demonstrated that the sustained thermal dissipation efficiency dominated the seasonal energy partitioning while the reversible heat dissipation dominated the diurnal leaf energy partitioning. These diurnal and seasonal variations in heat dissipation underlie, and are thus responsible for, the observed remote sensing-based link between far-red SIF and GPP.

中文翻译：

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揭示太阳诱导的叶绿素荧光与光合作用关系的生理基础

摘要。估算的陆地总碳吸收量具有很大的不确定性。太阳诱导的叶绿素荧光（SIF）与初级总产值（GPP）具有明显的近线性关系。这种关系将潜在地促进对来自太空的光合作用的监测。但是，SIF和GPP之间的确切机制连接仍然不清楚。为了探索它们之间关系的物理和生理基础，我们使用了一个独特的数据集，该数据集包括在整个生长季节对叶和冠层荧光以及玉米光合作用的连续野外测量。我们发现，在冠层范围内，SIF和GPP之间的正相关关系主要由吸收的光合有效辐射（APAR）决定，吸收辐射的变化和冠层结构的变化同样会影响该吸收。在统计上控制了这些潜在的物理效应之后，仅由于光化学水平上的荧光与光合作用之间的功能联系而导致的远红SIF和GPP之间的其余相关性要弱得多。积极的叶片水平荧光测量显示，日光照射的叶片的荧光发射效率与光化学效率之间存在中等相关性，而阴影的叶片的相关性较弱。在SIF和GPP的光使用效率（LUE）模型中区分阳光照射和阴影照射的叶子有助于更好地了解不同环境和树冠条件下的SIF-GPP关系。叶级荧光测量还表明，持续的散热效率主导了季节性能量分配，而可逆的散热主导了昼夜的叶片能量分配。这些散热的昼夜变化是远红外SIF和GPP之间观察到的基于遥感的链接的基础，因此是造成这种情况的原因。