Leaf angle adjustment improves apparent covariation between fluorescence and photochemical quantum yields across leaves

Solar-induced chlorophyll fluorescence (SIF) is commonly used to monitor photosynthetic responses to drought, but the SIF–photosynthesis relationship remains unclear due to the complex interactions between the quantum yield of photochemistry (ΦP[jls-end-space/]) and fluorescence (ΦF[jls-end-space/]), which reflect physiological energy partitioning and are indirectly influenced by structural factors through changes in absorbed light. To address this gap, we investigated the leaf angle distribution (LAD) shifts operating light regimes across leaves in sugar maple and white oak saplings, and how these shifts influence the apparent covariation between ΦP and ΦF across leaves using a growth-chamber drought experiment. By integrating classical techniques with remote sensing methods, we found that accounting for variation in LAD and absorbance made the apparent ΦP[jls-end-space/]-ΦF covariation across leaves more linear. Changes in LAD altered the operating light regime across leaves, resulting in a more linear positive apparent relationship between ΦP and ΦF (from not significant to R2 = 0.6). Additionally, we observed that the conductance of the Cytochrome b₆f complex to linear electron flow covaried with ΦF under drought and high light conditions (R2 = 0.82). Our findings suggest that considering both physiological and structural changes provides a more comprehensive understanding of the SIF-photosynthesis relationship under drought stress, particularly in relation to light energy partitioning. Focusing on only one aspect, either physiological or structural, can lead to an incomplete understanding of the mechanisms driving quantum yield relationships at the canopy scale and limit our ability to leverage SIF products to monitor how ecosystems are responding to changing environmental conditions.