Row orientation influences the diurnal cycle of solar-induced chlorophyll fluorescence emission from wheat canopy, as demonstrated by radiative transfer modeling

Abstract

Although multiple studies have used solar-induced chlorophyll fluorescence (SIF) of the crop canopy to estimate photosynthetic activity, the effect of canopy geometry (row orientation) on diurnal SIF remains uncertain. Using a combination of 3D canopy radiative transfer modeling and spectroscopic observations collected in a winter wheat field, we investigated (i) the relationships between observed SIF and factors including temperature, humidity, absorbed photosynthetically active radiation (APAR), and row orientation; (ii) the temporal pattern and multi-angle distribution of diurnal canopy SIF; and (iii) the mechanisms influencing components of diurnal SIF under the effect of row orientation. The relationship between observed SIF and APAR was relatively strong at the early growth period, but SIF was influenced by various factors (temperature, humidity, and row–sun angle) at the middle of the growth period. To test an asymmetric crop row effect as a potential cause of this skewness, we evaluated the modeled SIF with a 3D northwestward (315°) crop row alignment and other orientations. Each row orientation resulted in a trough-like depression parallel to the row in simulated canopy SIF, which could cause a diurnal asymmetry of canopy SIF, a fraction of fluorescence escape, and a fraction of APAR. View angles, the dimension of canopy objects (1D/3D), inter-row width, and leaf structure parameters impacted the representativeness of these simulations. An increased shade SIF, synchronized with the time when row–sun angles become zero at the incident sunlight being parallel to the crop row, was linked to decreasing canopy SIF emissions and would cause a non-linear SIF–APAR relationship at certain row orientations especially facing diagonally to the south. The concept of row orientation provides a novel viewpoint of diurnal canopy SIF from which to test the effect of row geometry in cropland, and will promote a mechanistic understanding of photosynthesis and canopy light use.