Mechanisms Underlying Diurnal Variations in the Canopy Spectral Reflectance of Winter Wheat in the Jointing Stage

Information regarding diurnal variations in vegetation canopy spectra and vegetation indices (VIs) is necessary for plant growth modelling. We analysed the diurnal change characteristics of canopy spectral reflectance and VIs of winter wheat in the jointing stage based on field-measured and simulated spectral data. The visible–near infrared reflectance showed a double peak followed by a deep trough. The double-peak period occurred from 11:00 to 13:00 h (UTC + 8), and reflectance fluctuated greatly during this period. This change was attributed to midday depression of photosynthesis caused by stomatal closure induced by strong solar radiation. We found that the vegetation canopy reflectance was mainly affected by photosynthesis rate, solar irradiation intensity and surplus leaf water content. All selected VIs (normalized difference vegetationindex (NDVI), photochemical reflectance index (PRI), water band index (WBI) and mSR705) exhibited distinct intraday variations, and VIs during the double-peak period tended to fluctuate strongly or decrease (NDVI, WBI and mSR705). Thus, field measurements during the double-peak period are not recommended for winter wheat in the jointing stage, with the exception of carotenoid content monitoring. A comparison of VIs showed that S_VNIR, NDVI and mSR705 were more sensitive to canopy structure in comparison with PRI and WBI.