Analysis of Hyperspectral Images of Hordeum vulgare L. Seedlings to Assess their Adaptation to Heat Stress

Abstract

Using hyperspectral imaging, the ontogenetic and heat-induced changes in the reflectance spectra of primary leaves of Hordeum vulgare L. seedlings of different ages in the visible and near-infrared (NIR) spectral regions we studied. It was found that, NIR light reflectance decreased and the mesophyll structure became less compact during leaf growing. Chlorophyll pigment absorption increased from 4 to 7 day of growth and then decreased. High temperature (40°C, 3 h, illumination of 120 µmol quanta · m^–2 · s^–1) effected insignificantly the reflectance coefficients of leaves of 4- and 7-day-old seedlings in NIR, but significantly reduced leaf density in 11-day-old plants. At the same time, the absorption of chlorophyll pigments significantly decreased in 4- and 11-day-old seedlings and remained unchanged in 7-day-old leaves after heating. Light absorption by water molecules in the studied seedlings of all age groups did not change significantly after heat exposure. A comparative and correlation analysis of eight commonly used vegetation indexes was carried out. It was found that the heat led to a significant decrease in the “photochemical reflectance index” (PRI) in 4-day-old leaves and the “red-edge normalised difference vegetation index” (NDVI705) and “structure-insensitive pigment index” (SIPI) in 11-day-old seedlings. It is proposed to use these vegetation indexes to assess heat stress in plants.