Reflectance of Wheat Cultivars as Related to Physiological Growth Stages

Crop yield predictions are based on a comparison between the rate of growth of the current crop and the normal crop-calendar. Remotely sensed reflectance data provide an indication of the amount of plant growth. In this investigation, we followed the seasonal changes in reflectance of three cultivars of wheat (Triticum aestivum L.) as related to their physiological development. Reflectances over the 0.45 to 2.50-µ/m wavelength intervals were measured with a ground-based spectroradiometer between planting and maturity of wheats planted on Hidalgo sandy clay loam (Typic Cakiustolls) soil in south Texas. After plants covered about 25% of the ground area, all reflectance curves had the characteristic shape for vegetated surfaces. Wheat plants seldom reach the eight leaf layers required for infinite reflectance in the near-infrared plateau (0.75 to 1.35 µ/m) wavelengths. Consequently, reflectance at these wavelengths were closely related to stage of physiological development as long as the plants were developing leaves, regardless of their rate of maturity. Dwarf, early Mexican cultivars (‘Anza’ and ‘Penjamo’) that were not daylength-sensitive progressed through physiological stages more rapidly than did a tall, later maturing, daylength-dependent cultivar (‘Nadadores’). Reflectance in the near-infrared plateau increased as plants of all cultivars developed physiologically until they reached Feekes growth Stage 9. Growth Stage 9 represents the stage at which the last ligule has appeared and the plant goes into the booting stage. The lower leaves began to turn brown, lose chlorophyll, and mature, resulting in a decrease in the number of actively reflecting leaf layers with a consequent decrease in reflectance within the near-infrared plateau wavelengths. Reflectance at other wavelength bands are not sensitive to the stage of physiological growth. Knowledge of the stage of plant development through a crop season will improve the accuracy of yield predictions made prior to harvest.