Leaf water stress detection utilizing thematic mapper bands 3, 4 and 5 in soybean plants

Abstract

Foliar water stress in a mature soybean canopy, manifested as wilt rather than as a reflectance shift, was studied using reflectance measurements for Thematic Mapper bands 3 (0.63-0.69 μm), 4 (0.76-0.90 μm) and 5 (1.55-1.75μm). Diffuse and total reflectances were determined using polarization measurements and compared statistically at a variety of look angles at 15min intervals from about 09.00 until 14.00 hours EST. Plots of the data from unstressed canopy show that the behaviour of both the diffuse and total reflectances mimics that of the solar radiance curve with time of day, whereas the stressed canopy reflectance data revealed a nearly linear behaviour with a small negative slope. For both the diffuse and total reflectances measured in the nadir position, TM4 was found to be the most responsive spectral band for foliar water stress detection when the water deficit was sufficient to cause wilting, implying that substantial changes in canopy leaf geometry can best be monitored by TM4. TM3 was not found as responsive to the level of foliar water stress which occurred during this investigation. TM5 showed a response intermediate between TM4 and TM3. The results of polarization calculations for TM4 indicated that during the process of wilting, the level of canopy polarization gradually decreased.