EARLY DETECTION OF PLANT STRESS BY DIGITAL IMAGING WITHIN NARROW STRESS-SENSITIVE WAVEBANDS

Digital images of soybean canopies [Glycine max (L.) Merrill] were obt ained within selected narrow wavebands (6-10 nm bandwidths) to determi ne their capability for early detection of plant stress. Images and ph ysiological measurements of stress were acquired 2 days, 4 days, and 7 days following application of control, drought, and herbicide [(3,4-d ichlorophenyl)-1, 1-dimethylurea, or DCMU] treatments. As a result of frequent rainfall, drought stress never occurred. However, exposure to herbicide rapidly induced plant stress. By day 4, the ratio of variab le to maximum of leaf fluorescence (F-v/F-m) decreased and leaf water potentials (psi(w)) increased in the herbicide treated soybean, indica ting damage to the photosynthetic apparatus and stomatal closure. Also , Munsell leaf color had increased from approximately 5GY 4.6/5.7 to a lighter green-yellow value. Canopy reflectances at 670 nm , 694 nm, a nd in the 410-740 nm band (R(vis)), as well as reflectance at 694 nm d ivided by reflectance at 760 nm (R(694)/R(760)), detected stress simul taneously with the physiological measurements and increased consistent ly with stress through day 7. Reflectances at 420 nm and 600 nm, toget her with R(600)/R(760) and R(vis)/R(760), did not increase until leave s were yellow or brown and wilted and canopies had begun to collapse o n day 7. None of the reflectance or reflectance ratio images detected stress prior to visible color changes. This was attributed primarily t o the rapid inducement of chlorosis by the herbicide. Reflectance in n arrow wavebands within the 690-700 nm region and its ratio with near-i nfrared reflectance should provide earlier detection of stress-induced chlorosis compared with broad band systems or narrow bands located at lesser wavelengths.