THE REFLECTANCE AT THE 950-970 NM REGION AS AN INDICATOR OF PLANT WATER STATUS

We present new remote sensing indices of plant water status: the ratio between the reflectance at 970 nm, one of the water absorption bands, and the reflectance at a reference wavelength, 900 nm (R970/R900); th e first derivative minimum in this near-infrared region (d(NIRminimum) ) and the wavelength where this minimum is found (lambda(NIRminimum). In order to evaluate them, we carried out three experiments. Daily irr igated gerbera plants were allowed to dry until almost wilting and the n daily irrigation was restarted; pepper and bean plants were grown fo r four months submitted to two different irrigation treatments; and be an detached leaves were submitted to progressive dehydration whereas p ressure-volume curves were being carried out. In gerbera plants, the t rough about 950-970 nm decreased as the drought was increasing. Theref ore, the R970/R900 index and the d(NIRminimum) closely tracked the cha nges in relative water content (RWC), leaf water potential, stomatal c onductance and the foliage-air temperature differences. The lambdad(NI Rminimum) tracked even better these changes in gerberas. However, thes e water status indices began to be significant when the water stress w as already well developed, at RWC smaller than 85 per cent. The same h appened to detached leaves of beans which did not present differences above - 1-55 MPa water potential. Beans and peppers growing at soil ma tric potentials larger than - 0.04 MPa presented higher R970/R900 valu es than those growing at soil matric potentials only larger than - 0.0 1 MPa. In all the cases, the maximum response of these indices was fou nd in the varieties or the species that lost cell wall elasticity in r esponse to drought stress. This could indicate an important structural component in these indices changes. Relative water content itself see med to be, however, the most important factor as shown by the highest correlation coefficients with these spectral indices. These spectral s ignals were more evident at canopy level than at leaf level. They seem to be useful as water status indicators at ground level, especially w hen there are not important changes of LAI and when plants wholly cove r the soil.