Reflectance index for early determination of water stress

The effect of soil drying on leaf expansion and reflectance index (RI) was studied in rape (Brassica napus L.) in 1991 and 1992 and in Lupin (Lupinus angustifolius L.) in 1995. The relative reflectance index (RRI) was defined as the ratio of the reflectance index of the droughted crops (RIa) to that of the fully irrigated reference crop (RIr). The drought was quantified as the number of stress days (SD) which is the integrated relative transpiration deficit. The crops were grown in sand and loam in lysimeters in the field. The plants were kept fully irrigated (FI); or exposed to drought (ED) during the vegetative and early flowering stages. The experiments were conducted in three seasons of different evaporative demand i.e. 2.5, 4.5 and 3.5 mm day-1 in 1991, 1992 and 1995, respectively. In rape, under low evaporative conditions in 1991, leaf area expansion was only slightly decreased by drought. Under high evaporative demands in 1992, drought during the vegetative growth halved the rape crop area. For the rape crop, RRI decreased as the number of stress days (SD) increased. The decrease in RRI was 0.053 and 0.023 per SD on sand and loam, respectively. However, in the lupin crop the decrease was only 0.0083 and 0.0047 per SD on sand and loam, respectively. Thus, the relationship between integrated growth, characterized by RRI, and the integrated evapotranspiration deficit, characterized by SD, was not affected by the evaporative demands but only by soil type and crop. For a rape crop grown in the field in 1993, RRI was decreased by 15% within three days after irrigation. From the results it was concluded that RRI is a non-destructive method to estimate the severity of water stress and that RRI is a sensitive index for determination of early water stress. Under field conditions RRI can be a useful tool for initiating irrigation.