SPECTRAL REFLECTANCE INDEX AS AN INDICATOR OF DROUGHT OF FIELD-GROWN OILSEED RAPE (BRASSICA-NAPUS L)

The effect of soil drying on leaf expansion and biomass accumulation w as studied in rape (Brassica napus. L) grown in loamy sand and sandy l oam in lysimeters in the field. Precipitation was prevented by an auto matic mobile glass shelter. The plants were kept fully irrigated (FI); or exposed to controlled soil drying to produce an early drought (ED) during the vegetative and early flowering stages or a late drought (L D) during the pod filling stage. The experiments were conducted in two seasons of contrasting evaporative demand: 1991 (3 mm day(-1)) and 19 92 (4-5 m m day(-1)). Under the low evaporative conditions of 1991, le af area expansion and biomass accumulation were only slightly decrease d by drought. In 1992, a year with a high evaporative demand, ED decre ased the leaf area and biomass accumulation at anthesis. The reflectan ce index (RI) was calculated from the reflectance index in the near in frared band (NIR) and in the photosythetically active band (PR) as RI= NIR/PR. During vegetative growth, the reflectance index (RI) was corre lated (R(2)=0.90) with crop green area index (CAI). The relative refle ctance index (RRI) was defined as the ratio of the reflectance index o f the droughted crops (RIa) to that of the fully irrigated reference c rop (RIr). RRI decreased as the number of stress days (SD) increased b y 0.053 and 0.023 per SD on loamy sand and sandy loam, respectively. I t was concluded from the study that RRI is a sensitive index for deter mination of early water stress. Under field conditions determination o f RRI from small non-irrigated and irrigated plots may be used for ini tiating irrigation when RRI<1.