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 was studied in rape (Brassica napus. L) grown in loamy sand and sandy loam in lysimeters in the field. Precipitation was prevented by an automatic 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 (LD) during the pod filling stage. The experiments were conducted in two seasons of contrasting evaporative demand: 1991 (3 mm day(-1)) and 1992 (4-5 m m day(-1)). Under the low evaporative conditions of 1991, leaf area expansion and biomass accumulation were only slightly decreased by drought. In 1992, a year with a high evaporative demand, ED decreased the leaf area and biomass accumulation at anthesis. The reflectance index (RI) was calculated from the reflectance index in the near infrared band (NIR) and in the photosythetically active band (PR) as RI=NIR/PR. During vegetative growth, the reflectance index (RI) was correlated (R(2)=0.90) with crop green area index (CAI). 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). RRI decreased as the number of stress days (SD) increased by 0.053 and 0.023 per SD on loamy sand and sandy loam, respectively. It was concluded from the study that RRI is a sensitive index for determination of early water stress. Under field conditions determination of RRI from small non-irrigated and irrigated plots may be used for initiating irrigation when RRI<1.