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An onion (Allium cepa L.) cultivar, Valenciana de Grano, was grown on a semi-arid site in Northeast Spain in drip irrigated field plots at three densities 30,60 and 90 plants m-2 as the main factor and at two levels of nitrogen fertiliser 240 and 420 kg N ha-1. These treatment combinations were arranged as a randomised split-plot design with three replicates.

The aim of the study was to assess the capability of field reflectance measurements for monitoring, under non-stressful conditions, the aboveground biomass and the leaf area index during the period of rapid leaf growth, from six leaves to onset of bulbing.

The shoot dry matter and leaf area index were obtained by destructive sampling. On sampling, in a subplot area, the fraction of intercepted photosynthetically active radiation (fpar) was measured with a ceptometer. Field reflectance measurements at different spectral bands were taken with a hand-held radiometer. Results showed that differences in the measured crop growth characteristics (LAI and biomass) were associated with densities. No differences,except in final yield, were associated with the amount of nitrogen fertiliser.

The ratio of near-infrared to red reflectance (RVI) and the normalised difference vegetation index (NDVI) were calculated to estimate crop growth characteristics.

The NDVI relationship with fpar gave a better prediction (R2= 0.90) than RVI. For estimating LAI, these indices saturate at LAI values close to 2.0.

LAI was related to fpar by a model (R2=0.80) that assumes that the extinction coefficient (K) is not constant during the observed period of vegetative growth. During this period, the aboveground biomass can be related to the accumulated intercepted PAR (AIpar). The relationship obtained shows that a 10 % AIpar increment corresponds to 10.3 % WA increment. The measurements of radiation reflectance are a suitable method for LAI and aboveground biomass estimation during a period of onion rapid leaf growth under a range of common field densities acid under non-limiting growth conditions.