A recirculating nutrient solution system was constructed to study N uptake by greenhouse 'Royalty' rose plants in relation to irradiance and the developmental stage of the crop. The rate of N uptake followed a cyclical pattern that was related to shoot development and harvest, but independent of transpiration rate. The N uptake rate changed four- to five-fold during a single cycle of flower shoot growth (e.g. 29-146 mg N per plant day-1). Following a flower harvest, the N uptake rate decreased even as the new flower shoots began to develop. The lowest N uptake occurred when the shoot elongation rate was at its maximum. Thereafter, uptake rates increased, with the highest rate occurring as the flower shoots reached commercial maturity. Potassium, Ca, Mg and P followed the same pattern of uptake observed for N. Irradiance did not control the periodicity of the N uptake cycles but did affect the average daily plant N demand. Uptake rates in summer days (approximately 60-70 mg N per plant day-1) were about twice of those in winter (approximately 30 mg N per plant day-1). The total annual plant N uptake (16.8 g N per plant year-1) was in close agreement with the yearly plant N demand calculated for container-grown roses.
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