CYCLIC NITROGEN UPTAKE BY GREENHOUSE ROSES

A recirculating nutrient solution system was constructed to study N up take 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, bu t independent of transpiration rate. The N uptake rate changed four- t o five-fold during a single cycle of newer shoot growth (e.g. 29-146 m g 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. Th ereafter, uptake rates increased, with the highest rate occurring as t he 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 ave rage 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 (app roximately 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.