EFFECT OF VARYING THE NITROGEN AND SULFUR SUPPLY ON THE FLOWERING OF POINSETTIA

Cuttings of poinsettia (Euphorbia pulchemma Willd. ex Klotzsch 'Dark Red Annette Hegg') were grown hydroponically to flowering at three levels of nitrogen (N) [64, 128 or 256 ppm] in combination with five levels of sulfur (S) [0, 8, 16, 32, or 64 ppm], Plants were kept vegetative for three weeks and then induced to flower using short days. Plants were observed weekly for formation of red bracts and cyathia. Leaves and roots were sampled for N and S content determination every four weeks. Treatments receiving no S showed typical foliage S-deficiency symptoms, flowered later and less completely, weighed less and were shorter than plants growing in treatments containing S. In terms of plant height and dry weight, there were no differences between treatments containing 128 or 256 ppm N. Leaf 8 content decreased over time, especially by the end of the experiment, at which time leaf N content had also decreased. This indicates that less fertilizer is needed once flowers approach anthesis. Root S decreased over time and showed an interaction between N and S. Overall, the N and S combinations that should be used in future investigations, since they produced plants of commercially saleable quality and had adequate levels of N and 8 in most plant parts, were 128 ppm N in combination with 16, 32, or 64 ppm S, and 256 ppm N in combination with 8, 16, 32, or 64 ppm S.