FOLIAR-APPLIED MOLYBDENUM FOR PREVENTING OR CORRECTING MOLYBDENUM DEFICIENCY OF POINSETTIA

'Annette Hegg Brilliant Diamond' poinsettia (Euphorbia pulcherrima Willd. ex Kl.) was grown in an unlimed growth medium consisting of equal volumes of sphagnum peat and perlite and received a fertilizer solution supplying all micronutrients except Mo. Plants were untreated or foliage was sprayed with solutions of 1, 10, or 100 mg Mo/liter 5, 8, or 11 weeks after pinching. Untreated plants developed foliar symptoms of Mo deficiency (marginal and interveinal chlorosis, marginal necrosis, and downward curling), and leaf tissue contained Mo below the critical level of 0.5-mu-g.g-1 and NO3-N > 1.0%. Treatment at 5 or 8 weeks largely prevented deficiency symptoms, increased tissue Mo, and reduced tissue NO3-N. Molybdenum deficiency symptoms were in the early stages of development on the day of treatment when plants were sprayed at 11 weeks. Molybdenum sprays at this time increased tissue Mo and reduced tissue NO3-N but did not eliminate the symptoms. However, when the experiment was completed 15 weeks after pinching, the number of leaves showing symptoms was about one-half that of untreated plants. Increasing the concentration of Mo in the spray solution increased the concentration of Mo in the leaves but had no effect on NO3-N.