POINSETTIA GROWTH, TISSUE NUTRIENT CONCENTRATION, AND NUTRIENT-UPTAKEAS INFLUENCED BY NITROGEN FORM AND STAGE OF GROWTH

Growth, development, and uptake of essential nutrients as influenced b y nitrogen (N) form and growth stage was evaluated for 'Freedom' poins ettias (Euphorbia pulcherrima Willd. Ex Klotz.). Treatments consisted of five nitrate (NH4+):ammonium (NO3-) ratios (% NH4+:% NO3-) of 100:0 , 75:25, 50:50, 25:75, and 0:100 with a total N concentration of 150 m g L-1. Plants were grown in solution culture for ten weeks under green house conditions. Nutrient uptake data was combined into three physiol ogical growth stages. Growth stage I(GSI) included early vegetative gr owth (long days). Growth stage II (GSII) began at floral induction and leaf and bract expansion (short days). Growth stage III (GSIII) was f rom visible bud through anthesis and harvest. Dry weights for all plan t parts and height increased as the ratio of NO, increased. Leaf area and bract area were maximized with 25:75 and 50:50 N treatments, respe ctively. Nitrogen treatments significantly affected foliar nutrient co ncentrations with calcium (Ca++) and magnesium (Mg++) being highest wh en NO, was the predominant N form. Uptake of each macronutrient was av eraged across all treatments and divided into physiological growth sta ges (GS) to identify peak demand periods during the growth cycle. The greatest uptake of NH4+ and NO3- was from the early vegetative stage t o floral induction(GSI). Phosphorus(P), potassium (K+), and Mg++ uptak e were greatest from floral induction to visible bud (GSII) and Ca++ u ptake remained relatively unchanged through GSI and GSII. Uptake was l owest for all nutrients from visible bud to anthesis (GSIII). Results from this study clearly indicate that peak demand periods for macronut rient uptake existed during the growth cycle of poinsettia.