Whole-plant response of six poinsettia cultivars to three fertilizer and two irrigation regimes

Six cultivars of poinsettia (Euphorbia pulcherrima Willd.), 'Angelika White', 'Celebrate 2', 'Freedom Red', 'Lilo Red', 'Red Sails', and 'Supjibi Red' 'were grown for 9 weeks during vegetative development under three constant-feed fertilizer treatments, 80, 160, or 240 mg N/liter and two irrigation regimes, well-watered (high irrigation) or water deficient (low irrigation). Plants fertilized with 80 or 240 mg N/liter were 10% to 18% shorter, while those fertilized with 160 mg N/liter were 25% shorter with low versus high irrigation. Leaf area and leaf dry weight increased linearly in response to increasing fertilizer concentrations. Low irrigation reduced leaf area, leaf, stem, and shoot dry weight 36% to 41%. Cultivate responded similarly to irrigation and fertilizer treatments in all components of shoot biomass production and no interactions between the main effects and cultivars occurred. Stomatal conductance and transpiration decreased with increasing fertilizer rates or sometimes with low irrigation. Highest chlorophyll contents occurred in leaves of 'Lilo Red' and 'Freedom Red'. Leaves of plants fertilized with 80 mg N/liter were deficient in leaf N and had 40% to 49% lower leaf chlorophyll content compared to plants fertilized with 160 or 240 mg N/liter. Irrigation had no effect on leaf N or chlorophyll content. At the end of the experiment leaves of 'Supjibi Red' and 'Angelika White' contained higher concentrations of soluble proteins than the other four cultivars.