Whole-plant Response of Six Poinsettia Cultivars to Three Fertilizer and Two Irrigation Regimes

Six cultivars of poinsettia (Euphorbia pulcherrima Wind.), '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 3670 to 41%. Cultivars 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. Fertilization and irrigation management are important compo- nents of producing a healthy crop. Applying liquid fertilizer at a constant rate with each irrigation has become a popular and economical practice for many poinsettia growers (Ecke et al., 1990). The advantage of such a program is ease of application, which supplies nutrients constantly at desired levels, while excess nutrients may be leached from the media. However, excess nutri- ents in the leachate are wasted. About 250 mg N/liter are recommended for constant fertigation of poinsettias (Ecke et al., 1990). In recent years fertilizer recom- mendations have been reevaluated because nitrate leaching from intensive greenhouse production has become a major environmen- tal concern (Biernbaum, 1992; Cabrera et al., 1993; Hershey and Paul, 1982). Poinsettias grown with a solution of 200 mg N/liter produced large plants with excessive foliar N levels (Tayama and Carver, 1992). Acceptable growth in poinsettias was achieved by lowering fertilizer applications via subirrigation to a constant rate of 125 mg N/liter (Rose and White, 1994), indicating that poinset- tias can be grown successfully with 50% less fertilizer than is currently recommended. Irrigation is a critical management tool when coupled with constant liquid fertilization to manipulate electrical conductivity (EC) levels of the growing medium. Poinsettias grown with constant application of 100, 200, or 400 mg N/liter and leaching fractions from 0 to 0.5 were of acceptable quality, but plant height and dry weight generally increased with decreasing root media EC (Yelanich and Biernbaum, 1993). Similar media EC, plant height, and fresh weight were produced with 100 mg N/liter and 0.2 to 0.3