Phosphorus restriction influences P efficiency and ornamental quality of poinsettia and chrysanthemum

Better synchronization of plant-available phosphorus (P) with crop P requirement is required to reduce P losses to the environment and to improve resource-efficiency of the exploitation of non-renewable phosphate rock. In horticultural plant production, a restricted availability of P may limit stem length and improve compactness, which are desirable characters for many ornamental plants. In the present study, we investigated the effect of reduced availability of P on plant quality, biomass production and phosphorus efficiency of poinsettia (Euphorbia pulcherrima cv. 'Mira Red') and chrysanthemum (Chrysanthemum x morifolium cv. 'Breeze Cassis'). Five P concentrations (6, 12, 18, 24 or 48 mg L-1)were applied as starter Pin the peat-based potting substrate as well as in the nutrient solution given during the experiment. Stem length of both plant species was strongly restricted at 6 mg P L-1 but was not significantly affected by higher P levels. For poinsettia, the optimum bract diameter was obtained at 18 mg P L-1. For maximum shoot dry biomass, branching and plant diameter, however, 24 mg L-1 was needed. Optimal plant diameter and shoot biomass of chrysanthemum was obtained at 18 mg P L-1 while 24 mg L-1 was required for maximum flower number. Increasing the P supply to 48 mg L-1 did not improve shoot dry matter, branching or flowering of either species, but induced luxury uptake of P. Total shoot P uptake increased linearly over the P fertilizer range tested. For optimal plant biomass combined with optimal ornamental quality, shoot P concentrations at 90 DAP was in the range of 0.30-0.35 % for poinsettia and 0.25-0.30 % for chrysanthemum. Chrysanthemum showed a higher phosphorus efficiency than poinsettia at low P levels, mainly related to a higher internal P utilization efficiency. The P acquisition efficiency was in the range of 55-60 0 /0 for both species, and was not significantly affected by the total amount of P applied. In conclusion, with the P fertilization strategy used, P restriction could not be used for plant height restriction of poinsettia "Mira Red" or chrysanthemum "Breeze Cassis" without negative effects on plant quality. However, P fertilization could be markedly reduced without negative effects on plant growth and development, improving phosphorus efficiency and recovery.