Nitrogen and potassium fertilization interactions with carbon dioxide enrichment in Hippeastrum bulb growth

Production of amaryllis (Hippeastrum spp.) flowers is highly affected by the bulb size. We studied the N and K fertilization interaction with CO₂ enrichment on the development of Hippeastrum bulbs grown in the greenhouse. Bulbs of two initial diameters, 3.5 and 5.4 cm, were grown for 233 days in the greenhouse on dune sand, either enriched with 1000 ppm CO₂ or with the ambient concentration. The plants were fertilized via the drip irrigation system with combinations of NH₄NO₃-N (0-25 mM) and K (0-6.4 mM), six levels of each nutrient. Carbon dioxide was applied during daytime to an insulated section of the greenhouse. The results indicated a significant contribution to either nutrients to bulb growth, but the optimal response of the larger bulbs was at a higher CO₂ concentration, for the two nutrients. The response in growth to both nutrients' concentration was curvilinear. Enrichment with CO₂ yielded an increase in bulb diameter, but the relative contribution was higher with the smaller bulbs.