Factors Influencing Premature Cyathia Abscission in Poinsettia ‘Annette Hegg Dark Red’

Abstract Low irradiance levels, high temperatures, and water stress all promoted premature cyathia abscission in poinsettia ‘Annette Hegg Dark Red’ ( Euphorbia pulcherrima Willd.). Abscission was greater in plants placed under 75% shade at 16°C night temperature (NT) than on plants placed under normal daylight (ND) at 16° or 21° NT. Water stress (0.6 MPa) promoted abscission on plants grown at an 18° NT and ND but did not promote abscission on plants grown at 16° NT and ND or on under 75% shade (13° to 21° NT). As plant density increased, transmission of photosynthetically active radiation (PAR) through the bracts to the leaf canopy decreased while cyathia abscission increased concomitantly. More than 90% of the PAR above the bracts was absorbed or reflected 5 cm below the bracts on 20 cm tall plants spaced at 65 or more plants m -2 . Reducing natural irradiation 75% by shading leaves of poinsettia promoted cyathia abscission, whereas removing immature bracts decreased abscission. Leaf removal on plants with intact bracts promoted abscission to a degree that 100% of the cyathia abscised prior to anthesis, whereas bract removal on plants with intact leaves resulted in only 23% abscission of the cyathia 25 days after first anthesis. Measurements of nonsoluble carbohydrate showed a significant increase in leaf carbohydrate on plants with bracts removed while carbohydrate decreased in leaves of plants with bracts intact. Carbohydrate depletion appears to be the primary factor responsible for premature cyathia abscission in poinsettia.