Poinsettia Bract Necrosis Is Affected by Scion/Rootstock Combinations and Arrested by a Benzyladenine Spray

Abstract

Poinsettia (Euphorbia pulcherrima Willd. ex. Klotzch) cultivars vary widely in susceptibility to bract necrosis, a disorder that has been associated with localized calcium deficiency. When `Supjibi Red' (SR) poinsettia shoots were grafted on `SR' stocks, a higher incidence of bract necrosis was observed (26.4% of bracts) 28 days after initial anthesis than on corresponding `SR' shoots grafted on `Annette Hegg Dark Red' (AH) stocks (13.8% of bracts). `AH' shoots developed few bract necrosis symptoms, regardless of rootstock. Calcium (Ca) concentrations were higher in whole bracts of `AH' than of `SR', but were unaffected by rootstock. In a second study, `SR' shoots were grafted on a double rootstock of one `AH' and one 'SR' to determine if removing either rootstock, at the start of flower initiation or at anthesis, affected bract necrosis. At 28 days after initial anthesis, `SR' scions on only `AH' roots from the beginning of flower initiation, or on `AH' roots from time of initial anthesis, had less bract necrosis (12.3% and 14.9%, respectively), than `SR' scions grafted on only `SR' roots from the onset of the study (24.2%). At harvest (36 days after anthesis), Ca concentrations in bract margin tissues were highest on `SR' shoots grafted to `SR' roots throughout the study [0.17% of dry mass (DM)], and lowest on shoots grafted to only `AH' from the time of flower initiation (0.15% of DM) or from time of initial anthesis (0.14% of DM). There was no significant correlation between Ca concentrations in bract margin tissue and bract necrosis. A single spray of benzyladenine to bracts, after bract necrosis symptoms first began to appear (at initial anthesis), totally arrested the further development of bract necrosis for up to 34 days. Similar treatment with daminozide increased the incidence of bract necrosis. This evidence suggests a new concept of bract necrosis etiology that includes a role for endogenous hormones. Chemical names used: N,N, dimethylaminosuccinamic acid (daminozide), N6-benzylaminopurine (benzyladenine).