Poinsettia Cultivars Differ in Their Response to Molybdenum Deficiency

Six cultivars of poinsettia (Euphorbia pulcherrima Willd. ex Kl.) were grown in unlimed growth medium consisting of equal volumes of sphagnum peat and perlite. Plants received fertilizer solutions supplying Mo at either 0.0 mg·liter -1 (-Mo) or 1.0 mg·liter -1 (+Mo). In the –Mo treatment, moderate to severe symptoms of Mo deficiency (marginal and interveinal chlorosis, marginal necrosis, and downward curling of the margins) developed on the middle-aged and some recently matured leaves of `Annette Hegg Brilliant Diamond' and `Eckespoint Lilo', while some interveinal chlorosis occurred on the same leaves of `Gutbier V-17 Angelika'. No symptoms appeared on `Gross Supjibi', `Peace Regal Velvet', and `Peace Noel'. All cultivars were symptomless in the + Mo treatment. In the –Mo treatment, upper, recently matured leaves of the symptomatic cultivars and two of three symptomless cultivars had Mo concentrations at or near the critical level for deficiency; however, nitrate reductase enzyme activity was higher and NO 3 -N was lower in the leaves of symptomless cultivars than of symptomatic cultivars.