EFFECT OF LIGHT REGIME ON MARGINAL BRACT NECROSIS IN POINSETTIA

Marginal bract necrosis in poinsettia (Euphorbia pulcherrima) is a physiological calcium deficiency disorder, which occurs irregularly and commonly unpredictable in susceptible cultivars. This may indicate that environmental factors play a decisive role. In the present study with cv. 'Angelika' it could be shown that particularly light intensity at certain development stages has an overwhelming impact on the occurrence of the disorder. Supplementary light in an early phase of bract development (short-day week 6 and 7) decreased or prevented marginal bract necrosis while a high light supply later on (short-day week 9 and 10) increased the occurrence of the disorder exponentially. The preventive and inductive effect of a high light supply in an early and a later phase of bract development, respectively, counteracted each other.

A factorial growth chamber experiment with two light and two temperature levels suggested that the induction of the disorder by a high light supply during a late phase of bract development is specific for light and not a consequence of temperature. Ca sprays prevented marginal bract necrosis owing to a high light supply during a late phase of bract development. By means of Ca translocation studies with strontium as Ca tracer it could be shown that the higher light supply in an early phase of bract development stimulated Ca accumulation in bracts while later on the light effect was virtual zero. A hypothesis based on the physiological Ca balance in bracts is proposed which explains the opposite light effects on marginal bract necrosis.