CULTIVAR DIFFERENCES IN NUTRIENT ABSORPTION AND SUSCEPTIBILITY TO BRACT NECROSIS IN POINSETTIA (EUPHORBIA-PULCHERRIMA WILLD EX KLOTZSCH)

Bract necrosis (BN) in poinsettia 'Eckespoint Lilo' was reduced signif icantly with foliar application of Ca. Plants of this and of the non-s usceptible cultivar'Annette Hegg Dark Red' (AHDR) were exposed to cond itions which affect Ca absorption and translocation. Varying the level of air humidity, partly by covering the plants continuously or only b y night, did not affect BN. Fertilization with nutrient solutions of v arying Ca content and ratios of nutrient elements affecting Ca availab ility and absorption gave the following results: 1. The NH4/NO3 ratio of the nutrient solution, normally influencing Ca absorption, had no e ffect on the Ca content and on BN. 2. A high K level of the nutrient s olution depressed the uptake of Ca and caused BN in 'Lilo', but not in AHDR. 3. The susceptible cultivar had, in all cases, a higher K conte nt and a lower Ca content of the bracts than the non-susceptible culti var. This was especially significant when the content of the marginal section of the bracts was compared. It is concluded that non-susceptib le poinsettia cultivars do not exhibit BN because they are able to fur nish the bracts with sufficient Ca under conditions which restrict Ca- absorption and causes BN in susceptible cultivars. There was a correla tion between total Ca content and K/Ca ratio of the marginal zone of t he bracts and BN. It is concluded that BN does not appear if the Ca co ntent of the marginal zone rises above 0.16 % of dry matter and the K/ Ca ratio falls below 20. The cultivar AHDR, in contrast to 'Lilo', exh ibits white patches ('rabbit tracks') along the midrib of the bracts. The bracts of AHDR proved to contain significantly less phosphorus tha n the bracts of 'Lilo'. In previous experiments on preventing BN with Ca-sprays white patches did not occur on AHDR plants sprayed with Ca p hosphate, indicating that the disorder might be related to the P statu s of the bracts.