Ethylene Evolution from Bracts and Leaves of Poinsettia,Euphorbia pulcherrimaWilld

Woodrow, L. and Grodzinski, B. 1987. Ethylene evolution trom bracts and leaves ol Poinsettia, Euphorbia pulcherrima Willd.—J. exp. Bot. 38: 2024–2032. Ethylene release from fully expanded, red and white bracts and leaves of poinsettia, Euphorbia pulcherrima Willd., was compared. On a laminar (area) basis leaves contained about 50 times more chlorophyll and demonstrated 10 times the photosynthetic rate of the bracts. Both tissues contained starch, however, soluble carbohydrate in the bracts consisted primarily of reducing hexoses while the leaves contained mainly sucrose for translocation. The total free alpha-amino nitrogen content of the bract tissue was twice that of the leaf tissue. The leaves contained more ACC (1-aminocyclopropane-1-carboxylic acid) and produced proportionally more endogenous C2H4 than either the red or white bracts. ACC-stimulated 2H4 release was also greatest from the green tissue indicating that the EFE (ethylene forming enzyme) was most active in the leaves. The specific activity of the 14C2H4/12C2H4 released from [2,3-14C]ACC confirmed ACC as the primary precursor of C2H4 in this tissue. Ethylene release from the non-photosynthetic, bract tissue was not markedly affected by alterations in CO2 or light conditions. In green leaf tissue endogeneous ethylene release increased from 1·5 to 6·0 pmol C2H4 cm–2 h–1 while ACC-stimulated ethylene release increased from 10 to 35 pmol C2H4 cm2– h1– as the CO2 partial pressure increased from 100 to 1 200 μbar.