A ROLE FOR ETHYLENE IN THE YELLOWING OF BROCCOLI AFTER HARVEST

Ethylene production from florets of 'Shogun' harvested broccoli (Brass ica oleracea L.var. italica) held at 20C in darkness increased as the sepal tissues yellowed. The pattern of respiration rate and ethylene p roduction from branchlets or entire heads was similar, although the ma gnitude of ethylene and carbon dioxide production appeared to be dilut ed by the other fleshy stem tissues. The reproductive structures, stam ens and pistil, may have a role in determining the rate of sepal degre ening, since removing them from florets reduced the yellowing rate. Th e pistil and stamens also had 7-fold higher levels of 1-aminocycloprop ane-1-carboxylic acid (ACC) oxidase activity and more than double the ethylene production of other tissues within the floret. Stamen ACC oxi dase activity was high on the first day after harvest, before yellowin g became obvious. Changes in ACC oxidase activity of the pistil and st amens mirrored changes in ACC content in these tissues. The climacteri c status of harvested broccoli was confirmed by exposure to 0.5% propy lene. Propylene stimulated respiration and ethylene production and acc elerated yellowing (measured as chlorophyll and hue-angle decline). Br occoli tissues did not respond to propylene immediately after harvest. In tissues aged in air before treatment, the time for response to pro pylene was shorter, a result suggesting a change in tissue sensitivity . Ethylene exposure induced a dose-dependent decline in hue angle, wit h 1 ppm ethylene giving the maximum response.