Ethylene Release from Leaves ofXanthium strumariumL. andZea maysL.

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Abstract
The release of ethylene into sealed Erlenmeyer flasks by intact leaves and leaf discs of Xanthium strumarium L. a C3 plant and Zea mays L. a C4 plant were compared both in white light and in darkness. The effects of the presence or absence of added CO2 (in the form of sodium bicarbonate) the photosynthetic inhibitor 3-[3,4-dichlorophenyl]-l, l-dimethyl urea (DCMU) and 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of ethylene in higher plants, were also investigated. The rate of ethylene release from leaf tissue of Xanthium in the absence of added CO2 was markedly reduced in the light (i.e. at the CO2 compensation point). Treatments that would enhance the CO2 availability to the tissue (i.e. added bicarbonate, darkness, treatment with DCMU) allowed higher levels of ethylene release. Incubation of the tissue with ACC considerably enhanced the release of ethylene compared to that from the corresponding control tissue without ACC. However, the pattern of ethylene release induced by the various treatments was similar with or without added ACC. When tissue, in the absence of added CO2, was transferred from light to darkness, and back to light for 90 min periods, the ethylene release rates Increased during the interposed dark period but resumed the lower rate during the final light period. The addition of CO2 in the light resulted in a similar rate of ethylene release to that found in the dark. The overall pattern of ethylene release from Zea leaf tissue subjected to light and dark in the presence or absence of added CO2 was similar to that of Xanthium. However, two or three times more ethylene was released from maize leaves in the light when CO2 was added compared to that generated in the dark. This is in marked contrast to Xanthium, where, under the light conditions used, the ethylene release rate in the dark equalled or exceeded that occurring in the light, even in the presence of high levels of CO2. A very low rate of ethylene release was observed at the CO2 compensation point of maize. A speculative model is presented to explain how photosynthetic activity might act as a key factor in regulating ethylene evolution from leaf tissue in these experiments. It invokes the concept of an inhibition by CO2 of ethylene retention or breakdown thus permitting more ethylene to be released from the leaves.

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