Ethylene emission from wild‐type Agrobacterium tumefaciens (C58)‐induced stem tumours of Ricinus communis was continuously measured with two different methods, process gas chromatography and photo‐acoustic spectrometry. Ethylene production was as high as 700 pmol g FW–1 h–1, namely 140 times greater than emitted by non‐tumourized control stems. It was highest in 5‐week‐old tumours, independent of light, depressed by anoxia and, during water deficit it was stimulated by rewatering. A remarkable concomitant CO‐production was discovered. Accumulation of 1‐aminocyclopropane‐1‐carboxylic acid (ACC), the substrate of ACC‐oxidase, preceded ethylene emission with a maximum 2 weeks after tumour induction. Simultaneously, the xylem in the tumour‐adjacent host stem underwent drastic changes: it increased two to three times in thickness, vessel diameters decreased, the rays remained unlignified and became multiseriate. With increasing emission of ethylene aerenchyma developed in the non‐transformed, tumour‐surrounding tissue that formerly was stem cortex. Cotyledons reacted with epinastic symptoms indicating induction of senescence. The present results reveal an important role of ethylene, in addition to cytokinin and auxin, for the differentiation and physiology of A. tumefaciens‐induced tumours.

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1. The transpiration in leaves of Bryophyllum daigremontianum exactly follows the changes in consumption of atmospheric carbon dioxide (caused by the Crassulaceen acid metabolism) during the light and dark periods. After removal of the epidermis no distinct rhythm in the course of transpiration can be observed any more, whereas the characteristic CO2 exchange continues in an unchanged matter. For this reason we assume that the changing rate of CO2 uptake from the atmosphere determines the concentration of carbon dioxide in the intercellular spaces of the leaves and in this way controls the opening of stomata. 2. CO2 uptake from the atmosphere in the light phase decreases faster than CO2 consumption in the dark when the plants are held under water stress conditions. At the endpoint CO2 is fixed only in the dark period. On the basis of the connection between CO2 uptake and movement of stomata we assume a closure of the stomata during the light period (since no extracellular CO2 is fixed). Since evaporation values in the light phase are high under natural conditions, this manner of gas exchange minimizes the loss of water during water stress conditions, and nevertheless guarantees a positive balance of carbon.

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