Daily variations of xylemic exudation rate in tomato

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Abstract
Tomato plants (Lycopersicum esculentum cv. Prisca) were grown on perlite or rock‐wool substrates in a greenhouse or in nutrient culture in a growth chamber. At the beginning of flowering and at the fruiting period, the plants were detopped at the base of the stem (1 plant every hour over the 24 hours), then exuded xylem sap was collected hourly for 24 to 48 hours. Exudation rates could be taken into account for about 2 to 10 hours after cutting the stem depending on the development stage of the plants. Cyclic variations of the rate of exudation were observed which followed a daily rhythm with a maximum at midday in the greenhouse as well as under the constant conditions of the growth chamber. The rhythmicity of these variations was also observed in continuous darkness, thus indicating its endogenous nature. The meaning of such a daily rhythm is discussed.