Daily variations of the mineral composition of xylemic exudates in tomato

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Abstract
Tomato plants (Lycopersicum esculentum cv. Prisca) were grown on perlite (or rock‐wool substrate) in a glasshouse. Xylemic sap was collected hourly for 24 to 48 hours after cutting the stem below the cotyledons, and the mineral composition of these xylemic exudates was determined. The influence of time between excision and collection on the mineral composition of xylemic exudates was studied, then, only the 4 first hours after excision were considered. Daily variations of mineral concentration were observed in exudates. Maximal values were reached during the day for NO3 and K, and during the night for Ca, Mg, H2PO4, and SO4. Mineral quantities exudated were maximal at midday for all the mineral elements as well as for the exudation rate. No relation with the watering volumes appeared. The daily cycles of mineral composition of xylemic exudates are discussed, pointing out rhythms in uptake or translocation of ions from roots to plant tops.