Abstract
The respective roles of carbon and water in radial growth were assessed using double girdling and water stress as experimental factors to induce variations of the carbon and water status of tissues of Juglans nigra x Juglans regia grown in France. The results show that structural radial growth (with its specific energy requirement) occurred mainly during the night-time, in relation with an improved water status, and consequently higher turgor pressure which allows tissue enlargement (cell division and/or expansion). Maximal respiration rate occurring at night, when transpiration was minimal, suggests no or very low involvement of dissolved CO2 originating from a distance, i.e. carried by the transpirational flux of xylem sap, in the total stem CO2 efflux rate. The observations point out the importance of current photoassimilate flux for active radial growth. Local stem reserves could only marginally contribute to radial growth, although sharp shortage in assimilate supply could induce, within a few days, significant mobilization of local parenchyma reserves which allowed growth resumption. However, the driving force of mobilization was apparently not organ-wide sugar deficiency, as mobilization was not associated with any significant drop of sugars at the organ level.
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