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Abstract Groups of apple trees within an orchard were irrigated by either releasing a fine mist within the canopy or spraying water on the soil. Diurnal changes in the water content of the xylem were inferred from measurements of density made with a gamma probe. Transpiration rates were estimated from leaf temperatures, and water potentials and changes in radial dimension of the stem were recorded. Xylem water content and water potential declined each morning and recovered in the afternoon. Radial dimensions of the xylem followed a similar pattern, but the percentage change was very small in relation to the change in water content of the same tissue. The decline in water content is unlikely to have been caused by cavitation, as it was readily reversed in the afternoon. It is more likely to have been caused by changes in the volume of water occupying already‐cavitated fibres or intercellular spaces. Water potential and xylem water content were highest in the mist‐irrigated trees and least in the unirrigated controls.
Brough et al. (Wed,) studied this question.