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Xylem development in relation to water uptake by roots of grapevine (Vitis vinifera L.).

Axial resistance (Ra ) was estimated from xylem diameter measurements obtained from periodic acid and toluidinc blue O (PAS-TBO) stained sections of Vitis vinifera L. ev. Shiraz. Multiple linear regression showed a strong negative relationship between axial resistance and either root diameter or distance from the root tip. Water stress treatments did not affect the relationships, but plant age significantly influenced the intercept of the regression. The use of both lignin and cytoplasm stain showed that some vessels retained degenerating protoplasm which would impede water flow. Inclusion of these vessels in calculations of axial resistance could account for the underestimation reported by some authors in comparison with experimental determinations. Calculations based on the assumption that all xylem vessels, large and small, are involved in axial water conduction showed that use of mean xylem diameter might result in overestimating values by a magnitude of between 1.7 and 4.4. The use of individual xylem vessel diameters gave more accurate estimation of axial resistance. Some adjacent secondary xylem and metaxylem vessels were observed to merge into single vessels as a result of breakdown of the wall between them. Implications of such a developmental phenomenon are discussed.