Water Flow Through Cotton Roots in Relation to Xylem Anatomy

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Abstract
The effect of root anatomy on water flow was studied in 7-d-old cotton (Gossypium hirsutum L.) seedlings grown in solution culture. The total water flux of the intact root system was measured using a pressure chamber. Then successive terminal root sections were removed at 2,6,10 and 12 cm behind the root tip and the flux was remeasured after each successive cut was made. Xylem development at different distances behind the root apex was studied with a microscope using sections cut free-hand and stained with toluidine blue. Water flux increased with the removal of successive terminal root sections and this coincided with the degree of basipetal primary xylem development. The large increase in water flux at 10 to 12 cm was associated with secondary xylem development and increased xylem vessel number. A comparison of water flow and xylem anatomy between roots with tetrarch (Stoneville 506 and Deltapine 41) and pentarch (T25 strain) vascular bundle arrangements showed no significant differences in the measured values of water flux for the primary root. Water flux, estimated using Poiseuille's equation and measured xylem dimensions, was greater for the tetrarch roots, primarily because of the larger diameter of individual vessel elements. The increased number of vessel elements in the pentarch primary root of T25 did not result in any apparent decrease in axial resistance to water flow.

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