Pressure -Flow Relationships, Xylem Solutes and Root Hydraulic Conductance in Flooded Tomato Plants

Author： Jackson M.B. Davies W.J. Else M.A.

ISSN： 0305-7364

Source： Annals of Botany, Vol.77, Iss.1, 1996-01, pp. : 17-24

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Abstract

Roots of month-old tomato plants ( Lycopersicon esculentum Mill.) were flooded for up to 36h. Shoots were removed just below the cotyledonary node, and the roots subjected to external pneumatic pressures ( Delta P =0.02 - 0.4MPa). Rates of resulting sap flow ( J v ) from the hypocotyl stump were measured and solute content assessed. Increasing Delta P raised J v and diluted sap osmolites. Dilution was proportional to flow in well-drained plants but less than proportional in flooded plants. Sap flow increased linearly over a Delta P range of 0.2 -0.4MPa; the slope of this line representing hydraulic conductance ( L P ), an estimate of inherent root hydraulic conductivity. Flooding for 24h did not change L P relative to well-drained controls. Despite this similar L P , J v at given values of Delta P were always faster through flooded root systems. This is explained by greater osmolality of xylem sap and thus more negative osmotic potentials ( pi sap ) creating an additional driving force promoting J v . Overall, solute osmolality and solute delivery in flowing xylem sap increased as flooding was prolonged from 6 to 36h flooding. On arrival in the shoot, these solutes would exert a negative effect on leaf water potentials down to -0.08MPa. The extra solutes exported from roots of flooded plants compared with controls, included potassium, sulphate, protein, serine, 1-aminocyclopropane-1-carboxylic acid and sucrose. Flooding decreased the delivery of nitrate, hydrogen ions, most protein amino acids, glutamine, and abscisic acid. These changes in delivery of solutes are systemic messages passing from flooded roots to shoots. Their osmotic and physiological properties may modify shoot growth and development and have adaptive significance.