The effect of phosphorus nutrition on water flow through the apoplastic bypass in cotton roots

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Abstract
A previous study comparing hydraulic conductivity of intact roots and cortical cells of cotton (Gossypium hirsutum L.) seedlings suggested that substantial water flow may bypass cell membranes entirely, following a completely apoplastic pathway, and also suggested that phosphorus deficiency might increase bypass flow. We used fluorescent apoplastic tracers to quantify flow through the apoplastic bypass and to assess the effect of phosphorus deficiency on bypass flow. Tracer concentration in shoot xylem sap was less than 0.25% of the concentration in the cultural solution for five different tracers. Phosphorus deficiency reduced this already low concentration even further, possibly by reducing the number of newly emerged lateral roots which can provide an alternative pathway for water entrance into the stele. No relationship existed between hydraulic conductivity and bypass flow. We concluded that the apoplastic bypass constituted only a small fraction of water movement into the stele. The contribution of the apoplastic bypass was not sufficient to explain differences in hydraulic conductivity between intact roots and cortical cells, or between plants receiving high or low phosphorus treatments.

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