The Contribution of an Apoplastic Pathway to Sodium Uptake by Rice Roots in Saline Conditions

An apoplastic pathway, the so-called bypass-flow, across rice roots to the xylem has been investigated and approximately quantified using the apoplastic tracer dye 8-hydroxy-l,3,6-pyrenetrisulphonic acid (PTS); former nomenclature 3-hydroxy-5,8,10-pyrenetrisulphonic acid. It was confirmed that PTS was confined to a compartment no greater than the apparent free space in living rice roots. Experimental handling did not contribute to bypass-flow. Rice roots recovered rapidly from severe damage: following root pruning, sodium and calcium uptake returned to steady values in about 6 h. Apoplastic flow declined after damage as a first-order kinetic with a half time of 75 min. Analysis of the pattern of elution of PTS from preloaded roots (intact, excised and heat-killed), and from cellulose, was followed to compare PTS movement in the extracellular compartment with that of water and small hydrated ions. Consideration is given to the factor by which the bypass-flow estimated with the dye would need to be corrected to reflect the proportion of the transpiration stream flowing in the apoplastic pathway. The data suggest that this factor would be at least 10 for transpiring rice plants. There was large individual variation in the transport both of sodium and of the apoplastic tracer PTS to the shoot. Plants with high sodium transport also had high PTS transport and it is concluded that some proportion of the sodium reaching the xylem in rice does so by a pathway which is also available to PTS, presumably direct apoplastic contact across the endodermis. A median value for the bypass-flow of water (corrected from PTS) would be 0.5 to 1.0 percent of the transpirational volume flow, but individuals with the highest sodium transport would have bypass-flow values of several percent. Evidence is discussed which suggests that apoplastic transport may increase in stress conditions, and it is argued that bypass-flow is a major contribution to sodium uptake in rice in saline conditions.