Water transport properties of roots and root cortical cells in proton- and Al-stressed maize varieties

Root and root cell pressure-probe techniques were used to investigate the possible relationship between Al- or H+-induced alterations of the hydraulic conductivity of root cells (L(Pc)) and whole-root water conductivity (L(Pr)) in maize (Zea mays L.) plants. To distinguish between H+ and Al effects two varieties that differ in H+ and Al tolerance were assayed. Based on root elongation rates after 24 h in nutrient solution of pH 6.0, pH 4.5, or pH 4.5 plus 50 mu M Al, the variety Adour 250 was found to be H+-sensitive and Al-tolerant, whereas the variety BR 201 F was found to be H+-tolerant but Al-sensitive. No Al-induced decrease of root pressure and root cell turgor was observed in Al-sensitive BR 201 F, indicating that Al toxicity did not cause a general breakdown of membrane integrity and that ion pumping to the stele was maintained. Al reduced L(Pc) more than L(Pr) in Al-sensitive BR 201 F. Proton toxicity in Adour 250 affected L(Pr) more than L(Pc). In this Al-tolerant variety L(Pc) was increased by Al. Nevertheless, this positive effect on L(Pc) did not render higher L(Pr) values. In conclusion, there were no direct relationships between Al- or H+-induced decreases of L(Pr) and the effects on L(Pc). To our knowledge, this is the first time that the influence of H+ and Al on root and root cell water relations has been directly measured by pressure-probe techniques.