l-Malate as an Essential Component of the Xylem Fluid of Corn Seedling Roots 1

AUTOR(ES)

Butz, Robert G.

RESUMO

Corn seedling xylem exudate has a pH of 5.30 ± 0.05 due to the presence of 10 millimolar malate which has a pKa of 5.13. This concentration of malate tends to buffer the xylem fluid at this pH. Exogenous treatment of corn seedling roots with CaCl2 caused a concentration-dependent decrease in the pH of the xylem fluid as well as a decrease in the volume of fluid secreted into the xylem. Exogenous 50 millimolar CaCl2 decreased exudate volume to 8% of control within 0.5 hour. Nitrate and malate deposition into the xylem was prevented by pretreatment in 5 millimolar CaCl2; nitrate deposition was resumed shortly after resumption of malate deposition. Fifty millimolar l-tartrate, an inhibitor of the Cl−, malate-activated ATPase (in vitro) of the tonoplast, also decreased exudate volume as well as slightly lowering exudate pH. The osmolality of the exudate was found to be constant at 70 ± 11 milliosmomoles per kilogram in all treatments. Dixon plots (log of exudation rate versus pH of exudate) indicated a pKa of 5.11 for the exudation process which is very close to the pKa of l-malate (5.13). In addition, a Dixon plot of the l-glutamine deposition (l-glutamine being the major form of reduced nitrogen in the exudate) versus pH also indicated a pKa near 5.15.

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