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

Corn seedling xylem exudate has a pH of 5.30 ± 0.05 due to the presence of 10 millimolar malate which has a pK a of 5.13. This concentration of malate tends to buffer the xylem fluid at this pH. Exogenous treatment of corn seedling roots with CaCl 2 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 CaCl 2 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 CaCl 2 ; 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 pK a of 5.11 for the exudation process which is very close to the pK a 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 pK a near 5.15. The pH optimum for glutamine transport into the xylem was 5.5. Deposition of glutamine into the xylem may be regulated by the xylem pH (5.30 ± 0.05) which in turn may be regulated by the presence of 10 millimolar malate. It is proposed that the transport of glutamine into the xylem may provide the driving force for the exudation process.