Xylem exudation is related to nitrate assimilation pathway in detopped maize seedlings: Use of nitrate reductase and glutamine synthetase inhibitors as tools

The xylem exudation of detopped 7-d-old seedlings of Zea mays L, doubled when KCl was present in the root medium compared to seedlings maintained on water. It was further enhanced when KCl was replaced by nitrogen compounds such as nitrate, ammonium and glutamine, The role of the nitrate assimilation pathway on the enhancement of xylem exudation rate was investigated using tungstate, an inhibitor of nitrate reductase (NR) activity, and phosphinothricin or methionine sulphoximine, inhibitors of glutamine synthetase (GS) activity, The sap levels of NO3- NH4+, glutamine, and asparagine was used to ascertain the in vivo inhibition of both enzymes. The tungstate effects were also checked by measuring leaf in vitro NR activity and NR protein content. Xylem exudation rate of detopped seedlings fed with KNO3 decreased when the nitrate assimilation pathway was blocked either at the NR or at GS sites, This decrease was prevented when urea (acting as NH4+ supply) was given simultaneously with tungstate, KNO3 does not act directly on exudation, but through the involvement of NH4+. The involvement of glutamine was also shown since GS inhibition resulted in a cancellation of the enhancing effect of KNO3 on exudation. As change of exudation rate was not linked to change in sap osmolarity, it is assumed that the assimilation chain could modify root water conductance. The role of glutamine was discussed.