The suitability of the infiltration-centrifugation method for collection of apoplastic fluid from intact leaves was evaluated for different plant species. Large differences with respect to infiltrability of the leaves, which correlated inversely with stomatal and mesophyll resistance, became apparent. Osmolality of infiltration medium (deionised water, 0.2 mM CaCl2, 10 mM KCl, 180 mM 2-[N-morpholino]ethane-sulphonic acid) and incubation time, time passed between onset of infiltration and end of centrifugation, revealed relatively little influence on the composition of the apoplastic washing fluid (AWF). In contrast, the pH of the infiltrated solution highly influenced the concentration of sucrose and hexoses. With increasing centrifugation force, hexosephosphate isomerase (HPI) activity in the AWF, which was taken as an indication for cytoplasmic contamination, increased. At the same time, Ca2+ concentration in the AWF increased even more. Since Ca2+ cannot originate from the cytoplasm, the suitability of HPI as marker for cytoplasmic contamination is questioned. From the composition of the AWF, it is concluded that, if centrifugation force does not exceed 1 000 g, cytoplasmic contamination is negligible and that the infiltration-centrifugation technique reveals an easy and inexpensive way to study apoplastic solutes. The infiltration-centrifugation method was also suitable to determine apoplastic air volume (Vair) and apoplastic water volume (Vwater), which are necessary for the calculation of the ion concentration in the leaf apoplast. It could be shown that the leaves of different species and the apical and basal leaves of single plants differ in Vair and Vwater.

Strategies for avoiding ion accumulation in leaves of plants grown at high concentration of NaCl (100 mol m(-3)) in the rooting media, i.e. retranslocation via the phloem and leaching from the leaf surface, were quantified for fully developed leaves of maize plants cultivated hydroponically with or without salt, and with or without sprinkling (to induce leaching). Phloem sap, apoplastic fluid, xylem sap, solutes from leaf and root tissues, and the leachate were analysed for carbohydrates, amino acids, malate, and inorganic ions. In spite of a reduced growth rate Na(+) and Cl(-) concentrations in the leaf apoplast remained relatively low (about 4-5 mol m(-3)) under salt treatment. Concentrations of Na(+) and Cl(-) in the phloem sap of salt-treated maize did not exceed 12 and 32 mol m(-3), respectively, and thus remained lower than described for other species. However, phloem transport rates of these ions were higher than reported for other species. The relatively high translocation rate of ions found in maize may be due to the higher carbon translocation rate observed for C(4) plants as opposed to C(3) plants. Approximately 13-36% of the Na(+) and Cl(-) imported into the leaves through the xylem were exported by the phloem. It is concluded that phloem transport plays an important role in controlling the NaCl content of the leaf in maize. Surprisingly, leaching by artificial rain did not affect plant growth. Ion concentrations in the leachate were lower than reported for other plants but increased with NaCl treatment.

In order to understand the metabolic processes governing the protein content in maize grains, we compared metabolic parameters of the two maize strains Illinois High Protein (IHP) and Illinois Low Protein (ILP), which dier largely in their relative starch and protein content. The activity of nitrate reductase (NR) and the transcription level of asparagine synthetase, two enzymes that play a central role in the¯ow of N into organic compounds, were also investigated. It was shown that IHP plants contained a higher activity of NR than ILP plants, which was in part due to an increased level of NR activity in the roots. The root-toshoot ratios in the ILP and IHP strains were 0.41 and 0.54, respectively. The total amino nitrogen contained in the amino acids of the leaves, phloem, xylem and grain was much higher in IHP plants than in ILP plants. The enhanced contents of amino nitrogen in IHP plants were mainly due to an increase in the asparagine level, which was 13 times higher in the xylem of IHP plants than in that of ILP plants. In the roots the asparagine synthetase was expressed constitutively. We conclude from these results that IHP maize diers from ILP maize in having a high capacity for delivering asparagine as a product of root metabolism in addition to the amino acids supplied by nitrate assimilation of the leaves.