Energy-Dependent Solute Transport from the Apoplast into the Symplast of Leaves during Transpiration

Inorganic and organic salts, amino acids, sugars, and phosphate esters (concentrations usually 25 mM) were fed via the transpiration stream through the petiole into detached leaves of Lepidium sativum and Solanum tuberosum. While water was lost by transpiration, solutes did not accumulate in the apoplast. Uptake into leaf cells was indicated by stimulation of respiration and by changes of membrane potential and apoplastic pH. Apoplastic alkalinization (followed by transient acidification) and membrane depolarization (followed by repolarization) indicated energization of transport at the expense of the proton motive force (PMF) across the plasma membrane in all examined cases. Loss of ATP in the symplast during proton extrusion into the apoplast by the plasmalemma ATPase is thought to be responsible for stimulation of respiration. Even unphysiological solutes such as β-morpholinoethane sulfonate (Mes), or potentially toxic salts such as CdCl2 or AlCl3, and metabolites involved in energy conservation such as AMP and NAD, were readily transported into leaf cells at the expense of metabolic energy. At the maximum stimulation of CO2 release by D-serine (which is unlikely to be metabolized) respiration exceeded basal respiration by an average of 33%. Occasionally, and with other solutes, basal respiration was almost doubled. The ratio of transported solute to released extra CO2 was 6.9 ± 1.1 (n = 11) in the case of D-serine. From this, maximum energized transport of D-serine was calculated to be close to 500 nmol/(m2 leaf area s). Solute/CO2 ratios similar to those observed with D-serine were also obtained for sucrose. Lower ones were observed with organic solutes such as L-glycine, pyruvate, malate or citrate where secondary metabolic conversions may contribute to CO2 release.