Leaf age-related differences in apoplastic NH4+ concentration, pH and the NH3 compensation point for a wild perennial

Abstract

Extracts of the foliar apoplast of leaves of different ages of Luzula sylvatica (Huds.) Gaud. were prepared by vacuum infiltration and centrifugation. Measurements of pH and NH4+ concentration were performed on extracts. From these bioassay measurements the relative magnitude of NH3 compensation points for leaves of different ages were inferred. Young leaves were found to have much higher apoplast pH than old leaves, leading to the calculation of 4-10-fold higher NH3 compensation points. Such age-related differences in the NH3 compensation point are considerably larger than those previously reported. Apoplast pH and NH4+ concentration were found to increase during leaf expansion before declining prior to senescence. Bulk foliar tissue pH, NH4+ and total N concentrations were also found to be generally higher in young leaves than in old leaves. Where a significant correlation was found, total foliar N, bulk tissue foliar NH4+ and the calculated NH3 compensation point were all found to increase with N supplied to roots, whilst apoplast and bulk tissue H+ concentrations were found to decline. The potential of bulk foliar tissue measurements to act as simple predictors of the NH3 compensation point is discussed.