Comparison of gas exchange and bioassay determinations of the ammonia compensation point in Luzula sylvatica (Huds.) Gaud.

Determinations of the NH, compensation point for the understory plant of se mi-natural woodlands Luzula sylvatica (Huds.) Gaud. were carried out by mea surements of gas exchange and by calculation from the NH4+ concentration an d pH of extracts of the foliar apoplast. Compensation. points determined by gas exchange measurements were among the lowest yet reported (0.51-1.10 mu g NH3 m(-3)) and those calculated from apoplast extracts were lower than an y yet reported (0.017-0.54 mug NH, m(-3)). Those determined by gas exchange were consistently found to be between 2 and 30 times higher than those det ermined from apoplast extracts. Consideration of possible causes of this di screpancy, which is not confined to this investigation, showed that all lik ely errors would result in an increase in the discrepancy, or were insuffic ient to account for observed differences; It is suggested that spatial vari ability of pH and NH4+ concentration within the foliar apoplast represents the most promising line for further investigation. It is also shown that th e foliar apoplast of L, sylvatica is sufficiently buffered to eliminate the need for correction of H+ concentration for dilution during extraction, bu t that it is necessary to correct the NH4+ concentration of apoplast extrac ts