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

Determinations of the NH(3) compensation point for the understory plant of semi-natural woodlands Luzula sylvatica (Huds.) Gaud. were carried out by measurements of gas exchange and by calculation from the NH(4)(+) concentration and pH of extracts of the foliar apoplast. Compensation points determined by gas exchange measurements were among the lowest yet reported (0.51-1.10 microg NH(3) m(-3)) and those calculated from apoplast extracts were lower than any yet reported (0.017-0.54 microg NH(3) m(-3)). Those determined by gas exchange were consistently found to be between 2 and 30 times higher than those determined from apoplast extracts. Consideration of possible causes of this discrepancy, which is not confined to this investigation, showed that all likely errors would result in an increase in the discrepancy, or were insufficient to account for observed differences. It is suggested that spatial variability of pH and NH(4)(+) concentration within the foliar apoplast represents the most promising line for further investigation. It is also shown that the foliar apoplast of L. sylvatica is sufficiently buffered to eliminate the need for correction of H(+) concentration for dilution during extraction, but that it is necessary to correct the NH(4)(+) concentration of apoplast extracts for dilution.