Factors controlling the emissions of volatile organic acids from leaves ofQuercus ilex L. (Holm oak)

Direct emissions and emission of precursor compounds of acetic and formic a cid from higher plants are a significant source of these acids in the atmos phere. To travel from the plant cell to the atmosphere, a gas molecule must first leave the liquid phase and then enter the internal leaf gas phase. T he apoplast (cell wall) is the last barrier before the molecule can escape through the stomata. During field experiments we monitored the gas exchange (H2O, CO2, organic acids) of Quercus ilex L. leaves. The exchange rates of acetic and formic acid under field conditions followed a typical diurnal p attern and ranged between -10 (uptake) and 52 (emission) nmol m(-2) leaf ar ea min(-1) with the maximum around noon. Growth chamber experiments indicat e that the emission is related to the stomatal conductance. We discussed th e exchange rate of organic acids between the cell wall and the atmosphere i n connection with Henry's law, and the physicochemical conditions in the ce ll wall. The evaluation showed that for apoplastic pH values between 4 and 5, 26-130% of the measured acetic acid emission based on leaf area could be predicted. (C) 1999 Elsevier Science Ltd. All rights reserved.