Influence of environmental factors on the emissions of gaseous formic and acetic acids from orange (Citrus sinensis L.) foliage

Gaseous acids can be emitted as well as taken up by plant foliage, but litt le is known about the influence of environmental factors on the exchange pr ocess. In a laboratory study we investigated the short-term effects of temp erature and light on the exchange of acetic and formic acids between orange foliage and the atmosphere. The results were compared with diurnal exchang e cycles obtained with the same species under field conditions. In the fiel d, the exchange of volatile acids showed pronounced diurnal variations with maximum emissions of about 0.15 nmol m(-2) projected leaf area s(-1) for a cetic acid and 0.3 nmol m(-2) s(-1) for formic acid during noon and afterno on and with acid deposition of up to 0.1 nmol m(-2) s(-1) for both acids du ring early morning and night. Under laboratory conditions no significant ac id deposition (< 0.01 nmol m(-2) s(-1)) could be observed, and emission rat es were lower than in the field and ranged around 0.035 nmol m(-2) s(-1) (S D = 0.013) for acetic acid and about 0.08 nmol m(-2) s(-1) for formic acid (SD = 0.033) at 30 degrees C and 1000 mu mol m(-2) s(-1) PAR. A clear posit ive response of acid release to light could be observed. Emissions were ver y low in darkness and strongly increased with light up to 1000 mu mol m(-2) s(-1) PAR. The emission response to light occurred within our hour and was also observed with other acids emitting plant species investigated. Respon se of emission to temperature was more variable, though, on the whole, the effect was also positive. Finally, we observed a diurnal rhythm in acid rel ease which was not related to other measured climatic or physiological para meters. We conclude that in the short-term the exchange of volatile acids b etween leaves and air under natural conditions is stimulated by light and, somewhat, by temperature, while a leaf-internal process promotes acid relea se in the morning and higher acid concentrations in ambient air promote aci d deposition.