IsopreneEmission fromAspenLeaves1 Influence ofEnvironment andRelation toPhotosynthesis andPhotorespiration

Isoprene emission rates fromquaking aspen(Populus tremu/oides Michx.) leaves weremeasured simultaneously withphotosynthesis rate, stomatal conductance, andintercellular CO2partial pressure. Isoprene emission required thepresence ofCO2or 02,butnotboth. Thelight response ofisoprene emission rate paralleled thatofphotosynthesis. Isoprene emission wasinhibited bydecreasing ambient 02from21%to2%,onlywhenthere wasoxygeninsensitive photosynthesis. Mannose(10millimolar) fedthrough cutstemsresulted instrong inhibition ofisoprene emission rateandisinterpreted asevidence thatisoprene biosynthesis requires either theexport oftriose phosphates from thechloroplast, orthecontinued synthesis ofATP.Light response experiments suggest thatphotosynthetically generated reductant orATPisrequired forisoprene biosynthesis. Isoprene biosynthesis andemission arenotdirectly linked toglycolate production through photorespiration, contrary toprevious reports. Isoprene emission ratewasinhibited byabove-ambient CO2partial pressures(640microbar outside and425microbar inside theleaf). Theinhibition wasnotduetostomatal closure. Thiswasestablished byvarying ambient humidity atnormal andelevated CO2 partial pressures tomeasureisoprene emission ratesovera rangeofstomatal conductances. Isoprene emission rates were inhibited atelevated CO2despite nochange instomatal conductance.Addition ofabscisic acidtothetranspiration streamdramatically inhibited stomatal conductance andphotosynthesis rate, witha slight increase inisoprene emission rate. Thus, isoprene emission isindependent ofstomatal conductance, and mayoccurthrough thecuticle. Temperature hadaninfluence on isoprene emission rate, withtheQ10being 1.8to2.4between 35 and450C. Atthesehigh temperatures theamountofcarbon lost through isoprene emission was between2.5and8% ofthat assimilated through photosynthesis. Thisrepresents asignificant carboncostthatshould betakenintoaccountindetermining midsummer carbon budgets forplants that areisoprene emitters. Isoprene (2-methyl- 1,3-butadiene) isoneoftheprimary nonmethane hydrocarbons emitted fromsomeplant leaves