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[1] Isoprene is the most dominant non-methane volatile organic compound (NMVOC) emitted by plants. NMVOCs play an important role in regulating the composition of atmospheric trace gases including global concentration of tropospheric ozone. Our present knowledge about NMVOCs emission is mainly from studies on temperate tree species. So far information on biogenic NMVOCs emission from tropical tree species is limited. In this study, isoprene emission rates from 40 tropical Indian tree species belonging to 33 genera and 17 families were measured for the first time using a dynamic flow through enclosure chamber technique. The isoprene emission rate from plants (30°C and PAR 1000 mmolm À2 s À1 ) ranged from undetectable to 81.5 mg g À1 h À1 and values were found to be comparable with other studies on tropical tree species. Tree species screened for isoprene emission in the present study may be grouped into the four categories, proposed by Karlik and Winer [2001], namely, 18 species were negligible or BDL isoprene emitting (<1 mg g À1 h À1 ), 6 species were low emitting (1 to <10 mg g À1 h À1 ), 5 species were moderate emitting (10 to <25 mg g À1 h À1), and 11 species were high isoprene emitting (!25 mg g À1 h À1 ). Maximum isoprene emission rate (81.5 mg g À1 h À1) was observed in the case of Dalbergia sissoo Linn. It was interesting to find that Citrus limon Linn., Citrus reticulata Linn., Citrus sinensis Linn., Grevillea robusta A. Cunn., and Morus alba Linn., which were earlier reported as BDL or non isoprene emitters in US [Winer et al., 1998;Karlik and Winer, 2001] were found to be appreciably high isoprene emitters (0.61-21.60 mg g À1 h À1 ) in the present study.

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