Methanol and isoprene emissions from the fast growing tropical pioneer species <i>Vismia guianensis</i> (Aubl.) Pers. (Hypericaceae) in the central Amazon forest
Isoprene (Is) emissions by plants represent a loss of carbon and energy
resources leading to the initial hypothesis that fast growing pioneer
species in secondary tropical forests allocate carbon primarily to growth at
the expense of isoprenoid defenses. In this study, we quantified leaf
isoprene and methanol emissions from the abundant pantropical pioneer tree
species Vismia guianensis and ambient isoprene concentrations above a diverse secondary
forest in the central Amazon. As photosynthetically active radiation (PAR)
was varied (0 to 3000 µmol m−2 s−1) under standard leaf
temperature (30 °C), isoprene emissions from V. guianensis increased without
saturation up to 80 nmol m−2 s−1. A nonlinear increase in
isoprene emissions with respect to net photosynthesis (Pn) resulted in the
fraction of Pn dedicated to isoprene emissions increasing with light
intensity (up to 2 % of Pn). Emission responses to temperature under
standard light conditions (PAR of 1000 µmol m−2 s−1)
resulted in the classic uncoupling of isoprene emissions (Topt, iso > 40 °C) from net photosynthesis
(Topt, Pn = 30.0–32.5 °C) with up to 7 % of Pn emitted as isoprene
at 40 °C. Under standard environmental conditions of PAR and leaf
temperature, young V. guianensis leaves showed high methanol emissions, low Pn, and low
isoprene emissions. In contrast, mature leaves showed high Pn, high isoprene
emissions, and low methanol emissions, highlighting the differential control
of leaf phenology over methanol and isoprene emissions. High daytime ambient
isoprene concentrations (11 ppbv) were observed above a secondary Amazon
rainforest, suggesting that isoprene emissions are common among neotropical
pioneer species. The results are not consistent with the initial hypothesis
and support a functional role of methanol during leaf expansion and the
establishment of photosynthetic machinery and a protective role of isoprene
for photosynthesis during high temperature extremes regularly experienced in
secondary rainforest ecosystems
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