Stimulation of isoprene emissions and electron transport rates as key mechanisms of thermal tolerance in the tropical speciesVismia guianensis

Tropical forests absorb large amounts of atmospheric CO(2)through photosynthesis, but high surface temperatures suppress this absorption while promoting isoprene emissions. While mechanistic isoprene emission models predict a tight coupling to photosynthetic electron transport (ETR) as a function of temperature, direct field observations of this phenomenon are lacking in the tropics and are necessary to assess the impact of a warming climate on global isoprene emissions. Here we demonstrate that in the early successional speciesVismia guianensisin the central Amazon, ETR rates increased with temperature in concert with isoprene emissions, even as stomatal conductance (g(s)) and net photosynthetic carbon fixation (P-n) declined. We observed the highest temperatures of continually increasing isoprene emissions yet reported (50 degrees C). WhileP(n)showed an optimum value of 32.6 +/- 0.4 degrees C, isoprene emissions, ETR, and the oxidation state of PSII reaction centers (q(L)) increased with leaf temperature with strong linear correlations for ETR ( = 0.98) and q(L)( = 0.99) with leaf isoprene emissions. In contrast, other photoprotective mechanisms, such as non-photochemical quenching, were not activated at elevated temperatures. Inhibition of isoprenoid biosynthesis repressedP(n)at high temperatures through a mechanism that was independent of stomatal closure. While extreme warming will decreaseg(s)andP(n)in tropical species, our observations support a thermal tolerance mechanism where the maintenance of high photosynthetic capacity under extreme warming is assisted by the simultaneous stimulation of ETR and metabolic pathways that consume the direct products of ETR including photorespiration and the biosynthesis of thermoprotective isoprenoids. Our results confirm that models which link isoprene emissions to the rate of ETR hold true in tropical species and provide necessary ground-truthing for simulations of the large predicted increases in tropical isoprene emissions with climate warming.