The Science

The increase in global temperature directly affects the net primary productivity of the forest. High temperatures can influence the rates of chemical reactions in cells, such as photosynthesis, electron transport, and isoprene emissions. In this study, researchers asked the following questions.

1) Are reductions in photosynthesis at high leaf temperatures in tropical forests linked to a reduction in g_s rather than direct negative temperature effects on photosynthesis?

2) Do current isoprene emission models that link photosynthetic electron transport rates to isoprene emissions rates as a function of temperature hold true in tropical species?

3) What is the role of isoprene on thermal tolerance of photosynthesis at high temperatures?

The research team discovered that in a thermophile early successional species in the Amazon, photosynthetic electron transport rates increased linearly with temperature in concert with isoprene emissions, even as stomatal conductance and net photosynthetic carbon fixation declined. The team observed the highest temperatures of continually increasing isoprene emissions yet reported and that blocking isoprene production induced a temperature-dependent loss of photosynthetic capacity.

The Impact

Tropical forests absorb large amounts of atmospheric CO₂, but substantial decreases in tropical forest gross primary productivity have been repeatedly demonstrated in the Amazon basin during periodic widespread drought associated with high temperature. Therefore, the physiological mechanisms through which tropical forests respond to high temperature are critically important to understand. While extreme warming will decrease stomatal conductance and net photosynthesis in tropical species, research 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. Results demonstrate that models which link isoprene emissions to the rate of ETR are ideal for tropical species and provide necessary “ground-truthing” for simulations of the large predicted increases in tropical isoprene emissions with climate warming.

Summary

Tropical forests absorb large amounts of atmospheric CO₂ 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 these phenomenon are lacking in the tropics and are necessary to assess the impact of a warming climate on global isoprene emissions. Here, the researchers demonstrate that in the early successional species Vismia guianensis in 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. The researchers observed the highest temperatures of continually increasing isoprene emissions yet reported (50°C). While P_n showed an optimum value of 32.6 ± 0.4°C, isoprene emissions, ETR, and the oxidation state of PSII reaction centers (q_L) increased with leaf temperature with strong linear correlations for ETR (ƿ = 0.98) and q_L (ƿ = 0.99) with leaf isoprene emissions. In contrast, other photoprotective mechanisms, such as non-photochemical quenching (NPQ), were not activated at elevated temperatures. Inhibition of isoprenoid biosynthesis repressed P_n at high temperatures through a mechanism that was independent of stomatal closure. While extreme warming will decrease g_s and P_n in tropical species, these 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. 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 large predicted increases in tropical isoprene emissions with climate warming.

Principal Investigator

Kolby Jardine
Lawrence Berkeley National Laboratory
kjjardine@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This material is based upon work supported as part of the Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics) funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research Terrestrial Ecosystem Science Program through contract No. DE-AC02-05CH11231 to Lawrence Berkeley National Laboratory, DE-AC05-00OR22725 to Oak Ridge National Laboratory, and DE-SC0012704 to Brookhaven National Laboratory. Additional funding for this research was provided by the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). Logistical and scientific support is acknowledged by the Forest Management laboratory (LMF), Climate and Environment (CLIAMB), and Large Scale Biosphere-Atmosphere (LBA) programs at the National Institute for Amazon Research (INPA).

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