Contribution of Various Carbon Sources Toward Isoprene Biosynthesis in Poplar Leaves Mediated by Altered Atmospheric CO2 Concentrations

Biogenically released isoprene plays important roles in both tropospheric photochemistry and plant metabolism. We performed a 13CO2-labeling study using proton-transfer-reaction mass spectrometry (PTR-MS) to examine the kinetics of recently assimilated photosynthate into isoprene emitted from poplar (Populus × canescens) trees grown and measured at different atmospheric CO2 concentrations. This is the first study to explicitly consider the effects of altered atmospheric CO2 concentration on carbon partitioning to isoprene biosynthesis. We studied changes in the proportion of labeled carbon as a function of time in two mass fragments, M41+, which represents, in part, substrate derived from pyruvate, and M69+, which represents the whole unlabeled isoprene molecule. We observed a trend of slower 13C incorporation into isoprene carbon derived from pyruvate, consistent with the previously hypothesized origin of chloroplastic pyruvate from cytosolic phosphenolpyruvate (PEP). Trees grown under sub-ambient CO2 (190 ppmv) had rates of isoprene emission and rates of labeling of M41+ and M69+ that were nearly twice those observed in trees grown under elevated CO2 (590 ppmv). However, they also demonstrated the lowest proportion of completely labeled isoprene molecules. These results suggest that under reduced atmospheric CO2 availability, more carbon from stored/older carbon sources is involved in isoprene biosynthesis, and this carbon most likely enters the isoprene biosynthesis pathway through the pyruvate substrate. We offer direct evidence that extra-chloroplastic rather than chloroplastic carbon sources are mobilized to increase the availability of pyruvate required to up-regulate the isoprene biosynthesis pathway when trees are grown under sub-ambient CO2

Potential contribution of exposed resin to ecosystem emissions of monoterpenes

Conifers, especially pines, produce and store under pressure monoterpene-laden resin in canals located throughout the plant. When the plants are damaged and resin canals punctured, the resin is exuded and the monoterpenes are released into the atmosphere, a process that has been shown to influence ecosystem-level monoterpene emissions. Less attention has been paid to the small amounts of resin that are exuded from branches, expanding needles, developing pollen cones, and terminal buds in the absence of any damage. The goal of this study was to provide the first estimate of the potential of this naturally-exposed resin to influence emissions of monoterpenes from ponderosa pine (Pinus ponderosa) ecosystems. When resin is first exuded as small spherical beads from undamaged tissues it emits monoterpenes to the atmosphere at a rate that is four orders of magnitude greater than needle tissue with an equivalent exposed surface area and the emissions from exuded beads decline exponentially as the resin dries. We made measurements of resin beads on the branches of ponderosa pine trees in the middle of the growing season and found, on average, 0.15 cm² of exposed resin bead surface area and 1250 cm² of total needle surface area per branch tip. If the resin emerged over the course of 10 days, resin emissions would make up 10% of the ecosystem emissions each day. Since we only accounted for exposed resin at a single point in time, this is probably an underestimate of how much total resin is exuded from undamaged pine tissues over the course of a growing season. Our observations, however, reveal the importance of this previously unrecognized source of monoterpenes emitted from pine forests and its potential to influence regional atmospheric chemistry dynamics.5 page(s

Differential controls by climate and physiology over the emission rates of biogenic volatile organic compounds from mature trees in a semi-arid pine forest

Drought has the potential to influence the emission of biogenic volatile organic compounds (BVOCs) from forests and thus affect the oxidative capacity of the atmosphere. Our understanding of these influences is limited, in part, by a lack of field observations on mature trees and the small number of BVOCs monitored. We studied 50- to 60-year-old Pinus ponderosa trees in a semi-arid forest that experience early summer drought followed by late-summer monsoon rains, and observed emissions for five BVOCs—monoterpenes, methylbutenol, methanol, acetaldehyde and acetone. We also constructed a throughfall-interception experiment to create “wetter” and “drier” plots. Generally, trees in drier plots exhibited reduced sap flow, photosynthesis, and stomatal conductances, while BVOC emission rates were unaffected by the artificial drought treatments. During the natural, early summer drought, a physiological threshold appeared to be crossed when photosynthesis ≅2 μmol m⁻² s⁻¹ and conductance ≅0.02 mol m⁻² s⁻¹. Below this threshold, BVOC emissions are correlated with leaf physiology (photosynthesis and conductance) while BVOC emissions are not correlated with other physicochemical factors (e.g., compound volatility and tissue BVOC concentration) that have been shown in past studies to influence emissions. The proportional loss of C to BVOC emission was highest during the drought primarily due to reduced CO₂ assimilation. It appears that seasonal drought changes the relations among BVOC emissions, photosynthesis and conductance. When drought is relaxed, BVOC emission rates are explained mostly by seasonal temperature, but when seasonal drought is maximal, photosynthesis and conductance—the physiological processes which best explain BVOC emission rates—decline, possibly indicating a more direct role of physiology in controlling BVOC emission.14 page(s

Appendix A. Specific leaf area (SLA; cm2·g-1·dry mass) values for the genotypes used in this study.

Specific leaf area (SLA; cm2·g-1·dry mass) values for the genotypes used in this study

Appendix B. Mean emission rates (nmol·m-2·s-1) of isoprene, methanol, and monoterpenes from new, young, mature, and old leaves from each of 30 genotypes.

Mean emission rates (nmol·m-2·s-1) of isoprene, methanol, and monoterpenes from new, young, mature, and old leaves from each of 30 genotypes

¹³CO₂ labeling of M41⁺ and M69⁺ as a function of CO₂ concentration and time.

<p>The number of labeled carbons present in both the M41⁺ fragment and M69⁺ parent isoprene molecule prior to and after ¹³C labeling with error bars representing the standard error of the mean (SEM). Change in the lines are evaluated in reference to the number of labeled carbons in both the fragment and parent molecule over time, with lines falling on one another representing labeling occurring simultaneously in both molecules and a divergence representing a faster label incorporated into M69⁺ that is not derived from M41⁺. Before leaves were exposed to ¹³CO₂ labeling at 1000 seconds, plants were exposed to the same ¹²CO₂ concentrations at which they were grown. As expected, no labeling occurred for either M41⁺ (closed circles) nor M69⁺ (open circles) before labeling. Immediately after labeling, one carbon was labeled in both the parent molecule and the fragment (demonstrated by the simultaneous increase in both lines), suggesting that the first carbon used to synthesize isoprene is contributed from the M41⁺ fragment. However, as time progressed and a second carbon becomes labeled on the parent molecule, the M41⁺ lines diverged for leaves grown in all three treatments, suggesting that the second labeled carbon on the isoprene molecule is not coming from the M41⁺ subunit.</p

¹³CO₂ labeling of carbon atoms in M69⁺ and M41⁺ and their isotopomers through time.

<p>(A) ¹³CO₂ labeling of carbon atoms in trees grown and measured in ambient CO₂ conditions (400 ppm CO₂) in the parent isoprene molecule, as characterized by a decrease in the M69⁺ signal (orange circles) and simultaneous increase in its isotopomers (denoted as sums) as labeled carbons were successively incorporated through time. Total emission (blue circles), sM70⁺ (red downward triangles), sM71⁺ (green triangles), sM72⁺ (yellow squares), sM73⁺ (sea green squares), sM74⁺ (purple diamonds) are represented. (B) ¹³CO₂ labeling of carbon atoms in trees grown and measured at 30°C in ambient CO₂ conditions (400 ppm CO₂) in the 3-C methyl-vinyl isoprene fragment, characterized by a decrease in the M41⁺ signal (light orange dotted downward triangles) with a simultaneous increase in its labeled isotopomers (denoted as sums). Total emission (blue dotted squares), sM42⁺ (pink crossed circles), sM43⁺ (green hexagons), sM44⁺ (yellow diamonds) are represented. Before leaves were exposed to ¹³CO₂ labeling at 1000 seconds, plants were exposed to the same ¹²CO₂ concentrations at which they were grown. The simultaneous labeling of the first carbon in the parent molecule (sM70⁺) and the fragment (sM42⁺) suggest that the first carbon contributing to the synthesis of isoprene comes from the M41⁺ fragment. However, while all of the isoprene molecules show the next two carbons labeled shortly after (sM71⁺ and sM72⁺), the next two carbons on the M41⁺ fragment (sM43⁺ and sM44⁺) are never fully labeled and may result from the incomplete labeling of pyruvate.</p

Rates of ¹³C transference between isotopomers as a function of CO₂ concentrations.

<p>Mean rates of loss (mean ± SEM) of the labeled isotopomers in units of molecules/cycle (cycle = detection every 30 seconds with a PTR-MS dwell time of 2 seconds) for both the parent molecule M69⁺ and its fragment M41⁺ (inset graph) among individuals grown at three different CO₂ concentrations (sub-ambient = 190 ppm (black triangles; dashed line); ambient = 400 ppm (dark gray circles; dashed line); elevated = 590 ppm (light gray squares; solid line)). In general, the photosynthetic pools of the leaves grown in sub-ambient CO₂ were labeled faster than leaves grown at ambient or elevated CO₂.</p

Proportion of isotopomers of the parent isoprene molecule labeled at the conclusion of the experiment.

<p>The mean proportion of the isotopomers of the parent isoprene molecule labeled at the conclusion of the experiment. Values were taken at stabilized conditions after ∼2 hr. Leaves grown in sub-ambient CO₂ demonstrated significantly lower proportions of total ¹³C labeling (M74⁺) compared to the high proportion of total labeled isoprene molecules from leaves grown in elevated CO₂.</p