Richard E. Dickson

The effect of elevated carbon dioxide and ozone on leaf- and branch-level photosynthesis and potential plant-level carbon gain in aspen

Abstract.Two aspen (Populus tremuloides Michx.) clones, differing in O3 tolerance, were grown in a free-air CO2 enrichment (FACE) facility near Rhinelander, Wisconsin, and exposed to ambient air, elevated CO2, elevated O3 and elevated CO2+O3. Leaf instantaneous light-saturated photosynthesis (PS) and leaf areas (A) were measured for all leaves of the current terminal, upper (current year) and the current-year increment of lower (1-year-old) lateral branches. An average, representative branch was chosen from each branch class. In addition, the average photosynthetic rate was estimated for the short-shoot leaves. A summing approach was used to estimate potential whole-plant C gain. The results of this method indicated that treatment differences were more pronounced at the plant- than at the leaf- or branch-level, because minor effects within modules accrued in scaling to plant level. The whole-plant response in C gain was determined by the counteracting changes in PS and A. For example, in the O3-sensitive clone (259), inhibition of PS in elevated O3 (at both ambient and elevated CO2) was partially ameliorated by an increase in total A. For the O3-tolerant clone (216), on the other hand, stimulation of photosynthetic rates in elevated CO2 was nullified by decreased total A.

Genetic control of responses to interacting tropospheric ozone and CO2 in Populus tremuloides

We exposed trembling aspen (Populus tremuK&s Michx.) clones differing in tropospheric ozone (O

Partitioning of 14C-labeled photosynthate to allelochemicals and primary metabolites in source and sink leaves of aspen: evidence for secondary metabolite turnover

tolerance in various opentop chamber studies for rhree growing seasons, and examined the effects of 03, COz, and O3 + CO2 on growth and physiological processes. Ozone in the range of 80 ppm hr (Sum 00) per growing season decreased height, diameter, and stem and leaf biomass slightly in a tolerant clone but severely in a sensitive clone. Elevated CO2 (150 ppm over ambient) did not compensate for the O3 effects. Antioxidant enzyme analysis showed elevated SOD levels in the tolerant clone but not in the sensitive clone following O3 exposure. Northern blot analysis indicated that the chloroplastic and cytosolic CwZn SOD’s were significantly increased in response to O3 in the tolerant but not the sensitive clone. Currently, we are conducting molecular analysis to determine the functional significance of SOD’s in regulaling O3 tolerance in aspen. 0 1997 Elsevier

Growth and crown architecture of two aspen genotypes exposed to interacting ozone and carbon dioxide

Abstract Theories on allelochemical concentrations in plants are often based upon the relative carbon costs and benefits of multiple metabolic fractions. Tests of these theories often rely on measuring metabolite concentrations, but frequently overlook priorities in carbon partitioning. We conducted a pulse-labeling experiment to follow the partitioning of 14CO2-labeled photosynthate into ten metabolic pools representing growth and maintenance (amino acids, organic acids, lipids plus pigments, protein, residue), defense (phenolic glycosides, methanol:water and acetone-soluble tannins/phenolics), and transport and storage (sugars and starch) in source and importing sink leaves of quaking aspen (Populus tremuloides). The peak period of 14C incorporation into sink leaves occurred at 24 h. Within 48 h of labeling, the specific radioactivity (dpm/mg dry leaf weight) of phenolic glycosides declined by over one-third in source and sink leaves. In addition, the specific radioactivity in the tannin/phenolic fraction decreased by 53% and 28% in source and sink leaves, respectively. On a percent recovery basis, sink leaves partitioned 1.7 times as much labeled photosynthate into phenolic glycosides as source leaves at peak 14C incorporation. In contrast, source leaves partitioned 1.8 times as much 14C-labeled photosynthate into tannins/phenolics as importing sink leaves. At the end of the 7-day chase period, sink leaves retained 18%, 52%, and 30% of imported 14C photosynthate, and labeled source leaves retained 15%, 66%, and 19% of in situ photosynthate in metabolic fractions representing transport and storage, growth and maintenance, and defense, respectively. Analyses of the phenolic fractions showed that total phenolics were twice as great and condensed tannins were 1.7 times greater in sink than in source leaves. The concentration of total phenolics and condensed tannins did not change in source and sink leaves during the 7-day chase period.

Fixation patterns of 14C within developing leaves of eastern cottonwood

To study the impact of ozone (O3) and O3 plus CO2 on aspen growth, we planted two trembling aspen clones, differing in sensitivity to O3 in the ground in open-top chambers and exposed them to different concentrations of O3 and O3 plus CO, for 98 days. Ozone exposure (58 to 97 microl l(-1)-h. total exposure) decreased growth and modified crown architecture of both aspen clones. Ozone exposure decreased leaf, stem, branch, and root dry weight particularly in the O3 sensitive clone (clone 259). The addition of CO2 (150 microl l(-1) over ambient) to the O3 exposure counteracted the negative impact of O3 only in the O3 tolerant clone (clone 216). Ozone had relatively little effect on allometric ratios such as, shoot/root ratio, leaf weight ratio, or root weight ratio. In both clones, however, O3 decreased the shoot dry weight, shoot length ratio and shoot diameter. This decrease in wood strength caused both current terminals and long shoots to droop and increased the branch angle of termination. These results show that aspen growth is highly sensitive to O3 and that O3 can also significantly affect crown architecture. Aspen plants with drooping terminals and lateral branches would be at a competitive disadvantage in dense stands with limited light.

Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests.

SummaryIndividual leaves of eastern cottonwood (Populus deltoides), representing an ontogenetic series from leaf plastochron index 0.0 to 8.0, were fed 14CO2 photosynthetically and then harvested at times ranging from 15 to 1440 min. The lamina of each fed leaf was sectioned from tip to base into 5 parts, and each part was quantitatively assayed for 14C activity. In young leaves, the percentage of the total 14C fixed (expressed in dpm/mg of dry leaf tissue) was high in the lamina tip and decreased almost linearly toward the base. With increasing leaf age, the percentage of 14C fixed decreased in the lamina tip and increased in the base. The relative activity in mature leaves was almost uniform throughout the lamina. No differences were detected in the 14C distribution patterns within leaves over the time series.On the basis of the data presented and of anatomical observations of developing cottonwood leaves, the hypothesis that the precociously mature lamina tip may provide photosynthates to the still-expanding lamina base was shown to be invalid. It is concluded that bidirectional transport in a developing cottonwood leaf results from simultaneous import to the immature basal region and export from the mature tip.

Interacting Effects of Multiple Stresses on Growth and Physiological Processes in Northern Forest Trees

Three young northern temperate forest communities in the north-central United States were exposed to factorial combinations of elevated carbon dioxide (CO2) and tropospheric ozone (O3) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO2 enhanced ecosystem C content by 11%, whereas elevated O3 decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO2 and O3. Treatment effects on ecosystem C content resulted primarily from changes in the near-surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r2 = 0.96). Elevated CO2 enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m−2) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O3 lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (ΔNPP/ΔN) decreased through time with further canopy development, the O3 effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O3 and less soil C from 0.1 to 0.2 m in depth under elevated CO2. Overall, these results suggest that elevated CO2 may create a sustained increase in NPP, whereas the long-term effect of elevated O3 on NPP will be smaller than expected. However, changes in soil C are not well-understood and limit our ability to predict changes in ecosystem C content.

Genetic implications for forest trees of increasing levels of greenhouse gases and UV-B radiation

Global climate chagnge is a complex and controversial subject, both technically and politically. Recently, the Intergovernmental Panel on Climate Change (IPCC) of the United Nations concluded that “the balance of evidence suggests a discernible human influence on global climate,” and that “further accumulation of greenhouse gases will commit the earth irreversibly to global climate change with its consequent ecological, economic, and social disruption” (Houghton et al., 1996; Brown et al., 1997; Kerr, 1997). One of the concerns is that changing climate will have major effects on future forest composition, productivity, sustainability, and biological as well as genetic diversity (Houghton et al., 1996).

Phloem Translocation from a Leaf to Its Nodal Region and Axillary Branch in Populus deltoides

Globally, the environment is changing and deteriorating as greenhouse gases such as carbon dioxide (CO2) and tropospheric ozone (O3) continue to increase at a rate of about 1% per year (Keeling et al. 1995, Chameides et al. 1995). The increase in these gases is directly related to anthropogenic activities (Chameides et al. 1995, Mooney et al. 1991) and is likely inducing subtle but substantial changes in the earth’s surface temperatures and weather (Cha 1997; Martin 1996). In addition, anthropogenic activities have been linked to decreasing levels of stratospheric 03 and concomitant increases in ultraviolet-B radiation (UV-B) passing through to the earth’s surface. Thus, man is creating a constantly changing global environment for which tree breeders must attempt to develop genotypes and races suitable for future forests. The purpose of this paper is to examine the genetic implications for forest trees of increasing greenhouse gases and UV-B and to suggest where tree breeders need to be concerned about the changing environment. Since very little is known about the impact of greenhouse gases and/or UV-B on genetic population structure, we will not discuss population structure or the effects of selection on population structure.

Tropospheric O3 moderates responses of temperate hardwood forests to elevated CO2: a synthesis of molecular to ecosystem results from the Aspen FACE project

Microautoradiography was used to follow the movement of 14C through the nodal regions and in the stems of young Populus deltoides Bartr ex Marsh plants when either entire laminae were photosynthetically fed 14CO2 or cut petioles were fed 14C-labeled amino compounds The leaf/branch gap of the central (C) leaf trace is filled with both thick- and thin-walled, heavily pitted parenchymatous cells that retain their protoplasts. The C-trace translocates little 14C to mature cells of the gap region because it lacks differentiated rays at this level in the stem However, the C-trace contributes 14C to the cambium-like region that adds cells to the gap. The C-trace also transfers 14C laterally to phloem of the branch traces in the nodal region. The branch traces in turn translocate 14C according to sink demands acropetally in the branch, or basipetally in the stem, or laterally to adjacent stem traces, or inward to the gap region The 14C translocated inward via the rays is presumably deposited in the gap cells, where it accumulates as stored starch during predormancy No evidence was found that gap cells functioned as transfer cells. Little cross transfer occurred between unrelated leaf traces in the stem Thus, gap cells at nodes other than the treated node received 14C only when a labeled trace lay immediately contiguous to branch traces at that node For example, the left trace of the treated leaf contributed 14C to the branch traces and nodal region but not to the leaf trace situated three nodes below Amino compounds fed through cut petioles transported 14C primarily upward in the transpiration stream. Xylem-to-phloem transfer via the rays occurred at all levels in the stem, but it was particularly pronounced at nodal junctures No labeled amino acids were transported into the gap region