Will photosynthetic capacity of aspen trees acclimate after long-term exposure to elevated CO2 and O3?

Photosynthetic acclimation under elevated carbon dioxide (CO(2)) and/or ozone (O(3)) has been the topic of discussion in many papers recently. We examined whether or not aspen plants grown under elevated CO(2) and/or O(3) will acclimate after 11 years of exposure at the Aspen Face site in Rhinelander, WI, USA. We studied diurnal patterns of instantaneous photosynthetic measurements as well as A/C(i) measurements monthly during the 2004-2008 growing seasons. Our results suggest that the responses of two aspen clones differing in O(3) sensitivity showed no evidence of photosynthetic and stomatal acclimation under either elevated CO(2), O(3) or CO(2) + O(3). Both clones 42E and 271 did not show photosynthetic nor stomatal acclimation under elevated CO(2) and O(3) after a decade of exposure. We found that the degree of increase or decrease in the photosynthesis and stomatal conductance varied significantly from day to day and from one season to another.

Leaf size and surface characteristics of Betula papyrifera exposed to elevated CO2 and O3.

Betula papyrifera trees were exposed to elevated concentrations of CO(2) (1.4 x ambient), O(3) (1.2 x ambient) or CO(2) + O(3) at the Aspen Free-air CO(2) Enrichment Experiment. The treatment effects on leaf surface characteristics were studied after nine years of tree exposure. CO(2) and O(3) increased epidermal cell size and reduced epidermal cell density but leaf size was not altered. Stomatal density remained unaffected, but stomatal index increased under elevated CO(2). Cuticular ridges and epicuticular wax crystallites were less evident under CO(2) and CO(2) + O(3). The increase in amorphous deposits, particularly under CO(2) + O(3,) was associated with the appearance of elongated plate crystallites in stomatal chambers. Increased proportions of alkyl esters resulted from increased esterification of fatty acids and alcohols under elevated CO(2) + O(3). The combination of elevated CO(2) and O(3) resulted in different responses than expected under exposure to CO(2) or O(3) alone.

Effects of decadal exposure to interacting elevated CO2 and/or O3 on paper birch (Betula papyrifera) reproduction.

We studied the effects of long-term exposure (nine years) of birch (Betula papyrifera) trees to elevated CO(2) and/or O(3) on reproduction and seedling development at the Aspen FACE (Free-Air Carbon Dioxide Enrichment) site in Rhinelander, WI. We found that elevated CO(2) increased both the number of trees that flowered and the quantity of flowers (260% increase in male flower production), increased seed weight, germination rate, and seedling vigor. Elevated O(3) also increased flowering but decreased seed weight and germination rate. In the combination treatment (elevated CO(2)+O(3)) seed weight is decreased (20% reduction) while germination rate was unaffected. The evidence from this study indicates that elevated CO(2) may have a largely positive impact on forest tree reproduction and regeneration while elevated O(3) will likely have a negative impact.

Wood properties of trembling aspen and paper birch after 5 years of exposure to elevated concentrations of CO(2) and O(3).

We investigated the interactive effects of elevated concentrations of carbon dioxide ([CO(2)]) and ozone ([O(3)]) on radial growth, wood chemistry and structure of five 5-year-old trembling aspen (Populus tremuloides Michx.) clones and the wood chemistry of paper birch (Betula papyrifera Marsh.). Material for the study was collected from the Aspen FACE (free-air CO(2) enrichment) experiment in Rhinelander, WI, where the saplings had been exposed to four treatments: control, elevated [CO(2)] (560 ppm), elevated [O(3)] (1.5 x ambient) and their combination for five growing seasons. Wood properties of both species were altered in response to exposure to the treatments. In aspen, elevated [CO(2)] decreased uronic acids (constituents of, e.g., hemicellulose) and tended to increase stem diameter. In response to elevated [O(3)] exposure, acid-soluble lignin concentration decreased and vessel lumen diameter tended to decrease. Elevated [O(3)] increased the concentration of acetone-soluble extractives in paper birch, but tended to decrease the concentration of these compounds in aspen. In paper birch, elevated [CO(2)] decreased and elevated [O(3)] increased starch concentration. The responses of wood properties to 5 years of fumigation differed from those previously reported after 3 years of fumigation.

Carbon gain and bud physiology in Populus tremuloides and Betula papyrifera grown under long-term exposure to elevated concentrations of CO2 and O3.

Paper birch (Betula papyrifera Marsh.) and three trembling aspen clones (Populus tremuloides Michx.) were studied to determine if alterations in carbon gain in response to an elevated concentration of CO(2) ([CO(2)]) or O(3) ([O(3)]) or a combination of both affected bud size and carbohydrate composition in autumn, and early leaf development in the following spring. The trees were measured for gas exchange, leaf size, date of leaf abscission, size and biochemical characteristics of the overwintering buds and early leaf development during the 8th-9th year of free-air CO(2) and O(3) exposure at the Aspen FACE site located near Rhinelander, WI. Net photosynthesis was enhanced 49-73% by elevated [CO(2)], and decreased 13-30% by elevated [O(3)]. Elevated [CO(2)] delayed, and elevated [O(3)] tended to accelerate, leaf abscission in autumn. Elevated [CO(2)] increased the ratio of monosaccharides to di- and oligosaccharides in aspen buds, which may indicate a lag in cold acclimation. The total carbon concentration in overwintering buds was unaffected by the treatments, although elevated [O(3)] decreased the amount of starch by 16% in birch buds, and reduced the size of aspen buds, which may be related to the delayed leaf development in aspen during the spring. Elevated [CO(2)] generally ameliorated the effects of elevated [O(3)]. Our results show that both elevated [CO(2)] and elevated [O(3)] have the potential to alter carbon metabolism of overwintering buds. These changes may cause carry-over effects during the next growing season.

Impacts of elevated atmospheric CO2 and O3 on paper birch (Betula papyrifera): reproductive fitness.

Atmospheric CO2 and tropospheric O3 are rising in many regions of the world. Little is known about how these two commonly co-occurring gases will affect reproductive fitness of important forest tree species. Here, we report on the long-term effects of CO2 and O3 for paper birch seedlings exposed for nearly their entire life history at the Aspen FACE (Free Air Carbon Dioxide Enrichment) site in Rhinelander, WI. Elevated CO2 increased both male and female flower production, while elevated O3 increased female flower production compared to trees in control rings. Interestingly, very little flowering has yet occurred in combined treatment. Elevated CO2 had significant positive effect on birch catkin size, weight, and germination success rate (elevated CO2 increased germination rate of birch by 110% compared to ambient CO2 concentrations, decreased seedling mortality by 73%, increased seed weight by 17%, increased root length by 59%, and root-to-shoot ratio was significantly decreased, all at 3 weeks after germination), while the opposite was true of elevated O3 (elevated O3 decreased the germination rate of birch by 62%, decreased seed weight by 25%, and increased root length by 15%). Under elevated CO2, plant dry mass increased by 9 and 78% at the end of 3 and 14 weeks, respectively. Also, the root and shoot lengths, as well as the biomass of the seedlings, were increased for seeds produced under elevated CO2, while the reverse was true for seedlings from seeds produced under the elevated O3. Similar trends in treatment differences were observed in seed characteristics, germination, and seedling development for seeds collected in both 2004 and 2005. Our results suggest that elevated CO2 and O3 can dramatically affect flowering, seed production, and seed quality of paper birch, affecting reproductive fitness of this species.

Identification and analysis of safener-inducible expressed sequence tags in Populus using a cDNA microarray.

Safeners are the chemicals used to protect plants from detrimental effects of herbicides, but their mode of action at the molecular level is not well understood. As an initial step towards understanding the molecular mechanism of safener action in trees, homologous genes in hybrid poplar (Populus nigra x Populus maximowiczii) that were induced by a safener were identified. We here describe the identification of differentially expressed genes in Populus that are induced by Concep-III, a herbicide safener. Expressed sequence tags (ESTs) enriched for transcriptionally induced genes were isolated by suppressive subtractive hybridization (SSH). The SSH library cDNA inserts were used to construct a cDNA microarray for high-throughput validation of the up-regulated expression of safener-induced genes. Single-pass and partial sequences of 1,344 safener-induced ESTs were assembled into 418 singletons and 328 clusters, but the putative functions of almost 53% of the ESTs are not known. Genes encoding proteins involved in all three different phases of safener action, viz., oxidation, conjugation, and sequestration, were found in the SSH library. Almost 75% of genes that showed greater than 2-fold expression upon safener treatment were redundant in the SSH library. The expression pattern for selected genes was validated by reverse transcription-polymerase chain reaction. A few safener-induced genes that were not previously reported to be induced by safeners, but which may have a role in herbicide metabolism, were identified. The newly identified genes could have potential for application in genetic engineering of plants for herbicide detoxification and tolerance.

Genome walking of large fragments: an improved method.

The PCR-based genome walking method has been commonly used to isolate upstream regions from known cDNA sequences. The limitation of this technique is based on the location of the restriction site upstream to the gene-specific primer in the genome; hence, different restriction enzymes have to be used to isolate larger upstream fragments. In this paper, we present the advantageous use of partial and size-selected DNA as templates for genome walking, in isolating larger upstream fragments. We have successfully tested this approach to isolate larger upstream fragments using the FailSafe PCR System. Use of partial digestion and size selection can provide better chances in obtaining larger flanking regions of known DNA sequence, when compared to use of total digested DNA.