Sphagnum fallax is generally found closer to the water table than S. nemoreum along the hummock—hollow gradient of an Adirondack bog. However, at low water contents S. fallax unexpectedly fixes carbon at higher rates than S. nemoreum, as we have previously reported. We here compare water and carbon balances for these species to resolve this puzzling contrast. S. nemoreum maintains a more favorable water balance by virtue of its greater water—holding capacity, lower rates of water loss from moist plants, and apparently more effective capillary conduction of water. The consequence is a significantly higher tissue water content in the field compared to adjacent patches of S. fallax for a wide range of dates and heights above the water table. We developed a simulation model, PEATMOSS, to evaluate the relative importance of contrasting physiological relationships and water balances of the two species. The model integrates physiological and field data to estimate 24—h carbon balance for these mosses in adjacent patches at two heights above the water table under different conditions. PEATMOSS estimated a greater carbon gain for S. fallax at the low site, where it is a clear dominant over S. nemoreum, and a markedly more favorable carbon balance for S. nemoreum at the hummock site, where S. nemoreum is common but S. fallax is infrequent. Morphological features of S. nemoreum which effect its more favorable water balance apparently confer an advantage over S. fallax when water availability is limited.

Chemical inhibition of the mitochondrial electron transport chain (mtETC) by antimycin A (AA) or the TCA cycle by monofluoroacetate (MFA) causes increased expression of nucleus-encoded alternative oxidase (AOX) genes in plants. In order to better understand the mechanisms of this mitochondrial retrograde regulation (MRR) of gene expression, constructs containing deleted and mutated versions of a promoter region of the Arabidopsis thaliana AOX1a gene (AtAOX1a) controlling expression of the coding region of the enhanced firefly luciferase gene were employed to identify regions of the AtAOX1a promoter important for induction in response to mtETC or TCA cycle inhibition. Transient transformation coupled with in vitro and in vivo assays as well as plants containing transgenes with truncated promoter regions were used to identify a 93 base pair portion of the promoter, termed the MRR region, that was necessary for induction. Further mutational analyses showed that most of the 93 bp MRR region is important for both AA and MFA induction. Sub-regions within the MRR region that are especially important for strong induction by both AA or MFA were identified. Specific mutations in a W-box and Dof motifs in the MRR region indicate that these specific motifs are not important for induction. Recent evidence suggests that MRR of AOX genes following inhibition of the mtETC is via a separate signaling pathway from MRR resulting from metabolic shifts, such as those that result from MFA treatment. Our data suggest that these signaling pathways share regulatory regions in the AtAOX1a promoter. Arabidopsis proteins interacted specifically with a probe containing the MRR region, as shown by electrophoretic mobility shift assays and Southwestern blotting. These interactions were eliminated under reducing conditions.

The vertical distributions of nine Sphagnum species and four associated mosses in two bogs (Adirondack Mountains, New York) were analyzed for interspecific and interbog differences. Based on 4300 sample points, Sphagnum mean heights ranged from 12 to 43 cm above the water table and the sequence of species was similar in both bogs (S. cuspidatum and S. majus) < (S. fallax and S. angustifolium) < (S. magellanicum, S. rubellum, and S. russowii) < (S. fuscum and S. nemoreum). However, at Bloomingdale Bog, a relatively dry mire with many well-developed hummocks and narrow hollows, mosses had significantly greater means (by 4-12 cm) than at Raybrook Bog, a relatively wet mire with wide Sphagnum carpets and fewer hummocks. South- and east-facing hummock slopes tended to be gentler than the north and west slopes, but the mosses’ mean heights did not vary with aspect. The abundance of hummocks at Bloomingdale seems responsible for not only the greater mean heights, but also for broader ranges and greater vertical overlap with other species. However, the ability of each species to dominate a certain array of height classes did not differ between bogs. Within each bog, hummock species tended to have larger vertical ranges than hollow species, implying a lesser ability of hollow mosses to tolerate the full range of conditions along the hummock-hollow gradient.

Sphagnum fallax and S. nemoreum differ in their vertical distribution above the water table on hummocks in Bloomingdale Bog (Adironkack Mountains, New York), and both exhibit decreasing water content with increasing height above the water table. We tested the prediction that S. nemoreum, a species better developed on hummock tops, would photosynthesize at greater rates when relatively dry than would S. fallax, which is more abundant in hollows and on hummock bases. Net photosynthesis declined sharply at tissue water contents (fresh—/dry—mass ratio) below 7.2 for S. fallax and 8.9 for S. nemoreum collected in spring. These values fell to 5.1 and 7.4, respectively, for plants collected in late summer. Thus both species exhibit adaptive seasonal acclimation, as reflected by the maintenance of relatively high net photosynthetic rates to lower water contents after a dry summer. Neither species showed significant response differences when collected from different heights above the water table. In direct contradiction to our hypothesis, Sphagnum fallax fixed carbon at greater rates than S. nemoreum at low water contents for both spring and summer collections. This physiological contrast appears inconsistent with observed field distribution patterns for these species.

Sphagnum fallax (Klinggr.) Klinggr., a moss growing in hollows close to the water table, is more desiccation tolerant than S. nemoreum Scop., a hummock former distributed high above the hollows. Sphagnum fallax recovered to a greater proportion of its predesiccation photosynthetic rate after one and five days of tissue dryness. Further, a greater percentage of S. fallax plants survived five and ten day periods at low tissue water contents. Longer desiccated periods and lower water contents during these periods decreased both photosynthetic recovery and survival.Water contents measured in Bloomingdale Bog (Adirondack Mountains, NY, USA) showed that S. fallax probably dries more frequently and for longer periods than S. nemoreum. These results support previous findings that the greater ability of S. nemoreum to remain moist in the field is the most important character in its success as a hummock former. Greater tolerance of desiccation helps S. fallax to compensate for its greater tendency to become dry, and is a key physiological feature enabling it to dominate hollows.