On the ecological and evolutionary significance OF BRANCH AND LEAF MORPHOLOGY IN aquaticSphagnum(Sphagnaceae)

Abstract

In aquatic environments difiusivity is low and CO2availability can limit plant growth. The hypothesis that natural selection has favored morphological features that reduce resistance to diffusion of CO2was tested using three phylogenetically independent species pairs from the genusSphagnum(S. macrophyllumand 5.strictum;S. portoricenseandS. papillosum;andS. trinitenseand S.recurvum). The aquatic (former) and the nonaquatic (latter) species were grown submerged and emerged in a common garden and used for studies of form and function. Aquatic taxa all had similar morphological features that included larger, thinner branch leaves arranged at lower densities and photosynthetic cells more greatly exposed at the leaf surface. The relationship between observed branch and leaf morphology and boundary layer resistance in the S.trinitense–S. recurvumspecies pair was assessed by measuring diffusion and convection of ions onto nickel‐plated models in a variable‐speed electrochemical fluid tunnel. For all flow speeds and orientations, the aquaticS. trinitensemodel had thinner boundary layers than the nonaquaticS. recurvummodel. Analysis of stable isotopes of carbon from the growth experiment corroborated results from the fluid‐tunnel experiments. The aquatic taxa all had lower δ13C values when grown submerged compared to their nonaquatic pair with the exception of the nonaquaticS. strictum, which was removed due to low growth rates. These results indicate that aquatic species did experience lower overall resistance to CO2uptake than nonaquatic taxa. Our observations suggest that aquatic habitats do select for morphological features that lower resistance to gas exchange.

Evolutionary and ecophysiological significance of sugar utilization by the peat moss Sphagnum compactum (Sphagnaceae) and the common charophycean associates Cylindrocystis brebissonii and Mougeotia sp. (Zygnemataceae)

“…Both Simola (1969) and Kajita et al (1987) indicate that the ability to use exogenous sugars is likely widespread among peat mosses. The sugar utilization trait should be particularly valuable to submerged Sphagnum species that inhabit peatland hollows, which are demonstrably carbon-limited ( Rice and Schuepp, 1995 ;Rice and Giles, 1996 ;Rice, 2000 ). Our results suggest that utilization of exogenous sugars could provide substantial benefi t even to hummock and lawn-forming mosses that extend above the water table and hence are able to access CO 2 in air, because even hummock-forming species may be carbon limited during wet conditions ( Smolders et al, 2001 ).…”

“…Our evidence that air-grown peat mosses engage in mixotrophy raises an issue with respect to the use of 13 C to evaluate species-specifi c responses to carbon availability constraints. Although the value of 13 C studies is undisputed (e.g., Proctor et al, 1992 ;Rice and Schuepp, 1995 ;Rice, 2000 ), not present. Sphagnum compactum ' s sugar preferences are consistent with the observation that the liverwort Plagiochila aspleniodes utilizes fructose and sucrose, but at about half the rate of glucose uptake ( Suleiman and Lewis, 1980 ).…”

Moss δ13C: an accurate proxy for past water environments in polar regions

“…Cell wall thickness was constant across water environments in the field for C. purpureus and B. pseudotriquetrum suggesting that the higher δ 13 C values displayed in wet environments were independent of morphological changes. Interestingly, continual submergence has been found to produce thinner boundary layers in Sphagnum species, which in turn may reduce resistance to CO 2 diffusion into the cell (Rice & Schuepp, ). We found that S. antarctici generated thinner cell walls under wet compared to dry growth conditions, implying there should be greater diffusional resistance (and less negative δ 13 C) in dry environments (the opposite of that found).…”

“…For example, morphological traits – such as cell wall thickness – may differ between growth environments and thus influence the assimilation of carbon into cells (Waite & Sack, ). Likewise, plant growth form may directly or indirectly influence diffusional resistance in cells by either altering boundary layers surrounding plant surfaces (Rice & Schuepp, ) or affecting the development of surface water film, respectively. Significant species‐specific δ 13 C variations have already been documented (Zhang et al ., ; Pancost et al ., ; Menot‐Combes et al ., ; Loader et al ., ; Moschen et al ., ; Brader et al ., ) especially in Antarctic mosses (Galimov, ; Wasley et al ., ).…”

Interactions between the mat‐forming algaDidymosphenia geminataand its hydrodynamic environment

“…However, the same traits tend to reduce the efficiency of nutrient acquisition by maximizing submergence in viscous boundary layers, where advective transport is limited (Raven 1992; Larned et al 2004). Traits that enhance nutrient acquisition in oligotrophic water include protrusion into turbulent flow (Larned et al 2004), wide‐spaced branching (Steinman et al 1992; Rice and Schuepp 1995), and flapping (Stevens et al 2003), yet these increase the risk of structural failure (Schutten and Davy 2000; Puijalon et al 2005).…”