Text by Kristiina Mark
Bryophytes – polyphyletic group consisting liverworts, hornworts and mosses – and lycophytes (also known as ‘fern allies’) are considered the earliest living relatives to ancient land plants that first migrated from aquatic environment to land. Both, bryophytes and lycophytes, are known for their simple structure, reproduction by spores, and tendency to prefer moist and shady environment. Being the second largest group of land plants (topped only by angiosperms), bryophytes contribute substantially to primary productivity in high latitude ecosystems where conditions for vascular plants are unsuitable.
Photosynthesis is the key process in primary metabolism and is often limited by CO2 concentration at carboxylation sites in chloroplast, which is determined by CO2 diffusion through plant tissues. Bryophytes and lycophytes show low photosynthetic capacity compared to vascular plants, however, contribution of specific constraining factors remained unexamined. Global collaborative study between seven research institutes from six countries (Spain, Australia, Chile, Estonia, USA, Indonesia) hypothesized that bryophyte and lycophyte lower rate of photosynthesis is largely due to constrained CO2 diffusion through photosynthetic tissues, specifically, limited by nonstomatal diffusion conductance (gnsd).
Research concluded that low photosynthesis rate in bryophytes and lycophytes is indeed related to their specific anatomical characteristics, especially their very thick cell walls and low chloroplast exposure to intercellular air spaces. Photosynthesis in mosses was mostly limited by gnsd and in lycophytes co-limited by gnsd and leaf photochemistry. These results support the suggested phylogenetic trend towards increasing photosynthesis and link to increasing stomatal and mesophyll/nonstomatal conductance.
Citation: Carriquí, M., Roig-Oliver, M., Brodribb, T. J., Coopman, R., Gill, W., Mark, K., Niinemets, Ü., Perera-Castro, A. V., Ribas-Carbó, M., Sack, L., Tosens, T., Waite, M., & Flexas, J. (2019). Anatomical constraints to non-stomatal diffusion conductance and photosynthesis in lycophytes and bryophytes. New Phytologist, https://doi.org/10.1111/nph.15675. (link to full text)
Photosynthesis in bryophytes and lycophytes has received less attention than terrestrial plant groups. In particular, few studies have addressed the non-stomatal diffusion conductance to CO2 (gnsd) of these plant groups. Their lower photosynthetic rate per leaf mass area at any given nitrogen concentration as compared to vascular plants suggested a stronger limitation by CO2 diffusion. We hypothesized that bryophyte and lycophyte photosynthesis is largely limited by low gnsd. Here we studied CO2 diffusion inside the photosynthetic tissues and its relationships with photosynthesis and anatomical parameters in bryophyte and lycophyte species in Antarctica, Australia, Estonia, Hawaii and Spain. On average, lycophytes and, specially, bryophytes had the lowest photosynthetic rates and non-stomatal diffusion conductance reported for terrestrial plants. These low values are related to their very thick cell walls and their low exposure of chloroplasts to cell perimeter. We conclude that the reason why bryophytes lie at the lower end of the leaf economics spectrum is their strong non-stomatal diffusion conductance limitation to photosynthesis, which is driven by their specific anatomical characteristics.