Study on the pH dependence of photosynthesis of Sphagnum spp.

1. Study on the pH dependence of photosynthesis of Sphagnum spp. 1.0 0.5 0.8 1.2 0.8 1.2 0.3 1.4 0.4 1.8 1.6 0.4 1.0 1.0 1.2 0.6 1.6 1.4 0.6 0.3 1.4 0.8 1.0 0.8 0.3 1.2 0.2 0.4 1.2 1.0 0.8 0.6 0.2 0.4 0.6 1.0 0.2 0.2 0.8 0.6 0.4 0.8 0.4 0.1 0.1 0.6 0.2 0.1 0.4 0.6 0.0 0.2 0.2 0.4 0.4 0.2 3.0 5.0 7.0 9.0 0.0 0.0 0.0 0.2 0.2 0.0 0.0 0.0 0.0 3.0 5.0 7.0 9.0 3.0 5.0 7.0 9.0 3.0 0.0 5.0 7.0 9.0 0.0 3.0 5.0 7.0 9.0 3.0 5.0 7.0 9.0 3.0 3.0 5.0 5.0 7.0 7.0 9.0 9.0 3.0 5.0 7.0 9.0 3.0 5.0 7.0 9.0 Gross photosynthetic rate Net photosynthetic rate Respiration rate Akira HARAGUCHI The University of Kitakyushu, Hibikino 1-1, Wakamatsu, Kitakyushu 808-0135, Japan E-mail: akhgc@env.kitakyu-u.ac.jp INTRODUCTION METHODS Sphagnum species usually show clear zonal distribution along environmental gradients. One of the factors that determine the distribution of Sphagnum species within a mire is water availability. Species differences in desiccation tolerance determine the distribution of each species along hydrological gradients in wetlands. Chemical properties of water are also thought to determine the distribution of Sphagnum species in mires. Among them, pH and nutrient concentration differ between ombrogenous and minerogenous sites. It has been clarified that growth of Sphagnum spp. respond to pH, and also clarified that the pH of the water is one of the factors that determine the distribution of Sphagnum species. Given the specific peat environment created by microtopography and community, we compared the photosynthetic rates between Sphagnum species from the aspect of peat chemical condition. The sampled plants were transferred to plastic vats 2 days before analysis. About 50 shoots with capitula (2.0 cm long) were arranged on the bottom of airtight 425-ml glass bottles, and pH-controlled buffer solution was added until only the capitula were above the surface. Buffers were prepared by mixing suitable ratios of 5.0 x 10-2 mol.dm-3 of Tris(hydroxymethyl) aminomethane and 5.0 x 10-2 mol.dm-3 of 2-(N-morpholino) ethane sulfonic acid. Air was introduced into the bottles at a rate of 0.3 L min-1 and the difference in CO2 concentrations between inlet and outlet was measured by Infrared Carbon Dioxide Analyser for 30 min. Six 60-W krypton lamps were used for the light source. The averaged photon flux density of PAR inside the chamber was around the light-saturation point of the plants (3.33 x 10-4 mol m-2 s-1 in each direction). After the photosynthetic rate was determined, bottles were covered with aluminum foil, and the change in CO2 was measured for 30 min. Then the plant material was dried at 110 C for more than 24 hours, and the dry weight was determined. Triplicate measurements were made for each species. Rates of photosynthesis and respiration are presented based on the dry weight. STUDY SITES The study area was in Cape Ochiishi Mire (43.16 N, 145.50 E), Nemuro City, eastern Hokkaido, Japan. The climate is cool temperate, characterized by frequent fog in summer (ca. 120 days year-1). Data of Sphagnum palustre and S. cuspidatum from Masukata Mire and Jyunsaiike Mire (Central Honshu) are presented as a reference. RESULTS and DISCUSSION Ochiishi Mire (ombrogenous mire) Sphagnum fuscum Sphagnum rubellum Sphagnum magellanicum Sphagnum girgensohnii Sphagnum nemoreum Sphagnum squarrosum CO2 exchange (mmol min-1 kg-1-DW) pH pH pH pH pH pH low habitat acidity high Masukata Mire (minerogenous mire) Jyunsaiike Mire (minerogenous mire) Sphagnum palustre Sphagnum cuspidatum Sphagnum palustre Sphagnum cuspidatum CO2 exchange (mmol min-1 kg-1-DW) pH pH pH pH high habitat acidity low high habitat acidity low S. fuscum and S. rubellum formed hummocks in open mire and had peaks of gross photosynthetic rate between pH = 5.0-7.0. Hummock forming species showed peaks of photosynthetic rate around the same pH of their habitats. S. magellanicum distributed in the lower mat and showed the peak at higher pH of 8.0. S. girgensohnii grew at the margin of the P. glehnii forest and showed the maximum photosynthetic rate at pH = 7.2. Hollow in Picea glehnii forest is highly acidic compared as the open mire in the Ochiishi mire, thus S. nemoreum and S. squarrosum grew in the acidic centre of the P. glehnii forest and showed the highest photosynthetic rate at pH = 3.8. Gross photosynthetic rate of hummock forming species of S. palustre in the higher pH range was lower than that in the lower pH range both in the two minerogenous mires. Peat in hollow in minerogenous mires are less acidic compared as hummocks. Photosynthetic rate of S. cuspidatum did not decrease in high pH solution. Thus hollow species adapted to their higher pH habitats in these minerogenous mires. Although peat chemistry showed different trends along micro-topography gradient between mires, photosynthetic capacity of Sphagnum spp. highly adapted to their own native habitats.