Crassulacean acid metabolism enhances underwater photosynthesis and diminishes photorespiration in the aquatic plant Isoetes australis

• Published in: The New phytologist 2011-04, Vol.190 (2), p.332-339
• Main Authors: Pedersen, Ole, Rich, Sarah Meghan, Pulido, Cristina, Cawthray, Gregory Robert, Colmer, Timothy David
• Format: Article
• Language: English
• Subjects: Acidity; Acids; Air; Aquatic Organisms - drug effects; Aquatic Organisms - physiology; Aquatic Organisms - radiation effects; Aquatic plants; Australia; Bryophyta - drug effects; Bryophyta - metabolism; Bryophyta - physiology; Bryophyta - radiation effects; Carbon dioxide; Carbon Dioxide - pharmacology; Carbon dioxide fixation; Carbon sequestration; Cell Respiration - drug effects; Cell Respiration - radiation effects; Crassulacean acid metabolism; crassulacean acid metabolism (CAM); Floodwater; Geologic Sediments; Isoetes; isoetids; leaf malate; Leaves; Light; Malate; Malates - metabolism; Metabolism; Organic acids; Oxygen - pharmacology; Photorespiration; Photosynthesis; Photosynthesis - drug effects; Photosynthesis - radiation effects; Plant Leaves - drug effects; Plant Leaves - physiology; Plant Leaves - radiation effects; Plants; Stomata; Submerged aquatic plants; submergence tolerance; Tidal pools; Time Factors; Titrimetry; Underwater; underwater photosynthesis; vernal pool; Water; wetland plant
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1111/j.1469-8137.2010.03522.x

Underwater photosynthesis by aquatic plants is often limited by low availability of CO₂, and photorespiration can be high. Some aquatic plants utilize crassulacean acid metabolism (CAM) photosynthesis. The benefits of CAM for increased underwater photosynthesis and suppression of photorespiration were evaluated for Isoetes australis, a submerged plant that inhabits shallow temporary rock pools. Leaves high or low in malate were evaluated for underwater net photosynthesis and apparent photorespiration at a range of CO₂ and O₂ concentrations. CAM activity was indicated by 9.7-fold higher leaf malate at dawn, compared with at dusk, and also by changes in the titratable acidity (μmol H⁺ equivalents) of leaves. Leaves high in malate showed not only higher underwater net photosynthesis at low external CO₂ concentrations but also lower apparent photorespiration. Suppression by CAM of apparent photorespiration was evident at a range of O₂ concentrations, including values below air equilibrium. At a high O₂ concentration of 2.2-fold the atmospheric equilibrium concentration, net photosynthesis was reduced substantially and, although it remained positive in leaves containing high malate concentrations, it became negative in those low in malate. CAM in aquatic plants enables higher rates of underwater net photosynthesis over large O₂ and CO₂ concentration ranges in floodwater, via increased CO₂ fixation and suppression of photorespiration.