2.1.7  Submerged aquatic macrophytes (SAM)

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Vascular plants often inhabit regions subject to tidal submergence while others carry out their entire life cycle under water. Examples of common submerged aquatic macrophytes are pond weeds and seagrasses. Once again, an evolutionary selective pressure for these plants has been availability of CO2. Low levels of dissolved CO2 are common in both inland and marine waters, particularly at more alkaline pH. In more productive inland lakes, CO2 content can vary enormously, requiring considerable flexibility in the actual mode of carbon acquisition. At high pH, HCO3 becomes the more abundant form of inorganic carbon, whereas dissolved CO2 will predominate at low pH (Section 18.2). Consequently, when SAM plants evolved from their C3 progenitors on land, there was some adaptive advantage in devices for CO2 accumulation because CO2 rather than HCO3 is substrate for Rubisco. The nature of this ‘CO2 pump’ and the energetics of carbon assimilation are not fully characterised in SAM plants but considerable CO2 concentrations do build up within leaves, enhancing assimilation and suppressing photorespiration (Badger 1987, and discussed further in Section 18.2).

Photosynthetic carbon gain — summary

Regardless of photosynthetic mode, and despite catalytic limitations, Rubisco is ubiquitous and remains pivotal to carbon gain in our biosphere. As a corollary, carbon loss via photorespiration is an equally universal feature of C3 leaves, and the evolution of devices that overcome such losses have conferred significant adaptive advantages to C4, CAM and SAM plants. Surprisingly, the biochemical sources for such massive dissipation of fixed carbon were not identified until biochemists combined forces in the early 1960s with leaf physiologists who were equipped with infrared CO2 analysers. An historic account of photosynthesis is given in Section 2.3.

 

 

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