Plasticity in the photosynthetic carbon metabolism of submersed aquatic macrophytes

Abstract

This review details the physiological and biochemical adaptations that enable submersed aquatic macrophytes to manage constraints associated with photosynthesis under water. The constraints are dissolved inorganic carbon (DIC), light and temperature, though pH, O 2 and mineral nutrients may be factors. Submersed aquatic macrophytes are characterized by low photosynthetic rates, with low light requirements and very high apparent K m (CO 2/HCO 3 −) values. These features are mainly caused by low DIC diffusion rates. High pH (10) may limit photosynthesis by decreasing free CO 2. Temperature responses vary, partly owing to growth adaptations, with photosynthesis reported under ice and at 35°C. Even though the photosynthetic carbon reduction (PCR), and photorespiratory carbon oxidation (PCO) cycles operate in submersed aquatic macrophytes, they possess mechanisms which preclude classifying them with C 3 or C 4 photosynthesis. These include: utilization of HCO 3 − ions, C 4 acids, and sediment CO 2. A major feature of submersed aquatic macrophytes is their plasticity, which is seen in variable CO 2 compensation points that indicate the photorespiratory (PR) state. Unlike unicellular phototrophs, submersed aquatic macrophytes usually exist in an O 2-sensitive, high-PR (C 3-like) state; but under daytime stress conditions of high light, temperature, O 2 and low CO 2 a low-PR state is induced. The active uptake of HCO 3 − ions, and/or fixation initially by phosphoenolpyruvate carboxylase in a C 4-like system, but without Kranz anatomy, concentrates CO 2 internally. The result is a low-PR state, with reduced O 2 inhibition of ribulose bisphosphate carboxylase-oxygenase. Not all species use HCO 3 − ions effectively. Some fix CO 2 at night in a CAM-like system, while other, rosette forms take sediment CO 2, via roots, and pipe it in lacunae to the leaves. These mechanisms all conserve carbon and improve DIC availability.