Growth and C/N metabolism of three submersed macrophytes in response to water depths

• Three common submersed macrophytes were grown at five water depths in situ in a lake.
• Low metabolism level in V. natans contributed to its high tolerance to deep depths.
• FAA contents increased with increasing water depth for all the plants.
• Carbon metabolic indices showed a unimodal curve along water depths in the leaves and stems.
• Concave curve of metabolite contents demonstrated complex mechanism of depth adaptation.

For submersed macrophytes, carbon (C) and nitrogen (N) metabolism is a central factor affecting growth and survival under low light availability. Three common submersed macrophytes, Vallisneria natans, Potamogeton maackianus, and Potamogeton malaianus, were grown at five water depths (1.0, 2.5, 4.0, 5.5, and 7.0 m) to explore mechanism of low light adaptation in terms of C/N metabolism by examining relative growth rate (RGR) and contents of soluble carbohydrate (SC), starch, total carbon (TC), free amino acid (FAA) and total nitrogen (TN) in leaf, stem and root of the plants. With increasing water depth, P. malaianus, P. maackianus and V. natans initiated dying at 4.0 m, 5.5 m and 7.0 m water depths, respectively. V. natans showed higher RGR than the other two species. For all the plants, the FAA contents increased with increasing water depth, except for the roots of P. maackianus. The TN contents showed a unimodal curve along water depths with the highest in moderate water depth, except for P. malaianus and the roots of V. natans. For V. natans and P. maackianus, the C metabolic indices (SC, starch, and TC) showed a unimodal curve along water depths with the lowest in moderate water depth in the leaves and stems, except for TC contents in the leaves of P. maackianus. Compared with P. maackianus and P. malaianus, lower SC and FAA contents and higher starch storage in V. natans contributed to its higher tolerance to deeper depths. The nonlinear changes in metabolite contents along water depths for V. natans and P. maackianus demonstrated complex mechanism for low light adaptation, and thus partly explained the wider ranges of colonizing water depths than P. malaianus.