A comparison of growth responses between two species of Potamogeton with contrasting canopy architecture

This study examines the response of two species of Potamogeton (Family: Potamogetonaceae), with differing canopy architectures, to an artificial light gradient. Potamogeton ochreatus Raoul and P. tricarinatus F. Meull. and A. Bennett were grown in water with an attenuation coefficient of 8.8 m^-1 at various depths (10-81 cm) to give initial instantaneous irradiances between 0.4 and 460 µmol m^-2 s^-1. The average daily water column irradiances I_ave between the planting depth and the water surface, over 15 daylight hours, ranged from 3.8 to 18.4 mol m^-2. After about 80 days all P. tricarinatus plantings, except those at 81 cm, formed dense surface canopies which could access atmospheric CO₂ and had a maximum relative growth rate (70 ± 4 mg g^-1 per day) and net assimilation rates (0.1-0.9 mg cm^-2 day^-1) significantly above those of P. ochreatus (57 ± 3 mg g^-1 day^-1 and , 0.1-0.5 mg cm^-2 day^-1, respectively). P. ochreatus, which had a more diffuse and fully submersed habit, had a lower specific absorption coefficient (0.1 m^-2 g^-1) and average daily light compensation point (37 µmol m^-2 s^-1) than P. tricarinatus (0.9-1.2 m^-2 g^-1 and 57 µmol m^-2 s^-1, respectively), but had a relative growth rate of approximately 25 mg g^-1 per day even at an initial instantaneous irradiance of 0.4 µmol m^-2 s^-1. In addition, P. ochreatus allocated about 80% of its biomass to leaves and stems irrespective of the light climate, whereas only small P. tricarinatus plants preferentially allocated biomass above ground. As energy levels increased, P. tricarinatus allocated a greater proportion of biomass to tissues capturing the limiting resource, light. As the light climate became more favourable, P. tricarinatus allocated more biomass to the rhizome. However, when compared to a wider range of submerged macrophytes, the two species optimised their respective growth rates by reacting to varying I_ave in a similar way. Both responded to lower than optimal I_ave by increasing photosynthetic area and to above optimal values of I_ave by decreasing photosynthetic area. © 2001 Elsevier Science B.V.