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

Abstract

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 CO 2 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.