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 mu mol m(-2) s(-1). The average daily water column irradiances ((I) over bar (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 mu mol m(-2) s(-1)) than P. tricarinatus (0.9-1.2 m(-2) g(-1) and 57 mu mol 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 mu mol 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. ha 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) over bar (ave) in a similar way. Both responded to lower than optimal (I) over bar (ave) by increasing photosynthetic area and to above optimal values of (I) over bar (ave) by decreasing photosynthesis area. (C) 2001 Elsevier Science B.V. All rights reserved.