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: Pot amogetonaceae), 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) a t various depths (10-81 cm) to give initial instantaneous irradiances betwe en 0.4 and 460 mu mol m(-2) s(-1). The average daily water column irradianc es ((I) over bar (ave)) between the planting depth and the water surface, o ver 15 daylight hours, ranged from 3.8 to 18.4 mol m(-2). After about 80 da ys all P. tricarinatus plantings, except those at 81 cm, formed dense surfa ce canopies which could access atmospheric CO2 and had a maximum relative g rowth 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, w hich had a more diffuse and fully submersed habit, had a lower specific abs orption 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 ap proximately 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, where as only small P. tricarinatus plants preferentially allocated biomass above ground. As energy levels increased, P. tricarinatus allocated a greater pr oportion of biomass to tissues capturing the limiting resource, light. ha t he light climate became more favourable, P. tricarinatus allocated more bio mass to the rhizome. However, when compared to a wider range of submerged m acrophytes, the two species optimised their respective growth rates by reac ting to varying (I) over bar (ave) in a similar way. Both responded to lowe r than optimal (I) over bar (ave) by increasing photosynthetic area and to above optimal values of (I) over bar (ave) by decreasing photosynthesis are a. (C) 2001 Elsevier Science B.V. All rights reserved.