Interactive effects of N and P on growth but not on resource allocation of Canna indica in wetland microcosms Zhenhua Zhang a , Zed Rengel a, *, Kathy Meney b a Soil Science and Plant Nutrition, School of Earth and Geographic Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia b Syrinx Environmental Pty Ltd., 12 Monger Street, Perth, WA 6000, Australia 1. Introduction The use of constructed wetlands for wastewater treatment has undergone dramatic development since the 1990s (Kadlec and Knight, 1996; Sundaravadivel and Vigneswaran, 2001). In constructed wetlands, plants not only take up nutrients from wastewater and substrates, but also act as catalysts for purification reactions by increasing the environmental diversity in the rhizosphere, and promoting a variety of chemical and biological reactions that enhance purification (Jenssen et al., 1993). A well-developed vegetation also shades out algae that cause a clogging effect at the surface of constructed wetlands (Cheng et al., 2002). The function of plants in nutrient stripping is a dynamic one, and requires an understanding of plant tolerances and nutrient requirements to optimize nutrient uptake. Plants not only grow at a slow rate at low nutrient supply compared with high nutrient supply, but also increase their biomass allocation to roots (Poorter and Nagel, 2000) and reduce the nutrient concentrations in the biomass (Aerts and Chapin, 2000). Like all plants, wetland plants require many macro- and micronutrients in proper proportions for healthy growth. Nitrogen (N) and phosphorus (P) are key nutrients in the life cycles of wetland plants (EPA, 2000). Therefore, the proper N and P availability are of principal concern in the growth of wetland plants in constructed wetlands. The tropical and subtropical plant species Canna indica Linn. is often grown in urban streets and parks because of its long Aquatic Botany 89 (2008) 317–323 ARTICLE INFO Article history: Received 10 June 2007 Received in revised form 3 December 2007 Accepted 7 March 2008 Available online 14 March 2008 Keywords: Biomass Constructed wetland Canna indica Interaction Nutrient Wastewater ABSTRACT The interactive effects of three levels of N (mM) (low 0.36, medium 2.1 and high 6.4) and two levels of P (mM) (low 0.10 and high 0.48) on growth and resource allocation of Canna indica Linn. were studied in wetland microcosms. After 91 days of plant growth, there was a significant interactive effect of N and P on plant growth, but not on resource allocation (except for allocation of N to leaves and allocation of P to the stems). The plant growth positively responded to the relatively higher nutrient availability (taller plants with more stems, leaves and flowers), but the growth performance was not significantly different between the medium N-low P and high N-low P treatments. At high P, the total biomass in the high N was about 51% higher than that in the medium N and about 348% higher than that in the low N. The growth performance was related to the physiological responses. The photochemical efficiency (F v /F m ) increased from 0.843 to 0.855 with an increase in N additions. The photosynthetic rate increased from 13 to 16 mmol m À2 s À1 in the low P levels and from 14 to 20 mmol m À2 s À1 in the high P levels with an increase in N applications, but significant difference was only between the low and medium N levels, regardless of the P levels. The tissue concentrations of N increased with an increase in N applications and decreased with an increase in P additions, whereas reverse was true for tissue concentrations of P. The highest concentrations of N and P in leaves were 30.8 g N kg À1 in the high N-low P treatment and 4.9 g P kg À1 in the low N-high P treatment. The percent biomass allocation to aboveground tissues in the high N was nearly twice that in the low N treatments. The N allocation to aboveground tissues was slightly larger in high N than in low N treatments, whereas the P allocation to aboveground tissues increased with an increase in the N addition. Although some patterns of biomass allocation were similar to those of nutrient allocation, they did not totally reflect the nutrient allocation. These results imply that in order to enhance the treatment performance, appropriately high nutrient availability of N and P are required to stimulate the growth of C. indica in constructed wetlands. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +61 8 6488 2557; fax: +61 8 6488 1050. E-mail addresses: zhenhuaz70@hotmail.com (Z. Zhang), zed.rengel@uwa.edu.au (Z. Rengel). Contents lists available at ScienceDirect Aquatic Botany journal homepage: www.elsevier.com/locate/aquabot 0304-3770/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2008.03.007