Response of Selected Wetland Plant Species to Varying Levels of Npk Fertilizer Application

Abstract

Nutrient enrichment is one of the most serious threats to wetland ecosystems. Increased nutrient loading to wetlands often results in an increase in emergent plant biomass and a decrease in species diversity. While these patterns are broadly predictable, the underlying mechanisms are not well understood. Information is lacking, particularly concerning growth and physiology of wetland plant species in response to nutrient enrichment. The overall objective of this study was to investigate the growth and physiological response of selected wetland plant species occurring in Lake Victoria basin in Kenya to NPK fertilizer application. Cyperus esculentus L., Aframomum angustifolium (Sonn.) K. Schum. and Phragmites australis (Cav.) Trin, ex Steud) seedlings and cuttings were grown outdoors in 3.5 litre pots. The pots were arranged in a 4x3 completely randomised factorial design in the Botanic garden at Maseno University. Plants were provided with four NPK fertilizer dosage levels of 0 mg [no fertiliser added], 50 mg, 100 mg, 150 mg fertilizer [N- P- K, 10:26:10] per pot and replicated five times. The plants were irrigated daily with tap water for eight weeks. Data on growth were collected and included shoot height, stem diameter, number of leaves, leaf width and leaf length, leaf area, number of tillers, shoot and root fresh and dry weights. Physiological parameters determined included gas exchange, chlorophyll fluorescence, photosynthetic pigment concentration and leaf Nand P concentration. Data collected from the study was subjected to analysis of variance (ANOY A) using SAS statistical program. Separation of means was carried out to compare NPK fertilizer application treatments and species. NPK fertilizer application significantly (P :s 0.05) influenced both growth and physiological parameters investigated. Shoot height, stem diameter, leaf area, leaf number, shoot and root weights, and number of tillers per pot were all increased by NPK fertilizer treatments. The number of tillers per pot increased with increasing NPK fertilizer levels and differed significantly among the three plant species. Root-shoot ratios reduced with increasing NPK fertilizer levels in C. esculentus and A. angustifolium, while that of P. australis increased up to 100 mg treatment, then slightly reduced at 150 mg treatment. Cyperus esculentus had higher CO2 assimilation rate compared to A. angustifolium and. P. australis. Stomatal conductance increased with increase in NPK fertilizer application levels, but P. australis and A. angustifolium had higher stomatal conductance compared to C. esculentus. Transpiration rates increased with increase in NPK fertilizer application level, but A. angustifolium and C. esculentus had significantly higher transpiration rates compared to P. australis. Intercellular C02 concentration increased as NPK fertilizer levels increased in all the species tested. NPK fertilizer application at medium levels, 50 and 100 mg, increased electron quantum yield of PSII. Electron transport rate in A. angustifolium, and C. esculentus increased with increasing NPK fertilizer levels, while that of P. australis decreased. NPK fertilizer application at medium levels, 50 and 100 mg caused an increase in maximum fluorescence yield from PSII. Photosynthetic pigments concentration significantly increased in all the three species. Phragmites australis had significantly higher fo liar P and N concentration compared to C. esculentus and A. angustifolium with increasing NPK fertilizer levels. There were significant interactions between NPK fertilizer treatments and species in most of the parameters measured, except in carotenoids concentration, maximum fluorescence yield and steady state yield of PSII. Phragmites australis exhibited a greater ability to sequester high concentrations of nitrogen and phosphorus in foliar tissues. Among the three species C. esculentus was found to be better adapted to increasing NPK fertilizer application. The results from this study may be used to predict the response of these species to future wetland eutrophication and may be useful in designing constructed wetlands to purify wastewater and to control the invasive species. The study adds to a small but growing number of physiological comparisons between wetland plant species.