Acacia holosericea (Fabaceae) litter has allelopathic and physical effects on mission grass (Cenchrus pedicellatus and C. polystachios) (Poaceae) seedling establishment Muhammad S. Quddus A,B , Sean M. Bellairs A and Penelope A. S. Wurm A A Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT 0909, Australia. B Corresponding author. Email: Muhammad.Quddus@cdu.edu.au Abstract. Invasion of grass weeds is a major threat for ecosystems. Mission grass (Cenchrus pedicellatus and C. polystachios) vigorously competes with native vegetation and has become a serious problem in northern Australian savanna. A lower density of mission grass has been observed under the canopy of stands of native Acacia holosericea. We used a series of laboratory and shade house experiments to assess the potential for allelopathy and the role of litter on germination, emergence and seedling growth of these two species of mission grass. Different concentrations of aqueous leaf extract of A. holosericea were used to assess allelopathic effects on germination. Various depths and types of litter were used to investigate the allelopathic and physical effects of litter on emergence and growth of mission grass seedlings in the shade house. Results indicate that extracts did not affect germination of either species of mission grass but root growth of seedlings was affected. Emergence of seedlings in the shade house was affected by physical litter treatments but not by allelopathy. After emergence no negative effects on seedling growth were detected. Overall we found that there was no allelopathic effect on germination and that the negative effect on emergence was due to the physical properties of the litter. This effect on emergence increased with increasing depth of litter. Allelopathy slightly inhibited root growth but once seedlings emerged, litter tended to facilitate growth. This has implications for the ecological management of mission grass on disturbed lands, using strategies such as manipulation of litter cover through Acacia establishment. Additional keywords: allelopathy, emergence, germination, grass weed, suppression. Received 6 December 2013, accepted 24 April 2014, published online 24 June 2014 Introduction Leaves of the Australian native species Acacia holosericea A.Cunn. ex G.Don may have an allelopathic or physical impact on establishment of two exotic species of mission grass (Cenchrus pedicellatus (Trin.) Morrone and C. polystachios (L.) Morrone). These two African grass species were introduced to Australia as fodder crops and have become environmental weeds (Douglas et al. 2006; Miller 2006). These exotic grass species have the ability to change the ecosystem by altering the fire regimes and nutrient dynamics (Miller 2006; Brooks et al. 2010). Woodlands of the Northern Territory are dominated by Eucalyptus and Acacia trees. Field observations show that mission grass understorey is reduced below stands of A. holosericea trees in northern Australia. This may be due to allelopathy or physical effects of the leaf litter underneath these stands. If so then A. holosericea could potentially be used to control mission grass establishment on managed disturbed sites, such as on topsoil stockpiles of mine sites. Plant allelopathy is defined as any process by which secondary metabolites produced by plants influence the growth and development of agricultural and biological systems (Romeo 2000; Macías et al. 2007). Plants can favourably or adversely affect other plants through allelochemicals, which may be released directly or indirectly from living or dead plants (Putnam and Duke 1978; Rice 1979, 1984; Putnam and Tang 1986; Kruse Strandberg et al. 2000; Zimdahl 2007). Allelopathy works in three steps. In the first step phytotoxic chemicals are produced, in the second step these chemicals are transported from donor to recipient, and in the last step target plants are exposed to chemicals for a sufficient time and concentration to have an effect (Aldrich 1984; Lovett and Ryuntyu 1992). Many types of allelochemicals are released from plants, affecting germination and growth of target plants. Plant parts that have been implicated in the production of phytotoxic chemicals include roots rhizomes, stems, leaves, flowers, inflorescences and seeds, and their impact varies from species to species. These chemicals are released to the environment through root exudation and leachates from litter (Rice 1984). For example, extracts from leaves of Acacia nilotica and Eucalyptus rostrata reduced the growth and germination of Zea mays and Phaselulus vulgaris (El-Khawas and Shehata 2005). Aqueous leachates of Eucalyptus globulus leaves reduce the chlorophyll content in leaves of Eleusine coracana (Padhy et al. 2000). Eucalyptus baxteri releases allelochemicals which suppress CSIRO PUBLISHING Australian Journal of Botany, 2014, 62, 189–195 http://dx.doi.org/10.1071/BT13294 Journal compilation Ó CSIRO 2014 www.publish.csiro.au/journals/ajb