Successful cotton growth in most soils depends on the interaction with mycorrhizal fungi (Australian Cotton Cooperative Research Centre 2002b; Cotton Catchment communities CRC 2002; Nehl & Allen 2004; Youssef & Mankarios 1974). The fungal species interacting with cotton roots, for example Glomus mosseae, grow intercellularly in the root cortex. They form arbuscules, highly branched, tree-like structures in intimate contact with the plant’s plasma membrane within the cortex cells of the plant. The arbuscules are characteristic of this type of endophytic symbiosis called vesicular arbuscular mycorrhizae (VAM) and are the sites of mineral exchange from the fungus to the plant and carbohydrate exchange from the plant to the fungus. For the plant, improvement of phosphate uptake is the main advantage in engaging in VAM (reviewed in Strack et al. 2003). VAM fungi are widespread in the environment.
The VAM fungal species Glomus mosseae, as many other VAM fungi, is capable of colonising a variety of plant species. For example, Giovannetti et al. (2004) demonstrated that an isolate of G. mosseae is able to colonise cotton (G. hirsutum), eggplant (Solanum melongena), carrot (Daucus carota), lettuce (Lactuca sativa) and leek (Allium porrum).
VAM fungi can influence the severity of plant diseases on cotton. Liu (1995) reported mutual inhibition of infection of cotton after simultaneous inoculation with VAM fungi and V. dahliae as well as reduced disease incidence and disease indices of plants sequentially inoculated with AM fungi and V. dahliae. In another report, Zhengjia and Xiangdong (1991) showed reduced severity of Fusarium wilt in G. hirsutum plants inoculated with G. mosseae.
Section 8 Weediness 8.1 Weediness status on a global scale
An important indicator of potential weediness of a particular plant is its history of weediness in any part of the world and its taxonomic relationship to declared weeds (Panetta 1993; Pheloung 2001). Cotton has been grown for centuries throughout the world without any reports that it is a serious weed. Worldwide, there are approximately 50 species of Gossypium (Craven et al. 1994; Fryxell 1992), none of which is listed as a serious weed (Groves et al. 2003; Holm et al. 1997; Holm et al. 1979; Randall 2002).
Modern cotton cultivars do not possess any of the attributes commonly associated with problematic weeds, such as seed dormancy, persistence in soil seed banks, germination under adverse environmental conditions, rapid vegetative growth, a short life cycle, very high seed output, high seed dispersal and long-distance dispersal of seeds (Keeler 1985; Keeler 1989).
8.2 Weediness status in Australia
Cotton is not considered to be a serious weed in Australia (Groves et al. 2000; Groves et al. 2003). No Gossypium species are recognised as problematic weeds in Australia, either agriculturally or environmentally (Lazarides et al. 1997; Tothill et al. 1982). Cotton has no relatives that are problematic weeds (Keeler et al. 1996), although locally G. sturtianum can be weedy (Lazarides et al. 1997).
In conservation areas, for example National Parks, where weeds may be defined as any naturalised alien/non native plant, cotton (G. hirsutum and G. barbadense) in the form of isolated populations may be considered as a weed (reviewed in Eastick 2002). G. hirsutum is for example listed under the category ‘moderate to minor weed usually in small infestations’ in Kakadu National Park (Cowie & Werner 1987; Storrs 1996). However, when grown in a glasshouse, seeds from these populations tend to have poor architecture and produce small bolls and seed with sparse, grey lint. They also produce mainly tufted rather than fuzzy seeds, which is a strong indication that they are not derived from modern cultivars which are all fuzzy seeded cotton plants (Curt Brubaker and Lyn Craven, CSIRO, pers. comm., 2005).
Tufted seeded G. hirsutum plants were originally used when hand delinting was required, before the advent of mechanical saw gins in the late 1700s. Tufted seeded G. hirsutum plants were subsequently replaced by fuzzy seeded varieties with better lint characteristics and disease resistance. It seems likely, therefore, that many naturalised G. hirsutum populations result from attempts in the early 1800s to establish cotton industries in northern QLD and the NT (Curt Brubaker and Lyn Craven, CSIRO, pers. comm., 2005) and there is no evidence that these isolated G. hirsutum populations are invasive or have become problematic weeds.
A small number of other G. hirsutum plants appear to be of more recent origin, but none seem to have originated from the current commercial types of G. hirsutum that have been cultivated since the 1970s (for example Eastick 2002). These naturalised G. hirsutum plants are confined to areas of disturbed land with at least a seasonal water supply; typical locations are above the high tide mark on beaches and near river banks in northern Australia.
Even though G. hirsutum has been grown previously in a number of places in northern Australia, only isolated G. hirsutum populations have been able to naturalise. For example, G. hirsutum has not persisted in the environment in the Ord River Irrigation Area following the abandonment of G. hirsutum farms, with actively growing G. hirsutum plants in the fields, in the 1960s and 70s (Eastick 2002).
Naturalised G. barbadense has been found in QLD and NT and data from the Australian Virtual Herbarium confirm that these specimens were collected primarily from the eastern coastal regions of QLD and northern areas of NT (ALA 2010). Unfortunately, few ecological data accompany the herbarium records. It is difficult, therefore, to assess the abundance or ‘weediness’ of G. barbadense in Australia, although specimen notes suggest that several of the collections were of ‘escaped’ or ‘naturalised’ plants growing in habitats such as roadsides and drainage lines. As G. barbadense is not regarded as a problematic weed, it is probable that the herbarium specimens highlight the existence of occasional individuals, and/or small ephemeral populations, rather than a significant weed problem.
Share with your friends: |
