The Biology of Triticum aestivum L. (Bread Wheat)

Other biotic interactions

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7.3 Other biotic interactions

Endophytic actinobacteria has been isolated from surface sterilized healthy wheat plants (Coombs & Franco 2003). Streptomyces caviscabies/Streptomyces setonii­-like and Streptomyces galilaeus isolates have been identified as the major components of the actinobacteria cultures isolated from root tissues (Coombs & Franco 2003). These isolates lacked pathogenicity gene and did not produce a toxin and the authors suggest that there is an important and possibly beneficial relationship between the plant and the microorganisms (Coombs & Franco 2003). It has been suggested that these endophytic actinobacteria have a role in disease resistance and maintaining the health of the plants (Conn & Franco 2004). Fungal endophytes have also been isolated from wheat cultivars (Coombs & Franco 2003).

Section 8 Weediness

Wheat shares some characteristics with known weeds, such as self- or wind-pollination and the ability to germinate or to produce some seed in a range of environmental conditions. However, it lacks most characteristics that are common to many weeds, such as the ability to produce a persisting seed bank, rapid growth to flowering, continuous seed production as long as growing conditions permit, very high seed output and seeds adapted for short and long range dispersal (Keeler 1989).

During domestication of the modern wheat plant, characteristics that benefited farmers were selected. This process also greatly reduced the ability of cultivated wheat to survive without the intervention of farmers (Eastham & Sweet 2002). Loss of seed shattering in wheat was selected in order to increase the ease of harvest, but this trait reduces the capacity for natural seed dispersal. Increased grain size was selected to improve germination rates under tillage, but this trait correlates with lower total number of seeds produced. Grains without hulls were selected to improve ease of threshing, however, hulled varieties have more reliable germination under environmental stresses (Purugganan & Fuller 2009).

8.1 Weediness status on a global scale

An important element in predicting weediness is a plant’s history of weediness in any part of the world (Panetta 1993; Pheloung 2001). Wheat has been grown for centuries throughout the world without any reports that it is a serious weed pest. There are a number of reports of wheat becoming naturalised in areas where it is not a native species, including California (Calflora 2015) and the Canadian prairies and North American central Great Plains (Harker et al. 2005 and references therein).

8.2 Weediness status in Australia

Wheat is not classified as a weed of national interest in Australia (Department of Environment National Weeds Lists; accessed 18 Jan 2016). In natural ecosystems, wheat is classified as a naturalised plant known to be a minor problem warranting control at three or fewer locations within a state or territory (Groves et al. 2003). It is considered a minor problem in a few natural environments in Tasmania (Glover 2002). In agricultural ecosystems, wheat is classified as a naturalised plant known to be a minor problem warranting control at four or more locations within a state or territory (Groves et al. 2003). Elsewhere, it is suggested that although volunteer wheat grows where cultivated seed is dropped, it is probably not truly naturalised in Victoria or South Australia (Walsh & Entwisle 1994; Jessop et al. 2006).

Some other species of Triticum and all species of the closely related genus Aegilops are prohibited for import into Australia as they have been assessed as posing a high risk of becoming weeds in Australia (Australian Biosecurity Import Conditions website; accessed 18 Jan 2016).

8.3 Weediness in agricultural ecosystems

Wheat is a naturalised non-native species present in agricultural ecosystems in all Australian states and territories with the exception of the Northern Territory (Groves et al. 2003). There are a few reports of volunteer wheat in the Northern Territory found along roadsides and in home gardens (Atlas of Living Australia, accessed 4 March 2015); these likely represent transient rather than naturalised populations. Wheat is known to be a minor problem with control warranted at four or more locations within a state or territory. Volunteers occur in follow-on crops and if not controlled can harbor disease (Groves et al. 2003).

Volunteer wheat is a recognised weed in agricultural fields in the Canadian prairies and North American central Great Plains (Harker et al. 2005 and references therein). A three year study of volunteer spring wheat emergence across the prairies found that most volunteer wheat emerged during the first year following dispersal. The overall volunteer wheat emergence rate in continuous cropping fields, in the first year of the study, was 3.3 plants m2. At the end of the three year monitoring period no viable wheat seeds were detected in the soil seed bank. From these results the authors suggest that volunteer wheat will not become a major agricultural problem (Harker et al. 2005).

8.4 Weediness in natural ecosystems

Wheat is not considered a problem weed of natural ecosystems (see Section 8.2).

8.5 Control measures

In Australia, volunteer wheat is often controlled to reduce the risk of pests and diseases surviving between seasons. The most effective control technique is herbicide spraying, though heavy grazing or tillage can also be effective (GRDC 2009). As of 18 Jan 2016, there were 266 herbicide products registered by the APVMA for use on volunteer wheat (APVMA website). These include herbicides from mode-of-action groups A, B, I, L, M and N. The National Variety Trials website has links to information on testing of wheat varieties for tolerance to commercially used herbicides, listing whether or not applications of herbicides at recommended or above recommended rates has an effect on wheat yield.

8.6 Weed risk assessment of wheat

The weed risk potential of wheat has been assessed (Appendix A) using methodology based on the Australia/New Zealand Standards HB 294:2006 National Post-Border Weed Risk Management Protocol. The National Post-Border Weed Risk Management Protocol evaluates weediness by relating the likelihood of risk to the feasibility of control methods for weeds (Auld 2012). The Protocol has been used as the basis for several weed management systems, for example, the South Australian weed risk management guide (Virtue 2004). These properties relate to invasiveness, impacts and potential distribution. The distribution of wheat is driven by economics, as well as factors such as climate and soil suitability.

In summary, as a volunteer (rather than a crop) wheat is considered to:

  • have a low ability to establish amongst existing plants

  • have a low tolerance to average weed management practices in cropping and intensive land uses

  • have a short time to seeding (less than one year)

  • have a low annual seed production and a low ability for volunteers to establish in any land use

  • not reproduce by vegetative means

  • be unlikely to undergo long distance spread by natural means

  • be commonly spread long distance by people from dryland and irrigated cropping areas, as well as from intensive land uses

  • have a limited ability to reduce the establishment or yield of desired plants

  • have a low ability to reduce the quality of products or services obtained from all land use areas

  • have a low potential to restrict the physical movement of people, animals, vehicles, machinery and/or water

  • have a low potential to negatively affect the health of animals and/or people

  • be able to act as a reservoir for a range of pests and pathogens

  • have a low effect upon soil nutrients, salinity, stability and the water table.

This is consistent with previous assessments of wheat in Australia summarised in Section 8.2, and provides a baseline for the assessment of GM wheat.

Section 9 Potential for Vertical Gene Transfer

Vertical gene transfer is the transfer of genetic material from parent to offspring by reproduction, either sexual or asexual. Gene transfer can be intraspecific, interspecific or intergeneric. This section deals with gene transfer by sexual reproduction only, as wheat does not reproduce by any asexual mechanism. Gene transfer requires sympatry of the cultivated and wild species, synchronous pollen emission of the donor and stigma receptivity of the recipient, as well as viability of the progeny (Zaharieva & Monneveux 2006).

The likelihood of wheat gene transfer and establishment of subsequent hybrids depends on a series of factors summarized by (Gustafson et al. 2005). Plant mating system and pollen characteristics are the two main factors influencing gene flow between populations (Waines & Hegde 2003). Details of pollen production and outcrossing are given in Section 4.2 including information about pollen shedding rates, pollen movement and outcrossing rates. Varying estimates are given for these parameters based on factors such as variety and environmental conditions.

The environmental conditions needed for maximising wheat pollen-mediated gene flow can be summarised as follows. A hot, dry period prior to flowering would have to be followed by cool temperatures, with high relative humidity and strong, prevalent winds at anthesis. This would allow maximum flower opening, pollen dispersal, pollen viability and stigma receptivity (Gustafson et al. 2005). Wheat has been described as a low risk crop for both intra- and interspecific gene flow (Eastham & Sweet 2002).

9.1 Intraspecific crossing

T. aestivum is a cultivated species, with no known wild or weedy strains (see Section 8). Thus, the potential for gene transfer to wild T. aestivum populations is low. However, cultivated varieties within the genome lineage of T. aestivum can successfully be cross-bred, naturally or under controlled conditions. Gene transfer may occur more frequently, as the parent lines are sexually compatible and may be grown in proximity to one another (Waines & Hegde 2003). The progeny is fertile, with fully developed endosperm (OECD 1999; Matus-Cádiz et al. 2004).

Intraspecific pollen-mediated gene flow has been studied at the field and commercial scales in Canada (Matus-Cádiz et al. 2004; Matus-Cádiz et al. 2007). The impact of 16 to 30 ha pollen blocks on neighbouring fields was examined within a 2.7 km radius (for the 16 ha pollen block) and 10 km radius (for the 30 ha pollen block) from the central pollinator source. The authors showed that intraspecific gene flow could be detected at trace rates (≤ 0.01 %) up to 300 m (for a 16 ha pollinator block) or 2.75 km (for a 30-ha pollinator block) (Matus-Cádiz et al. 2004; Matus-Cádiz et al. 2007). Gene flow was dependent on environmental conditions, with higher gene flow observed in cooler, more humid and wetter conditions (Matus-Cádiz et al. 2004). The authors suggest that the 0.01 % trace rate observed can be considered a worst-case scenario and a minor contribution to gene flow between cultivars (Matus-Cádiz et al. 2007). However, they conclude that, based on these results, a tolerance level of 0 % GM wheat in non-GM grains is unrealistic (Matus-Cádiz et al. 2004; Matus-Cádiz et al. 2007). Other authors have also concluded that a guarantee of zero gene flow is not possible for any plant that sheds pollen (Waines & Hegde 2003). A 1 to 5 % tolerance level was considered more realistic (Matus-Cádiz et al. 2004; Matus-Cádiz et al. 2007). Isolation distances of up to 45 m were recommended for wheat to reduce pollen-mediated gene flow to predictable levels (Hucl & Matus-Cádiz 2001; Hanson et al. 2005).

The rate of intraspecific pollen-mediated gene flow in south-eastern Australia has been shown to be lower than that observed overseas (Gatford et al. 2006). Using a series of small pollinator blocks, which has been shown to underestimate pollen flow (see above) these authors measured intraspecific gene flows far lower than those observed in similar conditions overseas, with a maximum rate of 0.055 % at 8 m from the pollen source (Gatford et al. 2006). This low gene flow could be explained by environmental and morphological factors. Low relative humidity and warmer temperatures could have accelerated pollen desiccation. Hot, dry weather conditions have been shown to lower pollen viability to less than 15 minutes (D'Souza 1970). It has also been suggested that as most Australian elite cultivars have a closed flower structure, floral morphology of the recipient could play a role in the gene flow rates observed (Gatford et al. 2006). Based on these results, they recommend a 12 m separation between GM and non-GM crops (Gatford et al. 2006).

Another study in Switzerland examined outcrossing between GM and non-GM wheat of the same and different lines. This study found that outcrossing from non-GM and GM lines to GM and non-GM lines varied between parental lines, with distance and with the location of crops in relation to one another (direction). In one experiment outcrossing rates declined from 0.7 % at 0.5 m to 0.03 % at 2.5 m (Rieben et al. 2011). A case-by-case approach was recommended in determining the likelihood of outcrossing between GM and non-GM crops due to the range of factors which might influence outcrossing rates (Rieben et al. 2011).

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