Ogtr document

Section 3 Morphology

3.1 Plant morphology

The morphology of B. napus is very similar to that of B. juncea, with few distinctive characteristics. They are annual (spring cultivars) or biennial (winter cultivars) plants, between 70-170 cm and 120-210 cm in height, respectively. In Australia, they are winter-growing crops, sown in autumn and maturing in spring, with a growing season of 5-6 months (Edwards & Hertel 2011).

A well-developed plant produces between 10 and 15 leaves (Colton & Sykes 1992). The oldest leaves at the base are the largest, forming a rosette which is up to 50 cm wide. They are lobed, bristly, dark bluish green waxy leaves with a rounded tip, about 100-300 mm long and 50-150 mm wide. Lobes are often completely separated towards the petiole. The terminal lobe is usually the largest one. The middle and upper leaves are smaller (up to 100 mm long), spear-shaped and smooth, sessile (no petiole) and not lobed (Bailey 1976; Kershaw 1998). Two main differences exist between B. napus and B. juncea leaves: B. napus upper leaves clasp the stem while B. juncea’s do not. The leaves of B. juncea are also a lighter green and have indented vein patterns (Edwards & Hertel 2011).

Leaves are attached to the stem at a node. Plants have one main supporting stem, with about 15-30 nodes at a spacing of 5-10 cm. Secondary stems (branches) bud from the axil of the leaves. Branches will support 1-4 leaves. Stems are polygonal in cross-section, with longitudinal striations often present on upper parts of the stem. Stems are important for photosynthesis during pod and seed growth, as the leaves are entering senescence.

Both species have a taproot system to a maximum depth of about 120 cm (Duke 1983).

3.2 Reproductive morphology

B. napus and B. juncea flowers are bisexual and develop in indeterminate simple inflorescences (or racemes). The flowers are regular with 4 sepals and 4 petals (see Figure 4 below), and are 6-25 mm wide. The diagonally opposite petals form a cross, which is where the original family name, Cruciferae (now Brassicaceae) stems from (OECD 2012). Petals are usually 8-15 mm long, white to pale yellow for B. napus, bright yellow for B. juncea. Petal colour variation from white to dark yellow or even pink has been recorded in different cultivars (Downey & Rakow 1987). Each flower contains 6 stamens and a pistil of 2 carpels. Nectaries are found at the base of the stamens.

Seeds develop in 2-celled, elongated capsules called siliques (or pods). Pods are 6-9 cm long and 5 mm wide, with a beak 1-2 cm long. They are smooth, almost cylindrical, with a prominent mid-vein and normally contain 15-25 seeds (Bailey 1976; Edwards & Hertel 2011). In B. juncea, pods are held more upright than in B. napus.

Seeds are spherical and about 1 - 2 mm wide. B. juncea seeds are generally smaller than B. napus seeds (2.0-3.0 g/1000 seeds for B. juncea compared to 3.0-4.0 g/1000 seeds for B. napus). Seed colour varies from light yellow to brown and black. The seed coat is sometimes slightly pitted (Edwards & Hertel 2011).

Figure 5. Flowering raceme of B. napus canola. Photo courtesy of Brian Weir.

Section 4 Development

4.1 Reproduction

Both B. napus and B. juncea reproduce through seeds. There are no reports of vegetative reproduction under field conditions (in vitro asexual reproduction is possible, see Section 2.4.2 for more details).

4.2 Pollination and pollen dispersal

B. napus and B. juncea have bisexual and entomophilous flowers (i.e. they can be pollinated by insects). The two species are largely self-compatible^a and mainly selfpollinating, with a self- to cross-pollination ratio of about 70:30 (Downey & Rakow 1987; Treu & Emberlin 2000). The importance of cross-pollination varies depending on variety and on prevailing environmental conditions (namely weather conditions – wind and temperature - and presence of pollinators) (see Section 9 for more details).

Brassica pollen grains are heavy and slightly sticky (Treu & Emberlin 2000). They are produced in large quantities, with more than 9 kilos emitted per ha per day over a period of 4-5 weeks (Damgaard & Kjellsson 2005; Westcott & Nelson 2001). Pollen can be dispersed by physical contact between neighbouring plants. Hoyle et al. (2007) suggested neighbour-to-neighbour plant contact is an important mechanism of pollination in commercial fields, where plant densities are very high.

Because of their small size (30-40 µm wide), canola pollen grains can become airborne and be transported by wind. Timmons et al. (1995) described Brassica pollen as moving rapidly from the source and not remaining airborne for significant periods of time. Pollination can also be mediated by insects, with a positive impact on canola seed weight and oil quality (Bommarco et al. 2012; Gavloski 2012; Steffan-Dewenter 2003). B. napus and B. juncea flowers produce nectar with relatively high concentrations of sugars which makes them particularly attractive to feral and managed honey bees (Apis mellifera) (Husken & Dietz-Pfeilstetter 2007). Australian native bees are thought to play only a minor role in canola pollination. Native stingless bees are the only native bees used for crop pollination in Australia. As they are only found in tropical and subtropical areas, they are unsuitable for canola pollination (Cunningham et al. 2002). Hoverflies have been described as alternative pollinators but their impact on canola pollination also appears to be quite low compared to honey bees (Jauker & Wolters 2008). Bumblebees (Bombus spp.) play a major pollination role in Europe (Cresswell 1999). However, since bumblebees only occur in Tasmania and are geographically discrete, these insects play a minor role in the pollination of B. napus and B. juncea crops in Australia.

Brassica pollen has been described as being viable for up to 5 days under natural conditions, with a viability rate of 20% measured 72 hours after emission (Bots & Mariani 2005; Ranito-Lehtimäki 1995). Pollen viability varies with environmental conditions, particularly temperature and humidity. B. napus pollen longevity and germinability is reduced in case of high temperature stress (Young et al. 2004). Under controlled conditions, pollen sterility can be induced at flowering by a temperature regime of 32°C/26°C day/night, with plants grown throughout their life cycle at 27°C/17°C found to be almost totally sterile (Edwards & Hertel 2011). B. juncea pollen is still able to germinate after up to 4 hours at 60ºC (Rao et al. 1992).

See Section 9 for more details regarding pollen flow.