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Figure 2. Effects of moisture and temperature on storage of canola. Source: Ref. 24.

No spoilage for 150 days

Figure 2. Effects of moisture and temperature on storage of canola. Source: Ref. 24.

Grain moisture content (%)

Spoilage will occur

No spoilage for 150 days

Conditioning required ripening, cultivar and seeding rate, have been shown to affect the chlorophyll level (26-28). Swathing at 35 to 37% seed moisture followed by drying is effective in reducing substantially the chlorophyll content of canola seeds (29).

BREEDING CANOLA FOR IMPROVED YIELD AND QUALITY—ROLE OF BIOTECHNOLOGY

Important objectives in canola breeding are both agronomic (seed yield, winter and frost hardiness, disease resistance, early maturity, herbicide tolerance, and resistance to shattering) and quality (oil and protein content and composition) traits (9-11,30,31). Although seed yield is one of the most desirable attributes, improvement through selection is a difficult task because of the strong influence from environmental factors.

Oil, being the most valuable component of the seed, is deposited in the form of small lipid droplets in the cytoplasm of embryonal cells. Protein bodies (storage proteins) are also stored in the cytoplasm of these cells (32). In Brassica spp., yellow-seeded cultivars have thinner seed coats and thus higher oil and protein content than seeds of darker color (31). It is, therefore, of great interest to producers and crushers to develop yellow-seeded forms or cultivars with increased seed size.

Oil quality is determined by the fatty acid composition. Fatty acid biosynthesis in oilseeds is carried out by assemblies of enzymes located in specific cell compartments (viz., plastids and endoplasmic reticulum of the cytoplasm) (33). Fatty acid synthesis via the fatty acid synthetase (FAS) complex in the plastids produce saturated aliphatic fatty acids (lauric, C12:0; palmitic, C16:0; stearic, C18:0) (Fig. 3). Production of the various saturated fatty acids depends on the presence and activity of specific thioesterases. In most oilseed plants, C18:0 is the primary product of the FAS pathway and it, in turn, is largely desaturated to oleic acid (C18:ln-9) by a specific (stearoyl-ACP) desaturase. Further desaturation of C18:ln-9 to linoleic acid (C18:2n-6) and linolenic acid (18:3n-3) and chain elongation to eicosaenoic acid (C20:ln-9) and erucic acid (C22:ln-9) occurs in the cytoplasm (8,33). Hence, changes in the fatty acid profile in Brassica can be achieved by modification of the biosynthetic pathways (Fig. 3). Changes in single genes can have profound effects on the fatty acid composition of the oil. For example, the amount of erucic acid depends on a series of alleles (at two gene loci in B. napus and a single locus in B. rapa) that influence the elongation of C18:ln-9; by varying the alleles present, it is possible to have erucic acid levels between 0 and over 50%. Interest in erucic acid for use as a feedstock in the oleo-chemical industry has stimulated interest in producing rapeseed cultivars with very high levels of erucic acid. In general, Brassica spp. do not incorporate erucic acid into the sn-2 position of the triacylglycerol molecule. However, identification of a genotype of B. oleracea, which can accumulate erucic acid in the sn-2 position, provides a possible route to B. napus lines with enhanced erucic acid levels (34). Another approach being investigated is the cloning of genes for sn-2 acyltransferases that recognize erucic acid, for example, from Limnanthes spp., and then inserting these genes into rapeseed (34).

Another oil quality breeding objective of canola has been to reduce the level of linolenic acid (C18:3n-3) from 8 to 10% to less than 2% for the purpose of improving oxidative stability of the oil. The first low-linolenic acid cultivar (Stellar), which was introduced in 1987, contained less than 3% linolenic acid (9,35). The oil from this cultivar exhibited improved stability at 60°C and showed negligible changes in sensory and chemical indices of rancidity, compared with regular canola oil (36); improved odor scores from frying tests were also reported for the low-linolenic canola oil (37). Cultivars with even lower levels of linolenic acid (eg, Apollo, avg. 1.7% C18:3n-3) have recently been released (9). Other modifications also have been made to the fatty acid composition of Brassica spp., for example, the development of cultivars with 86% oleic acid (C18:ln-9). These modified canola oils were produced in response to the demand for a frying oil with a low level of saturated and trans fatty acids and a high stability to oxidative changes without having to be hydrogenated. Another example is the development of cultivars with 10 to 12% palmitic (C16:0) plus palmitoleic (C16:ln-7) acids (15) in order to make margarines exclusively from low erucic acid rapeseed oils. For such products, the tendency to develop large crystals on storage would be diminished.

The application of biotechnology techniques (genetic engineering) to Brassica spp. has resulted in some novel oils. Scientists at Calgene Inc. have produced both a high stearic acid and a high lauric acid cultivar of rapeseed/canola using transgenic techniques. The high-lauric canola (>40% C12:0), which is intended for use in the manufacture of detergents and surfactants, was produced by adding the lauroyl-ACP thioesterase gene from the California Bay laurel plant into canola (38,39). The high-stearic canola was produced by inserting the antisense gene construct for stearoyl-ACP desaturase into canola (40). This high-stearic canola could be used in the production of margarine or the manufacture of cocoa butter substitutes (39). Hence, it is reasonable to expect the development of a broad range of rapeseed/canola oils having fatty acid composition appropriate for specific applications (Table 1).

In contrast to oil, relatively little attention has been paid to the protein quality of the meal. Indeed, the reduction of glucosinolate levels in the meal has been the primary objective in the breeding programs. Recessive alleles, responsible for the low-glucosinolate characteristics, have been introduced from B. napus var. Bronowski into cultivars of B. napus and B. rapa by backcrossing (43); several gene loci are involved in the inheritance of glucosinolate content (30,31). Additional improvement in the nutritional value and palatability of canola meal would also result if the 1.0 to 1.5% sinapine and phytic acids present in Brassica oilseeds are reduced or eliminated by breeding.

There is great interest among breeders and producers in developing hybrid cultivars of canola for increased yield. Cytoplasmic male sterility (CMS), self-incompatability (SI), and transgenic techniques are among the systems being investigated as means of producing hybrid seeds (10,11,31,39,44). For the CMS system, three inbred lines are used: (1) the female parent (male-sterile line owing to cytoplasmic components—incompatibility of "foreign" mitochondria and the nuclear material), (2) a maintainer line

[Plastid]

Acetyl - CoA

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