The experiments were carried out at the Sari Agricultural Experimental Station, 25 km, East of Sari, Iran in 2001–2002 and 2002–2003. The experimental design was a split plot with three replications. The three tillage practices, including no- (NT), minimum (MT) (residue return with two disk) and conventional tillage (CT) (residue return with plow and one disk) were allocated to main plots, and the combination of canola autumn genotypes (Hyola 401 (V1), PF (V2)) and sowing dates (including Sep. 8 and 23, and Oct. 7) were factorially randomized to sub-plots.

The 10 rows in each subplot were 7 m long, with 16 cm interspacing. The plots were 3 m apart and seeds planted at a 5 cm distance on the rows. Hence, the required area for each plot was 11.2 m². There were 18 plots in each replicate producing a 2000 m² experimental area.

Soil physical and chemical properties were determined. Soil texture (silty clay loam) was determined by the hydrometry method [23]. Acidity (7.7) of a saturated paste and electrical conductivity (0.6 dS/m) of a saturated extract were also measured [24]. Organic carbon (C) (0.63%) and total nitrogen (N) (0.055%) were measured using wet oxidation [25], and the Kjeldahl method [26], respectively. Phosphorous (P) (31 ppm) and potassium (K) (400 ppm) were determined by sodium bicarbonate extraction [27] and flame photometer (emission spectrophotometry), respectively [28]. Climate data, including temperature, humidity, precipitation, wind and degree-days, for the complete year are presented in Table 1.

Fertilization was performed according to the soil testing analyses. P, at 59 kg P₂O₅, and K, at 100 kg K₂O, fertilizers were completely applied at seeding according to canola fertilization requirements, while 30% (of the total of 150 kg) of N was applied at seeding and the remaining parts were applied at the stem and flowering producing stages.

Plants were thinned at the six-leaf stage. The field preparation, fertilization and chemical weeding were conducted in the August of each year. Surface irrigation was performed twice at seeding, with a 7–9-day interval, to let the seeds grow at their highest germination. The field was also irrigated at stemming and flowering along with N fertilization, and twice at the pod producing and grain filling stages.

Plants were harvested, when 40–50% of the seeds in the main pods and primary branches turned brown. After harvesting, the oil content and the fatty acid composition were determined. Oil content was determined by nuclear magnetic resonance analyser (Newport of North America, 1986). Fatty acid composition, including saturated (sum of palmitic (C_16:0), and stearic acid (C_18:0)) and unsaturated (sum of oleic (C_18:1), linoleic (C_18:2) and linolenic acid (C_18:3)) acids, was analysed using gas chromatography of methyl esters [29,20] by the following procedure.

Fifty milligram of extracted oil was saponified with 5 ml of methanolic NaOH (2%) solution by refluxing for 10 min at 90 °C. After addition of 2.2 ml BF₃-methanolic, the sample was boiled for 5 min. The FAMEs were extracted from a salt-saturated mixture with hexane. The FAMEs were then analyzed using a gas chromatograph (UNICAM model 4600, UK) coupled with a FID detector. The column used for fatty acid separation was a fused silica BPX70 column, 30 m × 0.22 mm i.d. × 25 μm film thickness (from SGE, UK). The oven temperature was held at 180 °C during separation; the injector and detector temperatures were 240 and 280 °C, respectively. The carrier gas (helium) flow ratio was 1 ml/min. One microliter of methyl esters of free fatty acids was injected into the split injector. The split ratio was adjusted to 1:10. The compounds were identified by comparison of their retention times with authentic compounds. The internal standard C_15:0 was used in the quantitative analysis of the separated fatty acids. Each fatty acid was expressed as a percent of the total fatty acids.

Data were analysed using the general linear model (GLM) procedure of the statistical analysis system, SAS [30]. When analysis of variance showed significant treatment effects, Duncan's multiple range test was applied to compare the means at P = 0.05. Bartlett's test determined the homogeneity of variance in all traits [31].

It is clear from our data that different tillage practices influenced the composition of fatty acids differently. The most appropriate method for the production of high quality canola oil, having the highest ratio of oleic acid and other unsaturated fatty acids, seems to be NT. The amounts of unsaturated acids under NT are significantly different from those of other tillage practices. This can also be another important advantage for using the NT method and can be very advantageous for the agronomical and pharmaceutical industries. According to Carvalho et al. [3], agronomical practices affect the quality of oil seeds.

Any situations resulting in the enhanced activity of enzymes, particularly at the flowering and seed-filing stage, can improve the amount and quality of seed oil. NT and MT can enhance soil productivity and properties through reducing soil erosion [11]. They may also increase seed yield by increasing plant available water [32,11] and also water use efficiency, due to creating a suitable microclimate, as a result of plant residues presence on the soil surface [33]. In addition, due to the presence of plant residues, the efficiency of plant uptake increases [34]. It also influences the distribution, and availability of nutrient to the plant, and hence plant growth [35]. Soil structure is a very important factor affecting the availability of nutrients in the soil [36–39]. Accordingly, tillage practices can also influence canola growth and oil production by affecting soil structure and hence nutrients uptake.

Plant residues extend the availability of N to the plant by immobilizing N, and then mineralizing it gradually [11]. NT is especially more efficient for plant growth in soils where the amount of organic matter is low and there is not a good soil structure, compared with soils with high amount of organic matter and good structure [40]. This is also of great significance under NT, since the change in nutrient availability to plant can alter seed yield and seed oil and fatty acid composition.

The two genotypes behaved differently under the different conditions of the experiments. The ratio of unsaturated fatty acid in V1 is significantly higher than V2, indicating that under the climatic conditions of the experiment this genotype can perform more efficiently. This enhanced performance may be attributed to the genotypic and physiological properties of V1, which is an early-mature genotype and can grow more efficiently in a shorter growth period. This can help the plant to reach the physiological maturity faster, compared with V2 and, hence would increase the plant efficiency. However, genotypes with a faster growth rate are more sensitive to the suboptimal air temperatures [15].

Genotypes with a higher rate of oleic acid are more resistant to oxidative changes and are of higher dietary values [21]. Fatty acid composition is very much affected by year, genotype, sowing date (earlier and standard) and irrigation. The amount of seed oil is a function to genotypic parameters [9]. Hence, significant amount of effort has been made to produce genetically modified canola genotypes with high amount of oil [16]. In addition to the significance of agronomical practices, which affect the quality of oil seeds, producing canola genotypes with high quality, which make them profitable like cereals, is also important [3].

The amount of unsaturated fatty acids in Sep. 23 is significantly different from other sowing dates. This can be attributed to the favorable climatic conditions (rain and temperature) at this time (Table 1). Long periods of high temperature during the seed-fill stage result in the production of seeds with a low quality, reducing the amount of seed oil content [9,41,42]. At maturity, high temperature significantly increased oleic acid and decreased linoleic acid content. This is in agreement with the results of Lagravere et al. [43] and Izquierdo et al. [6]. The reverse situation also takes place at low temperatures. Under high temperatures, the synthesis of linoleic acid from oleic acid, during the seed development is interrupted, due to the inhibition of oleate desaturase activity [44,45]. Heat stress alters the growth of embryo and hence the ratio of pericarp:embryo, resulting in the reduction of oil content. Also the oleic acid content is very much correlated with the temperature at the period of 21 to 70 days after flowering [46].

Heat stress considerably reduces the number of flowers. The optimum daily temperature for canola flowering is 20 °C (Chen et al., [15]). A daily temperature rise of 21 to 24 °C during flowering substantially reduced canola yield [47,15]. However, in areas with a temperate or even cold climate, seeds contain a high amount of oil. The optimum temperature for the germination of canola seed is in the range of 10 to 30 °C and the base germination temperature is between 0.4 to 5 °C [48,15].

Drought and high temperature reduces seed oil amount [10,16]. Hence, a 1-mm increase in rain and 1-°C reduction in daily mean temperature increased oil concentration by 0.04 and 0.27%, respectively, for all environments and canola genotypes. The effect of genotype, environment and their interaction on the amount of seed oil was very significant [16]. Salinity, high temperature and drought stress may increase the oleic/linoleic ratio due to a faster accumulation of lipid and, hence less available time duration for the activity of enzymes including Δ12 desaturase [49].

Canola is a good source of oleic or monounsaturated fatty acid and linoleic and linolenic acid or polyunsaturated fatty acid [5,3]. The desaturases change the saturated fatty acid into unsaturated ones, which is of great significance for the production of vegetable oils.

Climate and growing conditions are effective on the fatty acid composition, for example on the balance between n-6 and n-3 fatty acids. Canola is high in linolenic acid including 95 g/kg [3], which is very important for the pharmaceutical industry [50,11].

Increased water resulted in higher linoleic acid and lower oleic acid, which is reverse, compared with the effects of sowing date. Sowing date with regard to influencing soil moisture conditions and also temperature are very important and decisive parameters for canola production [9]. Early sowing (cooler temperature) decreased oleic and increased linoleic acid content. When the temperature was higher at the seed filing stage the oleic acid content was the highest and vice-versa. Irrigation resulted in higher and lower linoleic and oleic acid, respectively. Water stress at seed-filling stage increased oleic/linoleic ratio in high oleic acid genotypes [49].

According to ‘in vivo’ experiments for growing sun flower seeds, subjected to low and high temperature, the activity of oleate desaturase was stimulated and inhibited, respectively, and was re-stimulated when the temperature reduced once more [44,53]. Also, under lower moisture, the decreased cell water and also the nutritional situation may influence the synthesis and activity of oleate desaturase [9]. In addition, the synthesis of fatty acids takes place in different parts of the cell, up to 18:1 are synthesized in proplastids and from 18:1 to 18:2 and 18:3 are synthesized in cytosol [51]. Hence, the effect of environmental parameters on the ratio of fatty acids is through both affecting the activity of enzymes and their transporting to different parts of the cell [52]. The likely interaction effects between different parameters may also have significant effects on the fatty acid composition of seeds.

We can conclude that the agronomical, genotypic and environmental parameters, tested in these experiments substantially influenced the fatty acid composition of canola oil and must be taken into account when planning strategies for canola production for agronomical and pharmaceutical industries. The combination of NT, genotype Hyola 401 and second sowing date may be the most favorable cropping strategy for canola production under Mediterranean climatic conditions.