Evaluation of drought stress effect on seed oil yield and fatty acid composition in canola (Brassica napus L.) cultivars

Oilseed crops with a high relative amount of unsaturated fatty acids content are one of great significance for human health. Canola oil is considered one of the most health-promoting vegetable oils in terms of unsaturated fatty acid. Seed fatty acid composition and oil content of canola are also affected by drought stress. Large parts of the world are increasingly affected by drought. Drought stress is one of the most important abiotic factors which adversely affect growth, metabolism and yield of crops worldwide. In many parts of the Iran, canola is grown under rainfed conditions. Plant response to moisture stress has a negative effect on grain yield, which decreases significantly as a result of drought stress. Drought stress during flowering and seed-fill stages can alter oil contents and fatty acid compositions of canola seed. The present experiment studied the effect of drought stress on oil contents and fatty acid composition in canola genotypes at water stress under end-of-season drought (flowering and siliquing stages).

The experiment was conducted as split plot based on randomized complete block design with three replications. Irrigation regimes were considered as main plots and cultivars as subplots. Six winter canola cultivars including SLM046, Tassilo, Karun, Adriana, Cooper and Lilian were evaluated under irrigated and no irrigated. The irrigation regimes included: well-watered (irrigation during full season), water deficit at flowering stage and water deficit at siliquing stage.

The proportions (%) of fats (monounsaturated: polyunsaturated: saturated) in this study were approximately 68.0:26.0:6.0. The most abundant fatty acids in the current research were the oleic monounsaturated fatty acid (C18:1) and the polyunsaturated fatty acids, Linoleic acid (C18:2) and Linolenic acid (C18:3), in the proportions 67.96:17.21:8.75. The most abundant saturated fatty acid was Palmitic acid (C16:0), Stearic acid (18:0) and Myristic (14:0), in the proportions 3.87: 2.11: 0.044 % in samples. Based on analysis of variance, significant differences were observed between genotypes for oil yield (%), seed yield (kg ha-1), Palmitic acid, Palmitolic acid, Stearic acid, Oleic acid, Linoleic acid, Linolenic acid, total saturated fatty acid (TSFA) and ratio of total unsaturated to total saturated fatty acid (TU/TS). The results of analysis of variance indicated that drought stress at flowering and siliquing stages significantly affected the amount of Myristic, Palmitic, Palmitolic, Stearic, Linoleic acids. The results showed that the highest and the lowest Oleic acid and seed oil percentage were found in "Lilian" and "Tassilo" genotypes, respectively. Also, the highest and the lowest palmitic and linoleic acids content belong to "Tassilo" and "Lilian" genotypes, respectively. These results suggest significant associations between Oleic acid with Palmitic and Linoleic acids. Oleic acid had significantly and negatively correlated with Palmitic acid, Linoleic acid and Linolenic acid, but it had a significant and positive correlation with oil content. Strong and negative correlation between oleic and Linoleic acid suggest significant connections among these fatty acids, as expected by their close biochemical association. Under well-watered conditions (non-stress), the values of Palmitic, Myristic, Linoleic acids were significantly higher than drought stress at flowering (excepted for linoleic acid) and siliquing stages. At drought stress at flowering and siliquing stages, reduction in Palmitic acid content was 3.78 and 3.58%, in Myristic acid content was 17.06 and 21.67% and in Linoleic acid content was 2.24 and 3.44%, respectively. Under well-watered conditions (non-stress), the value of TSFA was significantly higher than drought stress at flowering and siliquing stages. Whereas, water stress under end-of-season drought increased TU/TS compared to the non-stress condition.

In this study, the water stress under end-of-season drought (flowering and siliquing stages) in canola crops considering fatty acid composition and seed oil stability has been studied. The changes in fatty acids composition in the present study were probably due to the combined effect of water deficits and high temperatures during the seed-filling period with the end-of-season drought treatment (flowering and siliquing stages). This study showed that drought decreases slightly seed oil percentage, alters fatty acid composition and affects fatty acid composition stability. In addition, development of drought tolerant canola genotypes with stable high oleic and low Linolenic acid genes is critical to maintain the stability of oil production and desirable fatty acid composition. In conclusion, Further work is needed to determine the cause of alters in oleic and in fatty acids concentration, under drought stress in canola. The highest Oleic acid and seed oil percentage in non-stress and water stress under end-of-season drought were found in "Lilian" genotype. Therefore, this genotype would be good parents in a breeding program to develop cultivars for stressed conditions, because their oil content remains unaffected but Oleic acid is increased by drought, thus improving the oil shelf-life and quality.