Molecular Soil Biology 2025, Vol.16, No.6, 325-334 http://bioscipublisher.com/index.php/msb 327 growth, flowering, and early maturity), achieved the highest seasonal evapotranspiration and seed yield, while also resulting in the highest protein, oil, and fatty acid content. In two growing seasons in Egypt, the "full irrigation" treatment with a total of 4800 m³/ha applied in five irrigations resulted in significantly higher seed and oil yields compared to treatments with reduced irrigation frequency and amount. 4 800 m³/ha irrigation, compared to only 1 920 m³/ha (two irrigations), increased plant height, number of branches, number of pods, single plant seed weight, and seed, oil, and protein yields by 40%~58% (Kandil et al., 2017). A population evaluation of 119 varieties under two water regimes (full irrigation vs. drought during the reproductive stage) over 3 years showed that average seed yield under full irrigation was significantly higher than under drought conditions, and the variation among varieties was more pronounced (Salami et al., 2024). 3.2 Irrigation strategies and yield trade-offs In Iran, a two-year experiment applying water stress during the reproductive stage after tillering (with irrigation stopped during flowering, pod development, and grain filling stages, respectively) showed that, compared to full irrigation throughout the growing season, water restriction during flowering reduced grain and oil yield by 29.5% and 31.7%, respectively, while water restriction during pod development and grain filling resulted in smaller yield reductions (Ahmadi and Bahrani, 2009). Compared to full irrigation, the 0.75 FI and 0.50 FI treatments reduced grain yield by 15.0% and 25.9%, respectively, while in the second year, the 0.65 FI and 0.35 FI treatments reduced grain yield by 20.8% and 33.0%, respectively. This indicates that within a 10%~13% reduction in irrigation volume, grain, oil, and protein yields can remain relatively stable, while larger reductions in water lead to significant yield losses (Shabani et al., 2012). Based on simulations using the APSIM-Canola model at 10 locations, with 3 varieties and 4 irrigation scenarios (full irrigation, irrigation stopped during flowering, pod development, and grain filling), the average potential yield at temperate western locations was 2 852.6 kg/ha, while at hot southwestern locations it was only 1 885.1 kg/ha. In all scenarios, stopping irrigation after the start of grain filling resulted in the smallest yield reduction, only 13.6%, while stopping irrigation during flowering and pod development resulted in more significant yield losses (Rahimi-Moghaddam et al., 2021). In a 2015~2017 field experiment in Iran involving 17 varieties and two irrigation treatments, withholding irrigation from silique formation to maturity reduced average grain yield, but some genotypes (such as HL3721) maintained a grain yield of 3892 kg/ha and an oil content of 437 g/kg under limited irrigation conditions (Eyni-Nargeseh et al., 2019). In a multi-variety study in the Karaj region, among three gradients of withholding irrigation starting from stem elongation, flowering, or silique formation, the earliest treatment (no irrigation after stem elongation) resulted in an average yield reduction of 30%~50%, while withholding irrigation after silique formation resulted in a relatively lower yield reduction (Rad et al., 2014). 3.3 Water stress during critical growth stages and yield reduction effects In a greenhouse pot experiment, drought stress was set at the flowering stage. Two soil water loss speeds were used, one slow and one fast. When water loss was slow, leaf relative water content only dropped by about 2% compared with the control. When water loss was fast, the drop reached about 6%. At the same time, stomatal conductance and CO2 assimilation both fell clearly. Grain dry weight and silique number also decreased a lot (Xiang et al., 2024). When irrigation was stopped from flowering to silique formation, the impact became more serious. Grain yield dropped by 38%~49%. Oil content was reduced by about 4%~9%. Fatty acid composition also changed. Linolenic acid, erucic acid, and glucosinolate levels increased under drought stress (Amiri et al., 2024). Field experiments carried out in Karaj from 2015 to 2017 showed similar results. Under normal sowing time and full irrigation, the highest grain yield reached 4505.6 kg/ha. However, when sowing was delayed and irrigation was stopped after flowering, yield fell sharply to only 1814.6 kg/ha. This means the yield was reduced by nearly 60% (Shafighi et al., 2022). Under the same drought conditions during flowering, different genotypes responded differently. Drought-tolerant genotypes, such as Nap9, RGS003, and SLM046, showed smaller reductions in yield and oil content. These genotypes usually had longer roots and higher relative leaf water content than drought-sensitive ones. 3.4 Irrigation efficiency and water-saving potential In the Trakya experiment, irrigation was applied only once after flowering. Under this condition, irrigation water
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