Effect of selenium and zinc on the physiological traits and yield of rapeseed genotypes at optimal and delayed sowing dates

Oilseed crops play a vital role in ensuring the food security of human societies and are the constant focus of agricultural sector researchers. Due to its favorable agronomic characteristics and high oil quality, rapeseed (Brassica napus L.) is one of the most important crops in the agricultural sector. Given that successful crop production depends on the climatic conditions of a given region, the degree to which plant growth stages adapt to the climatic conditions is a determining factor in achieving acceptable yields. In order to determine the optimal sowing date in each region, it is essential to keep in mind that during the crop's growing season, environmental conditions should be optimal and the crop should be unaffected by environmental stresses. The addition of micronutrients could enhance plant growth and development under various environmental conditions. In light of the importance of cultivating oilseeds, including rapeseed, it is necessary to increase the cultivated area and quality of rapeseed through the use of appropriate management strategies. Consequently, the purpose of the present study was to determine the response of rapeseed genotypes to foliar application of zinc and selenium at various sowing dates.

An experiment during the 2015-2016 growing season was performed at the Research Field of the Seed and Plant Improvement Institute (SPII), in Karaj, Iran. The experiment was conducted as a factorial-split plot in a randomized complete block design (RCBD) with three replicates. Three sowing dates of 7 (well-timed sowing), 17, and 27 October (delayed sowing dates) and four levels of foliar application with pure water (control), selenium (1.5%), zinc (1.5%), and selenium+zinc (1.5%) were factorial in the main plots and five genotypes of SW102, GKH2624, Okapi, Ahmadi, and GK-Gabriella were randomized in the subplots. Notably, sodium selenate (as a source of selenium) and chelated zinc (as a source of zinc) were applied in two stages of (i) 4-leaf (ii) and stem elongation for each sowing date and genotype, respectively. In this study winter survival, collar diameter, leaf relative water content, relative collar water content, proline content, carbohydrate content, chlorophyll content, seed yield, and oil content were measured.

The delayed sowing dates decreased winter survival, collar diameter, leaf relative water content, and chlorophyll content compared with the optimal sowing date, whereas relative collar water content, proline content, and leaf carbohydrate content increased under these conditions. When compared with the 7-Oct sowing date, the seed yield and oil content of rapeseed genotypes decreased by 44 and 6% on 17-Oct and by 51 and 14% on 27-Oct. The foliar application of selenium+zinc had the greatest effect on reducing the relative collar water content and increasing winter survival, collar diameter, leaf relative water content, and chlorophyll content. The selenium+zinc treatment increased rapeseed seed yield by 23% and oil content by 6% compared to the control treatment. The reduction in yield of genotypes sown on delayed planting dates can be attributed to a reduction in collar diameter, leaf relative water content, chlorophyll content, and the number of surviving plants, as well as an increase in relative collar water content. Correlation analysis also revealed a significant positive relationship between rapeseed seed yield and winter survival (r=+0.87, p value<0.01), collar diameter (r=+0.95, p value<0.01), leaf relative water content (r=+0.79, p value<0.01), and chlorophyll content (r=+0.96, p value<0.01), and a significant negative relationship with collar relative water content (r=-0.79, p value<0.01).

The Gk-Gabriella genotype produced the highest seed and oil yield on the optimal sowing date (7-Oct), while the GKH2624 genotype had the highest seed and oil yield on the delayed sowing dates (17 and 27-Oct). These genotypes are recommended for arid and semiarid regions. Our findings indicate that foliar application of selenium+zinc during the rosette and stem elongation stages increases seed yield and oil content in rapeseed genotypes sown at optimal and delayed sowing dates.