| چکیده انگلیسی مقاله |
Introduction and Objective: Sunflower is a cash crop with widespread cultivation worldwide due to its high adaptability to various climatic conditions. The cultivation of sunflower has accelerated due to the production and introduction of hybrid varieties that exhibit the phenomenon of heterosis. In Iran, the sunflower cultivation system has recently focused on second cropping, which requires tolerance to low temperatures during seed filling stage. The formation of yield in crop plants results from the interaction between two components: carbon-producing organs, source, and storage organs, sink. Identifying these components and regulating their relationships aids breeders in the direction of plant variety improvement. Although studies have been conducted on the relationships between source and sink and their interactions in sunflowers worldwide and in Iran, there is no report on the involvement of molecular components determining hybrid performance. Additionally, no study has been conducted on the physiological changes of hybrids during the hybrid breeding of sunflowers in Iran. This research aimed to identify strategies used in sunflower hybrids for yield formation under second cropping conditions, that is exposure to ambient temperature of 15̊ C, and to investigate the physiological molecular changes associated with yield formation in hybrids that have been bred and introduced over a 30-year period in Iran.
Materials and Methods: The research was conducted over two experiments (in different years and locations) on three hybrids: Azargol, Farrokh, and Ghasem under second cropping conditions. The cultivation conditions were set so that the seed filling period would be exposed to temperatures of 15˚ C. At the onset of pollination, leaf number, leaf area, dry leaf weight, dry weight of receptacle base, and five uppermost stem nodes were measured; at physiological maturity, capitulum dry weight, capitulum diameter, dry weight of five uppermost stem nodes, 1000 seed weight, number of achene (filled and unfilled) per capitulum, and yield per plant were measured and counted. Eight days after pollination began, invertase enzyme expression levels in the receptacle base tissue were measured using Real-Time PCR technology. Data analysis was performed through combined analysis and mean comparison after ensuring normality.
Findings: The results from combined analysis indicated no significant interaction effects between experiments and hybrids for all traits studied. Regarding source-related traits, the Ghasem hybrid was significantly higher than the other two hybrids, while Azargol hybrid had significantly lower dry leaf weight. Thus, in terms of source strength, Azargol hybrid had the lowest value among the three hybrids. The dry weight of receptacle base and upper stem nodes in Azargol hybrid were equivalent to Farokh hybrid and significantly higher than Ghasem hybrid. Additionally, there was no significant difference in the number of achenes per head between Azargol and Farokh hybrids. However, 1000 seed weight and seed yield for Azargol hybrid were significantly higher than for Farokh hybrid. The amount of non-structural carbohydrates stored in the receptacle base and five uppermost stem nodes in Azargol hybrid was significantly higher than in the other two hybrids, while the contribution of remobilization from receptacle base and the five uppermost stem nodes to seeds as well as current photosynthesis contribution to yield formation was statistically equal among all three hybrids. Invertase gene expression levels eight days after pollination began were significantly higher in Azargol hybrid compared to the other two hybrids. Despite having higher source strength in Ghasem hybrid, the dry weight of the receptacle base and the five uppermost stem nodes were lower than those of the other two hybrids, likely due to some resistance in translocating assimilates from leaves to sink. The equal number of akene and dry matter weights in receptacle base and the five uppermost stem nodes at pollination onset for Azargol and Farokh hybrids alongside a higher 1000 seed weight in Azargol hybrid likely resulted from greater invertase enzyme activity in its receptacle base tissue. This enzyme facilitates sustained phloem sap flow toward the capitulum as a temporary sink, ultimately leading to increased 1000 seed weight. One function of this enzyme is to protect tissues against low temperatures; therefore, special attention to high levels of this enzyme's expression in capitulum tissue could enhance breeding programs in second cropping system of sunflowers. On the other hand, examining physiological components affecting yield formation in hybrids introduced over a 30-year period indicates a focus on early maturity that has led to reduced yields. If performance formation relationships are considered from both physiological and molecular point of view as a roadmap for sunflower breeding, it will be possible to create early-maturing hybrids while maintaining yield.
Overall Conclusion: Focusing on molecular physiology aspects of biological processes leads to a shift in breeders' perspectives and increases the efficiency of breeding programs. Considering the activity of enzymes involved in starch and sugar metabolism, such as invertases, can increase yield despite low source strength while enabling tolerance to low temperatures during seed filling. |