Photosynthetic activity of early-ripening sunflower hybrids in the Western Forest-Steppe

Purpose. To determine the impact of seeding rates and foliar application of growth regulators on the growth and development of early-ripening sunflower hybrids. Methods. The research was conducted from 2022 to 2024 on the experimental fields of the Scientific Research Center Podillia of the Podillia State University. A three-factor scheme was used in the studies, which included: sunflower hybrids (‘P62LL109’, ‘MAS 81K’, ‘YeS Monaliza’), two seeding rates (60,000 and 65,000 seeds/ha), and foliar application of growth regulators at the four to six leaf stage [control, Deimos (1.5 l/ha), Mars ELBi (0.75 l/ha), Treptolem (15 ml/ha)]. The leaf area was measured, and photosynthetic parameters of the plants were determined. Results. The highest leaf area values for sunflower plants in the star leaf stage were observed in the hybrid ‘MAS 81K’ – 33.8 thousand m²/ha on average over the years of research. Slightly lower values were noted in the hybrids ‘YeS Monaliza’ and ‘P62LL109’ – 31.2 and 32.2 thousand m²/ha, respectively. The largest leaf area in the flowering stage was observed in the variants with a seeding rate of 65,000 seeds/ha and treatment with the growth regulator Deimos in the four-leaf stage. Specifically, under these conditions, the hybrid ‘P62LL109’ formed leaf area of 42.5 thousand m²/ha, ‘MAS 81K’ – 44.7, and ‘YeS Monaliza’ – 42.8 thousand m²/ha. With a seeding rate of 65,000 seeds/ha and the application of the growth regulator Deimos, the highest total content of photopigments (chlorophylls a + b) in the leaves of hybrids was 6.07 mg/g in ‘P62LL109’, 6.43 in ‘MAS 81K’, and 6.01 mg/g in ‘YeS Monaliza’. The application of other growth regulators also contributed to an increase in the content of photopigments, although they were slightly less effective. Conclusions. The highest photosynthetic potential in the flowering and ripening stages was observed in the hybrid ‘P62LL109’ at a seeding rate of 65,000 seeds/ha and foliar application of the growth regulator Deimos – 1.34 million m²/days. For hybrids ‘MAS 81K’ and ‘YeS Monaliza’ under similar conditions, these values were 1.28 and 1.18 million m²/days, respectively. In terms of the efficiency of different experimental variants in forming the net photosynthetic productivity, the highest indicators were achieved at a lower seeding rate of 60,000 seeds/ha. For the hybrid ‘P62LL109’, under these conditions, the most effective treatments were with the growth regulator Treptolem (4.37 g/m²·day) and Mars ELBi (4.29 g/m²·day), for ‘MAS 81K’ – Mars ELBi (4.21 g/m²·day) and Treptolem (4.17 g/m²·day), and for ‘YeS Monaliza’ – Treptolem (4.87 g/m²·day) and Deimos (4.81 g/m²·day).