Introduction
Forage sorghum (Sorghum bicolor (L.) Moench), a drought-tolerant C4 crop, is increasingly vital for sustainable livestock feed production in arid and semi-arid regions like Iran, where climate change and water scarcity threaten agricultural productivity. Despite its resilience, the limited availability of high-quality seeds hampers its widespread adoption. Plant growth regulators (PGRs), such as Medax Top (containing mepiquat chloride and prohexadione calcium), offer a promising approach to enhance grain yield and seed quality by modulating plant growth and assimilate allocation. This study aimed to evaluate the effects of Medax Top foliar application on grain yield, seed quality, and physiological traits of two open-pollinated forage sorghum cultivars under water-limited conditions, providing insights into optimizing seed production for sustainable agriculture.
Materials and Methods
A two-year field experiment (2023-2024) was conducted at the Seed and Plant Improvement Institute, Karaj, Iran, using a factorial split-plot of treatment arrangement in complete randomized block design with three replications. Main plots comprised factorial combination of Medax Top doses (0, 0.5, 1, and 2 L ha-1) and application timings (3-4 and 6-8 leaf stages), while subplots included two sorghum cultivars (Mansour and Behesht). The trial was established in a semi-arid climate (265 mm annual rainfall, 14°C mean temperature) with drip irrigation. Measured traits included leaf SPAD-value, leaf area index (LAI), grain yield, seed germination percentage, grain starch, total carbohydrates, soluble sugars, and forage yield. Data were analyzed using SAS 9.1, with means compared via LSD’s test (P ≤ 0.05) after confirming variance homogeneity across years.
Results and Discussion
Medax Top application significantly influenced physiological and agronomic traits. As the application rate of Medax Top increased, leaf SPAD -value, grain yield, grain starch content, carbohydrates, soluble sugars and seed germination percentage significantly increased. However, these positive changes were accompanied by a reduction in LAI and forage yield. Foliar application at the 3-4 leaf stage, compared to the 6-8 leaf stage, significantly increased starch content and grain yield while decreasing forage yield. Application at the 3-4 leaf stage yielded higher grain production (2976 kg ha-1) than the 6-8 leaf stage (2723 kg ha-1), likely due to reduced competition between vegetative and reproductive sinks in early development duration. The 2 L ha-1 of Medax Top maximized grain yield and germination percentage, alongside enhancing SPAD-value and carbohydrate content, reflecting improved photosynthetic efficiency and assimilate storage. However, forage yield decreased by 34% at this dose, indicating a shift in resource allocation from vegetative to reproductive growth. The Mansour cultivar demonstrated significant superiority over the Behesht in terms of starch content, carbohydrates, soluble sugars, and forage yield, showcasing superior genetic potential. The maximum grain soluble sugar content (6.88%) was observed in the Mansour cultivar with a ModaxTop application rate of 2 L ha-1 in the first year, while the highest grain yield (4542 kg ha-1) was achieved in the same cultivar with a dose of 1 L ha-1 in the second year. Lower doses (0.5-1 L ha-1) better supported forage production, balancing vegetative biomass retention.
Conclusion
This study demonstrates that Medax Top foliar application effectively enhances grain yield and seed quality in open-pollinated forage sorghum cultivars, with the 2 L ha-1 dose applied at the 3-4 leaf stage being optimal for seed production, particularly in the Mansour cultivar. For forage-focused systems, lower doses are recommended to minimize biomass loss. Overall, a Medax Top foliar application at 2 L ha-1 is recommended for producing seeds of appropriate quantity and quality, while a 1 L ha-1 dose is advisable for simultaneous seed and forage production. These findings underscore the potential of plant growth regulators in tailoring sorghum cultivation to specific goals—seed or forage—in water-scarce regions, contributing to agricultural resilience and food security.
Acknowledgement
The authors express gratitude to the Seed and Plant Improvement Institute, the Agricultural Research, Education, and Extension Organization (AREEO) for providing laboratory facilities and technical support in this research [Project number 03-03-0305-024-010312]. Also, thanks to the support of the Vice Chancellor for Research and Technology of Tarbiat Modares University. |
