Effect of Pinching, Spraying of Nitrogen and Plant Growth Regulators on Vegetative and Reproductive Characteristics of African Marigold

African marigold is one of the most popular annual flowers in almost of countries. In the most ornamental plants, the flowering and yield depends on the flowering branches that can be increased with pinching. Nitrogen spraying is beneficial on the growth, production and quality improvement of marigold flowers. Among plant growth regulators, auxin and gibberellin play a crucial role in regulating the development processes of African marigold.

This study was carried out in the research greenhouse of Velayat University of Iranshahr in two separate experiments in December 2017. The beds were prepared after mixing the soil with FYM (Farm Yard Manure) and the seeds of African marigold var. "African Double Orange" were sown on the bed at a distance of 10 cm between the row and 2 to 3 cm within the row. Transplanting of plantlets were performed in the 3 to 4 leaves stages in distances of 45 cm×35 cm. Treatments of the first experiment were contains of two stages of pinching (no pinching and pinching in 20 days after planting) and nitrogen spraying in four stages (0%, 2%, 4% and 6%). A factorial experiment was laid out in a complete randomized block design with three replications where each replication was comprised of six plantlets. Treatments of the second experiment consisted of three levels of indole acetic acid (50, 100 and 150 mgL-1) and three levels of gibberellic acid (100, 200 and 300 mgL-1) and distilled water as control. The first stage of auxin and gibberellin spraying, 20 days after planting and the second stage of spraying, was performed 50 days after planting. On the harvest time (when the last flower harvested), measurement factors were contains of vegetative parameters (plant height and diameter, branch number, leaf number and branch length) and reproductive parameters (flower number, flower diameter, flower fresh weight and flower dry weight). Days taken to flowering were counted from transplanting till the date of flower bud opening.

Analysis of variance showed that pinching and nitrogen spraying had significant effects on plant height and diameter, branch number, leaf number and branch length per plant (P< 0.01). The highest plant height (47.5 cm), plant diameter (2.5 cm) and branch length per plant (21.5 cm) obtained in no pinching with 6% N treatment. This result showed that spraying of nitrogen could increase apical dominance and growth of longitudinal growth of plant. The maximum leaf number (91) and branch number (10) obtained in pinching 20 days after harvesting with 6% N treatment. When the apical dominance is removed, the lowering of IAA (Indol acetic acid) concentration increased the new branches. Increasing in new branches and positive effect of nitrogen spraying created new leaves. Analysis of variance showed that pinching and nitrogen spraying were not effective on flower diameter but had significant effects on flower number, flower fresh weight and flower dry weight. The highest flower number (7) were created in pinching supplemented with 6% N treatment. The highest flower fresh weight (3.60 g) and flower dry weight (0.8 g) were obtained in no pinching and 6% N treatment. Sing et al. (2017) reported that no pinching increased availability of more food material and better allocation of energy pertaining to less number of flowers. Therefore, this action increased the fresh and dry weight of flower. Analysis of variance showed that pinching and nitrogen spraying had significant effects on the number of days to bud flower initiation. The highest number of days to flowering (75 day) were obtained in pinching supplemented with 6% N treatment. Pinching of apical bud prevents from cell division in the lateral meristem. Nitrogen spraying also increased vegetative growth of plant and therefore, resulted in delaying the initiation of flower bud. The quality of seedlings significantly increased with gibberellic acid spraying compared to indole acetic acid. As the concentration increased, seedling quality increased, so that the highest plant height (34.25 cm), the length of each branch (23 cm), Leaf number (42), the number of branches (14), flower diameter (3 cm), dry weight of flower (0.65 g), fresh weight of flower (3.13 g), flower number (6) and the minimum number of days to flowering (31 days) were sprayed with gibberellic acid at a concentration of 300 mgL-1. Increasing the number of flowers by application of gibberellic acid treatments resulted from an increasing in plant height and the number of leaves per plant. Increasing the number of leaves increased the production and accumulation of photosynthetic assimilates, and these assimilates were directed to the flowers and resulted in the production of more flowers.

Pinching and Spraying of nitrogen had significant effects on the vegetative and reproductive traits of African marigold. Pinching reduced the apical dominance and induced more lateral branches that ultimately increased the flower number of marigold., Nitrogen spraying was also effective on the growth and quality improvement of marigold flowers and the highest number of flower and fresh and dry weight of flowers induced with application of 6% N. In addition, the application of spraying of gibberellic acid had a direct correlation with height, the number of branches and the diameter of flowers and inversed relationship with the number of days to flowering. The concentration of 300 mgL-1 gibberellic acid had the greatest effect on the growth and reproductive characteristics of African marigold. Since, the African marigold is widely used in the garden and park landscapes, the application of 6% nitrogen or 300 mgL-1 of gibberellic acid is recommended to improve the reproductive characteristics of the flower.