Micronutrients and biostimulants for crop management in marigold (Tagetes erecta L.)

Abstract

Investigations on “Micronutrients and biostimulants for crop management in marigold (Tagetes erecta L.)” was undertaken at the Department of Floriculture and Landscaping, College of Agriculture, Vellanikkara during 2024 to 2025. Objectives of the study were to evaluate the influence of micronutrients, biostimulants, and plant growth promoting rhizobacteria (PGPR) on the growth, yield, and quality attributes of African marigold. Studies were carried out in the hybrid Royal Orange. The experiment was laid out in a randomized block design comprising of ten treatments with three replications each, involving combined application of recommended dose of fertilizers (RDF), boron (1 kg ha⁻¹), varying concentrations of humic acid (1%, 3%, and 5%), salicylic acid (50, 100, and 150 ppm), and PGPR Mix I and II (2% and 5%). Treatments were applied through soil application, foliar spray, soil drenching, and root dipping at different growth stages of the plants. Vegetative, floral, and biochemical parameters of the plants and soil nutrient characteristics of the experimental plot were studied. Significant variations among treatments were observed for all the traits studied except for the shelf life of flowers. Among the treatments, application of 5% humic acid along with RDF and boron (T5) recorded significant improvement in plant height (46.25 cm), plant spread (29.29 cm), number of branches per plant (15.67), and leaf count (15.5 per branch). This treatment also resulted in the highest fresh (464 g and 64 g respectively) and dry (95 g and 29 g respectively) shoot and root biomass, indicating enhanced nutrient absorption and root development. The same treatment (T5) also advanced the onset of flowering, with the earliest bud emergence (31.67 days) and 50% flowering (31.83 days), while the control treatment (RDF alone) showed delayed flowering (41.08 days). Floral attributes such as number of flowers per plant (18.67), flower diameter (6.03 cm), individual flower weight (8.8 g), and flower yield per plant (159.93 g) were also maximized in T5 (RDF + boron + 5% humic acid), followed by treatments with RDF + boron + PGPR (T9 and T10) and RDF + boron + salicylic acid (150 ppm) (T8). The duration of flowering (71.08 days) and field life of flowers (7.58 days) were prolonged under T5 (RDF + boron + 5% humic acid), reflecting improved physiological activity. Biochemical analysis revealed that treatments consisting of RDF + boron + salicylic acid and RDF + boron + PGPR improved biochemical composition of leaves and flowers. The treatment with RDF + boron + 150 ppm salicylic acid (T8) exhibited highest chlorophyll a (1.313 mg g⁻¹ FW), chlorophyll b (1.295 mg g⁻¹ FW), and total chlorophyll (2.61 mg g⁻¹ FW), suggesting improved photosynthetic efficiency. PGPR Mix II (5%), along with RDF and boron (T10), significantly increased phenolic (21.78 mg GAE/g and 54.23 mg GAE/g respectively) and flavonoid (0.307 mg QE/g and 4.51 mg QE/g respectively) contents in leaves and flowers, and carotenoid content (0.051 mg/g FW) in flowers, indicating higher antioxidant potential and stress tolerance. Nutrient uptake studies indicated that plants which received RDF + boron + 5% humic acid (T5) exhibited superior uptake of nitrogen (3.04 g plant⁻¹), phosphorus (0.238 g plant⁻¹), iron (0.219 g plant⁻¹), and zinc (0.029 g plant⁻¹). Potassium uptake was not significantly correlated with yield, suggesting that other nutrients played a more decisive role in flower production. Soil analysis after harvest of flowers showed improved macronutrient and micronutrient availability in plots receiving RDF + boron + PGPR and RDF + boron + biostimulants. Correlation studies demonstrated that yield per plant had a significant positive association with key vegetative and floral traits such as plant spread, number of branches and flowers, flower diameter and weight, duration of flowering, and total chlorophyll content, while being negatively correlated with the number of days to bud emergence and 50% flowering. Similarly, flower yield showed significant positive correlations with the uptake of nitrogen, phosphorus, iron, manganese, zinc, and boron, underscoring the role of balanced nutrition in the enhancement of productivity. Combined use of micronutrients and biostimulants improved growth, yield and quality attributes of African marigold. Integrated application of RDF + boron + humic acid (5%) enhanced the overall growth, yield and floral characteristics of African marigold. The use of RDF + boron + salicylic acid proved beneficial in enhancing chlorophyll content, flower longevity, and stress resilience, whereas RDF + boron + PGPR combinations improved secondary metabolite accumulation, contributing to superior flower colour and quality. Combinations and concentrations of micronutrients and biostimulants along with RDF identified in the present study could be utilized in further investigations.