Effect of bulb size, planting geometry and paclobutrazol on growth and phenological development of onion (Allium cepa L.) seed crop

A field experiment was conducted during Rabi season of the years 2023–24 and 2024–25 at the Instructional Farm, College of Agriculture, Jodhpur (India) to study the effect of bulb size, planting geometry and paclobutrazol on growth and phenological development of onion (Allium cepa L.) seed crop cv. NHRDF Red-4. The experiment was laid out in a split plot design with three replications. Treatments consisted of three planting geometries (45 × 15 cm, 45 × 30 cm and 45 × 45 cm) and two bulb sizes (40-60 g and 61–80 g) in main plots and four paclobutrazol levels (0, 250, 500 and 750 ppm) in sub-plots. The results revealed that wider spacing (45 × 45 cm) and larger bulb size (61-80 g) significantly improved growth parameters such as plant height, number of leaves per plant, flower stalk height and flower stalk diameter. Paclobutrazol application significantly reduced plant height but promoted earlier bolting, flowering and maturity. The earliest bolting, 50% flowering and physiological maturity were recorded with paclobutrazol @ 750 ppm, which remained statistically at par with 500 ppm. The interaction of larger bulb size, wider or medium spacing and paclobutrazol @ 750 ppm resulted in better crop regulation and optimum flowering behaviour.