The Effects of Deficit Irrigation, Nitrogen Levels and Bulb Size on Seed Yield and Reproduction Traits of Onion (Allium cepa var. Ghooli gheseh)

Water stress limits crop production throughout the world and contrary to other limiting factors (acidity, sodicity and salinity). It is highly variable within growing season and from year to year. Plants response to water deficit at morphological, anatomical and cellular levels by modifications allowing them to avoid stress or increase tolerance. When supply of water is limited, crop management practices that improve water stress resistance can benefit plant growth and improve water use efficiency.  Onion seed production has high requirements in inorganic fertilizers. The applied amount depends on the type and fertility status of the soil. The use of inorganic fertilizers is common in onion production. Onion is responsive to nitrogen fertilizer. The bulb refers to a fleshy structure serving as storage organ, and containing simple and sugars, sulfur, protein and nitrogen compound containing flavor precedents as well as a significant amount of water in the swollen cells producing the bulk of the bulb scales. In order to evaluate the effects of deficit irrigation, nitrogen levels and bulb size on seed yield and reproduction traits of onion (Allium cepa var. Ghooli gheseh), was conducted in Zanjan areas. In order to study the effects of nitrogen levels, bulb size and water tension at various growth stages on onion seed yield (Allium cepa var. Ghooli gheseh), an experiment was conducted in Zanjan University in 2014. The experiment was strip split plot with a randomized complete block design with three replications. Water tension was applied at the beginning of flowering, end of flowering, as well as the milking stage of seed by avoiding irrigation at those stages. The obtained seed yield was compared with that of control or treatment with normal irrigation (no water tension). The irrigation treatments including control were located on horizontal plots and vertical plots allocated to nitrogen levels (0, 75, 150 and 300 kg N ha-1) and bulb sizes (5-7 and 7-9 cm diameter). Traits such as days to inflorescence emergence, days to flowering, number of inflorescence per plant and seed yield were measured. All data were subjected to analysis of variance (ANOVA) using SAS 9.3 software. When F test indicated statistical significance at P< 0.01 or P < 0.05, the least significant difference (LSD) was used to calculate the means. The effect of water tension on seed yield and days to seed ripening were significant at 1% probability level. Water tension at milking stage of seed, end of flowering and beginning of flowering reduced seed yield in comparison with the control and yield decreased by 10.5, 12 and 39.5%, respectively. The highest seed yield (659.4 kg ha-1) was obtained in the control. Although the main effect of nitrogen was not significant, application of 150 kg N/ha increased inflorescence diameter, percentage of fertile florets and seed yield about 5.6, 4.9 and 20%, respectively in comparison with the control (N0). Application of nitrogen up to 150 kg ha-1 significantly increased days to seed ripening. Days to inflorescence emergence, days to flowering, number of inflorescence per plant and seed yield were affected significantly by bulb size. In onions, many factors such as bulb weight, cultivars, spacing, date of planting, climate, soil, besides fertilizer application seem to affect seed yield and quality. Moreover, according to Mishra, (1994), applying nitrogen has been revealed to enhance the number of umbels per plant in onion. Based on Islam et al. (1998), the larger sets associated with the closest spacing resulted in the highest bulb yield. Smaller set with zero nitrogen level led to the maximum number of single bulbs. The closest spacing with 120 N/ha resulted in the highest bulb yield, as well. The highest bulb yield from the combination of larger sets and closest spacing was equal to 120 kg N/ha. Smaller sets and the closest spacing zero N level yielded the highest number of single bulbs. For onion seed production in climatic conditions similar to those in Zanjan areas, application of normal irrigation, bulb diameter of 7-9 cm and 150 kg N ha-1 are recommended.