Abstract
Onion (Allium cepa L.) is the most widely cultivated species of the Allium genus. Onion is rich in many vital nutrients and sulfur (S)-containing several compounds important for human health. Recently, Allium species requirements have gained special attention in this special
time due to its export importance. The aim of this study is to compare the effects of the recommended dose of potassium (100% KRD) against three lower levels of potassium mixed with Bacillus circulans, potassium solubilizing bacteria (80% KRD+biofertilizer, 60% KRD+biofertilizer or
40% KRD+ biofertilizer, as well as foliar applications of copper (Cu), boron (B) or putrescine (Put) that have great impacts on yield, quality and onion attributes.The highest level of potassium (100% KRD) resulted in the highest growth parameters and the richest leaves content in N, P, K,
S, B, Cu, and carbohydrates, yield, as well as the best quality parameters of bulbs. All growth and yield parameters decreased gradually with reducing the levels of K fertilization. Foliar spraying of Cu, B, and Put combined with all different levels of K fertilizer improved all plant parameters.
The highest impact on plant growth was due to Put followed by B at all levels of soil fertilization. Plants that were exogenously sprayed with Put and B exhibited an increase in N, P, K, B, and carbohydrates under different levels of fertilizers, while foliar application with Cu highly increased
S and Cu in onion leaves with all levels of K fertilization. The combined treatment of B and all different levels of K fertilizer considerably improved all yield measurements, except with allyl propyl disulfide, which estimated a highly significant increase with Put foliar application coupled
with 40% KRD+Bio. In a nutshell, exogenous Put, B, and Cu foliar applications on onion plants increased bulb yield and its quality attributes with the recommended level of potassium or lower recommended levels of K mixed with biofertilizer.