The Effects of Increasing Boron on Growth, Yield, and Nutritional Value of Scallion (Allium cepa L.) Grown as a Bunch Harvest

Scallions are a highly valued leafy vegetable and are enjoyed worldwide due to their appealing taste and nutritional benefits. A combination of short cultivation cycles and high market demand not only enhances food security but also offers a profitable opportunity for growers. In our study, we aim to evaluate the effect of increasing boron (B) applications, specifically 0, 0.2, 0.4, 0.8, 1.2, and 1.6 mM B supplied as boric acid (H3BO3) in the nutrient solution, on several key physiological and agronomic parameters in scallions. Results showed that the effects of increasing B levels on biomass production were insignificant, but the root fresh weight (FW) significantly decreased with all B levels. Higher B levels (1.2 and 1.6 mM) caused decreases of 22.9% and 29.6%, respectively. The effects of all B levels on photosynthetic pigment contents [chlorophyll (Chl) a, b, a + b, and carotenoid (Car)], root and shoot membrane permeability (MP), and root, shoot, and leaf nutritional status [phosphorus (P), potassium (K), calcium (Ca), and sodium (Na) concentrations] were found insignificantly. However, all B levels caused a significant increase in the B concentrations of the root, shoot, and leaf of scallions and plants translocated the majority of applied B into their leaves. The translocation factor (TF) of B from the root to the leaf was found to be 138.2%, 133.3%, and 107.3% with 0.8, 1.2, and 1.6 mM B levels, respectively. Moreover, plants exposed to high levels of B showed no significant response or toxicity symptoms. We concluded that B is a phloem mobile element in onion, a non-graminaceous monocotyledonous plant, and therefore accumulates in the upper organs but illustrates partial toxicity symptoms in leaves. Studies with higher B concentrations could be recommended to determine critical B levels for green onion production in B-contaminated areas.