Production and Morphophysiology of Onion Grown Under Salinity and Fertilization with Silicon

Salinity limits agricultural yield in the arid and semi-arid regions of the world. Silicon (Si) is beneficial to plants and can be used as an attenuator of abiotic stresses. The beneficial effect of Si has not yet been fully clarified for onions. A field experiment was conducted in the Brazilian semi-arid region to evaluate the effects of fertilization with Si in onion cultivation, under irrigation with saline water, on aspects related to production, plant physiology and post-harvest quality of bulbs. The experiment was designed in randomized complete blocks, with four replicates, and treatments were arranged in split plots. The plots were four levels of electrical conductivity of irrigation water (EC: 0.65; 1.7; 2.8 and 4.1 dS m-1) and the subplots were five levels of fertilization with Si (SD: 0; 41.6; 83.2; 124.8 and 166.4 kg ha-1 of Si), in the form of fertilizer based on diatomaceous earth derived from the species Melosira granulata. Increase in irrigation water salinity significantly reduced onion yield and bulb fresh weight, while increasing the production of small bulbs (<50 mm). In addition, salinity hampered the physiology of onion plants (negatively affecting the membrane instability index, relative water content and contents of total chlorophyll and carotenoids) and reduced the post-harvest quality of onion bulbs (inducing reductions in firmness, pH, soluble solids concentration and SS/TA ratio). On the other hand, fertilization with Si increased onion yield, bulb fresh weight and production of large bulbs (≥ 50 mm), while attenuating the negative effects of salt stress under low salinity. The increase of Si in the soil also affected the physiology of plants and bulbs after harvest, improving the concentrations of chlorophylls and carotenoids, total sugars and soluble solids, and ascorbic and pyruvic acids. Thus, it was observed that salinity and fertilization with Si trigger acclimation responses related to the synthesis of soluble sugars, titratable and ascorbic acids and thio-compounds identifiable in PyA. In addition, fertilization with Si improved the acclimation responses of onion under stress of semi-arid climate and salinity, so the recommended Si doses are 27.8 kg ha-1, under cultivation with the use of good quality irrigation water (0.65 ± 0.3 dS m-1) or 166.4 kg ha-1, under cultivation with the use of saline water with salinity of 1.7 ± 0.3 dSm-1.