Effect of Salinity and Silicon Doses on Post-Harvest Quality and Shelf Life of Onion

Abstract

Salinity during pre-harvest limits the shelf life and post-harvest quality of produce. On the other hand, nutrition with silicon (Si) has been used to mitigate the effects of salinity on plants. The effects of salinity and nutrition with Si on plants are not yet well understood, especially in relation to post-harvest conservation and quality of onion bulbs. Thus, an experiment was conducted to evaluate the effects of salinity and fertilization with Si, in pre-harvest, on the morphophysiological characteristics of onion bulbs during shelf life. The experiment was set up in randomized complete blocks, with four replicates, and treatments arranged in split-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), the subplots were five levels of fertilization with Si (SD: 0; 41.6; 83.2; 124.8; and 166.4 kg ha^-1) and the sub-subplots were four shelf times (ST: 0; 20; 40; and 60 days after harvest). Irrigation water salinity and shelf time reduced firmness and increased the mass loss of onion bulbs during shelf life; while fertilization with Si did not significantly affect these variables. Salinity and Si did not alter tunic color, and this variable was dependent only on shelf time, which increased the expression of reddish colors. Salt stress reduced the contents of sugars and total soluble solids of onion bulbs during storage; however, Si supply improved the contents of these variables, with maximum values of 12.44% and 6.40 °Brix at Si doses of 127.0 kg ha^-1 and 121.8 kg ha^-1. Shelf time showed initial consumption of sugars, which had minimum values of 10.47% and 5.99 °Brix, at 13.2 and 16.7 days, increasing to 13.95% and 6.62 °Brix, at 47.6 days and 43.7 days, and decreasing to 12.25% and 5.65 °Brix, both at 60 days of shelf. Salinity reduced pH and increased titratable acidity during shelf life, while fertilization with Si did not affect these variables. In contrast, shelf time caused a significant increase in pH and a reduction in the total titratable acidity of the bulbs. Salinity, Si supply and shelf time increased the concentrations of pyruvic and ascorbic acids in onion bulbs during shelf life, as follows: the maximum concentrations of pyruvic acid were 6.93 and 6.75 μM g^-1 of FM, with EC of 2.81 dS m^-1 and SD of 78.3 kg ha^-1 both at 22.1 days of shelf life; and the maximum concentrations of ascorbic acid were 83.45 and 70.20 mg 100 g^-1 of FM, with EC of 4.00 dS m^-1 and SD of 166.4 kg ha^-1, respectively, both at 20 days of shelf life. Salinity stimulated the catabolism of metabolites during onion shelf life, while nutrition with Si attenuated the impacts caused by moderate salinity, contributing to increasing the synthesis of metabolites and prolonging the shelf life of onion bulbs.

Keywords: Allium cepa L., Physiology of vegetables after harvest, Mineral nutrition, horticulture, Diatomaceous earth, Melosira granulata