Effect of ultrasound on postharvest quality and stress markers in tomato fruit during storage

Tomato (Lycopersicon esculentum) is one of the most consumed fruit/vegetables, and its consumption is associated with many health benefits. However, it is a perishable crop and its storable life is limited. One approach that is gaining interest is to intensify the natural defenses of crops against diseases, and enhance plant protective phyto-compounds. That can also be health-beneficial by conditioning with hormetic doses of abiotic stresses such as ultrasound. Thus, the objective of this work was to determine the hormetic dose of US (Power 135 W, Frequency 42±5 kHz and treatment time 0-90 min at temperature of 16°C), and to monitor the early stress markers (respiration, ethylene, H₂O₂ production), following the exposure to US. Physiological and biochemical responses such as color change (ripening and senescence) and primary metabolites, especially precursors of secondary metabolites were also monitored during storage. The ripening rate of treated tomato treated with US for 45 min was the lowest, suggesting that this was hormetic for ripening response. Early responses, ethylene production and H₂O₂ accumulation, were dose-dependent and reached peak levels 4 and 18 h after US application, respectively. Amino-acids (AA) with high titer (>100 µg of ribitol equivalent g^-1 dry peel) were GABA, proline, and serine, increased up to the hormetic dose, but decreased above the hormetic dose. However, the titers of branched-chain AAs (valine, leucine, and isoleucine) in tomato, treated with hormetic US dose, were lower compared to those of the control, but they increased with higher US doses. Reduced titers of these AAs may indicate their utilization as carbon substrate during stress-induced altered metabolism and biosynthesis of volatiles. Reduced titers of oxalate, citrate, ketoglutarate and succinate may suggest fast-acting TCA cycle. Enhanced titers of phenylalanine indicate activation of shikimic acid-phenylpropanoid pathways. Results suggest that early events of ethylene and H₂O₂ production and changes in the levels of AAs could be used as stress markers to infer the severity of the stress. However, the effect of the hormetic dose of US on the changes in the free AA profile may provide some insights into US-induced modifications in the secondary metabolites derived from those AAs.