Ammonia accumulation in plant tissues: a potentially useful indicator of postharvest physiological stress

Ammonia accumulation in plant tissues is a consequence of protein catabolism and is reported to increase during senescence in various leafy and floral vegetables. Here we studied postharvest ammonia production to understand its relationship to the onset of senescence and to injury induced by modified atmospheres. The effect of storage temperature (0, 5 or 10°C) on ammonia accumulation and quality changes was studied in 5 cultivars of rocket leaves. The Weibull model was used to fit ammonia changes of each cultivar and storage temperature with a R² value higher than 0.98. A 2-way ANOVA on the kinetic constant obtained with fixed n factors, showed that temperature was the only significant factor affecting ammonia accumulation with time, and fitting data averaged across cultivars resulted in an even higher R² of 0.99. The Arrhenius model was used to calculate an energy of activation of 198 kJ when cultivar values were averaged. Moreover, a highly significant correlation was found with color changes (hue angle), indicating that ammonia accumulation may be directly or indirectly related to phenomena associated with senescence. The effect of CO₂ and O₂ concentrations was studied on mint leaves, where increasing CO₂ levels induced very evident browning; there were concurrent increases in ammonia with increasing CO₂ concentrations and time of storage at 5°C. A higher ammonia accumulation was also found with fresh-cut artichokes that browned during storage in atmospheres with CO₂ higher than 12%. In cauliflower florets, high CO₂ concentrations (15%) with 1 or 5% oxygen resulted in increased ammonia accumulation and product discoloration, whereas moderate CO₂ (7.5%) at 5°C helped preserve product quality and was associated with relatively small increases in ammonia. These results further demonstrate that ammonia may be an indicator of postharvest senescence and stressful conditions.