Phenylalanine ammonia-lyase, peroxidase, pyruvic acid and total phenolics variations in onion bulbs during long-term storage

Phenylalanine ammonia-lyase (PAL) and peroxidase (POD) enzyme activities, t otal phenolics and pyruvic acid (PA) (as pungency indicator) were measured in onion bulb tissues (c.v. Rouge Amposta) during storage at 4 and 20 degre es C. After 2 wk, PAL activity increased at 4 degrees C, but decreased at 2 0 degrees C and after 4 wk. PAL activity at 20 degrees C was higher than ac tivity observed at 4 degrees C. Peroxidase activity was high during the fir st weeks of storage, but at 4 degrees C this high activity lasted longer an d a decrease was noted at both temperatures, coinciding with sprouting appe arance. Total phenolics, which rose during the first 12 wk but fell during last weeks of storage, appeared to be linked to PAL activity. An inverse re lationship was observed between phenolic content and the amount of sproutin g development of bulbs. Pyruvic acid production appeared to be influenced b y temperature, but in the last period of storage, the effect of temperature on PA production was less. The data demonstrate that PAL activity, which i s linked to phenolic metabolism, and POD are highly involved in the sprouti ng of onion bulbs and these two enzymatic activities are much influenced by low temperature. (C) 2000 Academic Press.