ACTIVITY OF PHENYLALANINE AMMONIA-LYASE (PAL) AND CONCENTRATIONS OF ANTHOCYANINS AND PHENOLICS IN DEVELOPING STRAWBERRY FRUIT

Studies on regulation of production of phenolics in strawberry (Fragaria x ananassa Duch.) fruit were initiated by monitoring phenylalanine ammonia-lyase (PAL) activity and levels of anthocyanins, flavonoids, tannins, and other soluble phenols throughout fruit ontogeny in 'Tillikum'. PAL catalyzes the first step in the biosynthesis of phenylpropanoids, which are further modified into a wide variety of phenolic compounds. Peak in PAL activity (1 mol.s-1 = 1 kat) of 90 pkat.mg-1 protein was detected at 5 and 27 days after anthesis (DAA), when fruit was green and nearly ripe, respectively. PAL activity was only almost-equal-to 10% of peak values in the white berry stage, when fruit growth was most rapid. The second peak in PAL activity was followed by a rapid drop, to nearly zero in red-ripe fruit at 30 DAA. Total soluble phenols reached a maximum level soon after anthesis, just before the first peak in PAL activity, then declined to a low constant value well in advance of fruit ripening. Similar changes were observed in levels of tannins and flavonoids that, at anthesis, accounted for 44% and 51% of the soluble phenols, respectively. The concentration of anthocyanin was very low throughout most of fruit development, but beginning at 23 DAA it increased from < 0.03 to > 0.53 mg.g-1 fresh weight in 3 days. This accumulation paralleled the second rise in PAL activity. Accordingly, strawberry fruit have a developmental-dependent expression of PAL activity and accumulation of phenolic substances derived from the phenylpropanoid pathway.