PHENYLALANINE AMMONIA-LYASE AS A PRECURSORY ENZYME LEGUME STEM LIGNIFICATION

In some instances, lignin content may not be significantly correlated with phenylalanine ammonia lyase (PAL) activity because: (1) PAL is no t committed exclusively to lignin, and (2) the time of maximum PAL act ivity may not coincide with maximum lignin deposition. This study eval uates correlations and timing of PAL activity and lignin deposition du ring legume stem maturation. Three forage legumes, alfalfa (Medicago s ativa L.), birdsfoot trefoil (Lotus corniculatus L.), and red clover ( Trifolium pratense L.), were established, and basal stem regrowth was sampled, biweekly, for 10 wk, for dry weight (DW), cell wall (CW), lig nin, and PAL analyses. Nonlinear regression of lignin content by the G ompertz function indicated that lignin increased sigmoidally, and PAL activity by the third-order quadratic demonstrated rapid initial incre ases in activity, followed by decreases, as a function of regrowth day s. First derivative of the Gompertz function demonstrated that changes in lignin deposition closely resembled changes in PAL activity. Among species, peak deposition of DW and CW content occurred 3-11 d prior t o maximum lignin deposition. Time of maximum PAL activity occurred 8 d prior to maximum lignin deposition in birdsfoot trefoil and red clove r and 3 days after maximum lignin deposition in alfalfa. Across specie s, lignin content was not positively correlated with PAL activity on a protein basis. However, lignin deposition was positively correlated w ith PAL per unit protein (r = 0.76, P < 0.05) and lignin content was p ositively correlated with PAL on a per plant basis (r = 0.60, P < 0.05 ). These results indicate that the activity of PAL is related to ligni n deposition in a cause-and-effect relationship.