Peroxidases and lignification in relation to the intensity of water-deficit stress in white clover (Trifolium repens L.)

To investigate the lignification process and its physiological significance under drought-stressed conditions, the changes in enzymes responsible for lignification and the related physiological parameters were determined in white clover (Trifolium repens L.) leaves during 28 d of water deficit treatment. Water deficit gradually decreased leaf water potential (Psiw) to -2.33 MPa at day 28. For the first 14 d of water deficit, ascorbate peroxidase and phenylalanine ammonia-lyase were highly activated. Neither a change in the parameters symptomatic of oxidative stress nor growth inhibition was observed. The reduction of leaf biomass occurred from 21 d of water deficit treatment when Psiw was -2.27 MPa or less, and was concomitant with the increase of lipid peroxidation and lignin content. As Psiw decreased below -1.67 MPa from 14 d of water deficit, the enhanced activation of guaiacol peroxidase, coniferyl alcohol peroxidase, syringaldazine peroxidase, and benzidine peroxidase was involved in lignification rather than in protection of plant tissues against the oxidative damage. The data indicate that a high activation of lignifying enzymes during terminal stress may be a drought stress-induced injurious symptom, which leads to reduced forage growth and digestibility.