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 physiologicalsignificance under drought-stressed conditions, the changesin enzymes responsible for lignification and the related physiologicalparameters were determined in white clover (Trifolium repensL.) leaves during 28 d of water deficit treatment. Water deficitgradually decreased leaf water potential (_w) to –2.33MPa at day 28. For the first 14 d of water deficit, ascorbateperoxidase and phenylalanine ammonia-lyase were highly activated.Neither a change in the parameters symptomatic of oxidativestress nor growth inhibition was observed. The reduction ofleaf biomass occurred from 21 d of water deficit treatment when_w was –2.27 MPa or less, and was concomitant with theincrease of lipid peroxidation and lignin content. As _w decreasedbelow –1.67 MPa from 14 d of water deficit, the enhancedactivation of guaiacol peroxidase, coniferyl alcohol peroxidase,syringaldazine peroxidase, and benzidine peroxidase was involvedin lignification rather than in protection of plant tissuesagainst the oxidative damage. The data indicate that a highactivation of lignifying enzymes during terminal stress maybe a drought stress-induced injurious symptom, which leads toreduced forage growth and digestibility. Key words: Drought, lignification, peroxidases, phenylalanine ammonia-lyase, polyphenol oxidase, Trifolium repensReceived 23 July 2006; Revised 18 November 2006 Accepted 20 November 2006