Gravitational stress and lignification in aerial vs. submerged shoots of Hippuris vulgaris

Hippuris vulgaris L. is a heterophyllic aquatic plant that grows naturally under the different degrees of gravitational stress that are associated with submerged and aerial environments. This characteristic of H. vulgaris was exploited in order to study the interaction of gravitational stress with lignification processes. Lignin content was found to be 4.1% of aerial stem dry weight and 2.6% of submerged stem dry weight. The activity of phenylalanine ammonia-lyase (E.C. 4.3.1.5), an enzyme early in the lignin biosynthetic pathway, paralleled lignin content and was about 5 times higher in aerial than in submerged stems. Another lignin biosynthetic enzyme, peroxidase (E.C. 1.11.1.7), was studied, and although definite conclusions could not be drawn from measurements of total peroxidase activity, different isozyme patterns were observed in aerial and submerged-type shoots. Abscisic acid, which can induce the aerial-type shoot morphology on submerged shoots, probably is not involved in mediating changes in the lignin content of H. vulgaris . These results support the hypothesis that lignin biosynthesis is regulated by gravity.