Seasonal protease activity in storage tissue of the deciduous grass Molinia caerulea

Plants of Molinia caerulea were grown in sand culture for two seasons. All nitrogen (N) supplied to the plants was enriched with N-15 throughout the f irst season, and at natural isotopic abundance throughout the second season . A series of destructive harvests was taken during the second season. At e ach harvest, the N mobilized from roots and swollen basal internodes and th e protease activity (ability to degrade azocasein) of these storage tissues were measured. The pH response curves of protease activity of both basal i nternodes and roots exhibited optima at pH 5 throughout the season. The pro tease activity of roots and basal internodes increased in spring, both on a unit fresh weight and unit protein basis, concomitant with mobilization of N from both these tissues to new shoot growth. In absolute terms more N wa s mobilized from roots than from basal internodes. However, basal internode s which, compared with roots, showed the greatest protease activity (on a f . wt or unit protein basis) were also the tissues that mobilized a greater proportion of N present in the tissue over winter to new shoot material. In dividual clones of M. caerulea varied both in the amount of N mobilized fro m roots and in root protease activity (per plant). Individuals with a highe r protease activity mobilized more N compared with individuals of lower pro tease activity. Therefore in M. caerulea, relationships between N mobilizat ion and protease activity exist at several levels: (1) between different ti ssues; (2) temporally throughout the season; and (3) between individual clo nes.