Protein content and protease activity in senescing roots and leaves of wetland monocot species with contrasting root turnover strategies

Abstract

Perennial herbaceous monocots in Northern Ontario wetlands can be classified in two distinct types of root turnover strategies: those with overwintering roots, and those with complete root mortality at the end of the growing season. All species have autumnsenescing leaves. The present thesis is part of investigations to understand adaptive advantages of the two strategy types, focusing on nutrient remobilization from senescing roots. Existing data on nutrient remobilization from senescing roots is based on changes in element content in dying roots, and do not differentiate between remobilization and leaching out. Root protein content and aminopeptidase activity was assessed for gardengrown plants of six species from September to November, three species with autumnsenescing roots (Rhynchospora alba, Sagittaria latifolia, Sparganium americanum) and three with overwintering roots (Carex oligosperma, Iris versicolor, Scirpus microcarpus). We hypothesized that protein degradation and protease activity would be higher in autumn-senescing roots. The results confirm the existence of two root turnover strategies, species with annual roots showing a decline in root protein content, while species with perennial roots did not show such a decrease. Leaf protein content deceased in all species but C. oligosperma, known to senesce late. Total root aminopeptidase activity per fresh mass decreased in species with annual roots, but not in those with perennial roots. In contrast to expectation, specific aminopeptidase activity did not change over time and did not differ between the strategies. We conclude that nitrogen remobilization is an active process in senescing roots, and in autumn occurs only in annual roots. However, temporal characterization of root enzyme activities requires more detailed investigations