Throwing away to survive: rhizome senescence as a strategy adopted by an herbaceous clonal plant to remain young forever

BACKGROUND AND AIMS: Senescence is a final stage of plant development occurring from cells and organs to whole organisms. In clonal herbs, the senescence of a clonal growth organ (e.g. rhizome) causes physical separation of offspring rooting units (ramets) from one genetic individual (genet). Although well-documented in aboveground organs and fine roots, senescence in rhizomes has not been studied, despite its potential implications in clonal multiplication, plant carbon economy, and soil carbon cycles. This study investigates structural and functional changes related to senescence in belowground organs. METHODOLOGY: Twenty clonal fragments of the alpine species Rumex alpinus (7 to 14 years old) were collected at the Krkonoše Mountains, Vrchlabí - Czech Republic. Rhizomes and adventitious roots were subjected to morphological, anatomical, histochemical, and potential hydraulic conductivity analyses. KEY RESULTS: As rhizomes age, we observed colour shift from yellowish to dark brown or black, turgor loss and shrinkage in the blackened parts. Anatomical analysis showed progressive deterioration of inner integrity, with increased damage and hollows bordered by wound tissue in older segments. Calcium oxalate crystals in parenchyma cells and vessel elements sealed with phenolic, lipid compounds, and tylosis were more frequent in the middle and old segments compared to the young. The potential hydraulic conductivity of rhizome and roots was assured as it increased towards the oldest parts due to large and more lignified vessels. Abscission zone, like those found in other plant organs, was identified separating the oldest living portion from the decaying part. CONCLUSIONS: Our study is the first to investigate anatomical changes and abscission zone formation during rhizome ageing. Rhizomes thus undergo programmed death, like leaves and fruits. The drivers of programmed death in rhizomes require further studies that may contribute to our understanding of how rhizome longevity influences clonal plant performance and contributes to the plant and soil carbon cycle under varying environmental conditions.