Icons in the rhizosphere: the role of older roots.
North, Gretchen*,1, 1 Occidental College, Los Angeles, CA
ABSTRACT- In the rhizosphere, older roots may be considered as icons and younger roots as upstarts. Most textbooks identify young upstart roots as the primary instruments of resource uptake and carbon exchange. Due to the low surface area to volume ratio of large woody roots, this characterization may be partly accurate. However, older roots of smaller diameter can be major contributors to resource uptake, if only because they constitute much of the root system. For both dicotyledonous and monocotyledonous species, external root tissues generally become more suberized and lignified with age, the former developing a periderm and the latter an exodermis; cortical cells may also be sloughed off, collapse, or become lignified. Despite such changes, older roots about 5 mm in diameter or less maintain relatively great hydraulic conductivity in two dicotyledonous plant families with many arid-land representatives, the Cactaceae and the Rhamnaceae. Resource uptake in both is facilitated by breaks in the periderm and by aquaporin activity within the stele. In the monocotyledonous family Agavaceae, all agaves and yuccas examined to date have roots with a contractile region near the shoot base with features associated with resource uptake: root hairs, living cortical cells, and an endodermis with many unsuberized passage cells. Aquaporin opening and closing in this root region has been measured in one species of agave, indicating continued metabolic activity in older roots. The radial expansion accompanying longitudinal contraction ensures good root-soil contact, further underscoring the iconic role of the oldest roots in resource uptake and possible exchange.
Key words: water uptake, contractile roots, Agavaceae
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