The Aerenchymatous Phellem of Lythrum salicaria (L.): a Pathway for Gas Transport and its Role in Flood Tolerance

Coming up for air

Plants that are subject to flooding or which grow partially submerged often develop aerenchyma - air-conducting channels formed in the cortex - in order to supply air to the submerged parts of the plant. However, aerenchyma formation is problematic in plants that undergo secondary growth. In secondary growth the tissues in which aerenchyma normally forms are often lost, depriving the plant of a route for transporting oxygen to submerged organs. It has been widely suggested that phellem, a corky outer layer derived from the cork cambium (phellogen) may function as aerenchyma, but until now no-one had tested that hypothesis. However, this deficiency has been remedied by a group at Guelph University (Stevens et al., pp. 621-625). Laboratory-grown plants of Lythrum salicaria (purple loosestrife, a familiar waterside plant in temperate regions), were subjected to immersion of their roots and the lower parts of their shoots. The phellem was ringed at the stem base, breaking the continuity of the tissue. Gas exchange and growth were monitored during continued immersion. The results were clear: ringing the phellem reduced markedly the oxygen content of roots but not of shoots. This suggests that the phellem does indeed have a role in root aeration. However, the fact that the roots still contained significant amounts of oxygen suggests that there may also be other routes for air conduction. Another interesting feature is that ringed plants showed a reduced root biomass and an increased shoot biomass, the latter being very obvious in increased plant height. Waterside and amphibious plants that become completely submerged often respond with rapid shoot elongation and thus regain contact with the air. However, in these experiments, the shoots were never out of contact with the air so the function of this shoot growth is not clear. Further experimentation is obviously needed.