Shoot responses of six Lythraceae species to flooding

The large family Lythraceae has several genera and species that show tolerance to flooding; one species, Lythrum salicaria (purple loosestrife), is considered invasive in North American wetlands. It is not clear, however, which characteristic(s) contribute to the invasive nature of L. salicaria, but those that contribute to improved flood tolerance may be responsible. This study examined the response of the shoot system of several members of the Lythraceae, three Lythrum species (L. salicaria, L. hyssopifolia, L. alatum), Decodon verticillatum, Pleurophora anomala and Heimia myrticifolia, to flooding to determine if these species differ in their response in comparison to L. salicaria. All species, except L. hyssopifolia, responded to flooding by increasing total plant height. All species, except H. myrticifolia, formed a phellem of significantly wider diameter at the stem base of hooded plants compared to controls. This phellem consisted of alternating bands of smalt, isodiametric cells and radially elongated cells separated by large air lacunae forming a very specialized aerenchyma. The small cells had Casparian band-like wall modifications and occasionally displayed modifications that included all cell wall surfaces. The development of extensive aerenchymatous phellem in flooded plants may increase the air space continuum from shoot to root in shoots that have undergone secondary growth. Given that these species displayed similar responses to flooding, the purported invasiveness of L, salicaria cannot be attributed to presence of any of the characteristics studied.