Aerenchyma and barrier to radial oxygen loss are formed in roots of Taro (Colocasia esculenta) propagules under flooded conditions

Taro (Colocasia esculenta (L.) Schott) is cultivated primarily for its starchy underground stem (i.e., corm). It is adapted to both upland and wetland (i.e., flooded) conditions. Although taro is exposed to hypoxia that occurs in waterlogged soil, the mechanisms of its adaptation to hypoxia were unknown. To clarify the below-ground adaptation of taro to wetland conditions, we grew five taro cultivars/landraces hydroponically for 8 days under hypoxic conditions (n = 3) and analyzed: (1) the length of the longest root that emerged from the vegetative propagule; (2) aerenchyma (i.e., tissues containing air spaces); and (3) oxidation conditions around sides of root tips. Wild taro Āweu and the Chinese cultivar Bun-long had significantly longer roots than the Hawaiian cultivars/landraces Maui Lehua, Pi‘i‘ali‘i, and Ele‘ele Naioea (P < 0.05). Formation of aerenchyma, or air spaces that allow effective transportation of oxygen under hypoxic conditions, was observed consistently in roots of Āweu and Bun-long, but only occasionally in those of Hawaiian cultivars/landraces. In all cultivars/landraces, a pattern of radial oxygen leakage was detected only near root tips. In summary, taro appears to form aerenchyma and oxidize the rhizosphere around root tips under wetland conditions.