Elongation by primary lateral roots and adventitious roots during conditions of hypoxia and high ethylene concentrations

Soil flooding results in unusually low oxygen concentra­ tions and high ethylene concentrations in the roots of plants. This gas composition had a strongly negative effect on root elongation of two Rumex species. The effect of low >xygen concentrations was less severe when roots conained aerenchymatous tissues, such as in R. palustris Sm. thyrsiflorus Fingerh., which has little root porosity, was nuch more affected. Ethylene had an even stronger effect >n root elongation than hypoxia, since very small concenrations (0*1 cm3 m-3) reduced root extension in the two pecies, and higher concentrations inhibited elongation lore severely than did anoxia in the culture medium, hus, ethylene contributes strongly to the negative effects f flooding on root growth. An exception may be the highly erenchymatous, adventitious roots of R. palustris. erenchyma in these roots provides a low-resistance diffuon pathway for both endogenously produced ethylene id shoot-derived oxygen. This paper shows that exten­ on by roots of R. palustris in flooded soil depends almost impletely on this shoot-derived oxygen, and that renchyma prevents accumulation of growth-inhibiting 1 vels of ethylene in the root. A y-w ords : Rum ex pa lustris Sm .; Rum ex thyrsiflorus ngerh.; Polygonaceae; aeration; aerenchyma; diffusion; e lylene; flooding; oxygen; root elongation.