ELONGATION BY PRIMARY LATERAL ROOTS AND ADVENTITIOUS ROOTS DURING CONDITIONS OF HYPOXIA AND HIGH ETHYLENE CONCENTRATIONS

Soil flooding results in unusually low oxygen concentrations and high ethylene concentrations in the roots of plants. This gas composition h ad a strongly negative effect on root elongation of two Rumex species. The effect of low oxygen concentrations was less severe when roots co ntained aerenchymatous tissues, such as in R. palustris Sm. R. thyrsif lorus Fingerh., which has little root porosity, was much more affected . Ethylene had an even stronger effect on root elongation than hypoxia , since very small concentrations (0.1 cm(3) m(-3)) reduced root exten sion in the two species, and higher concentrations inhibited elongatio n more severely than did anoxia in the culture medium. Thus, ethylene contributes strongly to the negative effects of flooding on root growt h. An exception may be the highly aerenchymatous, adventitious roots o f R. palustris. Aerenchyma in these roots provides a low-resistance di ffusion pathway for both endogenously produced ethylene and shoot-deri ved oxygen. This paper shows that extension by roots of R. palustris i n flooded soil depends almost completely on this shoot-derived oxygen, and that aerenchyma prevents accumulation of growth-inhibiting levels of ethylene in the root.