Root aeration in rice (Oryza sativa):: evaluation of oxygen, carbon dioxide, and ethylene as possible regulators of root acclimatizations

Adventitious roots of rice (Oryza sativa) acclimatize to root-zone O-2 deficiency by increasing porosity, and induction of a barrier to radial O-2 loss (ROL) in basal zones, to enhance longitudinal O-2 diffusion towards the root tip. Changes in root-zone gas composition that might induce these acclimatizations, namely low O-2, elevated ethylene, ethylene-low O-2 interactions, and high CO2, were evaluated in hydroponic experiments. Neither low O-2 (0 or 0.028 mol m(-3) O-2), ethylene (0.2 or 2.0 mu l l(-1)), or combinations of these treatments, induced the barrier to ROL. This lack of induction of the barrier to ROL was despite a positive response of aerenchyma formation to low O-2 and elevated ethylene. Carbon dioxide at 10 kPa had no effect on root porosity, the barrier to ROL, or on growth. Our findings that ethylene does not induce the barrier to ROL in roots of rice, even though it can enhance aerenchyma formation, shows that these two acclimatizations for improved root aeration are differentially regulated.