Two varieties of rice (ev. Norin 36 and RB3) were either grown in stagnant or aerated 1/4-strength Hoagland's solution with or without exogenous ethene and a range of silver concentrations (an ethene antagonist), or were grown in flooded and drained soils.
With cultivar Norin 36, AgNO3 was very effective in reducing porosity by inhibiting aerenchyma development. This effect was antagonized by gassing with increasing concentrations (1 or 2 μ1−l) of ethene. The results are consistent, therefore, with reports that cavity formation in roots is controlled by endogenous levels of ethene. Also, porosity varied with root length, and if a correction was made for this, the inhibitory effect of the silver ion on aerenchyma development appeared to be even greater.
For the cultivar RB3, root porosities in solution culture appeared initially to be unaffected by ethene or AgNO2, but root lengths were reduced and root numbers increased by increasing ethene concentration. When these effects on root length were taken into account, it was concluded that ethene had increased root porosity, and AgNO3, had decreased it. Consistent with earlier studies, and with the ethene hypothesis, aerenchyma development in both varieties was also enhanced by soil waterlogging, with percentage porosities 12 units higher in flooded than in drained soil. The porosities of cv, RB3 were higher than those for cv. Norin 36, however, and the regression line of porosity against root length was steeper, indicating a greater predisposition to form aerenchyma in cv. RB3. This was confirmed in solution-grown roots, where AgNO3 had no significant effect on porosity in stagnant or in aerated roots of cv. RB3, and both 1 and 2 μ1−1 ethene increased the porosity in cv. RB3 roots to a similar level. Silver nitrate, however, did antagonize the effects of 1 and 2 μ1−1 ethene. In contrast, porosity in cv. Norin 36 was progressively reduced by AgNO3.
It is concluded that, although there are intervarietal differences in sensitivity, an ethene promotion of gas-space formation can occur in rice, and ethene should not be ruled out as the endogenous promoter.