SUMMARYNinety-one plant species from wetland (WL), intermediate (INT) and non-wetland (NWL) habitats were grown in flooded and drained soils and responses to flooding were assessed in relation to root anatomy and fractional root porosity (FRP).Flooding response and tolerance were related to FRP. Rooting depth increased with FRP in accordance with diffusion model predictions and emphasized the ventilating efliciency of aerenchyma. Major determinants of FRP were cortical cell conflgurations, closeness of cell packing, the relative proportions of conflguration types, porous: non-porous tissue ratios, aerenchyma development and the type and degree of secondary growth. A classification of cortical types based on cell and aerenchyma patterns is presented.Aerenchyma, both schizogenous and lysigenous, developed preferentially where preaerenchymatous cortical cell configurations (in TS) were radial and particularly, cubic and radial typical of WL and INT species. Aerenchyma rarely formed from hexagonal non-radial arrays which occur chiefly in NWL and in the outer cortical zones of WL and INT plants. The ventilating potential in non-aerenchymatous tissue was shown to be greater for cubic (fractional porosity, FP, max. 0-2146) than for hexagonal arrays (FP max. 0-0931); closer packing greatly accentuated the differences. It is suggested that cubic: hexagonal zonal ratios in roots may reflect a balance between respiratory and mechanical needs.In a majority of WL and INT species, shoot weights were unaffected by or increased with flooding and maximum rooting depths usually much exceeded 50 mm. Cubic packing raised the FRPs, as did aerenchyma which was often much greater under flooding. In the dicotyledonous species, a suppression of secondary growth in some, and a highly porous phelloderm in others, helped maintain high FRP. A minority of species were anatomically and responsively similar to NWL plants; survival under flooding was attributed to shallow rooting.Under flooding, the FRP of almost all NWL species was < 0-055 due to hexagonal packing, a lack of aerenchyma and, in dicotyledonous plants, secondary growth with scanty phelloderm. Shoot weights were reduced in 50 % of cases, rooting depths were < 50 mm, and some species died. Some species were exceptional in having cubic and radial packings; a lack of aerenchyma was associated with continuing meristematic activity in the primary cortex.

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SUMM.\RYTwo varieties of rice (cv, Norin 36 and RB3) were either grown in stagnant or aerated |-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, AgNO^ was very effective in reducing porosity by inhibiting aerenchyma developnnem. This effect was antagonized by gassing with increasing concentrations (1 or 2 jtX 1"') 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 AgNO^j, 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 AgNO^ had decreased it. Consistent with earlier studies, and with the ethene hypothesis, aerenchyma development in both \arieties was also enhanced by soil waterlogging, with percentage porosities 12 units higher in flooded than in drained soil. The porosities of c\', 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 AgNO^ had no signiflcant effect on porosity in stagnant or in aerated roots of cv. RB3, and hoth 1 and 2 /d 1"' ethene increased the porosity in cv. RB3 roots to a similar level. Silver nitrate, however, did antagonize the effects of 1 and 2 fi\ 1"' ethene. In contrast, porosity in cv. Norin 36 was progressively reduced by AgNOg, 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.