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Roots of intact wheat plants were grown for 7-12 d in stagnant nutrient solution, containing 0.1% agar, to mimic the lack of convection in waterlogged soil. Net K+ and P uptakes by seminal and nodal roots were measured separately using a split root system. For seminal roots in stagnant solution, net uptakes as a percentage of aerated roots were between 0% and 16% for P, while K+ ranged between 15% uptake and 54% loss. For the more waterlogging-tolerant nodal roots, net uptakes in stagnant nutrient solution, as a percentage of aerated roots, were 31-73% for P and 69-115% for K+. Elongation rates of nodal roots in stagnant nutrient were about 35-43% of those for roots in aerated solution. This partial inhibition occurred in these nodal roots despite their 15% porosity (v/v). Elevation of O2 partial pressures around the shoots to 40 kPa and then to 80 kPa substantially accelerated nodal root elongation in stagnant solution, demonstrating that most of the inhibition seen with ambient O2 around the shoots was associated with a restricted O2 supply to these nodal roots. Thus, in wheat nodal roots, with a partial pressure of 20 kPa O2 around the shoots, O2 diffusion from the shoots did not completely relieve the restrictions on elongation resulting from stagnancy in the nutrient solution. These results contrast with those in the literature for rice, in which roots function efficiently in stagnant solutions (0.1% agar). So, when wheat roots are aerenchymatous there are still restrictions to O2 diffusion in the gas space continuum between the atmosphere and the functional tissues of the roots. This poor acclimation must have been due to inefficiency of the aerenchymatous axes, which may include persistence of anoxic steles, and/or restricted O2 diffusion in other parts of the gas space continuum, in either the shoots and shoot-root junction or in the root tip.

Environment and plant measurements were made to determine what factors may limit gr-owth of deepwater and floating rice plants dur-ing par-tial or complete submergence. Field surveys included measurements of temperature, pH, light, Oj and CO, in floodwater in Thailand. In addition, measurements were made of Oj and CO, concentrations inside internodal laeunae of deepwater and floating tiee growing at 0.5-2.0 rn water depths.The bulk of measurements wet-e taken during periods when the changes in water level were less than 50 mm d~'. In the 0-0.02 rn surface layer of floodwater at any location there wer-e lar-ge changes in oxygen concentrations over diurnal eyeles: there were decreases during the nighl down to 0,02-0,18 mol m"^ Oj at 0600 h and increases during the day to 0.13-0.28 mol rn~-^ O, at 1500 h (0,28 mol rn"'' being 120% ofthe O, concentration of air satur-ated water at 30 "C). Dur-ing Ihe day oxygen concentrations deereased with inereasing water depth; coneentr-ations just above the soil surface were occasionally zero. Most of this gradient disappear-ed during the night, and at dawn the 0.6 rn surface layer of water had uniform low O, concentr-ations.O2 concentrations were also measured during flash floods in Thailand. In contrast to the conditions with only small inct-eases in water level, the O, eoncentrations in the water during flash floods were more unifor-m with depth and changed little over a diurnal cycle, the O2 ranging between 0.14-0,19 mol rn"-'.In most locations floodwater contained 0.2-1,9 mol m •' CO, and 0.7-1.6 rnol rn"-' bicarbonate; however, in a location with aeid sulphate soil COj was only 0.05 0.2 rnol rn"', and bicarbonate concentrations were several fold lower. Concentrations of COj in floodwater increased with increasing water depth.O2 and CO2 concentrations inside internodal lacunae of riee were determined in the field when waler depth were 12 rn. Concentrations of O2 in internodes at the water surfaee wete 16-20%, and •Present address: Rice Research Inslirure. Deparrnienr of Agriculture, Bangkhen, Bangkok, Thailand 10900.decr-eased to 10% and 5% at 0.8 and 1.8 rn water depth respectively. There was no diurnal cycle in O, concentr-ations inside internodes. In contrast, CO2 concentrations in the lacunae incr-eased with water depth and ranged fr-om 1-3% in internodes at the water sur-face to 5-10% in internodes at 1.8 rn water depth. There was evidence for a diurnal eyele in CO, concentr-ations in the basal internode near the soil surface, CO2 incr-eased during the day and decreased during the night.The above data are used to show that there is little or no r-elationship between gas concentrations in floodwater and internodal lacunae of t-ice plants.Results at-e diseussed in relation to O, supply to subtnerged portions of rice and tnetabolism of" these tissues at low O2 concentrations.