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.
S LI M M A R YGlass chambers were used to isolate 0-75 m deep water columns in situ in deepwater rice fields in 1'hailand to evaluate tbe contribution by various factors to the pronounced diurnal changes in O., concentrations in the floodwater. Chamber.s filled with distilled water provided estimates of Oj flow fron-i the air into the water, 'ndicating the importatice of eddy eurrents associated with wind action and evaporative cooling. Darkening the natural water gave estimates for Oj evolution due to photosynthe.sis, w-hile eomparisons between columns with and without rice distinguished between O., evolution and consumption by tbe rice and tbe bion-iass other that-i rice respectively. Removal of tbe above water portions of the rice shoots showed the in-iportat-ice of the emergent portion of the canopy to ()., movement frot-i-i the air into the aciuatic part of the ecosystem. This flow may be associated with O., flow through the lacunae and via air envelopes around the leaf, and with turbulence caused by swaying of the rice leaves in the wind.The changes in O, concentration were analysed c^uantitatively using several simplifying assut-nptiot-is. One major input was a net O., flow from the air into the water, about half being due to a net flow-aeross the air-water interface and the other half to flow-associated witb the rice plants. During the night, these flows were less than the ()., uptake bv tbe biomass, hence O., concentrations decreased. In the underwater portion of the riee stand, about 60 "" of the O, uptake was due to the rice and about 40",, due to tbe biomass otber tban rice. During the day, the O.r equirements by respiration were more tban compensated for by ()" evolutioi-i due to photosynthesis, resulting in increases in Oj concentration of the water.
The ability to withstand complete submergence was studied in R2 seedlings raised by self‐pollination from 158 R1 plants regenerated from callus of mature rice seeds (cultivars FR13A and Calrose). Compared to parental controls, significant improvement in submergence tolerance as assessed by percentage survival, number of new leaves and chlorophyll content of the third leaf, was found in 5 of the 115 cv. FR13A somaclones and 3 of the 43 cv. Calrose somaclones tested using an aquarium system in a controlled‐temperature glasshouse. With some exceptions these improvements were also observed in the R3 generation when tested under field conditions in Thailand. Variation in other agronomically important characters was observed in the R2 plants grown in a temperature glasshouse under 2 contrasting environments.
Concentrations of N, P and K were measured in floodwater and in floating rice cultivars growing at up to 2 m water depths in the central flood plain of Thailand. Concentrations of N, P and K in floodwater were often higher than those reported for oligotrophic lakes, nevertheless the floodwater contained 4-45 times less K and 15-90 times less N than concentrations reported in most soil solutions. P concentrations were similar in the floodwater and in most soil solutions.Concentrations of nutrients in leaves indicated there may be deficiencies of P at two sites and a deficiency of N at one of two sites. Data are discussed in terms of reduced nutrient uptake in rice fields at low 02 concentrations which have recently been measured in these areas.
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