Studies of silica in the oat plant

Summary Oat plants,Avena sterilis L., were grown on soils in which the concentration of monosilicic acid in the soil solution, that is the level of supply of silica, ranged from 7 to 67 ppm SiO₂. Analyses at intervals throughout the growing period showed that the level of supply affected the amount and concentration of silica in the plant but not the pattern of its distribution among the parts.At maturity the caryopsis contained only 0.5 to 0.8 per cent of the total silica in the tops while the other parts of the inflorescence contained 40.7 to 41.3 per cent. The leaves (blade and sheath) contained 42.5 to 45.0 per cent of the total silica and the stems contained 7.8 to 10.9 per cent; the remainder was present in small sterile tillers.The concentration of silica in the dry matter was highest in the palea, lemma glumes, awn, and leaves. Among the leaves, the flag leaf had the highest silica content, both in terms of concentration in the dry matter and amount per leaf. The distribution of silica along a leaf followed a hyperbolic curve, the concentration being highest at the apex and lowest at the base of the blade.The chemistry of silica and the pattern of its distribution in the tops suggest that monosilicic acid and water move concomitantly in the transpiration stream and that solid silica is deposited in greatest quantities in those parts and regions from which water is lost in greatest quantities.     

Studies of silica in the oat plant

Summary Silicon in soil solutions is present entirely as monosilicic acid and soils are capable of maintaining a steady concentration in solution despite repeated withdrawals. This concentration is always below the saturation figure of 120 ppm SiO₂ and varies amongst soils of the same pH.Both ferric and aluminium oxides adsorb monosilicic acid but among oxides of similar crystallinity, aluminium oxide is more effective than ferric oxide. When added to a soil, both ferric and aluminium oxides lowered the concentration of monosilicic acid in solution. Aluminium oxide again had the greater effect and one sample lowered the concentration of monosilicic acid from 48 to 9 ppm SiO₂.In pot experiments oats were grown in a soil to which ferric and aluminium oxides had been added and also in soils differing naturally in their content of free sesquioxides. The uptake of silica by the plants can be explained simply in terms of the concentration of silica in the soil solution and the amount of water transpired. This indicates that oats take up silica passively in the transpiration stream.     

A study of the transpiration surfaces of Avena sterilis L. var. Algerian leaves using monosilicic acid as a tracer for water movement

Summary The sites and pathways of transpiration from leaves of Avena sterilis L. var. Algerian were studied using the accumulation of monosilicic acid as a tracer for water movement. Seedlings of Algerian oats were grown under silicon free conditions and fed monosilicic acid, in a normal nutrient solution, via the roots. The silicon component of monosilicic acid was located in freeze substituted tissue by means of x-ray microprobe analysis. Methods of tissue fixation preventing post treatment movement of tracer were developed and it was determined that monosilicic acid is a suitable tracer for water.Sites of water loss were marked by accumulation of silicon. Internal evaporating surfaces having a high intensity of water loss were demonstrated. Evaporation from epidermal surfaces was most intense over the guard and subsidiary cells with very little evaporation from the cuticular surfaces of normal epidermal cells. Moderately high evaporation occurred from epidermal fibre cells located above the veins. Evaporation from all exposed walls of guard cells including the wall adjacent to the pore was intense. Smaller amounts of tracer were located in the unexposed anticlinal walls of epidermal cells as well as within the unexposed walls of mesophyll cells. The results are interpreted as demonstrating the extent of internal transpiration surfaces and that cuticular epidermal transpiration is low. Strong support is given to the existence of peristomatal transpiration. Internal pathways of water movement are defined and the occurrence of these is discussed in relation to cuticular transpiration and lateral water movement in the epidermis.     

Lead uptake from solution by perennial ryegrass and its transport from roots to shoots

Summary The uptake of lead by roots and its transport to the shoots was examined with perennial ryegrass in solution cultures. Root uptake as measured by the decrease in concentration of lead in an aqueous solution containing 1 mg Pb/l as Pb (NO₃)₂ was rapid, almost complete, and unaffected by removing the shoots or killing the roots. Lead bound in the roots was not released by exchange with Ca or Ba ions. The distribution of lead within the plant was examined at intervals after a single, 3-day exposure to various levels of lead added to a nutrient solution. The total uptake, or lead burden, increased with increasing rates of addition and ranged from 281 to 9969 g/Pb per 3 plants. The proportion of the lead reaching the shoots at the first harvest (7 days after adding lead) was 3.5 to 22.7 per cent of total uptake, the lower value being for plants with the greatest burden. Transport to the shoots continued throughout the experimental periods of 21 and 28 days but did not exceed 28.9 per cent of total uptake. The concentration of lead in shoots at the first harvest ranged from 0.2 to 58.4 ppm and that in the corresponding roots from 5.5 to 5310 ppm. At later harvests, and after cutting, the concentration in the shoots decreased; an exception was in plants with the greatest lead burden. It is concluded that roots of actively growing ryegrass provide a barrier which restricts the movement of lead to the above-ground parts of plants, and so to animals or man.     

Lead uptake from soils by perennial ryegrass and its relation to the supply of an essential element (sulphur)

Summary The lead status of 16 soils of England and Wales was studied by pot-culture and soil chemical procedures. Perennial ryegrass was grown on the soils, with and without added sulphur, in a controlled environment cabinet with carbonfiltered air. Plant-available lead comprised uptake in 4 successive harvests of tops plus that in roots at the final harvest. The concentration of lead in the tops of healthy plants, i.e. those with adequate sulphur, was lower than in the roots, e.g. at harvest 4 the means were 5.0 and 12.9 ppm, respectively. However, with sulphur-deficient plants the concentration of lead in the tops was often higher than in the roots, the means at harvest 4 being 16.3 and 13.0 ppm, respectively. The marked increases in the concentration of lead in the tops of sulphur-deficient plants coincided with decreases in dry-matter yield, but for any one soil the tops of such plants contained similar amounts of lead to those of healthy plants. The lead content of the tops was poorly correlated with soil lead whereas that of the roots, in terms of both concentration and total amount, was highly correlated. The amount of lead extracted by 0.5 M BaCl₂ or 0.05 M EDTA provided a slightly better assessment of availability than ‘total’ content or the amount extracted by 2.5 per cent acetic acid. The solutions of acetic acid, BaCl₂ and EDTA extracted, on average, 1.0, 16.3 and 32.7 per cent respectively, of the total lead in the soils. The greater replacement of lead by the Ba ion than by the H ion (acetic acid) is ascribed to valence and the similar radii of Pb⁺ and Ba⁺. It is concluded that in soil-grown ryegrass the roots restrict the movement of lead into the tops of high-yielding plants, but when growth is limited by sulphur deficiency the concentration in the tops increases markedly. re]19720922     

Active and passive ion transport in relation to transpiration in Helianthus annuus

Summary The effect of transpiration on the uptake of K, Ca, NO₃ and SO₄ from two strengths of culture solution by Helianthus plants was investigated. In the dilute solution transpiration had no effect on the uptake of all the ions studied. In the more concentrated solution the uptake of K, NO₃ and SO₄ were found to be sensitive to changes in water flux across the root. Calcium uptake was found to be completely independent of water uptake in all the experiments.The results are interpreted in the light of the results from an earlier study on excised Helianthus roots. It is suggested that transpiration only affects the nonmetabolic transport of the ions across the root.     

Cadmium uptake from solution by plants and its transport from roots to shoots

Summary The uptake of cadmium by the roots of plants, and its transport to shoots was examined using solution culture. Uptake by the roots of perennial ryegrass over a period of 4 hours from an aqueous solution containing 0.25 ppm cadmium as CdCl₂ was (i) enhanced by killing the roots and (ii) depressed when Ca²⁺, Mn²⁺ or Zn²⁺ were added to the solution. The distribution of cadmium between the roots and shoots of 23 species was examined at 4 days after a single, 3-day exposure to a nutrient solution containing 0.01 ppm added Cd. In all except 3 species, i.e. kale, lettuce and watercress, more than 50 per cent of that taken up was retained in the roots. The concentration in the roots was always greater than in the shoots, and in fibrous roots of fodder beet, parsnip, carrot and radish it was greater than in the swollen storage roots. When perennial ryegrass was similarly exposed to solutions containing 0.01, 0.05, and 0.25 ppm added cadmium, uptake, as measured at 3 days after adding cadmium, increased with increasing rates of addition, but the proportion retained in the roots was constant (approximately 88 per cent). There was no further transport from roots to shoots during the next 21 days, with the result that the concentration in the shoots decreased progressively with increasing growth. It is concluded that although the roots of several species can take up large quantities of cadmium from solution there are mechanisms which may restrict the movement of cadmium through plants, and thus to animals.     

The Relationship between Transpiration and the Absorption of Inorganic Ions by Intact Plants

The relationship between the rate at which water and the rubidiumand phosphate ions are absorbed by intact plants, and transferredto their shoots has been investigated in water culture undervarying conditions of transpiration and nutrient supply. When the external concentration and the nutrient status of theplants are sufficient low, wide variations in the rate of transpirationhave little effect on the transfer of nutrients to shoots; whenlittle water is being lost by transpiration the concentrationin the transpiration stream may exceed that in the externalmedium by factors exceeding 100. In contrast when the externalconcenration and the nutrient status of the plants are highthe rate of transfer of ions to shoots may vary closely withthe rate of transpiration and the concentration in the transpirationstream may be similar to, or less then, that in the externalmedium. The occurrence of concentrations of ions in the roots is transpirationstream which greatly exceed those in the medium external tothe roots is regarded as evidence that ions not transferredpassively across the roots of intact plants to a significantextent.     

A Comparison of the Uptake and Translocation of Some Organic Herbicides and a Systemic Fungicide by Barley: I. ABSORPTION IN EELATION TO PHYSICO-CHEMICAL PROPERTIES

A comparative study has been made of the uptake by and translocationfrom roots of intact barley plants of six herbicides and a systemicfungicide (four triazines, diuron, 2,4-dichloro-phenoxyaceticacid (2,4-D) and ethirimol). Relationships between uptake andtranspiration rate are discussed in the light of the physico-chemicalproperties of these compounds, notably their partition coefficientsin oil/water systems and their dissociation constants. Apartfrom 2,4-D, sorption of these compounds appears to be a passiveprocess. At pH4 the uptake of 2,4-D seems to be influenced bymetabolism; not only may the concentration of this compoundin the transpiration stream be considerably greater than thatin the medium surrounding the roots but absorption by rootsis markedly reduced at low temperatures and by sodium azide. The initial rate of uptake of these compounds correlates reasonablywell with their partition coefficients in olive oil/water orn-dodecane/water systems; likewise the concentration in thetranspiration stream is greater for lipophilic than for lipophobicsubstances. Whereas the hydrogen ion and calcium concentrations of the ambientmedium appear to have no effect on the uptake of compounds withlow pK's, the uptake of those substances which protonate betweenpH4 and pH6 is affected by them. These findings are discussedfrom the viewpoint that the pathways of transport of lipophilicand lipophobic compounds across the roots may differ. Although there is some evidence that retention by roots canlimit transport to shoots, there is no simple inverse correlationbetween the total concentration of the different substancesin the roots and that in the transpiration stream. This questionis discussed in a subsequent paper.     

Identification of the silicon form in xylem sap of rice (Oryza sativa L.)

Rice (Oryza sativa L.) is a typical silicon (Si)-accumulating plant, but the mechanism responsible for the translocation from the root to the shoot is poorly understood. In this study, the form of Si in xylem sap was identified by (29)Si-nuclear magnetic resonance (NMR) spectroscopy. In rice (cv. Oochikara) cultured in a monosilicic acid solution containing 0.5 mM Si, the Si concentration in the xylem reached 6 mM within 30 min. In the (29)Si-NMR spectra of the xylem sap, only one signal was observed at a chemical shift of -72.6 ppm, which is consistent with that of monosilicic acid. A (1)H-NMR study of xylem sap did not show any significant difference between the wild-type rice and mutant rice defective in Si uptake, and the components of the xylem sap were not affected by the Si supply. The Si concentration in the xylem sap in vitro decreased from an initial 18 mM to 2.6 mM with time. Addition of xylem sap to a solution containing 8 mM Si did not prevent the polymerization of silicic acid. All these results indicate that Si is translocated in the form of monosilicic acid through the xylem and that the concentration of monosilicic acid is high in the xylem only transiently.     

Effect of Changes of Temperature Around Roots in Relation to Water Uptake by Roots and Leaf Transpiration

Effects of changes in temperature around roots on water uptake by roots and leaf transpiration were studied in Leucaena leucocephala (Lam.) de Wit., a subtropical woody plant species, and in Zea mays L. When the temperature around roots was rapidly lowered from 25 ℃ to 15 ℃, the water uptake by the roots and leaf transpiration were stimulated significantly within a short period ( 14 min). However, this effect did not occur when the cooling time was prolonged neither did if occur when the temperature around the roots was resumed from 15 ℃ to 25 ℃. Both the hydraulic conductivity of roots and leaf transpiration were increased substantially at first (within 20 min)and then decreased steadily to a level lower than those of the control in which the roots were continuous exposed to a low temperature ( 15 ℃ ). Low temperature also promoted the biosynthesis of ABA in roots and enhanced the xylem ABA concentration, but such stimulation did not occur untill about 30 min after cooling treatment, leaf transpiration was reduced markedly, but the hydraulic conductivity of roots increased when the root system was treated with exogenous ABA. It was suggested that some mechanisms other than ABA may be involved in the short-time cryostimulation of water uptake by roots and leaf transpiration.     

Uptake and Distribution of Mercury within Higher Plants

The uptake and distribution of inorganic mercury (HgCl₂) within higher plants (Pisum sativum and Mentha spicata) was examined using solution culture and radiotracer techniques. Plants were found to tolerate an external level of 1 mgHg/kg of solution but both physiological and biochemical processes were affected at 5 mgHg/kg and 10 mgHg/kg. The uptake of Hg into plants grown in hydroponic solution was a function of external concentration. Over the concentration range considered the accumulation of Hg in the roots was linear on a log-log basis although the uptake of the element into the shoots appeared to be two-phased. The distribution of Hg in plants was asymmetrical with much greater amounts of the element in the roots than the shoots. Although the level of Hg increased generally in plant tissues with increasing external levels, the proportion retained in the roots, relative to the shoots, was constant (approximately 95%). Two binding characteristics of the Hg within plant tissue were detected. A major proportion of Hg was tightly bound, being unaffected by treatment with ethanol and hydrochloric acid. The remaining Hg in the tissue was removed by either water or hydrochloric acid treatment. Cell fractionation indicated that the major binding component of Hg in plant tissues was the cell wall.     

Changes in Transpiration, Net Carbon Dioxide Assimilation and Leaf Water Potential Resulting from Application of Hydrostatic Pressure to Roots of Intact Pepper Plants

Hydrostatic pressures varying from 0 to 6.0 bar were applied to roots of intact Capsicum annuum L. cv. California Wonder plants growing in nutrient solution and the rates of transpiration, and net CO₂ assimilation, apparent compensation point and leaf water potential measured. Increasing the pressure on the roots of plants with roots in solution with either -0.5 or -5.0 bar osmotic potential with 1 bar increments resulted in a decrease in transpiration. With the application of 1 or 2 bar pressure the rate of transpiration returned to near or above the original rate. An application of 3 or 4 bar pressure reduced the rate of transpiration of all plants. The transpiration of plants with roots in solution with -0.5 bar osmotic potential remained at the reduced rate for as long as these pressures were maintained. The transpiration of plants with roots in solution with -5.0 bar was only temporarily suppressed at these pressures. Changing the applied pressure from 3 or 4 bar to 0 resulted in a rapid increase in transpiration which lasted approximately 15 minutes. This was followed by a decrease in transpiration to a rate lower than before the pressure was applied. The pattern of response was similar for plants at low or high light intensity or at normal or low CO₂ concentrations. When leaf diffusive resistance was 6.0 s cm^?1 or greater, changes in net CO₂ assimilation were similar to those of transpiration. The apparent CO₂ compensation point increased as pressure was applied and decreased with a release in pressure. Leaf water potential increased with an increase in pressure and decreased with a decrease in pressure. The changes in leaf water potential were frequently but not always proportional to changes in pressure. It is postulated that the respouses noted were due to changes in resistance to flow of water from xylem terminals through the mesophyll cells and stomatal cavities to the atmosphere.     

Morphological and physiological characteristics of a root-hairless mutant in rice (Oryza sativa L.)

This paper reports morphological and physiological characteristics of a first root-hairless mutant (RH2) of rice (Oryza sativa L.), which can be useful in advancing knowledge on the role of root hairs in water and nutrient uptake, and genetics of root hairs. The mutant was selected among NaN₃ mutagenized progeny of the rice cultivar Oochikara. Microscopic observations showed absence of root hairs in RH2. At the seedling stage, RH2 showed shorter seedling height and shorter roots compared to the wild type variety Oochikara. Because of the differences in seedling growth, all comparisons between Oochikara and RH2 in uptake-related characters were made on the basis of values adjusted by the dry weight of either the shoot or the root. When grown at low water potential in soil, Oochikara and RH2 were similar in shoot water content and transpiration per unit shoot dry weight, and similarly, at low water potential in solution culture, there was no significant difference between Oochikara and RH2 in transpiration per unit shoot dry weight. These results suggest that at the seedling stage, root hairs do not significantly contribute to uptake of water. In solution culture, Oochikara and RH2 did not significantly differ in phosphate uptake per unit root dry weight. This result supports the previous work that root hairs do not contribute to phosphate uptake in solution culture. Regarding to response to plant hormones, RH2 showed a higher level of resistance to two synthetic auxins, 2,4-dichlorophenoxyacetic acid (2,4-D) and 1-naphthaleneacetic acid (NAA) than Oochikara. NAA treatment induced very short root hairs in RH2, suggesting that the absence of root hairs in RH2 may be due to a shortage of endogenous auxin. Genetic analysis showed that the root hairless character in RH2 is inherited as a single recessive gene.     

Soil solute concentration and water uptake by single lupin and radish plant roots

Application of computer assisted tomography to gamma and X-ray attenuation measurements and Na⁺-LIX microelectrodes were used to determine the spatial distributions of soil water content and Na⁺ concentrations respectively near single roots of eighteen day old lupin and radish plants. These quantities were monitored at root depths of 3, 6 and 9 cm and at zero, 2, 4, 6, and 8 hour intervals from the diurnal commencement of transpiration. The plants were subjected to two levels of transpirational demand and five Na⁺ soil solution concentration levels. Water extraction rates for the lupin and radish roots increased continuously with time but were substantially reduced with increasing Na⁺ concentration in the treatment. Water uptake was uniform along the length of the essentially constant diameter lupin roots but decreased along the tapering radish roots as the diameter and hence the surface area per unit length of the roots decreased. The accumulation of Na⁺ at the root surfaces of both plants increased gradually with time in a near linear fashion and was slightly higher under the higher transpiration demand. These increases were not exponential as would be expected with non-absorption by the roots and this is considered to be due to back diffusion at the relatively high water contents used. At these water contents matric potentials had a much smaller influence on transpiration than osmotic potentials. The relationships between leaf water potentials (Ψ₁) and osmotic potentials at the root surfaces were linear with the decreases in Ψ₁ almost exactly reflecting the decreases in Ψ_π indicating rapid plant adjustment. Leaf water potentials decreased progressively with time and the relationships between leaf water potential and the transpiration rate were also linear supporting the suggestion of constant plant resistances at any given concentration.     

Characterization of fluoride uptake by roots of tea plants (Camellia sinensis (L.) O. Kuntze)

Backgrounds and aims

Tea plants (Camellia sinensis (L.) O. Kuntze) accumulate high fluoride in the leaves whereas the mechanism on its uptake is poorly understood. The measured F^? uptake was compared to calculated uptake from transpiration rates assumuing no discrimination between F^? and water to characterize the property of F^? absorption by tea plant roots.

Methods

The F^? uptake was examined by depletion method under variable external F^? concentrations, pH, temperature, relative air humidity, anion channel blockers and metabolism inhibitors in solution experiments.

Results

Measured F^? uptake rates were significantly larger than those calculated from transpiration rates regardless of external F^? concentrations, uptake durations, relative humidity, and solution pH. The measured and net F^? uptake (subtracting that calculated from transpiration rate from the measured uptake) were reduced by low temperature and inhibited by anion channel and metabolism inhibitors anthracene-9-carboxylic acid (A-9-C), niflumic acid (NFA), and carbonylcyanide m-chlorophenylhydrazone (CCCP) but not by dihydro-4, 4′ diisothiocyanostilbene-2, 2′-disulphonic acid (DIDS). The F^? uptake showed biphasic response patterns, following saturable Michaelis–Menten kinetics in the range of low external F^? (below 100 μmol?L^?1) while increased linearly with external supply in the range of high concentrations.

Conclusion

The uptake of F^? by roots of accumulator tea plants was likely an active process and energy-dependent. This helps to explain why tea plants are able to accumulate considerably high F^?.     

Effect of environmental factors on water utilization and boron accumulation and translocation in sugarcane

Rates of boron (B) accumulation and/or water utilization of3 month-old sugarcane plants were altered by changes in temperature,relative humidity, light intensity and duration of exposureto light. The effects of the environment on boron accumulationwere not directly dependent upon effects on water utilization.Boron accumulation was affected more than water uptake by increasingroot and air temperatures from 8°C to 37°C, and by raisingthe pH of the external solution from 5.7 to 7.0. Contrarily,water utilization decreased more than B accumulation when therelative humidity was increased from 30±5% to 95±5%,when light intensity was decreased or daily exposure to lightwas shortened and when the plants were pretreated with 5x10^–5M phenyl mercuric acetate, an anti-transpirant. The absorption of B by roots was experimentally separated fromits subsequent translocation to the shoots. Absorption of Bby roots was at least partially under metabolic control, sinceuptake from a 2 mg/liter B solution could occur against a concentrationgradient. Translocation of B from the roots to the shoot occurredpassively in the transpiration stream. ¹ Journal Series No. 1427 of the Hawaii Agricultural ExperimentStation. (Received March 2, 1972; )     

Effect of seed pretreatments on phosphorus uptake by alfalfa

Summary A pot experiment was made to study the effect of two seed pretreatments on the phosphorus uptake by alfalfa in two different soils. The results showed that the dry weight of both the tops and the roots as well as the total P-uptake of alfalfa in both soils increased when the seeds were previously soaked in 0.5 mol/l KH₂PO₄ or 150 ppm indoleacetic acid for 6 hours when compared with the control. The increase was much higher with the seeds soaked in the former than in the later solution. Both soil and fertilizer phosphorus taken up by alfalfa increased by seed soaking in the above mentioned solutions and the increase was most pronounced in the fraction that derived from the soil.Professor of Soils, Professor of Plant Nutrition and a Lecturer in the Faculty of Agriculture, Ain Shams University, Cairo, respectively.     

Dynamics of water and ion transport driven by corn canopy in the Yellow River basin

Water and ion balance in a corn field in the semi-arid region of the upper Yellow River basin (Inner Mongolia, China) was analyzed with special reference to transpiration stream and selective nutrient uptake driven by the crop canopy. During the crop development stage (June 7 to July 17, 2005), crop transpiration and soil evaporation were evaluated separately on a daily basis, and concentrations of NO ₃ ^? , PO ₄ ^3? , K⁺, Na⁺, Ca²⁺, Mg²⁺ and Cl^? ions in the Yellow River water, irrigation water, ground water, soil of the root zone and xylem sap of the crop were analyzed.The crop transpiration accounted for 83.4% of the evapotranspiration during the crop development stage. All ions except for Na⁺ were highly concentrated in the xylem sap due to the active and selective uptake of nutrients by roots. In particular, extremely high concentrations of the major essential nutrients were found in the nighttime stem exudate, while these concentrations in the river water, the irrigation water, the ground water and the root-zone soil were lower. On the other hand, Na⁺, which is not the essential element for crop growth, was scarcely absorbed by roots and was not highly concentrated in the xylem sap. Consequently, Na⁺ remained in the ground water and the root-zone soil at higher concentrations. These results indicate that during the growing season, crop transpiration but not soil evaporation induces the most significant driving force for mass flow (capillary rise) transporting the ground water toward the rhizosphere, where the dynamics of ion balance largely depends on the active and selective nutrient uptake by roots.     

Role of root systems of eastern larch and white spruce in response to flooding

Abstract. Soil flooding causes rapid reductions in transpiration, stomatal conductance and photosynthesis of many woody plants, which can decrease growth and ultimately result in plant death. This study was conducted to determine the role of the root system in the flooding response. Eastern larch ( Larix laricina ) seedlings were grown in Plexiglas tubes in which water uptake by flooded and unflooded roots was measured independently. Further flooding studies were conducted with eastern larch and white spruce ( Picea glauca ) in which stems were girdled. Root hydraulic properties were analysed using pressure-flow relationships. Transpiration rates of partially flooded plants declined more slowly than fully-flooded plants. Water uptake by unflooded roots of partially flooded seedlings increased momentarily with flooding. After lOd, flooding caused little change in root hydraulic conductance, a decrease in root system reflection coefficient, and an increase in osmotic permeability. Stem girdling had little effect on stomatal conductance and transpiration in comparison to flooding effects. The response of plant tops to flooding appears to be xylem-mediated and in proportion to the amount of root system flooded. Root hydraulic conductance appears to be unaffected by flooding except for a possible temporary increase on the first day following flooding treatments.