The effect of high concentrations of growth substances on water uptake

Changes in water uptake immediately following the application of high concentrations of 2-methyl-4-chlorophenoxyacetic acid (MCPA) into the root medium of whole plants ofPisum sativum andVicia faba were investigated potometrically under controlled conditions. It was found that concentrations between 10^?2 and 10^?5 m bring about a sudden, pronounced and lasting inhibition of water uptake. Its rate depends directly on MCPA concentration. At 10^?2 m, sudden inhibition of water uptake becomes apparent as early as 10 min after application. With decreasing concentrations the appearance of inhibition is retarded but the inhibition is still sudden and well-pronounced. The inhibition of water uptake observed after MCPA application to the root medium of whole plants agrees with the sudden striking reduction in transpiration intensity observed byAllerup (1964) after the application of similar concentrations of 2,4-dichlorophenoxyacetic acid. The presence of MCPA in the nutrient medium causes inhibition of water uptake even in cut plants. Its rate again depends on the concentration of the growth substance. The inhibition does not appear here as quickly and is not as sudden and pronounced. High concentrations of MCPA during application to over-ground plant organs do not bring about immediate inhibition of water uptake by the root.     

Transpiration, water absorption, and internal water balance of cotton plants as affected by light and changes in saturation deficit

In controlled environment studies of cotton plants (Gossypium barbadense L.) a light-induced acceleration of transpiration upset the water balance established in the dark because of a lag in water absorption. A plant-water deficit could be generated either by sudden illumination at a given saturation deficit (sd) of the air, or by raising the sd in conjunction with illumination, without different effects.

Direct water balance measurements were confirmed in every experiment by beta ray gauge detection of changes in leaf-water content resulting from unequal gain and loss of water by the whole plant.

Recovery from the initial loss of turgidity always was faster and more complete at the higher than at the lower values of sd. Recovery occurred even in the light at the higher values of sd, but was enhanced by return to darkness and a lower sd, which at times resulted in superhydration.

Rehydration in the light could be attributed to at least 2 processes: A) a diminished transpiration rate if earlier water loss was sufficient to induce stomatal closure, and B) an increased rate of water absorption. The data suggest that a water deficit, temporary or persisting, does not cause a significantly lowered transpiration rate; thus, recovery must depend on increased absorption. The communicative link between the 2 processes appears weak, transmitting strong signals only.

     

Inhibition of water uptake by high concentrations of auxin-like substances

The intensity of water uptake by halves of splitroot cuttings fromColeus blumei Benth was measured by potometrical method. Inhibition of the water uptake induced by application of high concentrations of sodium salts of α-naftylacetic acid (NAA) and 2-methyl-4-chlorophenoxyacetic acid (MCPA) in the treated half of the root induced an increase of water uptake by the other half. The compensation is not complete. If only one half of the root treated by synthetic auxin was allowed to take up water, MCPA appeared to induce a smaller degree of reversibility than NAA.     

The Relationships between Transpiration and Potassium Uptake in Ricinus communis

The water uptake by individual water-culture-grown castor-oilplants (Ricinus communis) was varied and corresponding ratesof potassium uptake measured. The water flux was varied by changingthe rate of transpiration or by detopping the transpiring plants.Transpiration was altered by changing the atmospheric humidityat constant temperature and light intensity in a climatologicalwind tunnel. It was found that the uptake of potassium was divisibleinto two components: (a) an accumulation by the cells of theroot and (b) a passage through the root to the shoot via thevessels. These components were found to be entirely independentof one another. Also while (a) was unaffected by the water flux(b) was linearly related to it. The concentration of potassiumin the vessels was between 15 and 26 times that of the mediumand this ratio which was found to be similar in both intactand detopped exuding plants remained constant in the face ofwide changes in water flux. This essential similarity betweenexuding and transpiring plants and the finding that there wasa lag between the change in water flux and the response in potassiumuptake indicated that there is no continuous mass-flow pathwaybetween medium and xylem in these plants. Instead, increasedtranspiration seems in some way to increase the rate of theexudation process. These conclusions are discussed in relationto the results of other workers.     

Induction of Acid Metabolism in Portulacaria afra

Portulacaria afra, a succulent plant, shifts from a predominantly C₃ mode of gas exchange to a typical Crassulacean acid metabolism type CO₂ uptake in response to water or NaCl stress. Control plants in the absence of water stress assimilated CO₂ during the light (about 7-8 mg CO₂ dm⁻² hr⁻¹), transpiration (about 1.5 g dm⁻² hr⁻¹) was predominantly during the day, stomates were open during the day, and there was little diurnal organic acid fluctuation. Stressed plants showed only dark CO₂ uptake and dark water loss, nocturnal stomatal opening, and an increased diurnal fluctuation of titratable acidity. Within 2 weeks after rewatering, stressed plants returned to the control acid fluctuation levels indicating that the response to stress was reversible.     

The Effect of Light Intensity and Relative Humidity on Growth Rate and Root Respiration of Plantago lanceolata and Zea mays

Plants of Plantago lanceolata L. and Zea mays L., cv. ‘Campo’were grown at two levels of light intensity. Especially in theroots, the rate of dry matter accumulation decreased at lowlight intensity. The carbohydrate content of both roots andshoots of P. lanceolata was not affected by light intensity.The relative contribution of SHAM¹-sensitive respiration, thealternative chain, to total root respiration of both P. lanceolataand Z. mays, was not affected by light intensity during thedaytime. The alternative pathway was somewhat decreased at theend of the dark period, but not in the root tips (0–5mm) where it still contributed 56% in respiration. It was, therefore,concluded that photosynthesis is not a major factor in regulationof root growth in the species investigated. To see whether the effect of light intensity on root growthrate was via transpiration, plants of Z. mays were grown atdifferent air humidities. Both high humidity and low light intensityaffected the root morphology in such a way that the distancebetween the apex and the first laterals on the primary rootaxis increased. It is suggested that this effect on root morphologyis due to transpiration and the subsequent removal of root-producedinhibitors of lateral root growth; although light intensityalso affected the rate of dry matter accumulation of roots andthe rate was not affected by the humidity of the air. It is,therefore, concluded that the effect of light intensity on therate of dry matter accumulation of roots of Z. mays is not viaan effect on transpiration.     

Diurnal Pattern of Transpiration,Water Uptake and Water Budget of Succulents with Different CO2 Fixation Pathways

The water fluxes and the CO₂ exchange of three leaf succulents, Othonna opima, Cotyledon orbiculata and Senecio medley-woodii, with different leaf anatomy, growth form and CO₂ fixation pathways (C₃, CAM) were monitored with a gas exchange cuvette which was combined with a potometric system to quantify water uptake. Measurements, which are primarily valid for plants with a sufficient water supply, were made during 6 to 10 consecutive days under constant experimental conditions. Water uptake for 24 h exceeded water loss by transpiration only for a S, medley-woodii plant with 10 expanding but only 7 mature leaves. In this case the gained water evidently is put into leaf expansion. All other plants showed balanced transpiration and water uptake rates. O. opima and C. orbiculata have a similar life form, similar water storage volumes and the same natural habitat but their diurnal water uptake patterns differ significantly. In the C₃ plant O. opima water uptake increased when the transpiration increased or transpiration rates were higher than uptake rates and vice versa. On the contrary the CAM plant C. orbiculata transpired during the dark period at constant or decreasing rates but showed steadily increasing uptake rates. Senecio medley-woodii- and C. orbiculata are CAM plants with similar diurnal water uptake patterns with its maximum in uptake during or towards the end of the CO₂ dark fixation period. Water uptake of C. orbiculata was at its minimum at the end of the light period despite transpiration being maximal. The results were discussed considering the different CO₂ fixation pathways. In the investigated CAM succulents, C. orbiculata and S. medley-woodii, the CAM influenced water uptake throughout the whole day and not only during the CO₂ dark fixation period.     

Photosynthesis, Transpiration, Leaf Temperature, and Stomatal Activity of Cotton Plants under Varying Water Potentials

Cotton plants, Gossypium hirsutum L. were grown in a growth room under incident radiation levels of 65, 35, and 17 Langleys per hour to determine the effects of vapor pressure deficits (VPD's) of 2, 9, and 17 mm Hg at high soil water potential, and the effects of decreasing soil water potential and reirrigation on transpiration, leaf temperature, stomatal activity, photosynthesis, and respiration at a VPD of 9 mm Hg.

Transpiration was positively correlated with radiation level, air VPD and soil water potential. Reirrigation following stress led to slow recovery, which may be related to root damage occurring during stress. Leaf water potential decreased with, but not as fast as, soil water potential.

Leaf temperature was usually positively correlated with light intensity and negatively correlated with transpiration, air VPD, and soil water. At high soil water, leaf temperatures ranged from a fraction of 1 to a few degrees above ambient, except at medium and low light and a VPD of 19 mm Hg when they were slightly below ambient, probably because of increased transpirational cooling. During low soil water leaf temperatures as high as 3.4° above ambient were recorded. Reirrigation reduced leaf temperature before appreciably increasing transpiration. The upper leaf surface tended to be warmer than the lower at the beginning of the day and when soil water was adequate; otherwise there was little difference or the lower surface was warmer. This pattern seemed to reflect transpiration cooling and leaf position effects.

Although stomata were more numerous in the lower than the upper epidermis, most of the time a greater percentage of the upper were open. With sufficient soil water present, stomata opened with light and closed with darkness. Fewer stomata opened under low than high light intensity and under even moderate, as compared with high soil water. It required several days following reirrigation for stomata to regain original activity levels.

Apparent photosynthesis of cotton leaves occasionally oscillated with variable amplitude and frequency. When soil water was adequate, photosynthesis was nearly proportional to light intensity, with some indication of higher rates at higher VPD's. As soil water decreased, photosynthesis first increased and then markedly decreased. Following reirrigation, photosynthesis rapidly recovered.

Respiration was slowed moderately by decreasing soil water but increased before watering. Respiration slowed with increasing leaf age only on leaves that were previously under high light intensity.

     

Environmental Influences on CAM Activity of Cissus quadrangularis

Cissus quadrangularis, Vitacea, has a succulent stem and showsCrassulacean Acid Metabolism (CAM). Environmental control ofCAM is shown with respect to water supply, day temperature,and light intensity. C. quadrangularis, under moderate drought,has a high capacity for dark CO₂ fixation and high efficiencyof water use (nocturnal transpiration ratio = 8). Severely droughtedplants of C. quadrangularis still show dark CO₂ fixation althoughat a reduced rate, which allows a maintenance level metabolism.On the other hand, high temperature and light intensity promoteCAM activity in well-watered plants resulting in luxuriant andfast growth. Data indicate that under natural conditions CAMshould effectively contribute to the successful adaptation ofC. quadrangularis to its environment.     

Early responses of drought-resistant and -susceptible tomato plants subjected to water stress

Three-week-old seedlings of one drought-susceptible tomato cultivar (Lycopersicon esculentum cv. “New Yorker”) and two drought-resistant species of tomato (Solanum pennellii andLycopersicon chilense) were subjected to various degrees of PEG 8000-induced water stress from ?0.017 to ?1.0 MPa for a duration of 24 h so that their early responses to water stress could be compared. Such a comparison would determine if there was a relationship to root cytokinin levels following sudden induction of water stress in the drought-resistant species. Transpiration rates of leaves were monitored throughout the 24-h period, shoots were evaluated for leaf water potential (LWP), and roots were extracted for levels oft-zeatin riboside (t-ZR) and dihydrozeatin riboside (DHZR) using a monoclonal antibody enzyme immunoassay. Transpiration rates were evaluated gravimetrically by difference every 6 h up to 24 h. Transpiration rate decreased with increasing PEG levels and passage of time in all three species, measured at 6 and 12 h, logarithmically in the case of the twoLycopersicon species and linearly in the case ofSolanum. From 12–18 h (while plants were in darkness), transpiration rate was a function of the level of PEG only and not time in all three species. When light resumed from 18–24 h, only 5.pennellii showed no further decrease in transpiration rate over time with increasing PEG. Drought-susceptibleL. esculentum had a stronger linear decrease in LWP with increasing PEG 8000 concentration than the other two species.L. esculentum also had a higher initial transpiration rate than did either of the drought-resistant species. The two drought-resistant species showed less change in LWP with 5.pennellii having a small decrease andL. chilense having little change. OnlyS. pennellii exhibited a decrease in roott-ZR levels, which may imply a role for root cytokinin within the first 24-h exposure to water stress in this species.L. esculentum exhibited no change in roott-ZR. The levels oft-ZR inL. chilense were less than that ofL. esculentum but showed only a slight decrease with increasing PEG.S. pennellii andL. chilense, although both drought-resistant tomato species, showed different patterns of response with respect to pattern of decline in transpiration rate, LWP, and roott-ZR levels.     

Effects of Water Stress and Abscisic Acid on Transpiration Regulation in Wheat

The transpiration response to recurrent light periods was studied'n water-stressed wheat seedlings. Seedlings were stressed by three methods: addition of mannitol to the root medium, root cooling and drving of the roots in air. All three methods induced almost equal effects on transpiration regulation during alternating dark and light intervals. Exogenous abscisic acid supplied to the shoots of excised plants had qualitatively the same effect as water stress. Water stress and ABA increased the time lapse between light-on and the onset of transpiration increase and lowered the amplitude of transpiration increase in light. Weak light introduced before strong light shortened the delay times.     

Do root hydraulic properties change during the early vegetative stage of plant development in barley (Hordeum vulgare)?

Background and Aims

As annual crops develop, transpirational water loss increases substantially. This increase has to be matched by an increase in water uptake through the root system. The aim of this study was to assess the contributions of changes in intrinsic root hydraulic conductivity (Lp, water uptake per unit root surface area, driving force and time), driving force and root surface area to developmental increases in root water uptake.

Methods

Hydroponically grown barley plants were analysed during four windows of their vegetative stage of development, when they were 9–13, 14–18, 19–23 and 24–28 d old. Hydraulic conductivity was determined for individual roots (Lp) and for entire root systems (Lp_r). Osmotic Lp of individual seminal and adventitious roots and osmotic Lp_r of the root system were determined in exudation experiments. Hydrostatic Lp of individual roots was determined by root pressure probe analyses, and hydrostatic Lp_r of the root system was derived from analyses of transpiring plants.

Key Results

Although osmotic and hydrostatic Lp and Lp_r values increased initially during development and were correlated positively with plant transpiration rate, their overall developmental increases (about 2-fold) were small compared with increases in transpirational water loss and root surface area (about 10- to 40-fold). The water potential gradient driving water uptake in transpiring plants more than doubled during development, and potentially contributed to the increases in plant water flow. Osmotic Lp_r of entire root systems and hydrostatic Lp_r of transpiring plants were similar, suggesting that the main radial transport path in roots was the cell-to-cell path at all developmental stages.

Conclusions

Increase in the surface area of root system, and not changes in intrinsic root hydraulic properties, is the main means through which barley plants grown hydroponically sustain an increase in transpirational water loss during their vegetative development.     

Hydraulic regulation strategies for whole-plant water balance of two maize inbred lines differing in drought resistance under short-term osmotic stress

The conservation of water in agriculture requires an understanding of the mechanisms of plant–water relations. This study aimed to reveal hydraulic regulation strategies of maize (Zea mays L.) for maintaining the plant water balance during drought. The water relations of two maize inbred lines (Tian4 and 478) that differ in their resistance to drought in the field were investigated under well-watered conditions and osmotic stress induced with 10 % PEG 6000. The leaf transpiration rate and leaf water potential of 478 varied diurnally, but remained constant in Tian4, which is more drought resistant. Tian4 plants showed morphological, anatomical and physiological advantages that protected them from foliar water loss. The strategies of leaf hydraulics to regulate leaf water balance during the day and during short-term osmotic stress also differed between Tian4 and 478. The leaf hydraulic conductivity of Tian4 and 478 increased temporarily, but their root hydraulic conductivities were reduced under osmotic stress. However, the root hydraulic conductivity of Tian4 subsequently recovered. Lower and rapidly reduced leaf transpiration and the ability of root hydraulics to recover from short-term osmotic stress can help explain the strategies for plant water balance of drought-tolerant maize.     

Respiratory capacity,nitrogenase activity and structural changes ofFrankia,in symbiosis withAlnus incana,in response to prolonged darkness

Plants ofAlnus incana (L.) Moench in symbiosis with a local source ofFrankia were exposed to prolonged darkness under controlled climate conditions.Frankia vesicle clusters were prepared from the root nodules, and the condition ofFrankia was measured as respiratory capacity by supplying the preparation with saturating amounts of four different substrates. During darkness, nitrogenase (EC 1.7.99.2) activity decreased in intact plants and in the vesicle-cluster preparations. The respiratory capacity ofFrankia also decreased. After 4 d in darkness most respiration was lost, though all nitrogenase activity was already lost after 3 d. When the dark treatment was ended after 2 d and normal light/dark conditions restored, nitrogenase activity immediately started to recover. The respiratory capacity continued to decrease and no recovery was observed until the third day after the end of the dark treatment. Whole-plant nitrogenase activity slowly increased at a rate similar to the rate of increase observed in untreated plants. Transmission electron micrographs of the root nodules showed that the cytoplasm of infected host cells and the cells ofFrankia were structurally degraded in response to dark treatment, while young vesicles were frequent during recovery. Growth and differentiation ofFrankia cells were apparently important for recovery of the enzyme activities studied.     

A one-dimensional model of water flow in soil-plant systems based on plant architecture

The estimation of root water uptake and water flow in plants is crucial to quantify transpiration and hence the water exchange between land surface and atmosphere. In particular the soil water extraction by plant roots which provides the water supply of plants is a highly dynamic and non-linear process interacting with soil transport processes that are mainly determined by the natural soil variability at different scales. To better consider this root-soil interaction we extended and further developed a finite element tree hydro-dynamics model based on the one-dimensional (1D) porous media equation. This is achieved by including in addition to the explicit three-dimensional (3D) architectural representation of the tree crown a corresponding 3D characterisation of the root system. This 1D xylem water flow model was then coupled to a soil water flow model derived also from the 1D porous media equation. We apply the new model to conduct sensitivity analysis of root water uptake and transpiration dynamics and compare the results to simulation results obtained by using a 3D model of soil water flow and root water uptake. Based on data from lysimeter experiments with young European beech trees (Fagus silvatica L.) is shown, that the model is able to correctly describe transpiration and soil water flow. In conclusion, compared to a fully 3D model the 1D porous media approach provides a computationally efficient alternative, able to reproduce the main mechanisms of plant hydro-dynamics including root water uptake from soil.     

Experimental cloud immersion and foliar water uptake in saplings of Abies fraseri and Picea rubens

Key message

Frequent cloud immersion events result in direct uptake of cloud water and improve plant water potentials during daylight hours in saplings of two dominant cloud forest species.

Abstract

In ecosystems with frequent cloud immersion, the influence on plant water balance can be important. While cloud immersion can reduce plant water loss via transpiration, recent advances in methodology have suggested that many species also absorb water directly into leaves (foliar water uptake). The current study examines foliar water uptake and its influence on daily plant water balance in tree species of the endangered spruce–fir forest of the southern Appalachian Mountains, USA. These mountain-top communities are considered relic, boreal forests that may have persisted because of the benefits of frequent cloud immersion. We examined changes in needle water content, xylem water potentials, and stable isotope values in saplings of the two dominant tree species, Abies fraseri and Picea rubens before and after a 24 h period of experimental cloud immersion. Both species exhibited foliar water uptake following immersion, evidenced by substantial changes in stable isotope values of extracted needle water that reflected the composition of the fog water. In addition, total needle water content improved 3.7–6.4 % following experimental submersion and xylem water potentials were significantly greater (up to 0.33 MPa) in cloud-immersed plants over control plants. These results indicate that foliar water uptake may be an adaptive strategy for utilizing cloud water and improving overall tree vigor in these most southerly distributed boreal species.     

Phytotoxicity of Air Pollutants: Evidence for the Photodetoxification of SO(2) but Not O(3)

Pisum sativum L. cv Alsweet (garden pea) and Lycopersicon esculentum flacca Mill. (tomato) were used to evaluate the phytotoxicity of SO₂ and O₃ in the light and dark. Plants were grown in controlled environment chambers and exposed to SO₂ or O₃ in the light or dark at the same environmental conditions at which they were grown. The pea plants were treated with fusicoccin to ensure open stomata in the dark; the stomata of the tomato mutant remained open in the dark. Both species exhibited 64% to 80% less foliar necrosis following exposure to SO₂ (0.5 to 1.0 microliter per liter for 2 hours) in the light than in the dark. The decrease in SO₂ injury for light versus dark exposed plants was greater in fully expanded than expanding leaves. Both species exhibited 30% greater foliar necrosis following exposure to O₃ (0.2 microliter per liter for 2 hours) in the light than dark. The increase in O₃ injury in the light versus dark was similar for leaves at all stages of expansion. Leaf conductance to water vapor was 7% to 11% and 23% higher in the light than dark for fusicoccin-treated peas and tomato plants, respectively, indicating greater foliar uptake of both pollutants in the light than dark. Thus, the decreased SO₂ toxicity in the light was not associated with pollutant uptake, but rather the metabolism of SO₂. In contrast, the increased toxicity of O₃ in the light was at least in part associated with increased uptake or could not be separated from it.     

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.     

A model for nutrient and water flow and their uptake by plants grown in a soilless culture

The objective of this study was to develop a sensitive means of control to optimize nutrient concentrations in the root zone of a soilless system, considering plant water and nutrient uptake, and solution circulation rates. A model is proposed to simulate ornamental plants growth in a channel with a non-interacting soilless substrate, irrigated by point sources with constant discharge rates, spaced uniformly along the channel. The model accounts for compensation for transpiration water losses and consequent salinity buildup, and its interactions with plant growth and nutrient uptake. The added water may contain given concentrations of nutrients and/or toxic (saline) compounds, which would cause salinity buildup. Uptake of each solute is specific, according to a Michaelis–Menten kinetics mechanism, but passive uptake by the transpiration stream is also accounted for. Plant growth is affected by time/age and ionic balance in the solution. The model was calibrated with lettuce (Lactuca sativa L.) plants grown in volcanic ash. Simulation of potassium concentration change as a result of discharge rate and emitter spacing revealed that the two parameters could compensate one for the other, once a target lower limit is set. Potassium appeared to be most sensitive to sodium accumulation in the growth medium; this accumulation changed ionic concentration balance, which affected pH and bicarbonate concentration. Passive uptake of calcium by the transpiration stream is highly affected by the root fraction involved, but its calculated contribution is below published values is highly affected by the root fraction involved, but its calculated contribution is below published values.     

Diurnal regulation of NO3-uptake in soybean plants II. Relationship with accumulation of NO and asparagine in the roots

Experiments were performed with soybean plants to test the hypothesisthat the inhibition of NO₃^– uptake in darkness is dueto feedback control by NO₃^– and/or Asn accumulating inthe roots. Xylem export of N compounds was shown to depend onwater flux in both excised root systems and ¹⁵N-labelled intactplants, suggesting that the shortage of transpiration in darknessmay be responsible for the retention of NO₃^– and Asn inthe roots. This was verified in experiments where the light/darkpattern of transpiration was modulated in intact plants by changingthe relative humidity of the atmosphere. Any decrease of transpirationat night was associated with a concurrent stimulation of NO₃^–and Asn accumulations in the roots. However, the light/darkrhythmicity of NO₃^– uptake was only marginally affectedby these treatments, and thusappeared quite independent fromtranspiration and root NO₃^– or Asn levels. Typically,the maintainance of a constant transpiration during the day/nightcycle did not suppress the inhibition of NO₃^– uptake indarkness, whereas it almost prevented the dark increase in rootNO₃^– and Asn contents. These data strongly support theconclusion that the effect of light on NO₃^– uptake isnot mediated by changes in translocation and accumulation ofN compounds. Key words: Glycine max, light/dark, cycles, nitrate uptake, transpiration, transport of N compounds, accumulation of N compounds