Apoplastic transport across young maize roots: effect of the exodermis

The uptake of water and of the fluorescent apoplastic dye PTS (trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate) by root systems of young maize (Zea mays L.) seedlings (age: 11–21 d) has been studied with plants which either developed an exodermis (Casparian band in the hypodermis) or were lacking it. Steady-state techniques were used to measure water uptake across excised roots. Either hydrostatic or osmotic pressure gradients were applied to induce water flows. Roots without an exodermis were obtained from plants grown in hydroponic culture. Roots which developed an exodermis were obtained using an aeroponic (=mist) cultivation method. When the osmotic concentration of the medium was varied, the hydraulic conductivity of the root (Lp _r in m³ · m⁻² · MPa⁻¹ · s⁻¹) depended on the osmotic pressure gradient applied between root xylem and medium. Increasing the gradient (i.e. decreasing the osmotic concentration of the medium; range: zero to 40 mM of mannitol), increased the osmotic Lp _r. In the presence of hydrostatic pressure gradients applied by a pressure chamber, root Lp _r was constant over the entire range of pressures (0–0.4 MPa). The presence of an exodermis reduced root Lp _r in hydrostatic experiments by a factor of 3.6. When the osmotic pressure of the medium was low (i.e. in the presence of a strong osmotic gradient between xylem sap and medium), the presence of an exodermis caused the same reduction of root Lp _r in osmotic experiments as in hydrostatic ones. However, when the osmotic concentration of the medium was increased (i.e. the presence of low gradients of osmotic pressure), no marked effect of growth conditions on osmotic root Lp _r was found. Under these conditions, the absolute value of osmotic root Lp _r was lower by factors of 22 (hydroponic culture) and 9.7 (aeroponic culture) than in the corresponding experiments at low osmotic concentration. Apoplastic flow of PTS was low. In hydrostatic experiments, xylem exudate contained only 0.3% of the PTS concentration of the bathing medium. In the presence of osmotic pressure gradients, the apoplastic flow of PTS was further reduced by one order of magnitude. In both types of experiments, the development of an exodermis did not affect PTS flow. In osmotic experiments, the effect of the absolute value of the driving force cannot be explained in terms of a simple dilution effect (Fiscus model). The results indicate that the radial apoplastic flows of water and PTS across the root were affected differently by apoplastic barriers (Casparian bands) in the exodermis. It is concluded that, unlike water, the apoplastic flow of PTS is rate-limited at the endodermis rather than at the exodermis. The use of PTS as a tracer for apoplastic water should be abandoned. Received: 9 October 1997 / Accepted: 5 February 1998     

Photosynthesis, leaf conductance and water relations of in vitro cultured grapevine rootstock in relation to acclimatisation

Leaf net CO₂ uptake and leaf photosynthetic capacity were investigated in micropropagated 41B grapevine rootstock (Vitis vinifera‘Chasselas’×Vitis berlandieri, Mill. De Gr.) plants grown in the presence of four sucrose concentrations (6.25, 12.5, 25.0 or 37.5 g l^?1). Sucrose concentration in the medium during growth in vitro did not affect the leaf photosynthetic performance of plants neither before nor after transplantation. The maximum photosynthetic rate, measured as CO₂-dependent O₂ evolution, was 7.3 µmol m^?2 s^?1 before transplanting and 15.4 µmol m^?2 s^?1 one month after transplantation. The maximum quantum yield of O₂ evolution (on the basis of incident light) was about 0.07 for all sucrose treatments both before and after transplantation. Dry biomass before transplanting was highest in plants grown with 25.0 or 37.5 g l^?1 sucrose in the medium. One month after transplantation the highest dry biomass was also observed for the same treatments. Survival of plants was 100% for all treatments. Leaf conductance to water vapour was always higher in plants before than after transplantation. Both before and after transplanting it increased with increasing light intensity and decreased slightly with increasing CO₂ molar ratio in in vitro plants. Stomata of plants before transplantation were unresponsive to vapour pressure deficit. In vitro plants experience an acute water stress when they are maintained with the whole root system in water and exposed to ambient controlled conditions in a growth chamber. However, there was no wilting of the leaves when similar plants with roots cut off were left in the same conditions. Hydraulic conductivity was low at both root and shoot-root connection levels. It is likely that water supply could be limiting during transplantation because of the low root and root-stem connection conductivity. Water uptake by roots rather than water loss from the shoots would be of primary importance for the maintenance of water balance during acclimatisation.     

Analysis of the Relative Increase in Photosynthetic O2 Uptake When Photosynthesis in Grapevine Leaves Is Inhibited following Low Night Temperatures and/or Water Stress

We found similarities between the effects of low night temperatures (5°C–10°C) and slowly imposed water stress on photosynthesis in grapevine (Vitis vinifera L.) leaves. Exposure of plants growing outdoors to successive chilling nights caused light- and CO₂-saturated photosynthetic O₂ evolution to decline to zero within 5 d. Plants recovered after four warm nights. These photosynthetic responses were confirmed in potted plants, even when roots were heated. The inhibitory effects of chilling were greater after a period of illumination, probably because transpiration induced higher water deficit. Stomatal closure only accounted for part of the inhibition of photosynthesis. Fluorescence measurements showed no evidence of photoinhibition, but nonphotochemical quenching increased in stressed plants. The most characteristic response to both stresses was an increase in the ratio of electron transport to net O₂ evolution, even at high external CO₂ concentrations. Oxygen isotope exchange revealed that this imbalance was due to increased O₂ uptake, which probably has two components: photorespiration and the Mehler reaction. Chilling- and drought-induced water stress enhanced both O₂ uptake processes, and both processes maintained relatively high rates of electron flow as CO₂ exchange approached zero in stressed leaves. Presumably, high electron transport associated with O₂ uptake processes also maintained a high ΔpH, thus affording photoprotection.     

Uptake and transport of N,P and K in tomato supplied with nettle water and nutrient solution

Water extract of stinging nettle (Urtica dioica) has a growth stimulating effect on plants. This investigation elucidated effects of nettle water on uptake and transport of N, P and K. Tomato plants (Solanum lycopersicum L. cv. Dansk export) were grown in sand culture 6–8 weeks. Plants were supplied with nettle water and nutrient solution was used as a control medium. Uptake and transport of N, P and K⁺ were determined with isotopes (¹⁵N,³²P and⁸⁶Rb⁺ as a tracer for K⁺) and ion-selective electrodes and in exudation experiments. A 15% higher uptake of nitrogen (¹⁵N assay) was found after nettle water treatment compared with the nutrient solution control. The total amount of nitrogen was also higher in plants cultivated with nettle water. Transport of inorganic and organic nitrogen, measured in exudation experiments, was more than 50% higher for plants supplied with nettle water compared with plants supplied with nutrient solution. In contrast, nettle water had no effect on uptake, transport or total amount of phosphorus and potassium in the plants. Experiments in hydroculture showed that nettle water had a strong pH-elevating effect. Uptake of NH ₄ ⁺ was strongly stimulated by nettle water compared with nutrient solution. By holding pH at a constant level during the uptake period for 6 h, the uptake of NH ₄ ⁺ from nettle water was significantly lower when no adjustment of pH was made. Consequently a good deal of the NH ₄ ⁺ uptake enhancement by nettle water could be explained by pH-stimulation. Assays with the uncoupler/inhibitor 2,4-dinitrophenol (DNP) and dichlorophenyl-dimethyl-urea (DCMU) showed that uptake of nitrogen from nettle water was less metabolically-linked than uptake from a corresponding nutrient solution. All together, nettle water seems to stimulate the uptake of nitrogen, but not phosphorus or potassium.     

An overview of oxygen transport in plants: diffusion and convection

The movement of gases within plants is crucial for species that live in flood-prone areas with limited soil oxygen. These plants adapt to hypoxia/anoxia not by using oxygen more efficiently, but by ensuring a steady oxygen supply to their cells. Wetland plants typically form gas-filled spaces (aerenchyma) in their tissues, providing a low-resistance pathway for gas movement between shoots and roots, especially when the shoots are above water, and the roots are submerged. Oxygen movement in plant roots is mainly through diffusion. However, in certain species, such as emergent and floating-leaved plants, pressurized flows can also facilitate the movement of gases within their stems and rhizomes. Three types of pressurized (convective) flows have been identified: humidity-induced pressurization (positive pressure), thermal osmosis (positive pressure with air flow against the heat gradient), and venturi-induced suction (negative pressure) caused by wind passing over broken culms. A clear diel variation in pressurized flows exists, with higher pressures and flows during the day and negligible pressures and flows during the night. This article discusses some key aspects of these mechanisms for oxygen movement.     

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.     

Update on Boron in Higher Plants - Uptake, Primary Translocation and Compartmentation

Abstract: This review focuses on the uptake and primary translocation of boron (B), as well as on the subcellular compartmentation of B and its role in cell walls of higher plants. B uptake occurs via passive diffusion across the lipid bilayer, facilitated transport through major intrinsic proteins (MIPs), and energy-dependent transport through a high affinity uptake system. Whereas the first two represent passive uptake systems, which are constitutively present, the latter is induced by low B supply and is able to establish a concentration gradient for B between the root symplasm and the external medium. At high B supply, a substantial retention of B can be observed at xylem loading, and passive processes are most likely responsible for that. At low B supply, another energy-dependent high affinity transport system for B seems to be induced which establishes an additional concentration gradient between root symplasm and the xylem. The possible significance of all these processes at various B supplies is discussed. The role of soluble B complexes in uptake and primary translocation of B has been evaluated, but the few data available do not allow comprehensive conclusions to be drawn. In any case, there are no indications that soluble B complexes play a major role in either uptake or primary translocation of B. The subcellular compartmentation of B still remains a matter of controversy, but it is unequivocally clear that B is present in all subcellular compartments (apoplasm, cell wall, cytosol and vacuole). The relative distribution of B between these is dependent on plant species and experimental conditions and may vary greatly. Recent results on the well-established role of B in cell walls are summarized and their physiological significance discussed.     

A novel insight into the cost–benefit model for the evolution of botanical carnivory

Background The cost–benefit model for the evolution of botanical carnivory provides a conceptual framework for interpreting a wide range of comparative and experimental studies on carnivorous plants. This model assumes that the modified leaves called traps represent a significant cost for the plant, and this cost is outweighed by the benefits from increased nutrient uptake from prey, in terms of enhancing the rate of photosynthesis per unit leaf mass or area (A_N) in the microsites inhabited by carnivorous plants.Scope This review summarizes results from the classical interpretation of the cost–benefit model for evolution of botanical carnivory and highlights the costs and benefits of active trapping mechanisms, including water pumping, electrical signalling and accumulation of jasmonates. Novel alternative sequestration strategies (utilization of leaf litter and faeces) in carnivorous plants are also discussed in the context of the cost–benefit model.Conclusions Traps of carnivorous plants have lower A_N than leaves, and the leaves have higher A_N after feeding. Prey digestion, water pumping and electrical signalling represent a significant carbon cost (as an increased rate of respiration, R_D) for carnivorous plants. On the other hand, jasmonate accumulation during the digestive period and reprogramming of gene expression from growth and photosynthesis to prey digestion optimizes enzyme production in comparison with constitutive secretion. This inducibility may have evolved as a cost-saving strategy beneficial for carnivorous plants. The similarities between plant defence mechanisms and botanical carnivory are highlighted.     

Root water uptake and profile soil water as affected by vertical root distribution

Water uptake by plant roots is a main process controlling water balance in field profiles and vital for agro-ecosystem management. Based on the sap flow measurements for maize plants (Zea mays L.) in a field under natural wet- and dry-soil conditions, we studied the effect of vertical root distribution on root water uptake and the resulted changes of profile soil water. The observations indicate that depth of the most densely rooted soil layer was more important than the maximum rooting depth for increasing the ability of plants to cope with the shortage of water. Occurrence of the most densely rooted layer at or below 30-cm soil depth was very conducive to maintaining plant water supply under the dry-soil conditions. In the soil layers colonized most densely by roots, daytime effective soil water saturation (S _e) always dropped dramatically due to the high-efficient local water depletion. Restriction of the rooting depth markedly increased the difference of S _e between the individual soil layers particularly under the dry-soil conditions due likely to the physical non-equilibrium of water flow between the layers. This study highlights the importance of root distribution and pattern in regulating soil water use and thereby improving endurance of plants to seasonal droughts for sustainable agricultural productivity.     

Distinctive effects of cadmium on glucosinolate profiles in Cd hyperaccumulator Thlaspi praecox and non-hyperaccumulator Thlaspi arvense

We investigated the hypothesis that continuous water application allows favorable and steady water content and hydraulic conductivity in the root zone, thus enabling higher water potential in the soil–root interface (ψ_root). Elevated ψ_root increases transpiration (T) and prevents yield loss due to stomatal closure or to low root osmotic potential that develops in response to low ψ_root. We assume further, that the advantage of continuous water application is more pronounced for young plants, where water uptake per root length and competition on resources in the root system is higher. We investigated this hypothesis by examining the average water content of the root zone and T as a function of time for sunflowers grown under varied irrigation frequencies experimentally and in a modeled simulations, and by solving for the necessary effective root length and ψ_root for each case. High frequency water application was shown to positively affect root water uptake efficiency and yield, especially when plants were young. Irrigation frequency affected growth through the water content in the bulk soil (θ_soil) which in turn affects ψ_root. A low θ_soil and coupled low hydraulic conductivity decreased T and yield. Moreover, a decreased θ_soil caused low ψ_root, inefficient allocation of energy and carbohydrates and eventual yield loss. It was likely that these phenomena were more pronounced with young plants due to higher water uptake per root length.     

Water Relations,CO2 Exchange,Water-use Efficiency and Growth of Welwitschia mirabilis Hook. fil. in three Contrasting Habitats of the Namib Desert1

CO₂ exchange, transpiration and leaf water potential of Welwitschia mirabilis were measured in three contrasting habitats of the Namib desert. From these measurements stomatal conductance, internal CO₂concentration and WUE were calculated. In two of the three habitats photosynthetic CO₂ uptake decreased and transpiration increased with increasing leaf age while in the third habitat CO₂ uptake increased and transpiration decreased with leaf age. Except for the stomata of young leaf sections in this habitat, stomata closed with increasing δw leading to a pronounced midday depression of CO₂ uptake. The high stomatal limitation of photosynthetic CO₂ uptake of glasshouse-grown plants was verified in the natural habitat. Photosynthetic CO₂ uptake saturated between 800 and 1300 μmol photons m^?2 s^?1depending on leaf age and habitat. CO₂ uptake had a broad temperature optimum declining significantly beyond 32 °C. Predawn leaf water potential reflected water availability and atmospheric conditions in the three habitats and ranged from ? 2.5 to ? 6.2 MPa. There was a pronounced diurnal course of leaf water potential in all habitats. During the day a gradient in water potential developed along the leaf axis with the lowest potential at the leaf's tip. With respect to whole plant balances of CO₂ exchange and transpiration, there were marked differences between Welwitschias in the three habitats. Despite a negative CO₂ balance over a period of five months, leaves in the driest habitat grew constantly at the expense of carbon reserves in the plant. Only at the wettest site did carbon gain exceed carbon demand for growth. The WUE of whole plants was insignificant in all habitats. The results were as contrasting as the habitats and plants and did not allow generalisations about adaptational features of Welwitschia mirabilis.     

The Effect of temperature gradient on the transport phenomenon in roots of maize plants grown under salinity conditions. substance,heat, and ion flows

An analysis of flows through primary root and first node root tissues of plants grown under conditions of salinity and nutrient deficiency induced by temperature gradients was carried out using. a mathematical model. The results obtained show that high KNO₃ concentration in Knop’s nutrient solution (salinity) causes an inhibition of volume and heat flows and that the omission of KNO₃ from Knop’s nutrient solution (deficiency) stimulates these flows. The causes of the inhibition lay in the fact that salinity reduced hydraulic, electric, and osmotic conductivity when compared with the control (Knop’s solution), but relative to nutrient deficiency, it increased osmotic conductivity, electrodiffusion, diffusion, and filtration of heat flow induced by the electric and heat power. The causes of the stimulation were that deficiency partially decreased conductivities, similarly as salinity when compared with the control, and also decreased osmotic abilities of the system. By contrast, it increased heat conductivity and corresponding filtrations (diffusion-thermal, thermoosmotic). In first node root tissues, it increased all conductivities with the exception of electric conductivity, then osmotic, electroosmotic, diffusion, electrodiffusion, and filtration of heat flow and current flow, that is the number of possible ways of solution transport through root tissues increased. Part II.     

Fate of nitrate during initial nitrate utilization by nitrogen-depleted dwarf bean

The fate of nitrate and nitrogen-15 was followed during the apparent induction phase (6h) for nitrate uptake by N-depleted dwarf bean (Phaseolus vulgaris L. ev. Witte Krombek). Experiments were done with intact plants and with detached root systems. Qualitatively and quantitatively, xylem exudation from detached roots was a bad estimate of the export of NO^?₃ or NO^?₃-¹⁵N from roots of intact plants. In vivo nitrate reductase activity (NRA) agreed well with in situ reduction, calculated as the difference between uptake and accumulation in whole plants, provided NRA was assayed with merely endogenous nitrate as substrate (‘actual’ NRA). The majority (75%) of the entering nitrate remained unmetabolized. Both nitrate reduction and nitrate accumulation occurred predominantly in the root system. Some (< 25%) of the root-reduced nitrate-N was translocated to the shoot. Nitrate uptake occurred against the concentration gradient between medium and root cells, and probably against the gradient of the electro-chemical potential of nitrate. Part of the energy expended for NO^?₃ absorption came from the tops, since decapitation and ringing at the stem base restricted nitrate uptake.     

Low-temperature pretreatment of the root system of Brassica rapa L. plants: effects on the xylem sap exudation and on the nitrate absorption rate

Brassica rapa plants were exposed for a 52 h period (as pretreatment) to a differential temperature (DT) between roots (5°C) and shoots (20°C), while control plants were maintained with both shoot and roots at 20°C (warm grown = WG). Measured at 20°C, volume flow of xylem exudate from roots of DT plants was enhanced compared with that from WG plants, while transpiration flows were similar in pretreated and control plants. Both transpiration and exudation flows were dependent upon shoot/root ratio. Differences in the volume flow of exudate were principally related to increases in root hydraulic conductance. Anion fluxes (notably nitrate) into xylem exudate of DT plants were significantly greater than those into exudate of WG plants. This enhancement of nitrate flow from the pretreated roots was associated with a two-fold increase in nitrate uptake rate. The relationship of the cold-induced change in nitrate uptake capacity with shoot/root ratio is discussed in terms of control of nitrate absorption by shoot sink strength.     

Effect of Water Stress, Temperature, and Light on Photosynthesis in Alder Seedlings

Seedlings of Alnus incana (L.) Moench were studied in controlled environmental conditions. Net photosynthetic capacity of four plants simultaneously as well as light, temperature, and atmospheric water pressure deficit were continuously recorded during approximately two months. The potted plants were continuously given known quantities of water. Two different effects of water stress were found in the experimental plants. When sufficient amounts of water had been available to them, photosynthetic CO₂ fixation rates largely followed the variation in temperature and light. On the other hand, after prolonged water stress, higher temperatures caused a large decrease in net CO₂ uptake even if the plant apparently had sufficient water during the actual measurements. Possible mechanisms for this effect are discussed. Effects of water stress on photosynthesis were studied at a constant temperature as well as in conditions where temperature, light, soil water content, and atmospheric water vapor pressure were allowed to vary in a complex pattern. Mathematical models for expressing net CO₂ uptake as a function of environmental variables were constructed for both of these experimental situations. However, only the latter approach clearly demonstrated the fundamental role of temperature in controlling the photosynthesis of plants under water stress.     

Drought-induced root aerenchyma formation restricts water uptake in rice seedlings supplied with nitrate

Previous studies demonstrated that ammonium nutrition results in higher water uptake rate than does nitrate nutrition under water stress, and thus enhances the tolerance of rice plants to water stress. However, the process by which water uptake is related to nitrogen form under water stress remains unknown. A hydroponic experiment with simulated water stress induced by polyethylene glycol (PEG6000) was conducted in a greenhouse to study the relationship between root aerenchyma formation and water uptake rate, such as xylem sap flow rate and hydraulic conductance, in two different rice cultivars (cv. 'Shanyou 63' hybrid indica and cv. 'Yangdao 6' indica, China). The results showed that root aerenchyma tissue increased in water-stressed plants of both cultivars fed by nitrate. No significant difference was found in root hydraulic conductivity and/or xylem sap flow rate between the two rice cultivars fed by ammonium regardless of water status, whereas these parameters decreased significantly in water-stressed plants fed by nitrate. It was concluded that aerenchyma that formed in the root cortex impeded the radial transport of water in the root cylinder and decreased water uptake in water-stressed rice plants fed by nitrate. Water transport occurred mainly through Hg-sensitive water channels in rice roots supplied with ammonium.     

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.     

Amino acid content in xylem sap of regrowing alfalfa (Medicago sativa L.): Relations with N uptake,N2 fixation and N remobilization

During vegetative regrowth of Medicago sativa L., soil N, symbiotically fixed N₂ and N reserves meet the nitrogen requirements for shoot regrowth. Experiments with nodulated or non-nodulated plants were carried out to investigate the changes in N flows originating from the different N sources and in xylem transport of amino acids during regrowth. Exogenous N uptake, N₂ fixation and endogenous N remobilization were estimated by ¹⁵N labelling and amino acids in xylem sap were analysed. Removal of shoots resulted in great declines of exogenous N flows derived either from N₂ or from NH₄NO₃ during the first week of regrowth, thereafter recovery increased linearly. Mineral N uptake as well as N₂ fixation occurred mainly between the 10th and 18th day after removal of shoots while exogenous N assimilation in intact plants remained at a steady level. Nitrogen remobilization rates in defoliated plants increased by at least three to five-fold, especially during the first 10 days following shoot removal. Compared to control plants, contents of amino acids in xylem sap, during the first 10 days of regrowth, were reduced by about 72% and 82% in NH₄NO₃ grown and in N₂ fixing plants, respectively. Asparagine was the main amino acid transported in xylem sap of both treated plants. Its relative contents during this period significantly decreased from 75% to 59% and from 67% to 36% respectively in non-nodulated plants and in nodulated ones. This decline was accompanied by compensatory increase in the relative contents of aspartate and glutamine.     

Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth,transpiration, and water and potassium uptake and flow in tobacco

NH₄ ⁺-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO₃ ^–, NH₄NO₃, NH₄ ⁺ or NH₄ ⁺+6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO₃ ^–-N by NH₄ ⁺-N (NH₄NO₃) did not change growth, transpiration, uptake and flow of water and K⁺ compared with the NO₃ ^–-N treatment. However, NH₄ ⁺-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO₃ ^–-N and NH₄NO₃ treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (¹³C) compared with the NO₃ ^– treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K⁺ uptake to 10%. Under NH₄ ⁺ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K⁺ currently taken up, indicating massive mobilisation of K⁺ from lower leaves and even the roots. Phloem retranslocation of K⁺ from the shoot and cycling through the root contributed 67% to the xylem transport of K⁺, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K⁺) caused by NH₄ ⁺-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10^–8 M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH₄ ⁺-fed plants are discussed.     

The Relationship between Transpiration, Root Water Uptake, and Leaf Water Potential

The effect of changing the transpiration rate on leaf waterpotential and water balance has been examined to show if permeabilityof the plant (predominantly the roots) is constant or varieswith the transpiration rate. Measurements of leaf effectivethickness, water potential, transpiration, and uptake of waterby roots were made on sunflower, barley, and maize plants grownin solution culture and subjected to a range of atmosphericconditions and root treatments: cooling, low osmotic potential,and removal of part of the root system. Leaf water potential changed little under a wide range of atmosphericconditions and rates of water flux in the three species, sothat the root permeability to water increases as the rate oftranspiration, and therefore flow across the root surface, increases.Equality between uptake and loss of water and thereby maintenanceof constant leaf water potential is assisted by stomatal changes,which appear to be in response to conditions at or in the rootrather than a direct response to changes in bulk leaf waterpotential.