Root Density and Water Potential Gradients near the Plant Root

The models of Gardner (1960) and Cowan (1965) for water transferto the plant root are used to estimate the differences in waterpotential between the root and the bulk soil for a wide rangeof root densities and water extraction rates at a series ofmatric potentials for a Yolo light clay. For root densities and extraction rates reported both in theliterature and in this paper there is good evidence to suggestthat the large potential gradients originally predicted by Gardnerand Cowan are restricted to situations involving very low rootdensities and high extraction rates in relatively dry soil.     

Longitudinal Water Movement in the Primary Root of Zea mays

The rates of transfer of tritiated water (THO) along lengthsof excised primary roots of Zea mays have been measured undera variety of conditions. The following values of ‘apparentdiffusion coefficients’ for THO in the root tissue havebeen evaluated: 1.5±0.1x10^-5 cm² sec^-1 in roots boiledfor 3 min before use,0.5±0.03x10^-5 cm² sec^-1 in rootspoisoned with 10^-2 M NaF,0.9±0.07x10^-5 cm² sec^-1 in rootspoisoned with 10^-2 M NaN₃,and 2.1±0.2x10^-5 cm² sec^-1in normal roots. The bathing medium in all cases was 1.0 mMKCl/0.1 mM CaCl₂ with the addition of the inhibitors where appropriate.Thefourfold increase in the rate of THO transfer in normal rootscompared with poisoned ones is attributed to the existence ofa long-distance convective flow in the first case, which isterminated by the addition of inhibitors. Since experimentsshow that this convective flow must occur both acropetally andbasipetally with equal velocity, it is thought to occur in thephloem.By assuming the ‘streaming transcellular strands’model for phloem transport, the rate of movement required togive the observed transfer has been computed as approximately4.5x10^-2 cm sec^-1 (160 cm h^-1).The earlier report of the existenceof a highly impermeable barrier surrounding the xylem vesselshas been further substantiated by the experiments reported here.     

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.     

Decreased Growth-Induced Water Potential (A Primary Cause of Growth Inhibition at Low Water Potentials)

Cell enlargement depends on a growth-induced difference in water potential to move water into the cells. Water deficits decrease this potential difference and inhibit growth. To investigate whether the decrease causes the growth inhibition, pressure was applied to the roots of soybean (Glycine max L. Merr.) seedlings and the growth and potential difference were monitored in the stems. In water-limited plants, the inhibited stem growth increased when the roots were pressurized and it reverted to the previous rate when the pressure was released. The pressure around the roots was perceived as an increased turgor in the stem in small cells next to the xylem, but not in outlying cortical cells. This local effect implied that water transport was impeded by the small cells. The diffusivity for water was much less in the small cells than in the outlying cells. The small cells thus were a barrier that caused the growth-induced potential difference to be large during rapid growth, but to reverse locally during the early part of a water deficit. Such a barrier may be a frequent property of meristems. Because stem growth responded to the pressure-induced recovery of the potential difference across this barrier, we conclude that a decrease in the growth-induced potential difference was a primary cause of the inhibition.     

A Vesicle Containing Fibrous Materials Appearing in Water-Stressed Corn Root Cells

Ultrastructural changes induced by water stress were investigatedin corn root cells. Desiccation was imposed by withholding waterfrom the soil. Vesicles containing cellwall-like fibrous materialswere observed in the cortex cells of desiccated corn roots andnot in the controls. (Received April 7, 1981; Accepted June 15, 1981)     

Efflux of Potassium from Wheat Roots Induced by Changes in the Water Potential of the Root Medium

Growth of Plantago major L., ssp. major L. and ssp. pleiosperma Pilger and P. maritime L. was followed at two levels of mineral nutrition (low-salt and high-salt). In addition the response of transfer of plants from low-salt conditions to high-salt conditions and vice versa was studied. Growth of the studied Plantago species was strongly stimulated by high-salt conditions. The Ca²⁺- and Mg²⁺-stimulated ATPase activity of microsomal preparations of the roots was also studied. In P. major ssp. major and P. maritime a major pH optimum was observed at pH 6.5, and in addition a second pH optimum was found at pH 8.0. High-salt plants of these two species were characterized by biphasic stimulation curves for Ca²⁺ and Mg²⁺, whereas P. major ssp. pleiosperma showed a monophasic pattern. The ATPase activity per g dry weight of P. major and P. maritima was highest in high-salt plants. The species investigated here are adapted to relatively nutrient-rich conditions, and the properties of ATPases (K_m, K_max, protein content) and the growth responses are discussed in relation to this ecological property.     

A Robust Water Potential Parameterisation

Abstract

We compare molecular dynamics simulation results for the properties of liquid water predicted by four novel water potential models. These models are designed as a combination of parameters taken from the dedicated but brittle TIP3P water potential, and the more flexible but less accurate parameterisations such as the Dreiding and Universal force fields. We find that a hybrid of Dreiding and TIP3P delivers the best results, yielding a density, diffusion coefficient and radial distribution function in good agreement with experiment, performing in some respects even better than the dedicated reference TIP3P model. Another Dreiding based force field predicts semi-quantitative results for the water structure and dynamics while the Universal force field based models are incapable of simulating a condensed phase of water at all, continuing to expand indefinitely. These observations are useful for selecting and designing robust water force field parameterisations that can be used for general simulation purposes.     

Light Regulation of the Growth Response in Corn Root Gravitropism

Roots of Merit variety corn (Zea mays L.) require red light for orthogravitropic curvature. Experiments were undertaken to identify the step in the pathway from gravity perception to asymmetric growth on which light may act. Red light was effective in inducing gravitropism whether it was supplied concomitant with or as long as 30 minutes after the gravity stimulus (GS). The presentation time was the same whether the GS was supplied in red light or in darkness. Red light given before the GS slightly enhanced the rate of curvature but had little effect on the lag time or on the final curvature. This enhancement was expanded by a delay between the red light pulse and the GS. These results indicate that gravity perception and at least the initial transduction steps proceed in the dark. Light may regulate the final growth (motor) phase of gravitropism. The time required for full expression of the light enhancement of curvature is consistent with its involvement in some light-stimulated biosynthetic event.     

Potassium Transport in Corn Roots : II. The Significance of the Root Periphery

The relative transport capabilities of the cells of the root periphery and cortex were investigated using a variety of experimental techniques. Brief (30 seconds to 1 minute) exposures with the penetrating sulfhydryl reagent, N-ethyl maleimide (NEM), and the impermeant reagent, p-chloromercuribenzene sulfonic acid (PCMBS), dramatically reduced ⁸⁶Rb⁺ (0.2 millimolar RbCl) uptake into 2 centimeter corn (Zea mays [A632 × (C3640 × Oh43)]) root segments. Autoradiographic localization studies with [³H]NEM and [²⁰³Hg]PCMBS demonstrated that, during short term exposures with either reagent, sulfhydryl binding occurred almost exclusively in the cells of the root periphery.

Corn root cortical protoplasts were isolated, and exhibited significant K⁺(⁸⁶Rb⁺) influx. The kinetics for K⁺ uptake were studied; the influx isotherms were smooth, nonsaturating curves that approached linearity at higher K⁺(Rb⁺) concentrations (above 1 millimolar K⁺). These kinetics were identical in shape to the complex kinetics previously observed for K⁺ uptake in corn roots (Kochian, Lucas 1982 Plant Physiol 70: 1723-1731), and could be resolved into a saturable and a first order kinetic component.

The existence of a hypodermal apoplastic barrier was investigated. The apoplastic, cell wall binding dye, Calcofluor White M2R, appeared to be excluded from the cortex by the hypodermis. However, experiments with damaged roots indicated that this result may be an artifact resulting from the binding of dye to the epidermal cell walls. Furthermore, [²⁰³Hg] PCMBS autoradiography demonstrated that the hypodermis was not a barrier to apoplastic movement of PCMBS.

These results suggest that although cortical cells possess the capacity to absorb ions, K⁺ influx at low concentrations is limited to the root periphery. Cortical cell uptake appears to be repressed under these conditions. At higher concentrations, cortical cells may function to absorb K⁺. Such a model may involve regulation of cortical cell ion transport capacity.

     

Water Movement from Soil to Root Investigated through Simultaneous Measurement of Soil and Stem Water Potential in Potted Trees

Legge, N. J. 1985. Water movement from soil to root investigatedthrough simultaneous measurement of soil and stem water potentialin potted trees.—J. exp. Bot. 36: 1583–1589. Osmotic tensiometers implanted in the stems of three mountainash (Eucalyptus regnans F. Muell.) saplings growing in largeplastic bins recorded stem water potential, _w, while soil waterpotential, _w, was simultaneously recorded by instruments nearthe trees' roots and in the surrounding root-free soil Earlyin a drying cycle, with the soil still wet, the diurnal variationin ₁, was often slight, despite diurnal variations in _u approaching2.0 M Pa. Late in a drying cycle the diurnal fluctuations in₁, and _u were very similar although changes in ₁, still laggedup to 1.5 h behind changes in _u. ₁values at this time occasionallyreached –3.0 MPa with no apparent damage to the treesWatering the bins in daytime led to a response in ₁, valueswithin about 5 min, whereas _u, values did not respond for afurther 20 min. _u values then rose rapidly but after only 1h began to decline again, while ₁, values remained at or nearsaturation for the rest of the day. Water uptake hypotheseswhich attribute an important role to a soil-root interface resistanceare not supported by these data Key words: —Soil water potential, penrhizal gradients     

Rubidium Absorption by Corn Root Tissue after a Brief Period of Water Stress and during Recovery

Intact and excised samples of corn root tissue were subjected to water stress either by incubation in solutions of osmotica or by desiccation, after which they were transferred to CaSO₄ solution for various time periods for recovery. Osmotic agents used were either mannitol or polyethylene glycol 6000 at concentrations adequate to depress rubidium absorption to less than 30% of that of controls. During 6 h following release from osmotic stress, rubidium absorption by samples from intact seedlings treated with mannitol increased to 44% of that of controls, while those treated with polyethylene glycol increased to 79% of that of controls. Recovery of root samples excised prior to stress was very nearly the same as that of samples from intact roots. When water stress was produced by desiccation, recovery was about the same as from polyethylene glycol, attaining a rate of 76% of that of controls after 8 h. Whereas desiccated samples almost completely regained their water content during recovery, none showed correspondingly complete recovery of ion absorption capacity. Finally, during the recovery period, the submerged controls, but not controls in humid air, showed an increase in ion absorption capacity with time, confirming that the widely observed “aging effect” or “washing effect” is due to submersion and not to time per se.     

Water Deficit Rapidly Stimulates the Activity of a Protein Kinase in the Elongation Zone of the Maize Primary Root

The mechanisms by which plants detect water deficit and transduce that signal into adaptive responses is unknown. In maize (Zea mays L.) seedlings, primary roots adapt to low water potentials such that substantial rates of elongation continue when shoot growth is completely inhibited. In this study, in-gel protein kinase assays were used to determine whether protein kinases in the elongation zone of the primary root undergo activation or inactivation in response to water deficit. Multiple differences were detected in the phosphoprotein content of root tips of water-stressed compared with well-watered seedlings. Protein kinase assays identified water-deficit-activated protein kinases, including a 45-kD, Ca2+-independent serine/threonine protein kinase. Water-deficit activation of this kinase occurred within 30 min after transplanting seedlings to conditions of low water potential and was localized to the elongation zone, was independent of ABA accumulation, and was unaffected by cycloheximide-mediated inhibition of protein translation. These results provide evidence that the 45-kD protein kinase acts at an early step in the response of maize primary roots to water deficit and is possibly involved in regulating the adaptation of root growth to low water potential.     

Quantitative Assessment of Ultrastructural Changes in Primary Roots of Corn (Zea mays L.) after Geotropic Stimulation: I. Root Cap

The root cap is the site of gravity perception. In the study of caps of primary roots of corn (Zea mays L.), we compared the ultrastructure of geotropically responding roots that had received a 661 nm (red) irradiation (60 second) with nonresponding dark control roots kept in the dark, at comparable times following geotropic stimulation for a total of 150 minutes. The outstanding differences in the light-exposed root caps at the ultrastructural level were: (a) significantly more Golgi apparatus (dictyosomes) were found in the top than in the bottom of red-exposed cells; a random distribution is seen in the dark control cells; (b) the nucleus preferred the top in a greater number of the red-exposed cells; (c) the pattern of mitochondria localization was identical in both treatments, a greater preference for the top; however, the number of mitochondria was reduced in the bottom of red-treated cap cells as compared to the control cells. A lowering in number in the bottom of the red-treated cells was noted also in the dictyosomes; and (d) in a small percentage of cells that showed a preferential distribution of endoplasmic reticulum (ER), more red-exposed cells than controls, during the period 30 to 135 minutes after stimulation, had less ER in the top; however, a majority of the cells in both treatments showed no preferred position for ER distribution. Commonalities in ultra-structural behavior also existed between the red- and dark-treated root cap cells: (a) sedimentation of amyloplasts, with no difference in total number between treatments; and (b) a close association between amyloplasts and ER in both groups.

Polarization of organelles occurred in both the geotropically responding and nonresponding roots. The differences in dictyosome and nuclear localization, and dictyosome and mitochondrial number could be correlated with the tropic response in the red-exposed roots and no response in the dark roots, which in turn could be related to the reported hormonal events in the geotropism of roots.

     

Corn Root Protoplasts: ISOLATION AND GENERAL CHARACTERIZATION OF ION TRANSPORT

A method was developed for the large scale and rapid isolation of intact viable corn root protoplasts. Pure and metabolically active protoplasts were collected using a flotation technique. Vital staining tests, light and electron microscopy, and measurements of basic metabolic processes indicated that the isolated protoplasts were metabolically active, and that the plasmalemma and other organelles were well preserved. The isolated protoplasts performed normal, active ion transport functions. Time course of K⁺ and inorganic phosphate (H₂PO₄⁻) influx and the effects of external pH, carbonyl cyanide p-trifluoromethoxyphenylhydrazone, fusicoccin, and diethylstilbestrol on K⁺ and inorganic phosphate influx and net H⁺ efflux in isolated protoplasts correlated well with data obtained on root segments. Data presented indicated that isolated protoplasts from roots can be used to gain additional insights into the mechanism of ion transport in plant cells.     

Plant Growth in Polyethylene Glycol Solutions in Relation to the Osmotic Potential of the Root Medium and the Leaf Water Balance

The changes in leaf extension, plant dryweight, leaf area, netassimilation rate (E), relative growth-rate (R_W), and relativeleaf growth-rate (R_L), have been studied for four species grownfor 2 weeks in solutions of polyethylene glycol 4000 of controlledosmotic potentials. All aspects of growth were decreased bydecreasing the osmotic potential (_sol) of the root medium andthe leaf water potential (), and ceased when _/ was greater than— 10 bars in bean, cotton, maize. These plants were moresusceptible than ryegrass to water stress. Growth of bean stoppedat equal to about —6 bars, but even at —10 barsryegrass was capable of some growth. Slight decrease in fromthe values in the control plants decreased growth during thefirst week but partial recovery was apparent during the secondweek's growth in solution culture, when leaf extension, E, R_Land R_W increased in plants subjected to stress. Examinationof the water balance, water potential, osmotic potential andturgor of the leaf in relation to relative water content suggeststhat recovery was related to increased turgor and that the abilityof the plants to grow at reduced values of the osmotic potentialof the root medium and of the leaf water potential depend onthe maintenance of turgor.     

Cytokinin Inhibits Lateral Root Development at the Earliest Stages of Lateral Root Primordium Initiation in Maize Primary Root

Root architecture is basically controlled by auxin and cytokinin, which antagonize in the formation of lateral roots (LRs) along the primary root (PR) axis. Several mechanisms have been proposed to explain the interaction between these two hormones, cytokinin being the hormone that inhibits LR formation. The analysis of the cytokinin effect on LR formation using LRs in several stages of development could indicate which steps of LR formation are more sensitive to cytokinin. The application of cytokinin to maize PRs showed that the inhibitory effect of cytokinin on LR formation was greater in the zones in which the initial events to form new LRs are taking place. In the presence of cytokinin, the PR is not able to produce new LRs in the initiation zone; this inhibitory effect is permanent as this zone did not recover the capability to form LRs after removing cytokinin. However, the LR density in zones with appreciable LR primordia when cytokinin was applied was only slightly inhibited when a high concentration was used. These results showed that LR formation is more sensitive to the inhibitory effect of cytokinin in the earliest stages of LR development. However, the elongation of a LR primordium to emerge and the subsequent elongation of the new LR were only slightly affected by cytokinin.

     

The Intra-cellular Localization of the Herbicide Diphenamid in Corn Root .Tip

The intra-cellalar localization of the herbicide N,N-dimethyl–2,2-diphenylacetamide (diphenamid) was examined in 10 mm root tips of corn seedlings (Zea mays L. cv. Shawnee). Experiments are described which culminate in the observation that a significant, measurable effect on root growth occurs within 12 hours after treatment with diphenamid. Corn seedlings were then treated with 14-C-diphenamid for 12 hours and the 10 mm root tips excised and homogenized. By use of differential centrifugation and gel filtration, the root homogenate was separated into fractions; as follows; cell wall, mitochondria, microsomal, protein and eluate. The highest concentration of the herbicide was found in the root tips. Furthermore, most of the radioactivity was found in the eluate. Results are discussed as to localization of diphenamid in the microsomal and protein fraction and possible mode of action.     

初生根伸长对环境响应的数学模型

在根组织细胞分裂和细胞体积增大亚过程基础上,建立了一个综合根生长特征参数和环境影响在内的单根伸长的数学模型,同时实验观测了大豆根在不同的环境水势下的生长过程。模型计算结果与实验结果吻合较好。最后利用模型探讨了根伸长对环境因子变化的响应,以及根内水势分布。     

Frictional Potential Losses and Total Water Potential in Plants: a Re-evaluation

A theoretical treatment is given of isothermal potential lossesdue to frictional resistances against water flow in plants.It becomes apparent that in the past two main difficulties havebeen widely ignored: the proper choice of dimensions for fluxesand resistances deserves consideration, and the original vanden Honert concept seems to be especially prone to misinterpretation.A revised equation is presented for total water potential ata certain point in the plant, and some implications of thisapproach are outlined.