THE QUIESCENT CENTER IN CULTURED ROOTS OF CONVOLVULUS ARVENSIS L.

Cultured roots of Convolvulus arvensis were incubated in 0.2–0.3 μc/ml methyl-³H-thymidine for different intervals of time. In roots supplied with tritiated thymidine for 12 hr, 14 hr, 48 hr, or 14 hr followed by transfer to fresh medium for 24 hr, autoradiographs prepared of serial, longitudinal sections of the root tips showed the presence of a subterminal quiescent center in the root proper at the distal poles of the central cylinder and cortex. In addition, a zone of unlabelled cells in the columella, distal to the root cap initials, was present. In roots supplied continuously with tritiated thymidine for 64 hr, 96 hr, and 120 hr, the quiescent center was either reduced in size or was not present.     

Heterogeneity in Water Availability Alters Cellular Development and Hydraulic Conductivity along Roots of a Desert Succulent

Plants of the desert succulent Agave deserti were grown in partitionedcontainers to determine whether heterogeneity in soil moistureleads to differences in cellular development and hydraulic conductivityalong individual roots. Roots from containers with a dry distalcompartment (furthest from the shoot), a wet middle compartment,and a dry proximal compartment had distal regions (includingthe root tips) that were more suberized and lignified in theendodermis and adjacent cell layers than were root regions fromthe wet middle compartment. Proximal root regions about 40 mmfrom the succulent shoot base were also relatively unsuberized,suggesting that both external and internal supplies of waterdelayed tissue maturation. Root segments from wet middle compartmentsand from dry proximal compartments had higher hydraulic conductivitythan did the more suberized root segments from dry distal compartments.Unlike distal root segments from wet compartments, segmentsfrom dry compartments suffered no decrease in hydraulic conductivityafter immersion in mercuric chloride, suggesting that aquaporinactivity diminished for roots during drought. The possible closureof water channels could help limit root water loss to a dryingsoil. The delayed development of suberized cell layers may allowroot regions to maximize water uptake from wet soil patches(such as under rocks), and the relatively immature, absorptiveroot region near the base of the shoot may help A. deserti capturewater from a briefly wetted surface soil. Copyright 2000 Annalsof Botany Company Agave deserti, root plasticity, water uptake, aquaporins, suberization, endodermis, divided pots.     

DURATION OF THE NUCLEAR CYC LE IN TRADESCANTIA PALUDOSA ROOT TIPS AS MEASURED WITH H3-THYMIDINE

Wimber , Donald E. (Brookhaven National Lab., Upton, N. Y.) Duration of the nuclear cycle in Tradescantia paludosa root tips as measured with H³-thymidine. Amer. Jour. Bot. 47(10): 828–834. Illus. 1960.—The duration of the nuclear cycle and its various subdivisions were measured in Tradescantia root tips by autoradiographic techniques. H³-thymidine was used as a nuclear label and was supplied to the roots for 0.5 hr. After labeling, the roots were allowed to grow in the absence of label for periods up to 38 hr. By determining the percentage of divisions labeled at the various times of fixation, a reconstruction of the nuclear cycle could be made. The average cycle was determined as 20 hr. in duration, DNA synthesis 10.8 hr., presynthetic interphase 4 hr., postsynthetic interphase 2.7 hr., prophase 1.6 hr., metaphase 0.3 hr. and anaphase-telophase 0.6 hr. Approximate standard deviations for the duration of some of the subdivisions were calculated.     

THE TIME-COURSE OF POLAR MOVEMENT OF GIBBERELLIN THROUGH ZEA ROOTS

The polarity of movement of gibberellin through sections cut from near the root tips of Zea mays L. was studied, using methods like those we previously used in roots for auxin and in petioles for auxins, cytokinins, and gibberellic acid (GA-3). One μg GA-3 was added in a donor agar block and gibberellin activity in the receiver agar at the opposite end of the section was measured directly with a modified barley endosperm bioassay. The movement of gibberellin was away from the root tip (basipetal) and thus opposite in direction to the polarity of auxin through such root sections. The time-course of basipetal movement was dissimilar to that for gibberellin or auxin movement through petiole sections. It took 14-18 hr for gibberellin activity equivalent to 6 ng GA-3 to collect in the basal receivers on roots. Apical receivers showed activity equivalent to 1.6 ng GA-3 at 14-18 hr. Less than 0.01 ng equivalent GA-3 was collected from sections to which GA-3 was not added, so the 6 and 1.6 ng were almost entirely due to the added GA-3. These general conclusions were confirmed with an experiment using ¹⁴C-GA-3. A decline in activity in receivers was found in some experiments at 18 hr, paralleling earlier results with GA-3, IAA, and adenine in petioles and IAA in roots.     

Fertilization in a mature Scots pine (Pinus sylvestris L.) stand—effects on fine roots

The effects of liquid and solid fertilizers on fine-root development were studied in a 130-year-old Scots pine (Pinus sylvestris L.) stand. Ingrowth cores,viz. initially root-free mesh bags with sieved mineral soil taken outside the plots and driven to a depth of 30 cm from the soil surface, were subsequently resampled and the amount of fine roots was estimated. The total accretion of both fine-root length and dry weight was comparatively high in the liquid fertilization plot. The most substantial net accretion, however, during the 1984 period was in the control plot. The results of the study is that the ramification pattern of the fine roots was strongly influenced by fertilization. The average number of root tips per unit length was 9.9 cm⁻¹ in the control plot cm⁻¹, compared with both the solid (A and B) and liquid fertilization plots (2.3, 3.2 and 3.3 cm⁻¹, respectively) due to a greater occurrence of mycorrhizal aggregates (‘ball mycorrhiza’). The effects of fertilization on the mineral nutrient concentrations in the fine roots were limited—the strongest effects were in the liquid-fertilized area. The observed increase in the concentration of most mineral nutrients in the latter experimental area, however, may be due to a change in the growth pattern of the fine roots rather than an effect of the fertilizers themselves.     

Effects of defoliation and symbiosis on polyamine levels in pine and birch

 We report the effect of ectomycorrhizal fungi (Suillus variegatus, Paxillus involutus) and defoliation on polyamine concentrations in pine (Pinus silvestris) and birch (Betula pendula) foliage and roots. Symbiotic root tips showed consistently higher concentrations of putrescine than non-symbiotic roots. Partial defoliation had no effect on the polyamine levels in mycorrhizal pine or birch roots. The foliage of mycorrhizal pine seedlings had lower putrescine concentrations and higher spermidine than foliage of non-mycorrhizal plants, and defoliation reversed this pattern. The response to partial defoliation differed in birch foliage: mycorrhizal status had no effect and all new growth after defoliation had higher spermidine levels than in non-defoliated birch. The potential role of polyamines in mycorrhizal symbiosis is discussed. Accepted: 26 February 1997     

The flow of water into fruit trees. I. Resistances to water flow through roots and stems

Relative conductivity (K) to water in healthy apple trees ranged from maximum values of 18.2 cm³.100 s^-1.cm length.0.001 Pas.kPa^-1.cm^-2 xylem area, for major suberized roots to values of 1.6 for 1-yr-old twigs. The values for equivalent parts of healthy cherry trees were 26.3 and 3.3. Trees with roots affected by the larvae of the fruit tree root weevil (Leptopius squalidus) which causes either chronic growth decline or sudden wilting and death, had values as low as 1% of healthy trees, in those parts of the tree showing wilting and lack of growth. Water flow under pressure into the root systems of healthy apple trees increased linearly with increases in pressure from 200 to 800 kPa. Flows into dormant and active root systems respectively were 0.6 and 1.7 cm³.100 s^-1. 100 cm² root surface area. 100 kPa^-1.     

EFFECTS OF BORON DEFICIENCY ON MITOSIS AND INCORPORATION OF TRITIATED THYMIDINE INTO NUCLEI OF SUNFLOWER ROOT TIPS

Boron deprivation has multiple effects upon root growth within 6 hr after this essential micronutrient is withheld. Root elongation is inhibited and this response has been attributed to a cessation of mitosis and DNA synthesis. Our preliminary results using an autoradiographic analysis of sunflower roots labeled with [³H]-thymidine demonstrated no difference in label distribution between +/-B root tips. We found that mitosis in inhibited in -B roots but does not completely cease. Scintillation counting of whole root tips shows that boron-deficient roots up to 72 hr of treatment incorporate radioactive label at a level comparable to that of the controls. Because mitosis and presumably DNA synthesis are affected by prolonged boron deficiency, these results may be brought about by a change in membrane integrity or permeability. We propose that effects of boron deprivation on DNA synthesis and mitosis in sunflower are secondary and that primary events involve alterations in cellular membranes.     

Contrasting fine-root production,survival and soil CO2 efflux in pine and poplar plantations

Tree root activity, including fine-root production, turnover and metabolic activity are significant components of forest productivity and nutrient cycling. Differences in root activity among forest types are not well known. A 3-year study was undertaken in red pine (Pinus resinosa Ait.) and hybrid poplar (Populus tristis X P. balsamifera cv `Tristis no. 1') plantations to compare belowground root dynamics. We measured fine-root production, mortality and standing crop, as well as soil CO₂ efflux. Pine fine-root production was only 2.9% of that of poplar during three years; 85 pine roots were observed in minirhizotron tubes compared with 4088 poplar roots. Live-root density oscillated seasonally for both species with late winter minimum and autumn maximum. Poplar reached constant maximum live-root length within the first growing season, but pine continued to increase observed fine-root length for three growing seasons. Within the first 100 days following initial appearance, 22% of the pine roots disappeared and 38% of the poplar roots disappeared. Median fine-root longevity of pine was 291 days compared with 149 days for poplar roots. Fine-root longevity increased with depth in the soil, and was greater for roots with initial diameter >0.5 mm. The probability of poplar root death from late February to May was more than three times that in any other season, regardless of root age. Despite the greater poplar root production and live-root length, fine-root biomass and soil CO₂ efflux was greater in pine. Greater metabolic activity in the pine stand may be due to greater fine-root biomass or greater heterotrophic respiration. This revised version was published online in June 2006 with corrections to the Cover Date.     

THE ONSET OF MITOSIS AND DNA SYNTHESIS IN ROOTS OF GERMINATING BEANS

The time of onset of mitosis and DNA synthesis has been determined in roots of germinating seeds of Vicia faba. Mitosis is not initiated in all roots simultaneously. Dividing cells are seen 36 hr from the beginning of germination, but they are present in low frequency (0.02%). Dividing cells do not become frequent, i.e., occurring as 5% or more of all cells, until 56 hr, and it is not until 66–68 hr that all roots in a sample of 10 are mitotically active. DNA synthesis shows a similar sporadic beginning. It occurs in a few cells by 28 hr, and by 40 hr all roots exposed to ³H–thymidine show active incorporation. For most cells in these germinating roots DNA synthesis precedes mitosis. In one root in 10, however, some cells are unlabeled when they enter mitosis, indicating that they had spent the dormant period in the G₂ phase of the mitotic cycle. The presence of these cells determines whether or not roots show chromatid and chromosome aberrations following irradiation during germination.     

Induction of nitrate uptake and nitrate reductase activity in trembling aspen and lodgepole pine

¹³NO₃^– influx into the roots and in vivo nitrate reductase activity (NRA) in the roots and leaves have been measured in trembling aspen (Populus tremuloides Michx.) and lodgepole pine (Pinus contorta Dougl.) seedlings after exposure to either 0·1 or 1·5 mol m^–3 NO₃^– for varying periods up to 20 d. Both NO₃^– influx and NRA were inducible in these species and, in trembling aspen, peak induction of nitrate influx and NRA were achieved within 12 h, compared to 2–4 d for influx and 4–12 d for NRA in lodgepole pine. In trembling aspen, ≈ 30% of the total ¹³N absorbed during a 10 min influx period followed by 2 min of desorption was translocated to the shoot. In lodgepole pine, by contrast, translocation of ¹³N to the shoot was undetectable during the same time period. Root NRA as well as NO₃^– influx from 0·1 mol m^–3 NO₃^– were substantially higher in trembling aspen than in lodgepole pine at all stages of NO₃^– exposure, i.e. during the uninduced, the peak induction, and steady-state stages. In order to examine whether the lower rates of NO₃^– influx and NRA were related to proportionately fewer young (unsuberized) roots in lodgepole pine, we determined these parameters in young and old (suberized) roots of this species separately. Induction of influx and NRA were initially greater in young roots but at steady-state there were only minor differences between the young and the old roots. However, even the elevated initial rates in the young roots of lodgepole pine were substantially lower than those of aspen. In pine, influx at 1·5 mol m^–3 NO₃^– was ~ 6-fold higher than at 0·1 mol m^–3 NO₃^– and appeared to be mostly via a constitutive system. By contrast, in aspen, steady-state influxes at 0·1 and 1·5 mol m^–3 were not significantly different, being similar to the rate attained by pine at only the higher [NO₃^–]. In aspen, leaf NRA was ~ 2-fold higher than that of roots. In lodgepole pine NRA of the needles was below the detection limit. These results show that trembling aspen seedlings are better adapted for NO₃^– acquisition and utilization than lodgepole pine seedlings.     

A new approach for the quantification of root-cap mucilage exudation in the soil

Direct evidence on the functions of root-cap mucilage during plant root growth in soil is limited mainly due to the lack of a method for in situ measurements. In this paper, we offer a method that facilitates the measurement of mucilage exudation when roots are growing in soil. We observed the mucilage exudation directly through a transparent panel located on the side of a root box in which plant roots were growing. We used a CCD camera attached to a microscope to observe and record mucilage exudation. Using image analysis, the activity of mucilage exudation was evaluated based on the area occupied by the mucilage on the root tip. The area of mucilage observed on the root tips after 1-h growth in soil corresponded with the weight of mucilage that was originally observed on the tips before they were transplanted. This relationship suggests that the observed area on root tip relates to total exudation. The area of mucilage exudation on the root tips was high (0.48 mm²) at night and low (0.35 mm²) at midday, suggesting that the activity of mucilage exudation follows diurnal changes. Furthermore, the mucilage exudation positively correlated with the root elongation rate, implying that fast-growing roots exude more mucilage.     

Effect of mycorrhiza and nitrate nutrition on nitrate reductase activity in Scots pine seedlings

In vivo nitrate reductase (EC 1.6.6.1) activity was measured in seedlings of Scots pine ( Pinus sylvestris L.) inoculated with Cenococcum geophilum (Sow.) Ferd. & Winge, Paxillus involutus (Batsch:Fr) Fr, Piloderma croceum Erikss, & Hjortst, and Suillus variegatus (Fr.) O. Kuntze. The activity was higher in the mycorrhizal pine roots than was previously found in the fungus symbiont alone, but lower than in the roots of nonmycorrhizal pine seedlings. The differences observed in a previous study between the fungal species under pure culture conditions were not found in the present work for mycorrhiza synthezised with the same fungal species. An increase in the nitrate concentration of the nutrient solution increased the proportion of the nitrate reductase activity in the needles. The mycorrhizal root tips had higher nitrate reductase activity than nonmycorrhizal root tips in the same root system.     

Root growth of wetland plants with different root types

A wastewater culture system was designed to study the root growth of eight species of wetland plants with two different root types. The system included a plastic barrel for holding the wastewater and a foam plate for holding the plant. The results indicated that the root growth of the plants with fibril roots was faster than that of the plants with rhizomatic roots. The species with fibril roots had higher root number (1349 per plant) than species with rhizomatic roots (549 per plant) after ten weeks of cultivation. The average root biomass of plants with fibril roots was 11.3 g per plant, whereas that of plants with rhizomatic roots was 7.4 g per plant. Fine root biomass of diameter ≤ 1 mm constituted 51.9% of the total root biomass in plants with fibril roots, whereas it accounted for only 25.1% in plants with rhizomatic roots. The root surface area of the plants with fibril roots (6933 cm² per plant) was markedly larger than that of the species with rhizomatic roots (1897 cm² per plant). The species with rhizomatic roots showed a longer root lifespan (46.6 days) than those with fibril roots (34.8 days).     

OXYGEN TENSION AND THE RATES OF MITOSIS AND INTERPHASE IN ROOTS

The object of this work was to determine the influence of a wide range of oxygen tensions upon the relative rates of respiration, mitosis, and interphase in pea root tips, compared with the normal rates of these processes in air. From the rates of disappearance of mitotic figures in excised tips kept in various oxygen tensions, the relative rates of mitosis were found to decrease gradually from 122 per cent in 100 per cent oxygen to 24 per cent in 0.0007 per cent oxygen. From the mitotic indices of intact seedlings, the relative rates of interphase were found to decrease sharply from 82 per cent in 10 per cent oxygen to 6 per cent in 5 per cent oxygen. The data on relative rates of respiration, mitosis, and interphase in root tips were compared, and it was shown that the three processes are perfectly distinct in their quantitative relationships to low oxygen tensions.     

Root distribution of poplar at varying densities on pastoral hill country

Spaced poplar (Populus spp.) trees are used widely in New Zealand for soil conservation on erodible pastoral hill country. Their root distribution in this environment, and factors that affect it, are poorly understood. Robust recommendations on effective tree spacing depend on knowledge of root systems. This study determined the effect of tree density, position between trees, and soil depth (0–90 cm) on root number, root diameter distribution, root area ratio (RAR), and cross sectional area per root for young trees on slopes. Data were collected for lateral roots using trenches. Greater than 80% of roots were < 5 mm diameter and root attributes were highest in shallow soil. Trees at 770 stems per hectare (sph) had 3–12 times more roots and 3–9 times greater RAR than those at densities of ≤ 237 sph, representative of most tree-pasture systems. Mean cross sectional area per root was similar across densities. Positions close to trees had twice as many roots (46 vs. 23/m²) and RAR (109 vs. 52 mm²/m²) as positions midway between trees. The study provided quantitative understanding of variation in root distribution with tree density and information useful for supporting and strengthening recommendations on densities for effective erosion control.     

Effect of poplar genotypes on mycorrhizal infection and secreted enzyme activities in mycorrhizal and non-mycorrhizal roots

The impact of ectomycorrhiza formation on the secretion of exoenzymes by the host plant and the symbiont is unknown. Thirty-eight F(1) individuals from an interspecific Populus deltoides (Bartr.)×Populus trichocarpa (Torr. & A. Gray) controlled cross were inoculated with the ectomycorrhizal fungus Laccaria bicolor. The colonization of poplar roots by L. bicolor dramatically modified their ability to secrete enzymes involved in organic matter breakdown or organic phosphorus mobilization, such as N-acetylglucosaminidase, β-glucuronidase, cellobiohydrolase, β-glucosidase, β-xylosidase, laccase, and acid phosphatase. The expression of genes coding for laccase, N-acetylglucosaminidase, and acid phosphatase was studied in mycorrhizal and non-mycorrhizal root tips. Depending on the genes, their expression was regulated upon symbiosis development. Moreover, it appears that poplar laccases or phosphatases contribute poorly to ectomycorrhiza metabolic activity. Enzymes secreted by poplar roots were added to or substituted by enzymes secreted by L. bicolor. The enzymatic activities expressed in mycorrhizal roots differed significantly between the two parents, while it did not differ in non-mycorrhizal roots. Significant differences were found between poplar genotypes for all enzymatic activities measured on ectomycorrhizas except for laccases activity. In contrast, no significant differences were found between poplar genotypes for enzymatic activities of non-mycorrhizal root tips except for acid phosphatase activity. The level of enzymes secreted by the ectomycorrhizal root tips is under the genetic control of the host. Moreover, poplar heterosis was expressed through the enzymatic activities of the fungal partner.     

THE ISOLATION AND CULTURE IN VITRO OF THE QUIESCENT CENTER OF ZEA MAYS

Using 3-day-old seedling roots of Zea mays L., cv. Kelvedon 33, it was possible to remove the root cap by a simple surgical manipulation without damage to the root proper. By a further small cut, the quiescent center (QC) itself was isolated. This double-convex lens-shaped tissue piece 100 X 250 μm is composed of 1000–1500 cells representing only 0.25 mm³ in volume. The explant was demonstrated unequivocally by ³H-thymidine incorporation before excision and then by autoradiography to be composed of the specific cells usually designated the quiescent center. Using sterile techniques, the QC's were placed on nutrient agar slants and allowed to grow in culture. Of a number of nutrient media tested, only a medium supplemented with organic nitrogen components, indoleacetic acid, kinetin and inorganic nutrients plus sucrose (S2M + K -2,4-D) was effective in eliciting development. Thirty to 40 percent of the 150 isolated QC's grown on this medium formed elongated roots, up to 2 cm in length in 3–4 weeks. Roots developing on agar medium showed in their proximal portion a vascular pattern with 5–6 metaxylem elements or variations of this pattern, but as the root elongated, the vascular pattern was progressively reduced in complexity at the more distal end to a small central group of metaxylem elements. When agar-grown roots were transferred after one week in culture to a liquid nutrient medium of the same composition, the initially reduced vascular pattern evident in the proximal tissues became progressively more complex in the distal portion of the root and after 2 cm of elongation, showed an essentially normal primary vascular tissue pattern characteristic of the seedling root.     

Paxillus involutus mycorrhiza attenuate NaCl-stress responses in the salt-sensitive hybrid poplar Populus×canescens

In order to characterise the effect of ectomycorrhiza on Na⁺-responses of the salt-sensitive poplar hybrid Populus × canescens, growth and stress responses of Paxillus involutus (strain MAJ) were tested in liquid cultures in the presence of 20 to 500 mM NaCl, and the effects of mycorrhization on mineral nutrient accumulation and oxidative stress were characterised in mycorrhizal and non-mycorrhizal poplar seedlings exposed to 150 mM NaCl. Paxillus involutus was salt tolerant, showing biomass increases in media containing up to 500 mM NaCl after 4 weeks growth. Mycorrhizal mantle formation on poplar roots was not affected by 150 mM NaCl. Whole plant performance was positively affected by the fungus because total biomass was greater and leaves accumulated less Na⁺ than non-mycorrhizal plants. Energy dispersive X-ray microanalysis using transmission electron microscopy analysis of the influence of mycorrhization on the subcellular localisation of Na⁺ and Cl⁻ in roots showed that the hyphal mantle did not diminish salt accumulation in root cell walls, indicating that mycorrhization did not provide a physical barrier against excess salinity. In the absence of salt stress, mycorrhizal poplar roots contained higher Na⁺ and Cl⁻ concentrations than non-mycorrhizal poplar roots. Paxillus involutus hyphae produced H₂O₂ in the mantle but not in the Hartig net or in pure culture. Salt exposure resulted in H₂O₂ formation in cortical cells of both non-mycorrhizal and mycorrhizal poplar and stimulated peroxidase but not superoxide dismutase activities. This shows that mature ectomycorrhiza was unable to suppress salt-induced oxidative stress. Element analyses suggest that improved performance of mycorrhizal poplar under salt stress may result from diminished xylem loading of Na⁺ and increased supply with K⁺.     

Water Uptake along the Length of Grapevine Fine Roots: Developmental Anatomy,Tissue-Specific Aquaporin Expression,and Pathways of Water Transport

To better understand water uptake patterns in root systems of woody perennial crops, we detailed the developmental anatomy and hydraulic physiology along the length of grapevine (Vitis berlandieri × Vitis rupestris) fine roots from the tip to secondary growth zones. Our characterization included the localization of suberized structures and aquaporin gene expression and the determination of hydraulic conductivity (Lp_r) and aquaporin protein activity (via chemical inhibition) in different root zones under both osmotic and hydrostatic pressure gradients. Tissue-specific messenger RNA levels of the plasma membrane aquaporin isogenes (VvPIPs) were quantified using laser-capture microdissection and quantitative polymerase chain reaction. Our results highlight dramatic changes in structure and function along the length of grapevine fine roots. Although the root tip lacked suberization altogether, a suberized exodermis and endodermis developed in the maturation zone, which gave way to the secondary growth zone containing a multilayer suberized periderm. Longitudinally, VvPIP isogenes exhibited strong peaks of expression in the root tip that decreased precipitously along the root length in a pattern similar to Arabidopsis (Arabidopsis thaliana) roots. In the radial orientation, expression was always greatest in interior tissues (i.e. stele, endodermis, and/or vascular tissues) for all root zones. High Lp_r and aquaporin protein activity were associated with peak VvPIP expression levels in the root tip. This suggests that aquaporins play a limited role in controlling water uptake in secondary growth zones, which contradicts existing theoretical predictions. Despite having significantly lower Lp_r, woody roots can constitute the vast majority of the root system surface area in mature vines and thus provide for significant water uptake potential.In woody perennial root systems, the majority of water uptake is often attributed to unsuberized fine roots (Kramer and Boyer, 1995), even though woody portions can constitute the vast majority of root surface area for these plants at maturity (Nightingale, 1934; Kramer and Bullock, 1966). This assumption has likely been reinforced by the fact that most studies investigating root water uptake have been done with herbaceous species, whose roots function more like the tips of woody perennials. Although unsuberized fine roots typically have a greater ability to absorb water (i.e. they are more conductive per unit of surface area), it has been shown that older suberized portions of woody taproots can still contribute significantly to root system water uptake (Kramer and Bullock, 1966; Queen, 1967; Chung and Kramer, 1975; MacFall et al., 1990, 1991). Despite this knowledge and the fact that unsuberized roots of many woody perennials are scarce or absent during periods of the growing season when peak transpiration requires much water (MacFall et al., 1991), we still know little about how suberized portions of perennial rooting systems contribute to radial water absorption across species.The composite transport model (Steudle, 2001) is a conceptual framework describing water transport into plant roots. This model posits that water is able to flow into the root via multiple parallel pathways, traveling either in the cell walls (apoplastic) and/or from cell to cell (symplastic and/or transcellular). Transport across the cell-to-cell pathway can involve water crossing plasma membranes; thus, the rate of water uptake can be influenced by the abundance and activity of aquaporins (i.e. water channels). The contribution of aquaporins to root water uptake has been the focus of numerous studies, and the absolute magnitude of this contribution appears to be highly variable, ranging from 20% to 90% across species (for review, see Javot and Maurel, 2002). Steudle (2000) suggested that radial water flow would be dominated by aquaporin regulation in heavily suberized roots, as flow through the apoplast would be minimized. The localization of aquaporins should play a critical role in defining their impact on radial water uptake across suberized and unsuberized roots. For herbaceous species, peak aquaporin mRNA and/or protein levels have been found in root tips and the endodermis, pericycle, phloem, and xylem tissues (Schäffner, 1998; Otto and Kaldenhoff, 2000; Suga et al., 2003; Fraysse et al., 2005; Knipfer et al., 2011). Few aquaporin localization studies have been conducted in woody perennials (Vandeleur et al., 2009). Recent work from our laboratory revealed a precipitous drop in aquaporin expression between the grapevine (Vitis spp. rootstocks) root tips and older root portions (Gambetta et al., 2012). These observations led to this study, where we explore patterns of aquaporin localization in Vitis species fine roots and how they intersect with the structural anatomy and patterns of suberization to affect water uptake along the root length.Hydraulic conductivity (Lp_r) of the apoplastic pathway can be altered through changes in cell wall chemistry, especially through the deposition of suberin. Suberized apoplastic barriers in plant roots include the Casparian band of the endodermis and the suberin lamella of the endodermis, exodermis, and periderm in woody species (Esau, 1977). Casparian bands and suberin lamella are solute impermeable (for review, see Peterson and Enstone, 1996), but across studies, the extent to which they impede the flow of water is highly variable (Peterson et al., 1993; Steudle et al., 1993; Peterson and Enstone, 1996; Schreiber et al., 2005). Regardless, studies support the idea that in roots there is always some flow across the cell-to-cell pathway due to apoplastic barriers and/or an osmotic component to the driving gradient (Steudle et al., 1993; Miyamoto et al., 2001; Knipfer and Fricke, 2011). In the cell-to-cell pathway, Lp_r can be altered by intrinsic plasma membrane properties, plasmodesmata (Oparka and Prior, 1992; Roberts and Oparka, 2003), and/or the abundance and activity of aquaporins. Changes in aquaporin gene expression and protein activity remain potentially dynamic and can occur within hours, while alterations of suberized apoplastic barriers are permanent and would manifest over longer developmental time frames.The total water potential gradient across a fine root can be composed of both osmotic (ΔΨOs) and hydrostatic (ΔΨHy) pressure gradients. A purely ΔΨOs requires that some portion of the pathway be cell to cell. A purely ΔΨHy should drive flow through both pathways, and the proportion of flow through the two pathways will be determined by their Lp_r. Experimentally, Lp_r generated under ΔΨHy is typically greater than Lp_r generated under ΔΨOs, typically ranging from 2-fold to more than 100-fold greater (Steudle et al., 1987; Hallgren et al., 1994; Miyamoto et al., 2001; Knipfer and Fricke, 2011). In some cases, Lp_r is nearly equal under both types of gradients (Miyamoto et al., 2001; Knipfer and Fricke, 2011). These results suggest that if Lp_r through the apoplast were to be reduced by the presence of an apoplastic barrier, this would force flow across a cell-to-cell pathway regardless of the driving gradient (Steudle, 2000).In this study, we sought to provide a more detailed understanding of the localization of aquaporin expression and its contribution to radial water uptake in different zones of grapevine fine roots, from the unsuberized actively growing root tip to portions of the fine root undergoing secondary growth and containing a developed periderm. We characterized the developmental anatomy along the length of the fine root, including the localization of suberized structures, and quantified tissue-specific mRNA levels of plasma membrane aquaporin isogenes via a combination of laser-capture microdissection (LCM) and quantitative PCR. Finally, we determined the Lp_r of root tips and secondary growth root zones under both ΔΨOs and ΔΨHy while investigating the contribution of aquaporin activity to Lp_r via chemical inhibition.