Factors affecting the establishment and maintenance of ‘Titan’ red raspberry root organ cultures

Root organ cultures of red raspberry (Rubus ideaus cv. Titan) were established from two cm long terminal segments of adventitious roots induced in vitro on micropropagated shoot cultures. Growth of root organ cultures was almost entirely by initiation and elongation of lateral roots rather than by terminal elongation from the apical meristem of the original root explants. Indolebutyric acid (IBA) was required for lateral root initiation and elongation. The optimal IBA concentration for lateral root initiation decreased from 0.5 to 0.1 mg/L from the first to the fifth passage in culture. Two cm root explants initiated more lateral roots per unit length than four cm explants. Liquid Anderson's medium was superior to other basal nutrient and vitamin formulations tested. Root growth in liquid culture was stimulated by aeration. Root organ cultures also grew on media solidified with gelrite or agar. A rapid transfer technique was developed for subculturing these which involved cutting and transferring 1 cm discs of roots and underlying medium. Adventitious bud formation occurred spontaneously, but sporadically in liquid cultures, and was not influenced by cytokinin, auxin (spat) light, or chilling.     

Radial Oxygen Losses from Intact Rice Roots as Affected by Distance from the Apex, Respiration and Waterlogging

Radial oxygen losses (ROL) from the roots of intact rice plants were assayed by the cylindrical Pt electrode technique. At 23°C losses from roots grown in waterlogged soil proved to be about double those from non-waterlogged plants. Cooling which lowers respiratory activity led to increased ROL and it was estimated that at 23°C respiratory activity had been reducing oxygen loss by 8 to 10 10^–8g O₂ cm^–2 root surface min^–1 (c. 50 %) in the non waterlogged, and by 4.5 to 5.5 10^–8g O₂ cm^–2 min^–1 (2C–30 %) in the waterlogged roots. Lacunae formation occurred nearer to the apex and was eventually more extensive in the waterlogged roots while the presence of more intact and presumably functional tissue in the non-waterlogged roots coincides with the greater respiratory effect noted. Estimated flux rates at 23°C (respiration inactive) were respectively 15–17 × 10^–8g O₂ cm^–2 min^–1 (non-waterlogged) and 20–23 × 10^–8g O₂ cm^–2 min^–1 (waterlogged). A major part of this difference can probably be accounted for directly by the differences in root porosity, and Meakiness' superimposed upon lower porosity in the non-waterlogged plants may account for the remainder. ROL was also examined in relation to distance from the apex. With respiratory activity lowered by cooling, two patterns of oxygen loss were detected. Pattern I was a property of younger roots of length between 5–9 cm, while pattern 2 was found in longer roots 11–16 cm bearing numerous emergent laterals. In both, ROL fell rapidly towards the base and at 4–5 cm approached zero in pattern 1 and near zero to about 16% of the maximum in pattern 2. The rapid drop in oxygen loss in both patterns which indicates a concomitant decrease in root wall permeability was associated with the appearance of cortical lacunae at 2–3 cm from the apex. In pattern 2 a rise in ROL began at 5–6 cm from the apex. The presence of lateral root initials in both the pericycle and unbroken segments of cortex was associated with maintained permeability in this pattern as well as with the basal increase in ROL. With a 3-electrode system placed around the apical 3 cm regions of waterlogged roots, it was found that ROL was substantially affected by respiratory activity at 0.5 cm, a little less so at 1.7 cm, but much less or not at all at 3 cm from the apex. The drop in respiratory effect parallelled the formation of cortical lacunae.     

Isolation and culture of mesophyll protoplasts from Rosa hybrida

After 1 h plasmolysis in CPW13M solution, highly viable (>75%) protoplasts were isolated from leaves of axenic shoot cultures of Rosa hybrida L. cv. Abraham Darby using an enzyme mixture containing 1.0% (w/v) Hemicellulase, 0.1% (w/v) Macerozyme, 1.0 (w/v) Cellulase RS, 0.05% (w/v) Pectolyase Y23 and 1.0% (w/v) PVP-10 and from cv. Marie Pavié using an identical mixture but with Cellulase RS and Pectolyase Y23 at 0.7% (w/v) and 0.1% (w/v), respectively. With both cvs., sustained protoplast division was achieved after plating in agarose beads with modified KM8p medium containing 1.0% (w/v) polyvinylpyrrolidone (mol. wt. 10 000; PVP-10), 8.91 μM naphthaleneacetic acid (NAA) and 4.44 μM 6-benzyladenine (BA). Protoplast-derived callus gave rise to roots after transfer to SH medium containing 14 μM 2,4-D. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Influence of Growing Temperature on the Growth and Morphology of Cereal Seedling Root Systems

The root systems of cereal seedlings (wheat, rye, barley, oats),grown in a series of liquid and solid media at 5, 15 and 25?C, were measured at intervals between sowing and emergence(shoot length 4.0 cm). In all cases, the length of seminal axesincreased linearly with time, and the rate of root extensionwas increased significantly by each 10 ?C increment in growingtemperature. In general, extension rates were lower in the solidmedia than in water culture; however, there was a strong interactionbetween medium and growing temperature, indicating that highertemperatures can compensate partly for the detrimental effectsof mechanical impedance. Examination of the influence of growing temperature upon rootaxis diameter was hampered by the observation that, for allfour species, apical root diameter was inversely related toroot length. It was concluded that comparison of the diametersof roots grown at different temperatures was valid only if theroots were of the same length (but different ages). Followingthis principle, it was found that, although cereal roots grownat 5 ?C tended to be slightly thicker than those at 15 ?C or25 ?C, this effect was small and rarely exceeded 10%. Publishedreports of very large differences in root diameter at differentgrowing temperatures were examined and found to be generallyunreliable. Lateral roots did not represent a significant fractionof the root system in any of the treatments studied. Root: shoot relationships at different growing temperatureswere examined by plotting root system length against shoot lengthfor each sample. It was found that, for the same shoot length,the root systems at 5 ?C were much shorter than at 25 ?C, theresponse at 15 ?C being rather more variable. This finding,which indicates that plants grown at low temperatures have alower capacity for water and solute uptake than those grownat laboratory temperatures, has important implications for thedesign of laboratory experiments. The influence of root zonetemperature upon root growth and physiology can be determinedunequivocably only if the roots have been exposed to the experimentaltemperature throughout their growth. Key words: Barley, Oats, Rye, Wheat, Root length, Root diameter, Root, shoot ratio     

高寒草甸植物群落不同根序根系特征对降雨量变化的响应

根系是草原生态系统中最重要的碳库之一,分析高寒草甸植物群落生物量和地下不同径级根系碳分配特征及根系的生长特征对降雨变化的响应,有利于了解全球变化背景下高寒草甸植物根系、土壤碳氮循环及其过程。采用微根管技术原位监测5种降雨处理下(增雨50%:1.5P、自然降雨:1.0P、减雨30%:0.7P、减雨50%:0.5P、减雨90%:0.1P)高寒草甸植物群落和根系属性(现存量、生产量、死亡量、根系寿命和周转速率)的变化特征,结果表明:(1)降雨变化对地上植物群落生物量无显著影响,但0.5P和0.1P显著增加禾本科生物量(P<0.05)。(2)总根系现存量在处理间无显著差异,但随着降雨量减少呈先增加后降低的趋势。土层间不同径级根系现存量差异显著,0-10 cm土层1.5P和0.7P1级根现存量显著增加,2级和3级根现存量显著降低;在10-20 cm土层,1.0P2级根系现存量显著高于其余处理(P<0.05)。(3)总根生产量与死亡量随降雨减少而降低,在0-10 cm土层,1.0P总根生产量和死亡量最高,0.1P显著降低了1级根生产量(P<0.05)。(4)0.1P显著增加10-20 cm土层1级根和总根寿命(P<0.05)。(5)根系周转随降雨量减少呈降低趋势,但无显著差异(P>0.05)。(6)结构方程模型进一步表明:根系现存量和生产量受土层和水分的直接影响,土层和养分对根系周转有负效应。综上所述,降雨量的变化并未显著改变地下总根系生物量,但少量降雨变化(0.7P、1.5P)会降低植物对2、3级根生物量的分配,投入更多资源以促进1级根的生长;而水分下降至轻度水分胁迫(0.1P),植物会减少地下各径级根系生物量的分配,保持低根系生物量消耗和低根系生长来维持其正常的生长状态,完成其正常的生态功能。     

Influence of the Sting Nematode, Belonolaimus longicaudalus,on Young Citrus Trees

The sting nematode, Belonolaimus longicaudatus, was associated with poor growth of citrus in a central Florida nursery. Foliage of trees was sparse and chlorotic. Affected rootstocks included Changsha and Cleopatra mandarin orange; Flying Dragon, Rubidoux, and Jacobsen trifoliate orange; Macrophylla and Milam lemon; Palestine sweet lime; sour orange; and the hybrids - Carrizo, Morton, and Rusk citrange and Swingle citrumelo. Root symptoms included apical swelling, development of swollen terminals containing 3-5 apical meristems and hyperplastic tissue, coarse roots, and a reduction in the number of fibrous roots. Population densities as high as 392 sting nematodes per liter soil were detected, with 80% of the population occurring in the top 30 cm of soil; however, nematodes were detected to 107 cm deep. Although an ectoparasite, the nematode was closely associated with citrus root systems and was transported with bare root nursery stock. Disinfestation was accomplished by hot water treatment (49 C for 5 minutes).     

Iron absorption by wheat roots as a function of distance from the root tip

Summary The apex of the root has been proposed as the site of maximum nutrient uptake, a view based on the accumulation of some ions in the cells of apical meristem⁴. In this study, intact roots were grown across a thin agar sheet containing a radioisotope of iron. Uptake was then measured quantitatively by autoradiography of the dried agar once the root had been removed. Uptake patterns were not observed to occur around the roots of plants that had been supplied with sufficient iron. Around the roots of plants showing severe irondeficiency symptoms, patterns consistently showed depletion of iron in the agar starting at 3.5 cm behind the tip and continuing to the point of complete suberization of the endodermis. No uptake was observed around the apical 3.5 cm of the root, although the highest concentrations within the root have centered on the apex. Agronomy Paper No.869, New York State College of Agriculture.     

Effects of soil structural discontinuity on root and shoot growth and water use of maize

The role of roots penetrating various undisturbed soil horizons beneath loose layer in water use and shoot growth of maize was evaluated in greenhouse experiment. 18 undisturbed soil columns 20 cm in diameter and 20 cm in height were taken from the depths 30–50 cm and 50–70 cm from a Brown Lowland soil, a Pseudogley and a Brown Andosol (3 columns from each depth and soil). Initial resistance to penetration in undisturbed soil horizons varied from 2.5 to 8.9 MPa while that in the loose layer was 0.01 MPa. The undisturbed horizons had a major effect on vertical arrangement of roots. Root length density in loose layer varied from 96 to 126 km m^-3 while in adjacent stronger top layers of undisturbed horizons from 1.6 to 20.0 km m^-3 with higher values in upper horizons of each soil. For specific root length, the corresponding ranges were 79.4–107.7 m g^-1 (on dry basis) and 38.2–63.7 m g^-1, respectively. Ratios of root dry weight per unit volume of soil between loose and adjacent undisturbed layers were much lower than those of root length density indicating that roots in undisturbed horizons were produced with considerably higher partition of assimilates. Root size in undisturbed horizons relative to total roots was from 1.1 to 38.1% while water use from the horizons was from 54.1 to 74.0%. Total water use and shoot growth were positively correlated with root length in undisturbed soil horizons. There was no correlation between shoot growth and water use from the loose layers.     

Pre-germination effects on mechanically impeded root growth of barley (Hordeum vulgare L)

The effects of conditions pre-dating germination on growth rate of impeded barley cv. Harrington roots were measured using an agar-capillary tube technique. Seedling root tips were directed into glass capillary tubes twothirds filled with agar at eight concentrations ranging from 1.6 to 9.6%, equivalent to penetrometer resistances of 25 to 1240 kPa. The rate of unrestricted root elongation (growth in air) of seed stored for 13 months (old seed), and of seed grown for a second generation without subsequent storage (new seed) was compared with growth in agar over a 24-hour interval. Root elongation rate of old and new seed was identical in the absence of resistance. At low to intermediate agar concentrations, elongation was significantly slower in roots from old, compared with new seed. At high agar concentrations root growth of old and new seed was the same. In both old and new seed, root growth through agar was greater in seed that germinated after 24, compared with 48 h. Differences in impeded root growth between old and new seed were lost in progeny of the test seed. Environmental factors that pre-date germination are an important influence on the ability of seedling roots to elongate through soil.LRS Contribution no. 3879349LRS Contribution no. 3879349     

Larger adenylate energy charge and ATP/ADP ratios in aerenchymatous roots of Zea mays in anaerobic media as a consequence of improved internal oxygen transport

Internal transport of O₂ from the aerial tissues along the adventitious roots of intact maize plants was estimated by measuring the concentrations of adenine nucleotides in various zones along the root under an oxygen-free atmosphere. Young maize plants were grown in nutrient solution under conditions that either stimulated or prevented the formation of a lysigenous aerenchyma, and the roots (up to 210 mm long) were then exposed to an anaerobic (oxygen-free) nutrient solution. Aerenchymatous roots showed higher values than non-aerenchymatous ones for ATP content, adenylate energy charge and ATP/ADP ratios. We conclude that the lysigenous cortical gas spaces help maintain a high respiration rate in the tissues along the root, and in the apical zone, by improving internal transport of oxygen over distances of at least 210 mm. This contrasted sharply with the low energy status (poor O₂ transport) in non-aerenchymatous roots.Abbreviation AEC adenylate energy charge     

The contribution of different regions of the seminal roots of wheat to uptake of nitrate from soil

A technique was developed to determine the physiological activity of defined sections of seminal roots of wheat grown in sand. Wheat plants were grown for 2 weeks in narrow columns of N-deficient sand to which all other nutrients had been added. The columns were split longitudinally and ¹⁵N-labelled nitrate, in an agar medium, supplied to 2 cm sections of root. Shoots and roots were analysed after 24 h to determine the uptake of ¹⁵N. Three sections were examined on either the secondary or tertiary seminal root: 1 cm from the seed (basal segment), 35 cm from the seed (middle segment) and 4 cm from the root apex (apical segment). Total uptake was greatest from the basal and middle segments, declining by 50% from the apical segment. However, uptake per unit root length, including exposed sections of lateral roots, was not significantly different along the root.     

Oxygen requirements and mass transfer in hairy-root culture

Oxygen mass transfer in clumps of Atropa belladonna hairy roots was investigated as a function of root density and external flow conditions. Convection was the dominant mechanism for mass transfer into root clumps 3.5 to 5.0 cm in diameter; Peclet numbers inside the clumps ranged from 1.4 x 10(3) to 7.1 x 10(4) for external superficial flow velocities between 0.4 and 1.4 cm s(-1). Local dissolved-oxygen levels and rates of oxygen uptake were measured in aflow chamber and in bubble column and stirred bioreactors. When air was used as oxygen source, intraclump dissolved-oxygen tensions ranged from90% to 100% air saturation at high external flow velocity andlow root density, to less than 20% air saturation in dense root clumps. Specific oxygen-uptake rate declined with increasing root density. When external boundary layers around individual roots were eliminated byforcing liquid through the clumps at superficial velocities between 0.2 and1.0 cm s(-1), internal dissolved-oxygen tension was maintained at 95% to 100% air saturation and rate of oxygen uptake at 1.6 x 10(-6) g g(-1) s(-1) dry weight. Liquid culture of single A. belladonna hairy roots was used to investigate the effect of dissolved-oxygen tensionon root growth and morphology. Total root length and number of root tips increased exponentially at oxygen tensions between 70% and 100%air saturation. Specific growth rate increased with oxygen tension up to 100% air saturation; this result demonstrates that hairy roots aeratedwithout oxygen supplementation are likely to be oxygenlimited. No growth occurred at 50% air saturation. Growth of hairy roots proceeded with an average length per tip of about 1 cm; this value was essentially independent of dissolved-oxygen tension between 70% and 100% air saturation. (c) 1994 John Wiley & Sons, Inc.     

Root growth, developmental changes in the apex, and hydraulic conductivity for Opuntia ficus-indica during drought

Developmental changes in the root apex and accompanying changes in lateral root growth and root hydraulic conductivity were examined for Opuntia ficus-indica (L.) Miller during rapid drying, as occurs for roots near the soil surface, and more gradual drying, as occurs in deeper soil layers. During 7 d of rapid drying (in containers with a 3-cm depth of vermiculite), the rate of root growth decreased sharply and most root apices died; such a determinate pattern of root growth was not due to meristem exhaustion but rather to meristem mortality after 3 d of drying. The length of the meristem, the duration of the cell division cycle, and the length of the elongation zone were unchanged during rapid drying. During 14 d of gradual drying (in containers with a 6-cm depth of vermiculite), root mortality was relatively low; the length of the elongation zone decreased by 70%, the number of meristematic cells decreased 30%, and the duration of the cell cycle increased by 36%. Root hydraulic conductivity ( L _P) decreased to one half during both drying treatments; L _P was restored by 2 d of rewetting owing to the emergence of lateral roots following rapid drying and to renewed apical elongation following gradual drying. Thus, in response to drought, the apical meristems of roots of O. ficus-indica near the surface die, whereas deeper in the substrate cell division and elongation in root apices continue. Water uptake in response to rainfall in the field can be enhanced by lateral root proliferation near the soil surface and additionally by resumption of apical growth for deeper roots.     

Exploring the Radial and Longitudinal Aeration of Primary Maize Roots by Means of Clark-Type Oxygen Microelectrodes

Clark-type oxygen microelectrodes were used to measure the radial and longitudinal oxygen distribution in aerenchymatous and nonaerenchymatous primary roots of intact maize seedlings. A radial intake of oxygen from the rooting medium was restricted by embedding the roots in 1% agar causing aeration to be largely dependent upon longitudinal internal transport from the shoot. In both root types, oxygen concentrations declined with distance from the base, and were lower in the stele than in the cortex. Also, the bulk of the oxygen demand was met internally by transport from the shoots, but a little oxygen was received by radial inward diffusion from the surrounding agar, and in some positions the hypodermal layers received oxygen from both the agar and the cortex. Near to the base, the oxygen partial pressure difference between the cortex and the center of the stele could be as much as 6–8 kPa. Nearer to the tip, the differences were smaller but equally significant. In the nonaerenchymatous roots, cortical oxygen partial pressures near the apex were becoming very low (< 1 kPa) as root lengths approached 100 mm, and towards the center of the stele values reached 0.1 kPa or lower. However, the data indicated that respiratory activity did not decline until the cortical oxygen pressure was less than 2 kPa. Mathematical modeling based on Michaelis–Menten kinetics supported this and suggested that the respiratory decline would be mostly restricted to the stele until cortical oxygen pressures approached very low values. At a cortical oxygen pressure of 0.75 kPa, it was shown that respiratory activity in the pericycle and phloem might remain as high as 80–100% of maximum even though in the center of the stele it could be less than 1% of maximum. Aerenchyma production resulted in increases in oxygen concentration throughout the roots with cortical partial pressures of ca. 5–6 kPa and stelar values of ca. 3–4 kPa near the tips of 100 mm long roots. In aerenchymatous roots, there was some evidence of a decline in the oxygen permeability of the epidermal–hypodermal cylinder close to the apex; a decline in stelar oxygen permeability near the base was indicated for both root types. There was some evidence that the mesocotyl and coleoptile represented a very significant resistance to oxygen transport to the root.     

Number,position, diameter and initial direction of growth of primary roots in Musa

To understand soil colonization by a root system, information is needed on the architecture of the root system. In monocotyledons, soil exploration is mainly due to the growth of adventitious primary roots. Primary root emergence in banana was quantified in relation to shoot and corm development. Root emergence kinetics were closely related to the development of aerial organs. Root position at emergence on the corm followed an asymptotic function of corm dry weight, so that the age of each root at a given time could be deduced from its position. Root diameter at emergence was related to the position of the roots on the corm, with younger roots being thicker than older ones. However, root diameters were not constant along a given root, but instead decreased with the distance to the base; roots appear to be conical in their basal and apical parts. Root growth directions at emergence were variable, but a high proportion of the primary roots emerged with a low angle to the horizontal. Further research is needed to evaluate whether these initial trajectories are conserved during root development. Results presented in this study are in good agreement with those reported for other monocotyledons such as maize and rice. They give quantitative information that will facilitate the development of models of root system architecture in banana.     

Optimization of culture conditions for in vitro rooting of argan (Argania spinosa L.)]

The root system produced of in vitro organ plantlets is of poor quality and not efficient for the transfer to out-door conditions. To overcome such problems, experimentation was undertaken where the effects of growth regulators, nitrogen, sugar, activated charcoal and coconut fiber were tested on root induction and elongation. Modified Murashige and Skoog with half strength salt was used as a basal medium. Root induction (85%) with a mean of 16 roots per explant was obtained when shoots were grown, under dark conditions for 14 days, with a combination of two auxins (IBA and NNA), added at equal concentrations (5 mg.L-1). Secondary roots, 10 cm long, were initiated in 12% of the cultures in presence of 5 g.L-1 activated charcoal. Further improvements in the growth of the primary and secondary roots were obtained when semi-solid medium was substituted with a substrate composed of coconut fibers (80 g) mixed with semi-solid medium (35 mL) and agar (2.5 g.L-1).     

Oxygen Distribution in Wetland Plant Roots and Permeability Barriers to Gas-exchange with the Rhizosphere: a Microelectrode and Modelling Study with Phragmites australis

Adventitious roots of intact Phragmites plantlets were securedhorizontally 2–3 mm below the surface of an oxygen-depletedfluid agar across which oxygen-free nitrogen was gently streamedto create a constant oxygen sink; the leafy shoot was fullyexposed to air. Radial oxygen profiles through rhizosphere androot at different distances from the apex were obtained polarographicallyusing Clark-type bevelled microelectrodes servo-driven in stepsof 10 µm (root) or 10–50 µm (rhizosphere).The pattern of radial oxygen loss (ROL) typical of wetland plants,viz. high at the apex and declining sharply sub-apically, wasrelated to synergism between ROL, and oxygen consumption andincreasing impedance to diffusion within the epidermal/hypodermalcylinder rather than to a surface resistance. The smallest oxygendeficit (2 kPa) to develop across the 80 µm thick epidermal/hypodermalcylinder was within the apical 10 mm and was consistent withtissue oxygen diffusivities similar to water. At 100 mm fromthe apex, consumption and impedance had increased the deficitto about 15 kPa and reduced ROL almost to zero. The developingimpedance within the epidermal/hypodermal cylinder was leastin cell layers immediately adjoining the cortex and increasedmost in the hypodermal cell layer abutting the epidermis. Thesub-apical decline in ROL appeared to coincide with the appearanceof aerenchyma in the cortex but thin walled ‘passage areas’(windows) in the hypodermal/epidermal cylinder persisted locallyand remained leaky to oxygen to some degree. It is through thesewindows that lateral roots emerge and the cortex in line withthe windows remains non-aerenchymatous. The radial and longitudinaloxygen profiles were consistent with modelling predictions.The shapes of the stelar oxygen profiles were consistent witha higher oxygen demand in the outer region (viz. pericycle,phloem, protoxylem and early metaxylem cylinder) than in theinner core (late metaxylem cylinder and medulla), but the deficitswere relatively small (     

Root Development and Aerenchyma Formation in Two Wheat Cultivars and One Triticale Cultivar Grown in Stagnant Agar and Aerated Nutrient Solution

Stagnant nutrient solution containing 0.1% agar and with anextremely low oxygen level (‘stagnant agar solution’)was used to simulate the gaseous composition and slow gas diffusionof waterlogged soils. Comparisons were made between the growthof two wheat cultivars(Triticum aestivum,cvs. Gamenya and Kite)and one triticale cultivar(Triticosecale,cv. Muir) grown instagnant relative to aerated solution. For all genotypes tested,immersion of roots in stagnant agar solution resulted in thedeath of the entire seminal root system and led to profuse branchingof the laterals of the nodal roots. In the stagnant agar solutionaerenchyma, as a percentage of the total cross sectional areaof nodal roots, was 18% for Muir, 14% for Kite and 12% for Gamenya;the roots of species with more aerenchyma also attained a longermaximum root length as predicted by the model of Armstrong (in:Woolhouse HW, ed.Advances in botanical research, vol. 7. London:Academic Press, 1979). Muir also had a nodal root/shoot freshweight ratio of 0.5 compared with 0.2–0.3 in Kite andGamenya. The greater number and length of nodal roots of Muirdid not lead to better shoot growth than in the other genotypes;one possible reason for this lack of improvement is a low efficiencyof aerenchymatous roots in wheat.Copyright 1998 Annals of BotanyCompany Root development; aerenchyma; stagnant agar;Triticum aestivumcv. Gamenya;Triticum aestivumcv. Kite;Triticosecalecv. Muir.     

Nitric oxide is involved in nitrate-induced inhibition of root elongation in Zea mays

BACKGROUND AND AIMS: Root growth and development are closely dependent upon nitrate supply in the growth medium. To unravel the mechanism underlying dependence of root growth on nitrate, an examination was made of whether endogenous nitric oxide (NO) is involved in nitrate-dependent growth of primary roots in maize. METHODS: Maize seedlings grown in varying concentrations of nitrate for 7 d were used to evaluate the effects on root elongation of a nitric oxide (NO) donor (sodium nitroprusside, SNP), a NO scavenger (methylene blue, MB), a nitric oxide synthase inhibitor (N(omega)-nitro-L-arginine, L-NNA), H(2)O(2), indole-3-acetic acid (IAA) and a nitric reducatse inhibitor (tungstate). The effects of these treatments on endogenous NO levels in maize root apical cells were investigated using a NO-specific fluorescent probe, 4, 5-diaminofluorescein diacetate (DAF-2DA) in association with a confocal microscopy. KEY RESULTS: Elongation of primary roots was negatively dependent on external concentrations of nitrate, and inhibition by high external nitrate was diminished when roots were treated with SNP and IAA. MB and L-NNA inhibited root elongation of plants grown in low-nitrate solution, but they had no effect on elongation of roots grown in high-nitrate solution. Tungstate inhibited root elongation grown in both low- and high-nitrate solutions. Endogenous NO levels in root apices grown in high-nitrate solution were lower than those grown in low-nitrate solution. IAA and SNP markedly enhanced endogenous NO levels in root apices grown in high nitrate, but they had no effect on endogenous NO levels in root apical cells grown in low-nitrate solution. Tungstate induced a greater increase in the endogenous NO levels in root apical cells grown in low-nitrate solution than those grown in high-nitrate solution. CONCLUSIONS: Inhibition of root elongation in maize by high external nitrate is likely to result from a reduction of nitric oxide synthase-dependent endogenous NO levels in maize root apical cells.     

Inhibition of maize root growth by high nitrate supply is correlated with reduced IAA levels in roots

The plant root system is highly sensitive to nutrient availability and distribution in the soil. For instance, root elongation is inhibited when grown in high nitrate concentrations. To decipher the mechanism underlying the nitrate-induced inhibition of root elongation, the involvement of the plant hormone auxin in nitrate-dependent root elongation of maize was investigated. Root growth, nitrogen and nitrate concentrations, and indole-3-acetic acid (IAA) concentrations in roots and in phloem exudates of maize grown under varying nitrate concentrations were analyzed. Total N and nitrate concentrations in shoots and roots increased and elongation of primary, seminal and crown roots were inhibited with increasing external nitrate from 0.05 to 5 mM. High nitrate-inhibited root growth resulted primarily from the reduced cell elongation and not from changes in meristem length. IAA concentrations in phloem exudates reduced with higher nitrate supply. Inhibition of root growth by high nitrate was closely related to the reduction of IAA levels in roots, especially in the sections close to root tips. Exogenous NAA and IAA restored primary root growth in high nitrate concentrations. It is concluded that the inhibitory effect of high nitrate concentrations on root growth may be partly attributed to the decrease in auxin concentrations of roots.