Split Root Research Articles

Partitioning of carbon in split root systems of barley: effect of temperature of the root

The time-course of partitioning of recently fixed photosynthate to a barley root was followed during a step change in the temperature of the root. The induced change in partitioning had a response time of about 1 h, whilst the change in respiratory loss from the root of labelled and unlabelled CO was much faster. These observations suggest that while a sudden temperature change has an immediate effect upon the metabolic rate of the roots it is not the metabolic rate as such which influences import (sink strength). The reported temperature responses are consistent with the supposition that the size of the cytosolic pool of sugar in the root, which is supplied directly by phloem import and used as a substrate for root metabolism, appears to control import into the root.

Carbon import into barley roots: stimulation by galactose

Galactose was applied to one-half of a split root system of barley. It stimulated import of photoassimilate labelled with C-11 from leaf 2 into that root half at concentrations from 0.2-50 mol m -3 . The lag before an increase in import was detected decreased from >60 to <30 min as concentration of galactose was increased across this range. In the presence of 10 mol m -3 glucose, 2 mol m -3 galactose did not increase import of C-11. Whilst 2 and 50 mol m -3 galactose had no effect on respiratory oxygen uptake of roots, they caused a rise in the rate of evolution of 11 CO 2 in <5 min

Responses to salinity of grapevine plants with split root systems.

Grape yield, shoot and root vigour and water use by grapevine plants with split root systems were investigated. Some plants had both root parts continuously irrigated either with fresh or with saline water. Some plants got a dual treatment; one portion got fresh water and the other saline water. The irrigation water of a third group was changed during the experiment from fresh to saline water or vice versa. Fruit yield and root and shoot viability were positively correlated with the actual water use. Water ascent along the stem of the grapevines was found to be sectorial. Most of the water was supplied by the freshwater roots. Only small quantities of water were supplied by salt-affected roots to their respective twigs. Changes in the water quality of the root medium induced a dual effect: (a) a fast response, caused by the direct change in the ambient w7 ater potential; and (b) a long-term response that developed over several weeks. The latter response was induced by the development of new roots, or by death of others, upon a change in the quality of the irrigation water. The commonly used grapevine plants of the Arava valley are negatively affected by NaCl already at concentrations below 100 mM. Under such conditions, shoot growth and fruit yield were seriously inhibited, even when one part only of the root system was exposed to saline water.

Auxin Dependent Growth and Auxin-Binding Proteins in Primary Roots and Root Hairs of Corn ( Zea mays L.)

Summary Cell elongation in shoots and roots as well is regulated by auxins. The auxin sensitivity of corn roots is very high. Root segments and split roots respond optimally at 10 −9 M IAA and 10 −10 M NAA, respectively. The split root tip test demonstrates that the rhizodermis and perhaps the outer cortex cells react actively by cell elongation. To search for auxin-binding proteins (ABP) in roots, related to those from corn coleoptiles (Lobler and Klambt,1985 a, b), similar methods were employed. On the basis of immuno-Western blots corn roots contain about the same amounts of ABP as corn coleoptiles. The molecular size of ABP is apparently a little smaller than those from coleoptiles. There are some indications that along the axis root hair region, root elongation region and root tip the molecular masses of ABPs become smaller. Root hairs were prepared and collected in liquid N 2 (Rohm and Werner,1987). Freezing and thawing caused severe leakage of the ER membrane system for ABP, which in these cases is only found within the cytosolic fraction. ABP freshly prepared from total root tips including the root hair regions shows two different dissociation constants for NAA: K DI = 4.1 × 10 −10 and K DII = 6.5 × 10 −9 . Both K D values reflect the higher auxin sensitivity of roots.

Leaf Expansion in Field‐Grown Sunflower in Response to Soil and Leaf Water Status

AbstractLeaf responses to soil water deficits in controlled environments may be mediated by nonhydraulic root signals (RS). The aim of this study was to evaluate in the field the effects of RS and leaf water potential (Ψ1) on leaf expansion rate (LER) of sunflower (Helianthus annuus L.). Two experiments were performed on a deep sandy‐loam soil (Typic Xerofluvent) in a Mediterranean environment under spring (Exp. 1) and summer conditions (Exp. 2). WET and DRY treatments were established at the 16th‐leaf stage in both experiments. WET plots were irrigated daily with an amount of water equal to the reference evapotranspiration of the previous day. DRY plots received no water during the treatment period which was maintained until LER declined to about 30% of the WET controls. A split‐root (SR) treatment with the root zone divided in dry and wet sections was included in Exp. 2. Soil water content (θa), LER and Ψ1 were measured. Responses of LER to soil drying were better described by Ψ1 than by plant available water (PAW); i.e., the response of LER to PAW depended on evaporative demand, whereas a single regression of Ψ1 on LER fitted all the data (P &lt; 0.001). Leaf expansion rate and Ψ1 of DRY plants began to decline with respect to that of WET plants at similar PAW thresholds. The soil water content in the dry soil zone of the SR plots was similar to that in the DRY plots during the first week of treatment whereas θa in the wet zone allowed SR plants to maintain a water status similar to that of WET controls. Split‐rooted plants did not behave as DRY controls, as expected in the case of a significant root signal effect, but maintained LER similar to the WET plants. It is concluded that, under the conditions of the present experiments, hydraulic effects were probably of more importance than nonhydraulic root signals in the regulation of sunflower leaf expansion.

Separation of Vesicular-Arbuscular Mycorrhizal Fungus and Root Effects on Soil Aggregation

Mycorrhizae influence soil stability, but relative contributions by plant or fungal endophyte to aggregation are little known. We studied the effects of both symbionts together and of each alone on waterstable soil aggregate (WSSA) formation. Split-root soybean [Glycine max (L.) Merr.] plants were grown in containers. One side of the split root was colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe; from here the VAM hyphase penetrated through a screen (44 − μ openings) to a root-free chamber. On the other side of a solid barrier, the non-VAM portion of the root was in contact through a screen with control soil free of roots and VAM hyphae. Thus, the four treatments, VAM roots (roots and hyphae), VAM hyphae, non-VAM roots, and control, were contained in the same experimental unit. Root length and mass were greater in the VAM-root than in the non-VAM-root chamber, whereas the density of VAM hyphae in the soil was lower in the VAM-root than in the VAM-hyphal chamber. The relative amount of WSSA was highest with VAM roots, lowest in the control chamber, and intermediate and similar with non-VAM roots and VAM hyphae. This incidence of WSSA was lower in all of the chambers at harvest (8 mo.) than in the starting material due to slaking. The composition of WSSA showed that the differences in WSSA between treatments was due to a variable slowing of the slaking process by the mycorrhizae. Non-VAM roots and VAM hyphae had similar effects on soil aggregation.

Nitrogen Utilization in N-limited Barley During Vegetative and Generative Growth

Barley (Hordeum vulgare L., cvs Golf and Laevigatum) was grown under nitrogen limitation in solution culture until near maturity. Three different nitrogen addition regimes were used: in the ‘HN’ culture, the relative rate of nitrate-N addition was 0·08 d−1 until day 48 and then stepwise decreased to, finally, 0·005 d−1 during late grain-filling; the ‘LN’ culture received 45% of the nitrogen added in HN; the ‘CN’ culture was maintained at RA 0·0375 d-1 throughout growth. At four different growth stages(vegetative, anthesis, and twice during grain-filling), 15N-nitrate was fed to the plants. In some cases (‘split root cultures’), label was fed only to one-half of the root system. These were harvested directly after labelling, whereas ‘standard cultured’ plants were harvested at termination of the experiment (day 148). Absorption of added nitrate was nearly complete in the HN and LN cultures, and translocation of nitrogen within the plants could thus be studied independently of differences in nitrate absorption. Cycling of nitrogen absorbed by vegetative plants accounted for up to 50% of the nitrogen recovered in the roots. The sink strength of the roots for cycling nitrogen, however, declined during post-anthesis growth, and net loss of nitrogen from both roots and vegetative shoot tissue occurred concomitantly with incorporation of labelled 15N-nitrogen. The nitrogen of the vegetative shoot tissue was substantially less labelled than the nitrogen entering the ears, indicating that translocation of recently absorbed nitrogen to ears occurs with minor prior exchange with the bulk nitrogen of shoots. In cases where the sink strength of the ears was weak, as in LN-cultured Laevigatum (due to high frequency of sterile flowers) and in CN-cultured Golf, nitrogen translocated from roots appeared to be incorporated into the vegetative shoot tissue. There were also indications that a fraction of the remobilized nitrogen was actually lost from the plants in these cases. It is concluded that the root remains efficient in translocation of nitrogen to the aerial parts throughout ontogeny and that nitrogen taken up during grain-filling is preferentially directly translocated to the developing grains. The further translocation of nitrogen received by vegetative shoot parts to ears appears mainly related to the potential of the ear to accumulate nitrogen. Nitrogen absorbed/remobilized in excess of the sink strength of the ears is either invested in continued shoot growth, or is irreversibly lost from the plants.

Evidence against the Regulation of Grain Set by Spikelet Abscisic Acid Levels in Water-Stressed Wheat

A possible role of abscisic acid (ABA) in the regulation of grain set in water-stressed wheat (Triticum aestivum L.) was investigated using a split root system to dry half the roots while the remainder were kept watered. Water uptake by the wet roots maintained the leaf water potential at the normal level, whereas the ABA produced in the dry roots was transported to the spike. This caused the spikelet ABA level to increase to the same extent as when the entire root system was stressed to permit a drop in the leaf water potential. In spite of this, the former treatment did not induce a reduction in grain set, whereas the latter did. Thus, contrary to previous reports, water stress-induced changes in spikelet ABA level alone do not appear to regulate grain set.

Transfer of water from roots into dry soil and the effect on wheat water relations and growth

This investigation was performed to study the effect on plant water relations and growth when some of roots grow into dry soil. Common spring water (Triticum aestivum) plants were grown from seed in soil in 1.2 m long PVC (polyvinyl chloride) tubes. Some of the tubes had a PVC partition along their center so that plants developed a split root system (SPR). Part of the roots grew in fully irrigated soil on one side of the partition while the rest of the roots grew into a very dry (-4.1 MPa) soil on the other side of the partition. Split root plants were compared with plants grown from emergence on stored soil moisture (STOR) and with plants that were fully irrigated as needed (IRR). The experiment was duplicated over two temperature regimes (10°/20°C and 15°/25°C, night/day temperatures) in growth chambers. Data were collected on root dry matter distribution, soil moisture status, midday leaf water potential (LWP), leaf relative water content (RWC) and parameters of plant growth and yield.

Feedback regulation of nodule formation in Hippopha� rhamnoides

Seedlings of Hippophae rhamnoides possessing two equally infectible root systems (‘split roots’) were used in conjunction with specific Frankia strains to investigate plant control over nodulation. When a wild-type Frankia strain was inoculated onto both root systems simultaneously or 1, 2, 4, or 8 weeks apart, an inhibitory response occurred which retarded nodulation on the root exposed to the delayed inoculum. Similar suppressive responses were also observed when two different wild-type Frankia inocula were applied onto opposite sides of a split-root system at different times. The depressed response shown by the delayed inoculum was more pronounced as the delay period was increased. The roots exposed to the delayed inoculum displayed a complete lack of nodulation when the delay was 4 or 8 weeks. The nodulation response on the root inoculated first depended on subsequent inoculation of the second root system of the plant, so that maximum nodulation of the first root was observed when the second root was unnodulated. These results provide evidence that sea buckthorn has an active, systemic mechanism for feedback control of nodulation that suppresses further nodule formation and prevents excessive nodulation. The significance of these results to the understanding of nodule ontogeny is discussed.

Effect of Phloem-Translocated Malate on NO3− Uptake by Roots of Intact Soybean Plants

In soybean (Glycine max L. Merr. cv Kingsoy), NO(3) (-) assimilation in leaves resulted in production and transport of malate to roots (B Touraine, N Grignon, C Grignon [1988] Plant Physiol 88: 605-612). This paper examines the significance of this phenomenon for the control of NO(3) (-) uptake by roots. The net NO(3) (-) uptake rate by roots of soybean plants was stimulated by the addition of K-malate to the external solution. It was decreased when phloem translocation was interrupted by hypocotyl girdling, and partially restored by malate addition to the medium, whereas glucose was ineffective. Introduction of K-malate into the transpiration stream using a split root system resulted in an enrichment of the phloem sap translocated back to the roots. This treatment resulted in an increase in both NO(3) (-) uptake and C excretion rates by roots. These results suggest that NO(3) (-) uptake by roots is dependent on the availability of shoot-borne, phloem-translocated malate. Shoot-to-root transport of malate stimulated NO(3) (-) uptake, and excretion of HCO(3) (-) ions was probably released by malate decarboxylation. NO(3) (-) uptake rate increased when the supply of NO(3) (-) to the shoot was increased, and decreased when the activity of nitrate reductase in the shoot was inhibited by WO(4) (2-). We conclude that in situ, NO(3) (-) reduction rate in the shoot may control NO(3) (-) uptake rate in the roots via the translocation rate of malate in the phloem.

OBSERVATION OF A SELECTIVE HYDRATION AND CYCLIC WATER MOVEMENT IN LYCOPERSICON LYCOPERSICUM ROOTS USING MAGNETIC RESONANCE IMAGING

A top-and-bottom split root system of Lycopersicon lycopersicum `Burpee's Pixie' was developed using a nonferromagnetic phenolic foam growing medium. The objective of the study was to observe hydrodynamic activity in the roots and substrate when one side of the split root system was dehydrated. After withholding water for 22 days from the top block, the plant and substrate were scanned for 46.5 hours every 30 min using a Siemens 1.5 tesla magnetron whole body imaging system operating at 63 MHz. Resulting images were compiled into a time lapse movie and clearly showed selective root hydration and dehydration on the dry side of the split root system. Those changes in the root MRI signal intensity suggest a cyclic hydration of the roots and a partitioning of water among roots in dry environment.

Translocation of nitrogen in osmotically stressed wheat seedlings

ABSTRACTWheat (Triticum aestivum L., cv. Drabant) seedlings were grown in a ‘split root’ system where either the whole root system or one root half was subjected to osmotic stress for 24 h, using 200 g polyethylene glycol (PEG, molecular weight 4000) dm−3 nutrient solution. 15N‐Labelled nitrate was fed to one of the root compartments and total N and 15N‐labelling were measured in plant material and xylem sap. Untreated plants translocated 87% of the N taken up to the shoot, and 10% of this was then retranslocated back to the root. Recalculated on a root nitrogen basis, 36% of the label recovered in the root after 24 h had passed through the shoot. Significant labelling of xylem sap collected from non‐labelled roots indicated cycling of organic N through the roots. PEG‐treatment of the whole root system caused significant water loss in both roots and shoots. Uptake of nitrate and retranslocation of N to roots were inhibited, whereas cycling of organic nitrogen through the root was still measurable. Treatment of half the root system with PEG had minor effects on shoot water content, but reduced the water content of the treated root part. The total uptake of nitrate by the root system was unaffected, and the effect on the treated root half was comparatively small. Nitrate reductase activity (NRA) declined in PEG‐treated roots even if high nitrate uptake rates were maintained. Shoot NRA was unaffected by osmotic stress. The data indicate that the reduction in water content of the root per se has only small effects on nitrate uptake. Major inhibition of nitrate uptake was observed only after treatment of a sufficiently large portion of the root system to given an effect on shoot water content.

Nitrate assimilatory properties of barley grown under long‐term N limitation: Effects of local nitrate supply in split‐root cultures

The effect of nitrate availability on characteristics of the nitrate assimilatory system was investigated in N‐limited barley (Hordeum valgare L. cv. Golf), grown with the seminal root system split into initially equal‐sized halves. The cultures were continuously supplied with nitrate‐N at a relative addition rate (RA) of 0.09 day−1, which resulted in relative growth rates (RG) that were ca 85% of those observed under surplus nitrate nutrition. The total N addition was divided between the subroots in ratios of 100:0, 80:20, 70:30, 60:40, and 50:50. For comparison, standard cultures were grown at RAs ranging from 0.03 to 0.18 day−1. Initially, biomass and N partitioning to the subroots responded strongly and proportionally to the nitrate distribution ratio. After 12‐14 days no further effect was observed. The Vmax for net nitrate uptake and in vitro nitrate reductase (NR) activity were measured in acclimated plants, i.e., after &gt; 14 days under a certain nitrate regime. In subroots fed from 20 to 100% of the total N addition, Vmax for net nitrate uptake increased slightly, whereas NR activity was unaffected. Uptake and NR activities were insignificant in the 0%‐subroot. Uneven nitrate supply to individual subroots had negligible effect on the whole‐plant ability for nitrate uptake, and the relative Vmax (unit N taken up per unit N in whole plant tissue and time) remained about 7‐fold in excess of the demand set by growth. Balancing nitrate concentrations (the resulting external nitrate concentrations at a certain RA) generally ranged between 2 and 10 μM at growth‐limiting RA, both when predicted from uptake kinetics and when actually measured. When comparing split root and standard cultures when acclimated, it appears that uptake and NR activities in roots respond more strongly to over‐all nitrate availability than to nitrate availability to individual subroots.

In VitroSomatic Embryogenesis and Plant Regeneration inClitoria ternatea

A sustainable plant regeneration system in vitro through somatic embryos from mature sexual embryos has been reported in Clitoria tematea. Somatic embryos developed through callus from seedling roots on hormone-free MS medium (MS,). Addition of growth hormones, KN 0-5 mg dm-3 (MS2) or K.N + 1AA 0-5 mg dm^3 of each (MS3) induced direct somatic embryos, in high frequency, on split root and hypocotyl systems. The embryogenie potential varied with the organ, roots or hypocotyls, and also with the medium. The morphogenetic capacity of the somatic embryos is retained for more than 2 years by subculturing at intervals of 4 weeks on MS3 in complete darkness. Somatic embryos, under the appropriate subculture conditions (16 h light/8 h dark photoperiod at 24+ 1 °C on media MS3, MS4 and MS5), resulted in recurrent-somatic embryogenesis and was profuse at the shoot and root apices of the somatic embryos. Mature somatic embryos were transplanted to MS, to stimulate germination and plantlet regeneration. Plantlets, developed from primary and secondary embryos on MS,, were successfully hardened and grown in natural outdoor conditions. The morphology and histology of the somatic embryo and plantlet and the culture conditions for continuous production of plantlets through direct somatic embryogeny are discussed.

Nitrate-Regulated Growth and Cytokinin Responses in Seminal Roots of Barley

The influence of nitrate availability on growth of seminal roots, and root cytokinin levels, was studied in barley (Hordeum vulgare L. cv Golf). Nitrate was continuously supplied to initially N-starved seedlings at relative addition rates (RA) of 0.03 to 0.21 per day (standard cultures) or at RA 0.09 per day in split root cultures with the nitrate additions distributed in ratios of 100:0 or 80:20 to the two subroots. Data were collected both during a phase of acclimation (first 10 days of N additions) and in the acclimated stage (>10 days after onset of N additions). Limitation of whole-plant growth was observed at RA <0.15 per day. The lateral root frequency increased with RA in plants of equal chronological age. However, the lateral root frequency was related to root size rather than to RA; roots of uneven age but having comparable total root lengths also had comparable lateral root frequencies. Growth of individual subroots in split root systems during acclimation was proportional to the fraction of the total N addition that was fed to the root. All subroots had comparable relative growth rates in acclimated plants, and their lateral root frequency correlated with total root length in the same manner as in standard cultures. Onset of N additions in a 80:20 split root culture resulted in doublings of zeatin riboside (ZR) levels in shoots and in the "80" root, whereas the response of the "20" root was small. No effect of perturbed nitrate availability on xylem translocation of ZR was observed. The ZR levels remained higher in the "80" root during acclimation but returned to the level of the "20" root after acclimation. Root cytokinin levels and xylem translocation in acclimated standard cultures were unaffected by RA in the lower range but increased at high RA. Arguments for involvement of cytokinins in the nitrate-regulated growth response are discussed.

Carbon Partitioning in Split Root Systems of Barley: Relation to Metabolism

We tested four hypotheses for the control of partitioning of photoassimilated C-ll between the two halves of split root systems of young barley plants. Our data supported the hypothesis that phloem is unloaded without the use of metabolic energy, since several metabolic inhibitors applied to one half of a split root system reduced respiratory oxygen uptake without altering import of C-ll. The hypothesis that rate of import C-ll is directly related to metabolic activity in the root was rejected, since (a) certain inhibitors reduced respiration but not import and (b) exogenous sucrose reduced import into the root half to which it was supplied. Our data were consistent with the hypothesis that import is related to the total ability (metabolism plus storage) of the sink to use sucrose. Treatments that would have led to greatly decreased use of sucrose (iodoacetate inhibition) decreased import before those which would have led to a smaller decrease in sucrose use (FCCP inhibition). These data, and the reduction in import to a root half supplied exogenously with sucrose, supported the hypothesis that the size of soluble sugar pools within the roots is, in the short-term, inversely proportional to rate of import, the soluble sugar pools thus acting as a mediator between rate of sucrose use and supply from the phloem.

The Distribution of Mg, P and K in the Split Roots of Subterranean Clover

The aim of these experiments was to determine whether uninhibited root growth is possible in a soil lacking Mg, provided that the plant is adequately supplied with Mg from another region within the soil. Two experiments were undertaken using split root systems in free-draining sand or gravel irrigated daily with nutrient solution. Magnesium supply to one half of the root system was varied and root growth in the deficient part of the system was measured. Magnesium redistribution was compared to that of calcium, phosphorus and potassium.

Analysis of Nitrogen Accumulation in Tulip Roots during Winter Season by Split Root Experiment

Analysis of Nitrogen Accumulation in Tulip Roots during Winter Season by Split Root Experiment

Zinc translocation to wheat roots and its implications for a phosphorus/zinc interaction in wheat plants

Abstract Seven‐day‐old wheat plants (Triticum aestivum L. cv. Gamenya) were grown for a further 11 and 29 days using a split root technique such that 2.5 μM Zn was added to both (Zn++), one (Zn+0), or neither (Zn00) of paired half‐root systems in an otherwise complete nutrient solution. When compared with Zn++ plants, the Zn+0 treatment had no effect on dry matter (DM) of wheat plants or plant parts. Indeed, in all respects, Zn+0 plants appeared identical to Zn++ plants. By contrast, at final harvest, Zn00 plants had symptoms of severe Zn deficiency and P toxicity, and depressed whole plant DM. The Zn+0 treatment was used to calculate translocation rates of Zn from one half‐root system to the other while all three treatments were used to study the effect of Zn supply on P accumulation. Zinc translocation from day 18 (D18) to D36 from the half‐root system supplied with Zn (Ra) to the half‐root system not supplied with Zn (Rb) was 1.0 to 1.5 mg Zn/kg DM RB/day. This rate of translocation could maintain a co...