Role of root hairs in phosphorus depletion from a macrostructured soil

One rape (Brassica napus cv. Wesroona) plant and four cotton (Gossypium hirsutum cv. Sicot 3) plants were grown in plastic cells containing soil labelled with 407 kBq of³³P g⁻¹ soil. After 5–8 days of growth, the³³P depletion zones of all plants were autoradiographed and³³P uptake by plants was measured. The autoradiographs were scanned with a microdensitometer and the optical densities at several places within the³³P depletion zones of roots were obtained. The volume of soil explored by root hairs was estimated from measurements of root diameters and lengths of roots and root hairs. About half of the total³³P depleted by cotion roots came from outside the root hair cylinder whereas most of³³P taken up by rape was from within the root hair cylinder. Plants grown in a macrostructured soil may have roots growing in voids, within aggregates or on the surfaces of aggregates. The results of this study demonstrate that root hairs have a strong influence on the accessibility of phosphorus to roots in such a soil, and thus on the phosphorus nutrition of plants.     

Phosphorus nutrition of barley, buckwheat and rape seedlings. I. Influence of seed-borne P and external P levels on growth, P content and 32P/31P-fractionation in shoots and roots

Schjørring, J. K. and Jensén, P. 1984. Phosphorus nutrition of barley, buckwheat and rape seedlings. I. Influence of seed-borne P and external P levels on growth, P content and ³²P/³¹P-fractionation in shoots and roots. Seedlings of barly (Hordeum vulgare L. cvs Salka and Zita), buckwheat (Fagopyrum esculentum Moench) and rape (Brassica napus L. ssp. napus ev. Line) were grown at 8 or 10 different external P levels in the range 0-2000 μM. Apart from P, the nutrient solutions were complete. In some experiments with barley and rape, ³²P-labelled phosphate was used. Root fresh weights of buckwheat and rape decreased when the external P supply exceeded the level required for maximal root development. In all three species, the roots constituted a decreasing proportion of the total plant fresh weight as the external P level increased. The shoot/root fresh weight ratio increased linearly with the P concentration of the roots. The ratio between the P concentration in shoots and roots increased with the P status of the seedlings grown at low to intermediate external P levels, but decreased at higher P levels. The proportion of total seedling-P held in roots consequently reached a minimum value and thereafter increased as the P status of the seedlings increased. This indicates that some control mechanism counteracted the accumulation of harmful P levels in the shoots. ³²P-Phosphate uptake by seedlings of barley and rape grown in solutions with 2 μM P overestimated the actual net phosphorus uptake by a factor of 6 to 7, indicating a marked fractionation of ³²P and ³¹P. For seedlings grown in solutions with 25 μM P (barley) or 50 μM (rape) no fractionation occurred. The relative excess of ³²P in high P seedlings accumulated in the roots. It is suggested that the fracionation was caused by efflux of low specific activity phosphorus and by diffusion of free phosphate ions across the plasmalemma of the root cells in response to a difference in the concentration gradient between the two P isotopes.     

Transformations and movement of P in the rhizosphere

Summary Wheat plants labelled with³³P were grown in thin layers of soil amended with³²P-labelled fertiliser. Roots were separated from the soil during plant growth by a porous membrane to overcome difficulties in measuring microbial P in rhizosphere soil. Over the 22 day growth period, net movement of³³P out of healthy growing roots varied from 0.9–4.9% of the total³³P translocated to the root. Over the same period the plants took up 12.0% and the microbial biomass 14.1% of the fertiliser³²P. On drying and rewetting of the soil after the plants were harvested, a large proportion of root P moved into soil fractions while³²P appeared to accumulate in the biomass and stable P forms.     

Contribution by two arbuscular mycorrhizal fungi to P uptake by cucumber (Cucumis sativus L.) from32P-labelled organic matter during mineralization in soil

An experiment was set up to investigate the role of arbuscular mycorrhiza (AM) in utilization of P from organic matter during mineralization in soil. Cucumber (Cucumis sativus L.) inoculated with one of two AM fungi or left uninoculated were grown for 30 days in cross-shaped PVC pots. One of two horizontal compartments contained 100 g soil (quartz sand: clay loam, 1:1) with 0.5 g ground clover leaves labelled with³²P. The labelled soil received microbial inoculum without AM fungi to ensure mineralization of the added organic matter. The labelling compartment was separated from a central root compartment by either 37 m or 700 m nylon mesh giving only hyphae or both roots and hyphae, respectively, access to the labelled soil. The recovery of³²P from the hyphal compartment was 5.5 and 8.6% for plants colonized withGlomus sp. andG. caledonium, respectively, but only 0.6 % for the non-mycorrhizal controls. Interfungal differences were not related to root colonization or hyphal length densities, which were lowest forG. caledonium. Both fungi depleted the labelled soil of NaHCO₃-extractable P and³²P compared to controls. A 15–25% recovery of³²P by roots was not enhanced in the presence of mycorrhizas, probably due to high root densities in the labelled soil. The experiment confirms that AM fungi differ in P uptake characteristics, and that mycorrhizal hyphae can intercept some P immobilization by other microorganisms and P-sorbing clay minerals.     

Carbon distribution in grass (Festuca pratensis L.) during regrowth after cutting—utilization of stored and newly assimilated carbon

Distribution of net assimilated C in meadow fescue (Fectuca pratensi L.) was followed before and after cutting of the shoots. Plants were continuously labelled in a growth chamber with ¹⁴C-labelled CO₂ in the atmosphere from seedling to cutting and with ¹³C-labelled CO₂ in the atmosphere during regrowth after the cutting. Labelled C, both ¹⁴C and ¹³C, was determined at the end of the two growth periods in shoots, crowns, roots, soil and rhizosphere respiration. Distribution of net assimilated C followed almost the same pattern at the end of the two growth periods, i.e. at the end of the ¹⁴C- and the ¹³C-labelling periods. Shoots retained 71–73% of net assimilated C while 9% was detected in the roots and 11–14% was released from the roots, determined as labelled C in soil and as rhizosphere respiration. At the end of the 2nd growth period, after cutting and regrowth, 21% of the residual plant ¹⁴C at cutting (¹⁴C in crowns and roots) was found in the new shoot biomass. A minor part of the residual plant ¹⁴C, 12%, was lost from the plants. The decreases in ¹⁴C in crowns and roots during the regrowth period suggest that ¹⁴C in both crowns and roots was translocated to new shoot tissue. Approximately half of the total root C at the end of the regrowth period after cutting was ¹³C-labelled C and thus represents new root growth. Root death after cutting could not be determined in this experiment, since the decline in root ¹⁴C during the regrowth period may also be assigned to root respiration, root exudation and translocation to the shoots. ei]{gnH}{fnLambers} ei]{gnA C}{fnBorstlap}     

The effects of lime and phosphorus on the function of wheat roots in acid top soils and subsoils

Two wheat varieties with differing aluminium tolerance were grown in pots of acid soil. Liming did not change significantly the amounts of chemically extractable P and K, but caused improved vegetative growth, increased inflow of P and K and reduced uptake of Al. Without lime, roots had a higher content and concentration of P than shoots; liming reversed this. Without lime the sensitive variety with a shorter root length had an Al inflow ten times that of the tolerant one: tolerance involves a mechanism for exlcuding Al. The inflow of P per unit inflow of Al (mol ratio) without lime was three times greater for the tolerant variety which therefore has more P to counteract the effects of Al. The same varieties were grown in two-layer soil columns, with a low P status and a limed topsoil and acid subsoil. Liming the subsoil improved plant growth but this was still restricted by low P availability. Addition of P to the topsoil caused good growth regardless of subsoil acidity: root growth increased in both layers and P (labelled with³²P) taken up from the topsoil was translocated to roots in the subsoil. This P inactivated root Al and allowed the roots to grow and take up more P from the acid subsoil with however a reduction in inflow. The sensitive variety was affected more by the acid subsoil and low P availability, had a similar ability to translocate P to subsoil roots but could not attain the growth rate of the tolerant wheat even with P and lime.     

Phosphorus transport to the xylem and its regulation by water flow

Summary The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of ³²P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the ³²P taken up by the plants was confined to the roots. Only about half of this ³²P was later translocated to shoots; there was very little translocation after 4 hours.Experiments after long-term labelling showed that only a small part of the total P in the root is readily translocated to shoots. This P appears to be in part of the symplast and contributes about 75% of the P transported to the xylem sap. The rest is presumably derived by leakage from vacuoles.A slow rate of water flow reduced both uptake into the symplast and the translocation to the shoots of P which had already been absorbed by the roots. This was conclusively demonstrated by giving a ³²P pulse before reducing the rate of water flow; ³²P not translocated to shoots was partly retained by the roots and partly lost to the external solution. Water flow also accelerates transport to the xylem of previously-absorbed P in excised roots.It is concluded that the major effect of water flow on phosphorus transport to shoots occurs after phosphorus uptake by the roots, probably during radial transport to the xylem.     

Physiological changes in, and phosphate uptake by potato plants during development of, and recovery from phosphate deficiency

The development of phosphate deficiency (P-stress) was observed in rooted sprouts of Solanum tuberosum L. cv. Desiree growing in solutions without phosphate. Shoot growth was inhibited by P-stress within 3 to 5 days of terminating the phosphate supply, while significant effects on root growth were not recorded until 7 to 9 days. Thus, the shoot:root dry weight ratio decreased from 4.3 to 2.6 over a 10-day period. Growth in the absence of an exogenous phosphate supply progressively diluted the phosphorus in the plant. The proportional decrease in concentration was similar in roots and shoots over a 7-day period, even though the former were growing more quickly. The potential for phosphate uptake per unit weight of root increased rapidly during the first 3 days of P-stress. When the plants were provided subsequently with a labelled, 1 mol m^?3 phosphate solution, the absorption rate was 3 to 4-fold greater than that of control plants which had received a continuous phosphate supply. The increased rate of uptake by P-stressed plants was accounted for by an increase (3-fold) in the V_max of system 1 for phosphate transport and by a marked increase in the affinity of the system for phosphate (decrease in K_m). In the early stages of P-stress, before marked changes in growth were measured, the proportion of labelled phosphate translocated to the shoots increased slightly relative to the controls when a phosphate supply was restored. In the later stages of stress a greater proportion was retained in the root system of P-stressed plants than in that of controls. In plants with roots divided between solutions containing or lacking a phosphate supply, the increased absorption rate was determined by the general demand for phosphate in the plant and not by the P-status of the particular root where uptake was measured. By contrast, the poportion translocated was strongly dependent on the P-status of the root. The restoration of a phosphate supply to P-stressed plants was marked by a rapid increase in the P concentration in snoots and roots which returned to levels similar to unstressed controls within 24 h. The enhanced uptake rate persisted for at least 5 days, resulting in supra-normal concentrations of P in both shoots and roots, and in the formation of extensive necrotic areas between the veins of mature leaves. Autoradiographs showed accumulations of ³²P in these lesions and at the points where guttation droplets formed on leaves.     

32P uptake and transport to shoots in Pinuus serotina seedlings under aerobic and hypoxic growth conditions

We examined the effects o long-term hypoxic growth conditions on net uptake and transport of P to shoots of pond Pine (Pinus serotina Michx.), a moderately flood-tolerant southern pine. Seedlings were grown under aerobic orhypoxic solution conditions for 4–5 weeks in continuously flowing solution culture containing 100 μM P. Short – and long-term ³²P. experiments were then concluded with intact seedlings to determine rates of ³²P influx, efflux and net transport to the shoot. Shoot fresh weight/root fresh weight ratios were significantly higher under hypoxic gorwth conditions, reflecting the larger reduction in root growth than shoot growth, despite extensive aerechyma formation in roots. Estimates for the unidirectional influx of ³²P in aerobic and hypoxic seedlings were 1.43 and 3.20 μmol P (g_FW root)_?1 h_?1, respectively. However, ³²P accumulation between the two treatments became similar within 8 h, suggesting that efflux was also higer in seedlings from the hypoxic treatment. Indeed in a separate experiment, hypoxic growth conditions increased efflux by over 60%. Transport of ³²P to shoots was significantly reduced under hypoxic growth conditions, despite higher root P concentrations and lower shoot P concentrations. After 48 h, ³²P accumulation in roots was similar between the two treatments. Yet total accumulation of seedling ³²P decrcased by 31% under the hypoxic treatment, largely because of reduced transport of ³²p to the shoot. The lower accumulation of ³² by shoots of seedlings in the hypoxic treatment may be the result of a direct inhibition on the transport process in O²-defident tissues, but could also reflect a slower turnover or labeling of the ool available for transport. Indeed, the percentage of total ³²P in. roots present in the soluble P. (or transportable form of P) was about 33% lower in seedlings from the hypoxic treatment, probably reflecting increased assimilation into organic compounds as well as chelation with iron. Our results suggest that P transport to the shoots of acclimated seedlings may be more sensitive to hypoxic solution conditions than influx at the root Plasmalemma.     

Shoot P status regulates cluster-root growth and citrate exudation in Lupinus albus grown with a divided root system

The present study was carried out to investigate whether the P concentration in the roots or the shoots controls the growth and citrate exudation of cluster roots in white lupin (Lupinus albus L). Foliar P application indicated that low P concentration in the shoots enhanced cluster‐root growth and citrate‐exudation rate more so than low P concentration in the roots. In the split‐root study, the P concentration in the shoots increased with increased P supply (1, 25 or 75 mmol m^?3 P), to the ‘privileged’ root halves. Roots ‘deprived’ of P invariably had the same low P concentrations, whereas those in the ‘privileged’ roots increased with increasing P supply (1, 25 or 75 mmol m^?3 P). Nevertheless, the proportion of the total root mass allocated to cluster roots, and the citrate‐exudation rates from the root halves were always similar on both root halves, irrespective of P supply, and decreased with increasing shoot P concentrations. Peak citrate exudation rates from developing cluster roots were significantly faster from cluster roots on the ‘deprived’ root halves when the ‘privileged’ half was exposed to 1 mmol m^?3 P as compared with 25 or 75 mmol m^?3 P. The possibility that changes in the concentrations of P fractions in the root halves influenced cluster‐root growth and citrate exudation was discounted, because there were no significant differences in insoluble organic P, ester‐P and inorganic P among all ‘deprived’ root halves. The results indicate that cluster‐root proportions and citrate exudation rates were regulated systemically by the P status of the shoot, and that P concentrations in the roots had little influence on growth and citrate exudation of cluster roots in L. albus.     

Root growth,macro-nutrient uptake dynamics and soil fertility requirements of a high-yielding winter oilseed rape crop

Shoot growth, root growth and macro-nutrient uptake by a high-yielding (5t/ha grain) winter oilseed rape crop have been measured. Maximum rooting density in the top 20cm of soil was 9.4 cm cm⁻³ and roots reached a depth of at least 1.8 m. Maximum nutrient uptakes were 364 kg ha⁻¹ for N, 43 kg ha⁻¹ for P, 308 kg ha⁻¹ for K, 287 kg ha⁻¹ for Ca and 16 kg ha⁻¹ for Mg. A 30-day drought coincided with the flowering period and root and shoot growth, as well as nutrient uptake rates, were reduced. Nutrient concentrations in the soil solution necessary to sustain the nutrient fluxes into the root system by diffusive supply have been calculated. Peak values were in the range 10 μM for P to 87 μM for N, lower than the observed concentrations, and it was concluded that nutrient transport to roots was not a limitation to uptake by this rape crop.     

Studies of iron transport by arbuscular mycorrhizal hyphae from soil to peanut and sorghum plants

 The influence of an arbuscular mycorrhizal (AM) fungus on phosphorus (P) and iron (Fe) uptake of peanut (Arachis hypogea L.) and sorghum (Sorghum bicolor L.) plants was studied in a pot experiment under controlled environmental conditions. The plants were grown for 10 weeks in pots containing sterilised calcareous soil with two levels of Fe supply. The soil was inoculated with rhizosphere microorganisms only or with rhizosphere microorganisms together with an AM fungus (Glomus mosseae [Nicol. & Gerd.] Gerdemann & Trappe). An additional small soil compartment accessible to hyphae but not roots was added to each pot after 6 weeks of plant growth. Radiolabelled P and Fe were supplied to the hyphae compartment 2 weeks after addition of this compartment. After a further 2 weeks, plants were harvested and shoots were analysed for radiolabelled elements. In both plant species, P uptake from the labelled soil increased significantly more in shoots of mycorrhizal plants than non-mycorrhizal plants, thus confirming the well-known activity of the fungus in P uptake. Mycorrhizal inoculation had no significant influence on the concentration of labelled Fe in shoots of peanut plants. In contrast, ⁵⁹Fe increased in shoots of mycorrhizal sorghum plants. The uptake of Fe from labelled soil by sorghum was particularly high under conditions producing a low Fe nutritional status of the plants. These results are preliminary evidence that hyphae of an arbuscular mycorrhizal fungus can mobilise and/or take up Fe from soil and translocate it to the plant. Accepted: 6 March 1998     

Phosphorus nutrition of barley, buckwheat and rape seedlings. II. Influx and efflux of phosphorous by intact roots of different P status

Seedlings of barley (Hordeum vulgare L. cvs Salka and Zita), buckwheat (Fagopyrum esculentum Moench) and rape (Brassica napus L. ssp. napus cv. Line) were raised at 8 or 10 different extenral P concentrations in the range 0–2000 μM. Apart from P, the nutrient solutions were complete. Phosphate influx in roots of different P status was determined by use of a nutrient solution containing 0.1 mM³²P-labelled phosphate. A double labelling technique was used for simultaneous determination of influx (³³P) and efflux (³²P) of phosphorus by roots of barley and rape with three selected P levels. Flux determinations were also done in presence of a metabolic uncoupler (2,4-dinitrophenol) and a protein synthesis inhibitor (cycloheximide). Influx of phosphate was maximal at a certin optimal P level of the roots and decreased at both lower and higher P levels. Maximum phosphate influex [μmol (g root)-^?1 h^?1] were: rape 4,4, buckwheat 2.2, barley cv. Salka 1.6, barley cv. Zita 1.5. Both Hill plots and plots of the untransformed decreasing phosphate influx vs root P concentrations above the optimal were linear and had high correlation coefficients. The Hill coefficient varied between -3.1 and -4.2. The decrease of phosphate influx from the maximum to the lowest value at the highest P concentration of the root was 60–70%. Hence, phosphate influex appeared to be regulated through negative feedback by the internal level of phosphorous in the roots. The regulation mechanism seems bascially similar for the three species and may be of an allosteric type. P efflux from roots of low and optimal (with regard to P influx) P status was 15–20% of the simultaneous P influx. Contary to P influx, P efflux increased at high P status and almost eliminated (barley) or halved (rape) net P uptake. 2,4-Dinitrophenol reduced both P influx and P efflux by low P roots and gave linearly increasing P efflux with increasing root P status. This indicates that P efflux partly occurred by counter transport and ion exchange at the uptake sites, partly by passive P efflux along an electrochemical potential gradient. Phosphate influx was not affected by inhibition of barley root growth with cycloheximide, but P efflux increased considerably.     

Maintenance of 32P uptake and transport in Pinus serotina seedlings under hypoxic growth conditions

In the present study, we examined the effects of long- and short-term hypoxia on net uptake and transport of phosphorus to shoots of pond pine (Pinus serotina Michx.), a moderately flood-tolerant southern pine, and the influence aerenchyma formation might have in maintenance of P uptake and transport. Seedlings were grown under aerobic (250 μM O₂) or hypoxic (≤50 μM O₂) solution conditions for 5.3 weeks in continuously flowing solution culture containing 100 μM P. Intact seedlings were then labeled with ³²P for up to 24 h to determine how short- and long-term hypoxic solution conditions affected rates of unidirectional influx and the accumulation of ³²P in roots and shoots. Seedlings in the long-term hypoxic treatment were grown for 5.3 weeks in hypoxic solution and also labeled in hypoxic uptake solution. The short-term hypoxic treatments included a 24-h hypoxic pretreatment followed by time in labeled hypoxic uptake solution for seedlings grown under aerobic or hypoxic conditions; in the latter case, diffusion of atmospheric O₂ entry into stem and root collar lenticels was blocked, thus removing any influence that aerenchyma formation might have had on enhancing O₂ concentrations of root tissue. Although unidirectional influx rates of ³²P in roots of seedlings grown under long-term hypoxic conditions were 1.4 times those of aerobically grown seedlings, accumulation of ³²P in roots was similar after 24 h in labeled uptake solution. These results suggest that ³²P efflux was also higher under hypoxic conditions. Higher shoot/root fresh weight ratios and lower shoot P concentrations in seedlings grown under hypoxic solution conditions suggest that the “shoot P demand” per unit root should be high. Yet accumulation of ³²P in shoots was reduced by 50% after 24 h in hypoxic uptake solution. Both short-term hypoxic treatments decreased accumulation of ³²P in roots by more than 50%. Short-term hypoxia decreased shoot accumulation in seedlings grown under aerobic and hypoxic conditions by 84 and 50%. respectively. Short- and long-term hypoxic conditions increased the percentage of root ³²P in the nucleic acid and chelated-P pools, resulting in a significantly smaller percentage of ³²P in the soluble inorganic phosphate (p_i) pool, the pool available for transport to the shoot. However, a reduction in pool size or in labeling of the pool available for transport cannot fully account for the large reduction in accumulation of ³²P in shoots, particularly in the short-term hypoxic treatment of aerobically grown seedlings. Our results suggest that both influx and transport of ³²P to shoots of pond pine seedlings are O₂-dependent processes, and that the transport of ³²P to shoots may be more sensitive to hypoxic solution conditions than influx at the cortical and epidermal plasmalemma, with aerenchyma formation supporting a substantial amount of both ³²P uptake and transport.     

Impact of defoliation intensities on plant biomass,nutrient uptake and arbuscular mycorrhizal symbiosis in Lotus tenuis growing in a saline‐sodic soil

The impact of different defoliation intensities on the ability of Lotus tenuis plants to regrowth, mobilise nutrients and to associate with native AM fungi and Rhizobium in a saline‐sodic soil was investigated. After 70 days, plants were subjected to 0, 25, 50, 75 and 100% defoliation and shoot regrowth was assessed at the end of subsequent 35 days. Compared to non‐defoliated plants, low or moderate defoliation up to 75% did not affect shoot regrowth. However, 100% treatment affected shoot regrowth and the clipped plants were not able to compensate the growth attained by non‐defoliated plants. Root growth was more affected by defoliation than shoot growth. P and N concentrations in shoots and roots increased with increasing defoliation while Na⁺ concentration in shoots of non‐defoliated and moderately defoliated plants was similar. Non‐defoliated and moderately defoliated plants prevented increases of Na⁺ concentration in shoots through both reducing Na⁺ uptake and Na⁺ transport to shoots by accumulating Na⁺ in roots. At high defoliation, the salinity tolerance mechanism is altered and Na⁺ concentration in shoots was higher than in roots. Reduction in the photosynthetic capacity induced by defoliation neither changed the root length colonised by AM fungi nor arbuscular colonisation but decreased the vesicular colonisation. Spore density did not change, but hyphal density and Rhizobium nodules increased with defoliation. The strategy of the AM symbiont consists in investing most of the C resources to preferentially retain arbuscular colonisation as well as inoculum density in the soil.     

The fate of carbon and fertiliser nitrogen when dryland wheat is grown in monoliths of duplex soil

Triticum aestivum L. (cv. Gutha), a short-season wheat, was grown to maturity in large monoliths of duplex soil (sand over sandy-clay) in a daylight phytotron mimicking field conditions. Either ¹⁵N-labelled ammonium sulphate ((NH₄)₂SO₄) or urea was banded into the soil at a rate of 30 kg N ha^–1: even though roots were about 20% heavier when grown in the presence of (NH₄)₂SO₄ for 86 d (P<0.05), above-ground mass was not affected by the source of nitrogen. At four times through crop development up to grain-filling (50, 56, 70 and 86 d after sowing) shoots were labelled heavily with ¹⁴CO₂ with two purposes. First, to trace `instantaneous' assimilate movement over 24 h, revealing relative sink strengths throughout plants. This, in turn, allowed precise measurements of live root mass and the proportion of recent photoassimilates deposited in the rhizosphere. Although root systems were sparse, even in surface soil layers, they were strong sinks for photoassimilates early in development (0–50 d), supporting the conversion of inorganic applied nitrogen (N) to soil organic forms. In the presence of roots, up to 28% of ¹⁵N was immobilised, whereas only 12% of labelled ammonium sulphate was immobilised in unplanted plots in spite of a favourable moisture status in both treatments. The effect of plants on rates of ¹⁵N transformation is ascribed to recently imported photoassimilates sustaining rhizosphere metabolism. Not more than 15% of recently fixed carbon imported by roots was recovered from the rhizoplane, suggesting that a highly localised microbial biomass supported vigorous immobilisation of soil N. Thus, more than twice as much applied N was destined for soil organic fractions as for root material. By these processes, root- and soil-immobilised N become substantial stores of applied N and together with shoot N accounted for all the applied N under dryland conditions.     

Regenerative ability of somatic single and multiple embryos from cotyledons of Korean ginseng on hormone-free medium

Cotyledon explants of Korean ginseng (Panax ginseng C. A. Meyer) produced somatic embryos directly on growth regulator-free medium. Somatic embryos developed as either multiple or single-state forms, depending on the degree of maturity of the cotyledons. Cotyledon explants from midmature zygotic embryos formed multiple embryos, while cotyledons from fully mature zygotic embryos formed single embryos. Somatic single embryos regenerated into normal plantlets with both roots and shoots, while multiple embryos did not produce roots but regenerated only into multiple shoots. In full-strength MS basal medium, the root growth of plantlets derived from single embryos was weak compared to that of shoots. Deletion of ammonium nitrate from the MS medium promoted the root growth of the plantlets. The ginseng plants with well-developed shoots and roots regenerated from single embryos were successfully acclimatized in a greenhouse when they were planted in soil. Received: 19 July 1997 / Revision received: 6 October 1997 / Accepted: 3 October 1997     

Increased root exudation of14C-compounds by sorghum seedlings inoculated with nitrogen-fixing bacteria

Summary Organic components leaked fromSorghum bicolor seedlings (‘root exudates’) were examined by recovering¹⁴C labelled compounds from root solutions of seedlings inoculated withAzospirillum brasilense, Azotobacter vinelandii orKlebsiella pneumoniae nif-. Up to 3.5% of the total¹⁴C recovered from shoots, roots, and nutrient solutions was found in the root solutions. Inoculation with Azospirillum and Azotobacter increased the amounts of¹⁴C and decreased the amounts of carbohydrates in the root solutions. When sucrose was added as a carbon source for the bacteria, the increase of¹⁴C in the solutions did not occur. Quantities of¹⁴C found in the root solutions were proportional to amounts of mineral nitrogen supplied to the plants. Bacterial growth also was proportional to nitrogen levels. When sorghum plants were grown in soil and labelled with¹⁴CO₂, about 15% of the total¹⁴C recovered within 48 hours exposure was found in soil leachates.     

The influence of some soil and plant factors on the concentration of copper in perennial ryegrass

Summary The absorption and transport of Cu were studied in perennial ryegrass grwon on 21 soils under controlled environment conditions. Neither the concentration, nor the total amount, of Cu in the shoots was related to available Cu in the soils as assessed by extraction with 0.05M EDTA, 0.005M DTPA, or 1.95 per cent HNO₃. The concentration in the roots and, more especially, absorption per unit weight of root (i.e. μg Cu g dry wt⁻¹) were, however, highly correlated with available soil Cu. This suggests that, unless the extent of exploitation of the soil by roots is taken into account, measurements of available Cu will not be effective in predicting uptake by plants. On average, 63 per cent of the Cu absorbed by the roots was retained in the roots, and variation in the proportion retained was related to the transport of nitrogen from roots to shoots. On some soils the concentrations of N and Cu in the shoots were significantly correlated, and variation in N concentration accounted for a considerable proportion of the variance in the Cu concentration at later harvests. The relative importance of the measured soil (pH, organic matter) and plant (dry weight, N content) factors changed markedly over 6 successive harvests.     

Direct evidence on participation of root hairs in phosphorus (32P) uptake from soil

Root hairs substantially extend root surface for ion uptake. Although many reports suggest a relationship between root hairs and phosphorus (P) uptake of plants, the role of root hairs in phosphorus uptake from soils is still debated. We measured uptake of phosphorus from soil directly via root hairs. Root hairs only were allowed to penetrate through a tightly stretched nylon screen (53 µm) glued to the bottom of a PVC tube. The penetrating root hairs grew for 2 and 4 days in soil labelled with radioisotope phosphorus (P) tracer ³²P (185 kBq g^-1 dry soil) filled in another PVC tube. Transparent plastic rings of thickness ranging from 0.25 mm to 2.0 mm were inserted between the two PVC tubes. This provided slit width for microscopic observations in situ, which confirmed that only root hairs were growing into the ³²P labelled soil. In some cases no rings were inserted (slit width = 0) where both root hairs and root surface were in contact with the labelled soil (total ³²P uptake). The uptake of³² P from soil via the root hairs only was quantified by measuring activity of ³²P in the plant shoot (³²P uptake only via root hairs).The results showed that when 70 percent of the root hairs grew into the labelled soil, they contributed to 63 percent of the total P uptake. With decreasing number of root hairs growing into the ³²P labelled soil, the quantity of ³²P in the plant shoot decreased. In this study, P uptake via root hairs was measured in a soil-based system, where root hairs were the only pathway of ³²P from soil to the plant shoot. Therefore, this study provides a strong evidence on the substantial participation of root hairs in uptake of phosphorus from soil.