Analysis of Phosphate Acquisition Efficiency in Different Arabidopsis Accessions

HA-PAE of Five Divergent Arabidopsis Accessions

A set of 36 different Arabidopsis accessions was subjected to a pre-screen for differences in phosphorus accumulation on HA medium. The five most divergent accessions selected for this study wereC24, Co, and Cal with the highest andCol-0 and Te with the lowest abilities of HA-PAE. These accessions were studied in detail for PAE-related characteristics.

The five selected Arabidopsis accessions were evaluated in detail for their PAEs. To this end, shoot dry masses (TableI) and shoot phosphate contents (TableII) were measured for plants grown in modified MS medium (0.15% or 0.4% [w/v] agarose) containing HA (0.5 g L⁻¹) or soluble phosphate (1 mm KH₂PO₄), respectively. The shoot dry mass values for accessions C24,Co, and Cal grown in HA medium solidified with 0.4% (w/v) agarose were higher than those of Col-0 andTe. When calculated as a percentage of those accumulated on KH₂PO₄ medium, the shoot dry masses of C24, Co, and Calaccessions ranged between 30% and 31%, whereas those ofCol-0 and Te were significantly lower and ranged between 21% and 25%. In accordance with this, the latter two accessions were classified as inefficient.

This classification was confirmed by the analysis of the shoot phosphate contents accumulated when the plants were grown under phosphate-limiting conditions (in hydroxylapatite medium solidified with 0.4% [w/v] agarose). C24, Co, andCal accumulated significantly more phosphate thanCol-0 and Te (Table II). Although C24and Co acquired more phosphate when supplied with HA, they showed relatively low phosphate contents after growth in soluble phosphate medium (phosphate-sufficient conditions). In comparison to the other accessions, Cal attained relatively high phosphate contents and Te relatively low phosphate contents regardless of the phosphate sources provided. C24 and Coshowed the highest efficiencies in phosphorus acquisition from HA (expressed as a percentage of phosphorus accumulated from HA/phosphorus accumulated from KH₂PO₄), which were approximately 2.5 times greater than those of the less efficient accessions Col-0 and Te (Table II). The PAE of the accession Cal was intermediate between the two groups.

To study the effect of growth medium agar density on phosphate efficiency, the five Arabidopsis accessions were grown on low-density (0.15% [w/v] agarose) modified MS medium containing HA (0.5 g L⁻¹) or soluble phosphate (1 mmKH₂PO₄). The shoot dry masses of efficient accessions C24 and Co grown in HA calculated as a percentage of those in KH₂PO₄ medium under these conditions were, however, lower than those of Cal, Col-0, and Te (Table I).

All accessions accumulated more phosphate when grown in low-density (0.15%) agarose HA medium as compared with their phosphate contents on high-density agarose (0.4%) HA medium. The relative differences in phosphorus accumulation from the two media were more dramatic in the inefficient accessions, Col-0 and Te, than in the efficient accessions, C24 and Co. While the latter accumulated 37% and 54% more phosphate from the 0.15% (w/v) agarose medium, the increases in Col-0 andTe were 213% and 211%, respectively. The relative differences in phosphate concentration in the shoot tissue (nmol mg⁻¹) of the plants grown on the two media were, however, unchanged (Table II). When grown in the low-density medium, the less efficient accession Col-0acquired 203 nmol phosphate shoot ⁻¹ from sparingly soluble sources, which is approximately 50% more than that of efficient accessions C24 and Co. Furthermore, the differences in PAE between the accessions were less pronounced under these conditions and the ranking of the accessions was different.

Characterization of Morphological and Physiological Attributes Related to PAE among the Five Divergent Arabidopsis Accessions

Previous studies on plant responses to phosphate deficiency have shown that plant characteristics likely to be involved in the determination of the PAE include seed size (Sadeghian, 1991;Liao and Yan, 1999), root system size (Lynch and van Beem, 1993;Gahoonia et al., 1997), phosphate uptake rates (Schachtman et al., 1998), proton release (Moorby et al., 1988), and organic acid excretion (Van den Boogaard et al., 1992). In this study experiments were carried out to evaluate these attributes and to determine their relative contributions to the PAEs of the five divergent Arabidopsis accessions.

Seed Size

The comparison of seed masses (mg⁻¹ 1,000 seeds) of the five accessions shows that the efficient accessionCal has slightly higher seed mass. The other four accessions did not differ significantly in their seed masses (TableIII). Therefore, variation in seed size could not account for the difference in dry masses observed for the accessions grown in hydroxylapatite medium.

Shoot-to-Root Ratios

It has been generally observed that relative root growth appears to decrease with increasing phosphorus levels, whereas an adequate supply of inorganic phosphate significantly increases the formation of shoot dry matter. To determine whether the differences in PAE among the five Arabidopsis accessions are due to unequal growth partitioning, the shoot-to-root dry mass ratios of the accessions were compared. The ranking of shoot-to-root dry mass ratios of the five Arabidopsis accessions was found to be: Cal > C24≈ Co > Col-0 ≈ Te. The accession Cal had the highest shoot-to-root ratios in all measurements. When grown in HA medium all accessions showed a decreased shoot-to-root ratio in comparison to the situation in the soluble phosphate medium. The decrease in the efficient accessions (C24, Co, and Cal), however, was lower in comparison to that of the inefficient accessions (Col-0and Te). In general, the efficient accessions appeared to have higher shoot-to-root ratios than the inefficient accessions (TableIII).

Root Morphology

A number of root traits, including the size and structure of the root system, appear to affect PAE (Bates and Lynch, 1996). Therefore, it was desirable to determine the morphological differences in the root system among selected accessions to provide basic architectural information and to detect its potential correlations with the observed differences in PAE. The accessions differed significantly in root length, root hair length, and root hair density (Fig.1; TablesIV andV).

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Fig. 1.

Root hairs in the primary roots of five Arabidopsis accessions. Seedlings were germinated and grown for 16 d in phosphate-free medium solidified with 0.15% (w/v) agarose. The regions of the roots where root hairs ceased to increase and had a uniform maximum size were observed under a binocular microscope and photographed. Bar represents 0.5 mm.

When grown in 0.4% (w/v) agarose medium, the ranking of accessions according to their root length was Cal >Col-0 > C24 > Co >Te. The efficient accessions C24 and Co had relatively short roots, but their root hairs were the longest and had the highest density (Tables IV and V). Root hair length and density was intermediate in Cal and low for the less efficient accessions, Col-0 and Te. The high phosphate efficiency observed in C24 and Co thus coincided with the presence of long, dense root hairs.

When grown in 0.15% (w/v) agarose medium, plants of all accessions developed longer roots, but the relative differences in root length were accession specific (Table IV). Thus Te displayed the largest difference with 2.3-fold longer roots established in the 0.15% (w/v) agarose medium as compared with the roots grown in 0.4% (w/v) agarose medium. In the 0.15% (w/v) agarose mediumCol-0 had the longest roots, approximately 2 times longer than those of C24 and Co. In accordance with this, the ranking of accessions with respect to their root lengths wasCol-0 ≈ Cal > Te >C24 ≈ Co. When grown in 0.15% (w/v) agarose medium only Cal plants developed considerably longer root hairs as compared with the 0.4% (w/v) agarose medium (Table V). Only very small differences were observed in the root hair densities of plants grown in high and low strength medium within the accessions (Table V). The increased root lengths (and root hair length inCal) in 0.15% (w/v) agarose medium coincided with strong increases in the PAEs of the accessions Cal,Col-0, and Te (Table II).

The observed differences in root lengths established upon growth in the 0.15% and 0.4% (w/v) agarose media were used to derive a measure for the substrate penetration ability of the roots of the different accessions. Determined as the ratios of the root lengths in 0.4% (w/v) agarose medium to the root lengths in 0.15% (w/v) agarose medium, substrate penetration abilities were higher for roots of the phosphate efficient accessions C24, Co, and Calthan for the inefficient accessions Col-0 and Te(Table IV).

Phosphate Uptake Kinetics

Accessions may vary in their effectiveness in acquiring phosphate through different levels of expression of transport proteins in roots (I_max ), through differences in the affinity (K_m ) for phosphate, or through variation in C_min (the minimum phosphate concentration in the growth medium at which no net uptake occurs into the roots) for phosphate. Phosphate uptake of the five Arabidopsis accessions was measured using seedlings grown in liquid culture for 7 to 8 d. The results of short-term uptake experiments performed with the plants grown in phosphate-sufficient (0.5 mmKH₂PO₄) or phosphate-deficient (2.5 μmKH₂PO₄) medium are shown in Table VI. Phosphate uptake over a concentration range of 0 to 300 μm can be described by Michaelis-Menten kinetics (Barber, 1995). Phosphate uptake was measured for concentrations ranging from 2.5 to 100 μmKH₂PO₄ (Fig.2). Michaelis-Menten parameters describing the kinetics of phosphorus uptake were estimated separately for each accession by linear regression analysis.

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Fig. 2.

Lineweaver-Burke plots of phosphate uptake into roots of five Arabidopsis accessions. For each measurement, 300 seedlings (on three nylon meshes) were grown in a modified 0.5× Hoagland medium containing 2.5 μmKH₂PO₄. Phosphate concentrations in the uptake solutions ranged from 2.5 to 100 μm. Plotted values are means of three replicate measurements. Y_C24 = 42.344X_C24 + 6.1877, R² _C24 = 0.9481; Y_Co = 42.162X_Co + 7.0285, R² _Co = 0.9723; Y_Cal = 78.462X_Cal + 11.086, R² _Cal = 0.993; Y_Col-0 = 61.345X_Col-0 + 9.6375, R² _Col-0 = 0.9262; Y_Te = 69.503X_Te + 9.3792, R² _Te = 0.9829.

The apparent K_m of the phosphate transporters of all five accessions were determined to be approximately 6.0 to 7.0 (±2.0) μm, which is characteristic for high affinity transporters. Although there were no significant differences among accessions for the estimated apparentK_m value for phosphate-starved plants, the estimated I_max (pmol h⁻¹ cm⁻¹ root) values forC24 and Co were greater by a factor of about 1.5 than those of Cal, Co, and Col-0. Thus phosphate-starved C24 and Co plants show the highest phosphorus uptake rates per unit root length. On a plant biomass basis, however, the uptake rates of Cal,Col-0, and Te are higher (TableVI). This difference is due to the variation in root lengths, which, under the condition used, were greater for Cal, Col-0, and Te than for C24 and Co.

Another physiological parameter C_min that can affect nutrient absorption was determined for the five accessions, but no obvious differences were observed. All accessions were able to deplete the 0.5-μmKH₂PO₄ solutions to between 7 and 11 nm (Table VI).

Organic Acid Exudations

Organic acids released from the roots of phosphorus-efficient plants increase the availability of phosphorus by mobilizing sparingly soluble forms of phosphate such as calcium-phosphate (Dinkelaker et al., 1989). To determine if a similar mechanism operates in Arabidopsis, the accessions were examined for organic acid root exudation triggered by phosphate deficiency. Citric, malic, and succinic acids were measured in the root exudates of 8-d-old plants. The amount of organic acids released from roots of phosphorus-starved plants (grown at 10 μmKH₂PO₄) differed from those of plants grown in phosphorus-sufficient conditions (0.5 mmKH₂PO₄). When plants were supplied with adequate phosphorus, organic acids were released in very low amounts by all accessions. In contrast, these acids were present in higher concentrations in the bathing solutions when plants were pre-cultivated for 8 d under phosphate deficient conditions. The amounts of citric and malic acids released from the roots differed among accessions (Table VII; Fig.3).

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Fig. 3.

Organic acid exudations (citrate and malate) by the roots of five Arabidopsis accessions. For each measurement, 400 seedlings (on four nylon meshes) were grown for 8 d in modified 0.5× Hoagland nutrient solution containing 10 μmKH₂PO₄. Root exudates were collected for 1 h, concentrated, and analyzed using HPLC. Each bar represents the total micromoles of organic acid released per gram seedling dry weight. Data are means ± sd of three replicate measurements.

In all accessions citrate and malate are the predominant organic acids released. The kinds and amounts of exuded organic acids appeared to be accession specific. In the accessions C24, Co, and Col-0, citrate exudation is slightly higher than is the exudation of malate, whereas in the accessions Cal andTe, malate is released in higher amounts than is citrate (inTe, malate exudation is six times higher than citrate).

In the efficient accessions C24 and Co the amounts of citrate and malate (μmol h⁻¹g⁻¹ dry mass plants) in root exudates are higher than those of Cal, Col-0, and Te. When calculated on the basis of root lengths, these differences are even more dramatic. The accumulation of organic acids in the rhizosphere ofC24 and Co plants is therefore expected to be drastically higher than that of Cal, Col-0, andTe plants (Table VII).

pH Changes and Its Effect on Phosphate Solubility

In alkaline soils, phosphorus mobilization occurs predominantly through rhizosphere acidification. To test whether organic anions are released in association with protons and whether rhizosphere acidification is correlated with organic anion secretion in Arabidopsis, pH changes in the growth media were measured.

As expected, since nitrogen was supplied exclusively in the NO₃ ⁻ form, cultivation of plants from all accessions resulted in pH increase of the medium. The observed pH changes in the growth medium, however, did not correlate to organic acid exudation. No significant differences in the pH of growth medium of high organic acid exuding, phosphate-efficient accessions (C24 and Co) and low organic acid exuding, phosphate-inefficient accessions (Col-0 andTe) were observed. Of all the accessions onlyCal showed a slightly enhanced release of protons (Table VIII). Consistent with this observation, the accession Cal showed the highest PAE at higher buffer concentration (100 mm MOPS [3-(N-morpholino)-propanesulfonic acid], pH 7.0; data not shown).

Effect of Root Morphology and Penetration Ability on Phosphate Efficiency

Root hairs substantially extend the root surface area available for nutrient uptake from the soil. Theoretical calculations (Nye, 1966), autoradiographs (Lewis and Quirk, 1967), mechanistic models (Föhse et al., 1991), and experimental evidence (Gahoonia and Nielsen, 1998) suggest a relationship between root hair length and density and phosphate uptake from the soil. The Arabidopsis accessions tested for root morphology differed widely in their root hair parameters. The accessions C24, Co, andCal have long, dense root hairs, whereas Col-0and Te have shorter and fewer root hairs. The contribution of root hairs to phosphate acquisition depends upon the diffusion of nutrients (Barber and Silberbrush, 1984). Long root hairs may intercept the phosphate diffusing toward the root at some distance. If the root hairs are short and develop in pre-existing zones depleted by the absorption of phosphate by the rhizodermis, they contribute only a little to phosphate acquisition. The presence of long and dense root hairs in C24, Co, and Cal might well explain the superior PAE exhibited by these accessions when grown in HA medium. A wide variation in root hairs of cereal varieties and the correlation of long root hairs to high phosphate uptake has been reported (Gahoonia et al., 1997). Root hair growth appears to be controlled by multiple genetic loci and to be strongly influenced by environmental conditions (Schiefelbein and Somerville C, 1990).

Root parameters were measured under two different conditions since it was found that they changed with changes in growth medium density (Tables IV and V). Root elongation of the five accessions decreased when the growth media agarose concentration increased (0.15%–0.4%). The high acquisition of phosphate from sparingly soluble phosphate medium, as well as from full phosphate medium in Cal is probably due to its long roots and long root hairs (i.e. high general uptake efficiency). The comparison of the root lengths of the accessions furthermore showed that the efficient accessionsC24 and Co have slightly shorter roots as compared with the less efficient accession Col-0. C24 andCo, however, display highest root hair densities and root hair lengths among all accessions tested when grown in 0.4% (w/v) agarose medium. The inefficient accessions Col-0 andTe have much shorter and fewer root hairs. However, when grown in low-density agarose medium (0.15% [w/v] agarose),Col-0 and Te showed strongly increased PAEs. In case of Col-0 this results in a PAE equivalent to those exhibited by the efficient accessions C24 and Co. The change in phosphate efficiency of accessions Col-0 andTe under low agarose density is obviously due to the dramatic increase in their root lengths, and consequently their root surface areas and, therefore, the volume of the medium, which they can exploit.

The development of a root system capable of anchoring the shoot and obtaining sufficient water and nutrients is essential for the survival of most terrestrial plants (Bengough et al., 1997). In the soil, roots experience a range of penetration resistances depending upon soil type. Since the selected accessions were sampled from different habitats, their adaptations potentially included differences in soil penetration abilities. The differential changes in the root growth of the different accessions with varying densities of the agarose medium reflect differences in their penetration abilities. The root penetrating abilities of C24 and Co were found to be approximately two times higher than those of Col-0 andTe. In accordance with this, the C24 andCo accessions showed higher PAEs when grown in high-density agarose medium (0.4%).

Mucilage exudation and sloughing of root cap cells may be involved in decreasing the frictional resistance in soil penetration by roots. Increases in the exudation of mucilage are found in roots growing in compressed soil when compared with those in loose soil (Boeuf-Tremblay et al., 1995). Whether similar mechanism influences the root penetration ability in Arabidopsis is presently unknown.

Diversity among Accessions in Phosphate Uptake

The results of uptake measurements show that the accessions differed in their phosphate uptake characteristics. Differences in the phosphate uptake have also been identified among crops plants (Barber, 1995; Horst et al., 1996; Römer and Schenk, 1998). In the uptake experiments described here all Arabidopsis accessions showed single-phase Michalis-Menten kinetics when supplied with phosphate in the range of 2.5 to 100 μm. The apparentK_m values of phosphate uptake, determined for plants of the accessions C24, Co,Cal, Col-0, and Te grown under phosphate deficient conditions, are characteristic for high affinity transporters (Schachtman et al., 1998). Similar results for the high affinity system have been reported in Arabidopsis despite differences in experimental procedure and age of plants used (Dunlop et al., 1997;Dong et al., 1999). It is surprising that slightly higher affinities were observed for C24, Co, Cal,Col-0, and Te as determined by apparentK_m values (which could be determined here with lower precision) under phosphate-sufficient conditions.

I_max values, under phosphate-deficient conditions, were within the range of 90 to 152 pmol/h⁻¹ cm⁻² root, which is approximately 1.5-fold higher than theI_max values determined under sufficient phosphate supply. Increases in I_max have also been reported for various species under phosphate deficiency by several researchers (Lee 1982; Drew et al., 1984; Dunlop et al., 1997) and have been attributed to increased expression of Pi transporters in tobacco suspension cells (Shimogawara and Usuda, 1995).

Dunlop et al. (1997), however, suggested the existence of a low-affinity system (two-phase kinetics) in Arabidopsis at higher phosphate concentrations. Under phosphate-deficient conditions the efficient accessions C24 and Co have approximately 1.5 times higher I_max (pmol h⁻¹ cm⁻¹ root) than those of Col-0 and Te and, consequently, they can acquire more phosphate per centimeter of root than the latter (less efficient) accessions. Because no obvious differences were observed in the apparent K_m values among the different accessions, the differences in the phosphorus uptake systems of the accessions are probably manifested in different frequencies of active phosphate transporters (difference inI_max ) rather than in expression of different kinds of phosphate transporter genes. Variation in the numbers of functional ion transporters per unit length of root may be caused by differential rates of synthesis and breakdown of the transport proteins, a process likely to cause changes in the maximum transport capacity of the root (Lee, 1982). The tomato phosphate transporter genes LePT1 and LePT2 show predominant expression in the root epidermis and the root hairs (Daram et al., 1998; Liu et al., 1998). The phosphate transport activity may therefore also be influenced by the frequencies and sizes of the root hairs, which differ in the various accessions and are highest inC24 and Co. The lower phosphate transport activity per centimeter of root in Cal, Col-0, andTe is over-compensated by the elevated root lengths in these accessions when grown in low strength medium. This results in higher phosphate transport activity per gram of plant fresh weight inCal, Col-0, and Te under these conditions in comparison to C24 and Co. The superior PAEs displayed by C24 and Co when grown in high-strength medium, therefore, are most probably conferred by a combination of high root penetration abilities, high root surface areas, and high phosphate transport activities per centimeter of root.

Another physiological parameter, C_min , the concentration of phosphate in the growth medium at which no net uptake or release of ions occurs, is a measure of the ability of a plant to acquire phosphate at low concentrations and of the degree to which the soil solution can be depleted. This threshold may be set by a limited affinity of the carrier sites for the ions or it may be the point at which influx is balanced by the efflux (Marschner, 1988). This parameter of ion uptake is also strongly affected by the plant nutritional status (like I_max andK_m ). Measurement of phosphate-starved plants showed that all tested accessions (C24 andCo, Cal, Col-0, and Te) displayed C_min between 7 and 11 nm. In accordance with this, this measure does not contribute substantially to the differences in phosphate efficiency in Arabidopsis as in other plant species (Jungk et al., 1990). TheC_min values determined in this study were four to six times lower than those measured by Krannitz et al. (1991)in 25 Arabidopsis genotypes. The reason for this discrepancy may reside in the different measurement conditions used.

Phosphate Mobilization by Root Exudates

In addition to root morphology and uptake activity, the release of organic anions from roots of can contribute to PAE through the dissolution of sparingly soluble phosphates (Dinkelaker et al., 1989). It is widely believed that the efficiency of uptake is of minor importance for phosphorus acquisition from soils because availability of Pi to the root surface rather than its uptake is the limiting factor (Barber, 1995). The efficient accessions C24 andCo exude four to five times more citrate than the less efficient accessions Col-0 and Te. The increased exudation of organic anions, particularly citrate and malate, in the accession C24 and Co may enable them to acquire phosphate from hydroxylapatite. Differences in the exudation of citrate and malate among the members of the Brassicaceae, which correlated with their PAEs, have been reported previously (Van den Boogaard et al., 1992). Consistent with these data, elevated root citrate and malate exudation has been observed in the efficient Arabidopsis accessions C24 and Co in this study.

Polar substances like organic acids can diffuse into the rhizosphere due to the high electrochemical potential gradient existing between the cytoplasm of root cells and the soil solution (Jones et al., 1994). This passive diffusion of citrate, however, is slow ranging from 14.4 to 93.6 pmol h⁻¹ (cm root length)⁻¹ for wheat and tomato and 100 to 200 pmol h⁻¹ (cm root length)⁻¹ from non-proteoids roots of white lupin as compared with the active citrate exudation from the proteoid roots of phosphorus-deficient white lupin, which is approximately 30- to 70-fold higher (Neumann and Römheld, 1999; Neumann et al., 1999). Citric acid exudation rates in white lupin depend upon the age of proteoids roots and reaches up to 6,700 pmol h⁻¹ (cm root length)⁻¹ in mature proteoid roots (Neumann et al., 1999). Exudation of organic acids at a high level in response to phosphorus stress has been documented also for other species such as oilseed rape (Hoffland et al., 1992) and alfalfa (Lipton et al., 1987). The carboxylates that are most effective in mobilizing phosphate from sparingly soluble sources are citrate > oxalate > malate (Bar-Yosef, 1996).

The root exudation of citrate and malate in the efficient accessionsC24 and Co is lower as compared with white lupin proteoid roots, but it is comparable with oilseed rape (Hoffland et al., 1989). In contrast to white lupin and oilseed rape, however, the high citrate and malate exudation in the Arabidopsis accessionsC24 and Co are not accompanied by strong proton extrusion. All accessions displayed a net alkalinization of the growth medium. To monitor potential spatial variation of root-induced pH changes, other experimental procedures need to be applied such as the use of pH indicators and H⁺-selective microelectrodes (Marschner and Römheld, 1983; Plassard et al., 1999). Rhizosphere acidification would be required to cause the dissolution of HA resulting in efficient phosphate mobilization. Lowest pHs of the growth media were observed for the accessions Caland Col-0, which showed intermediate or low root organic acid exudation, respectively. Therefore, the main mechanism governing the proton release in Arabidopsis probably is the balancing of cation-anion uptake (Jeong and Lee, 1996; Schöttelndreier and Falkengren-Grerup, 1999). Rhizosphere acidification in response to phosphorus starvation in oilseed rape has been attributed to unbalanced cation-anion uptake (Hedley et al., 1982). The independence of the rate of organic acid anion release and the degree of HA utilization by the Arabidopsis accessions is furthermore supported by the observation that the differences in their PAEs are minor when they are grown in low strength media. The ability to exploit a large substrate volume therefore appears to be of much higher importance than activities related to an enhanced phosphate mobilization from the sparingly soluble HA.

Secretion of organic acids without concomitant release of protons has been observed in phosphorus-efficient plants adapted to acid soils (H. Lambers, personal communication). It remains to be tested whether the Arabidopsis accessions C24 and Co display high PAEs under such conditions.

This study highlights the fact that inherent differences in PAE exist among Arabidopsis accessions. The adaptations evolved in these accessions to cope with low phosphate availability in soils are dominated by root growth and morphology and by phosphate uptake rates per unit root length. Rhizosphere acidification, root organic acid exudation, and affinity of the phosphate transporters appear to be of minor relevance. Two extreme sets of accessions were identified: The first, represented by C24 and Co, develops relatively short roots with high substrate penetration abilities, long root hairs at high densities, and high phosphate uptake efficiencies per unit root length. The second, represented by Col-0 andTe, produces long roots (under favorable conditions) with low substrate penetration abilities, short and sparse root hairs, and low uptake efficiencies per unit root length. Cal appears intermediate for these characteristics, but displays slightly enhanced proton extrusion.

Plants with larger root systems have a greater potential to acquire phosphate than do plants with small roots. However, the uptake experiments show that the accessions with small root systems can be more efficient because they can acquire more phosphate per unit length of root. This finding is in close agreement with the conclusions reached through other studies (Evans 1977; Caradus, 1980; Rao et al., 1997). In most soils the phosphorus available to the plant is concentrated primarily in the upper soil horizons and decreases with the soil depth (Anderson, 1980; Keter and Ahn 1986; Pothuluri et al., 1986; Lynch 1995). When this is the case plants with short roots, but long root hairs at high density and with root systems of branched architecture would be more efficient than accessions with long roots and short and sparse root hairs. The accessions C24 andCo with their slow, but sustainable (high penetration ability) root growth and high phosphate uptake per unit of root are apparently better adapted to compact soils, whereas the less efficient accessions with fast growing roots such as Col-0 andTe are better suited to loose soils. The latter cope with phosphate deficiency through elongation of their roots so that they can access a larger soil volume, which may be of lower richness.

Taking into consideration multiple soil parameters, Barber (1995)predicted that nutrient uptake is likely to be most strongly affected by the root growth rate. This is supported by the observed strong drop in the PAEs of the Col-0 and Te accessions upon increased growth medium density, which caused severe root growth retardation. Under these conditions, the accessions C24 andCo, with their higher root penetration ability and their long root hairs and high uptake per centimeter of root are more efficient.

In conclusion this study represents a detailed morphological and physiological analysis of the mechanisms governing PAEs of Arabidopsis accessions. Based on the results presented here the excellent resources available for this molecular genetic model system can be used to isolate genes that determine phosphate efficiency traits. Furthermore, favorable/unfavorable allelic variants of these genes may be identified that shall provide detailed insight into the molecular mechanisms underlying the PAE phenomenon.