Journal of Experimental Botany, Vol. 52, No. 361, pp. 1703-1710,
August 1, 2001
© 2001 Oxford University Press
Original Papers |
Genotypic differences in the presence of hairs on roots and gynophores of peanuts ( Arachis hypogaea L.) and their significance for phosphorus uptake
National Institute of Agro-Environmental Sciences, 3-1-1 Kannondai, Tsukuba, Ibaraki 305, Japan
Received 11 December 2000; Accepted 10 April 2001
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Abstract |
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Root hairs substantially increase the surface area of plant roots with positive effects for phosphorus (P) uptake, but the ability of peanuts to form root hairs has been questioned. The aim was to examine hair development on roots and gynophores of a variety of peanut genotypes and to relate genotypic differences in hair formation to differences in P uptake. Five out of eighteen genotypes completely lacked hairs on both organs whereas others consistently developed hairs on roots and gynophores, although with considerable variation in hair density. The ability to form root hairs as well as root hair density concurred with the presence and density of hairs on gynophores, suggesting a possible connection between both developmental processes. The contribution of root hairs to P uptake was studied in three genotypes differing in hair density. The final amount of P taken up by roots did not differ between genotypes but two distinct P uptake strategies could be identified. The genotype lacking root hairs maintained P uptake due to the development of a large root system whereas densely covered roots of genotype ‘Wasedairyu’ were three times as efficient in extracting P from a P-deficient soil. Furthermore P uptake through gynophores contributed about 20% to the total P uptake of Wasedairyu but only insignificant amounts to other genotypes. The ability to form hairs on roots and gynophores can therefore be seen as an adaptation to low P availability and if combined with a large root system, could substantially increase the tolerance of peanuts to P deficiency.
Key words: Gynophore, root hairs, phosphorus uptake, phosphorus deficiency, root efficiency.
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Introduction |
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The concentration of phosphorus (P) in the soil solution of P-deficient soils is extremely low because most soil-P is bound in forms of low solubility (Sanchez and Uehara, 1980
). As a result, the mobility of P in the soil solution is far smaller than that of other major plant nutrients (Barber, 1984). Due to this low mobility, P uptake is generally considered to be proportional to the surface area of plant organs involved in P uptake (Jungk and Barber, 1974; Sattelmacher et al., 1994). Genotypic differences in the number and length of root hairs have been identified as one important factor that improves P uptake due to an increase in root surface area (Caradus, 1982; Itoh and Barber, 1983; Föhse et al., 1991).
In a previous study genotypic differences were detected among peanut (Arachis hypogaea L.) genotypes in the amount of phosphorus taken up from a highly P-deficient growth medium (Wissuwa and Ae, 1999). These differences only became apparent at later stages of plant development. During pod filling daily uptake rates increased 4-fold in genotype ‘Kintoki’ and 12-fold in ‘Wasedairyu’ but remained low in ‘ICGV 87358’. Because P uptake increased only after pod setting and because differences in root development failed to explain the observed changes in P uptake, it was concluded that genotypic differences in P uptake were due to direct P uptake by fruiting organs. In addition to having a greater number of fruits, gynophores (pegs) of Wasedairyu were characterized by being densely covered with root hair-like outgrowths whereas ICGV 87358, the genotype without any increase in the daily P uptake rate, completely lacked hair development on pegs. Kintoki was intermediate in hair development. The presence of peg hairs could substantially increase the surface area of fruiting organs, which could be of similar significance for P uptake as the presence of root hairs. The high number of hairs on pegs of Wasedairyu may thus explain how direct P uptake via fruiting organs could have contributed a major portion to the total P uptake of Wasedairyu.
As early as 1895 botanists detected root hair-like structures on the subterranean parts of the groundnut peg (Pettit, 1895, as quoted by Bledsoe and Harris, 1950). Their existence was later confirmed (Harris, 1949; Webb and Hansen, 1989), but their involvement in nutrient uptake remained speculative. Despite the early discovery of peg hairs and their potential importance for nutrient uptake, a recent study (Wissuwa and Ae, 1999) was the first to show that genotypic variation exists among groundnuts for the ability to form such hairs. As yet it is unknown if the presence and density of peg hairs is related to genotypic differences in root hair formation. The presence of hairs on groundnut roots is itself a matter of discussion. It appears that groundnuts form two distinct types of root hairs: a rather long (
4 mm) rosette-type surrounding lateral root initiates, and shorter ‘normal’ hairs (1 mm) near the tip of lateral roots (Allen and Allen, 1940; Meisner and Karnok, 1991). The rosette-type seems to be commonly formed whereas reports on the presence of ‘normal’ type root hairs near root tips are inconsistent. In several studies this type was detected in small numbers (Reed, 1924; Allen and Allen, 1940; Meisner and Karnok, 1991), however, their presence varied greatly with environmental conditions. Others failed to detect ‘normal’ root hairs altogether (Pettit, 1895; Chandler, 1978; Krishna and Bagyaraj, 1984). Genotypic differences in the ability to form root hairs, if present, could offer an explanation for these contradictory reports.
One objective of this study therefore was the assessment of genotypic variation in peg and root hair density and to test the hypothesis that genotypic differences in the ability to form peg hairs could also be observed in root hair formation. A second objective was to determine if genotype-specific changes in hair density could be observed at different growth stages. Finally, a third objective was to test the hypothesis that the lack of hairs on pegs and roots would lead to less efficient P uptake if plants are grown in a P-deficient medium.
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Materials and methods |
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Experiment I
Genotypic variation for the presence and density of peg and root hairs was assessed on a set of 18 groundnut genotypes of diverse origin and plant type. Root hairs were observed on 14-d-old seedlings that had been germinated between moistened filter paper. Hairs were counted on the first 25 mm from the root tip because root hairs were only found in high density on young, growing roots. Four plants per genotype were analysed and for each plant the average hair density of three roots was determined (in hairs mm-1). Peg hairs were counted on plants that had been grown in a field for a 120 d period. Plants were dug out and quickly brought to the laboratory where pegs were cut and stored in tap water until longitudinal sections could be observed under a light microscope. The highest density of hairs was found 1–2 cm from the base of the pod. The number of hairs per millimetre was counted on three pegs per plant with four plants per genotype.
Experiment II
Changes in hair density over time were determined for the genotypes Wasedairyu, Kintoki, and ICGV 87358. These genotypes mature within 110–120 d and therefore enter phenological stages at the same time. Seeds were germinated on moist filter paper for 3 d. On May 17, 1998, five seeds per genotype were sown into 20 l boxes filled with silica sand. Plants were grown in a greenhouse and watered with Arnon solution (Arnon, 1938) twice per week and with tap water on other days, if needed. Three sets of boxes were prepared to sample plants at different growth stages: 50 d after sowing (DAS) at the flowering stage, 75 DAS at the pod formation stage, and 100 DAS at the pod filling stage. In order to sample whole plants without injuring peg and root hairs, boxes were flooded and tilted and the sand was washed out by letting water run through the box. Plants were kept in beakers filled with tap water until pegs and roots could be examined under a light microscope.
Because pegs had not yet formed, peg hairs were only counted 75 and 100 DAS. Pegs were sectioned longitudinally with a razor blade to facilitate focusing during microscopic examinations. The number of hairs on a 1 mm part of high density was counted on both half-sections of three pegs per plant. In addition the length of the hairy area on pegs was measured and the presence of hairs on pod cross-sections recorded. For statistical analysis the average of these three pegs was used to get the hair count per plant (in hairs mm-1). Five plants per genotype were analysed and treated as replications in a completely randomized design. Root hairs were counted 50, 75, and 100 DAS. Per plant five randomly chosen root tips were analysed and the average number of root hairs (hairs mm-1) used in statistical analysis. Root hair length was measured on micrographs taken 50 DAS. The five longest root hairs per root were measured and averaged. Again five plants per genotype were treated as replications in a completely randomized design. All data were subjected to analysis of variance using PROC GLM of SAS (SAS Institute, 1989). To determine if genotypic differences were significant, Tukey's HSD test was used and the significance threshold set at P0.05.
Experiment III
The separate contribution of roots and pegs to P uptake of genotypes Wasedairyu, Kintoki, and ICGV 87358 was studied by dividing root and peg zones and altering the P supply to each zone. The effect of seed-P reserves was minimized by cutting cotyledons once the first leaf had emerged during germination. To study the P uptake through pegs, plants were grown with their roots contained in 1.0 l plastic soft drink bottles filled with Perlite. These bottles were placed in 10 l pots that were subsequently filled with silica-sand (Fig. 1A). Bottles were watered twice weekly with half-strength, P-free Arnon solution. Phosphorus was supplied to roots inside bottles in a total amount of 5 mg per plant (as KH2PO4): an initial dose of 2 mg P followed by additional doses of 1 mg P at the end of the first, second, and third month. During peg development care was taken that pegs developed in the sandy outside part rather than inside the bottle. Per genotype two sets with four replications per set were grown. In one set the sandy part surrounding the bottle was watered with half-strength, P-free Arnon solution. Since pegs developed in a medium lacking P, all P taken up by plants in this set must have originated from the P supplied to their roots inside the bottle. The second set was watered with half-strength Arnon solution containing P. Any additional P uptake of plants in the second set relative to plants in the first set must therefore have been taken up through pegs. After a 120 d growth period the sand was washed away, bottles were cut open and roots, shoots, pegs, and pods were harvested. Their respective dry weight was determined after oven-drying samples at 65 °C for 4 d. Samples were then ground in a Wiley mill to pass a 1 mm mesh. The tissue-P concentration in 1 mg of root, shoot and peg/pod sample was determined colorimetrically by the phosphovanadate method (Hanson, 1950) after digestion in a mixture of HNO3, HClO4, H2SO4 (3 : 1 : 1, by vol.). Total P uptake was calculated as the product of dry weight and tissue-P concentration. The P uptake through pegs and pods was calculated by subtracting the P content of plants that had pegs growing in sand without addition of P from plants whose pegs grew in sand supplied with P.
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A second experiment was conducted to determine genotypic differences in P uptake through roots. Plants were grown in 3.0 l pots for a 50 d period and in 10 l pots for a 120 d period. Pots were filled with soil (Humic haplic andosol) of high P-fixing capacity that had been collected from two fields. (1) A P-deficient field plot that had not received P fertilizer for 30 years and was characterized by low levels of plant available P of 0.5 mg P kg-1 (Truog-P) or 4.5 mg P kg-1 (Bray2-P) (Otani and Ae, 1996
). (2) A plus-P field that had been amended with 60 kg P ha-1 annually. The concentration of available P in the plus-P field was 7.3 mg P kg-1 (Truog-P) or 25.6 mg P kg-1 (Bray2-P). The soil in 10 l pots (120 d experiment) was overlaid with a 10 cm thick layer of silica-sand. This arrangement made sure that roots developed mostly in soil whereas pegs and pods developed in the sandy layer without contact with soil (Fig. 1B
). Any P taken up must therefore have been solely supplied by roots. Plants were watered twice weekly with half-strength, P-free Arnon solution to provide all essential nutrients to developing pegs and pods with the exception of P. At the end of the respective growth periods shoots and pegs/pods were harvested and roots were sampled by carefully cleaning them from attached soil. The root surface area (not including root hairs) was analysed using a digital scanner and the software package WinRhizo (Regent, Canada). A subsample of lateral roots was then stained in Tri-Pan Blue to determine root infection with vesicular arbuscular mycorrhiza (VAM). The number of arbuscles per 2 cm piece of root were counted and averaged over five roots per plant. Dry weight and P content was determined as described above. All data were subjected to analysis of variance using PROC GLM of SAS (SAS Institute, 1989). Tukey's HSD with a threshold of P0.05 was used throughout the experiment for the comparison of genotypic means.
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Results |
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Experiment I
All genotypes developed the rosette-type of root hairs surrounding lateral root initiates, but the ability to form hairs on pegs and along the axis of lateral roots (‘normal’ root hairs) varied among genotypes (Table 1
). In addition to ICGV 87358, four other genotypes completely lacked hair development on pegs and roots axis. Of the ones that did form hairs, the variation for hair density was large, ranging from a sparse cover of just a few hairs mm-1 (Starr, Java 13, Kintoki) to more than 20 hairs mm-1 (ICG 10107, ICGV 86031, Wasedairyu). The presence of hairs on pegs and roots appeared to be a linked phenomenon since no genotype developed hairs on only one organ. Furthermore, the density of hairs on pegs was correlated to root hair density (r=0.76). The ability to grow hairs could not be attributed to taxonomic groups. Some Spanish (A. hypogaea subsp. fastigiata var. vulgaris) and Valencia type peanuts (A. hypogaea subsp. fastigiata var. fastigiata) lacked hairs whereas others had hairs in high density. The Virginia type peanuts (A. hypogaea subsp. hypogaea var. hypogaea) used in this study, however, all did grow hairs at rather high density.
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Experiment II
Qualitatively observations of the more detailed study in experiment II were in agreement with results from experiment I: ICGV 87358 lacked hairs on roots and pegs regardless of sampling date whereas Kintoki and Wasedairyu developed hairs on both (Fig. 2). Using 14-d-old seedlings for root hair assessment and field-grown plants for peg hair counts in experiment I therefore reliably identified genotypic differences in hair formation even though some quantitative changes occurred. Peg hair density of Kintoki and Wasedairyu did not differ significantly on day 75 but on day 100 Wasedairyu showed a slightly higher hair density compared to Kintoki (Table 2). The difference between both genotypes further increased when sampled 120 DAS (experiment I). This could indicate the presence of genotypic differences in longevity of peg hairs or that Wasedairyu was able to form new peg hairs for a prolonged period of time. That both genotypes had lower peg hair density when grown in the field (experiment I) was probably due to some losses of hairs during removal of pegs from soil and subsequent washing in addition to a possible ageing process. In addition to differences in peg hair density, large differences were observed for the length of the hairy zone. Because pods of Kintoki formed at very shallow depth, pegs themselves were short and the hairy zone never exceeded 1 cm in length. Wasedairyu on the other hand developed longer pegs and this significantly increased the hairy zone to a length that was four times as long as in Kintoki. Furthermore, individual hairs tended to be longer in Wasedairyu (Fig. 2).
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As for pegs, ICGV 87358 completely lacked the ability to form hairs on roots regardless of sampling time (Table 2
). Both Kintoki and Wasedairyu developed root hairs but hair density changed depending on genotype and growth stage at sampling. During the flowering stage of development (50 DAS), root hair density of Wasedairyu was significantly greater compared to Kintoki. This gradually changed during later growth stages. Roots of Wasedairyu began to vary greatly in hair density during pod development (75 DAS) when hair density on some root tips was reduced to 3 hairs mm-1 while other root tips were densely covered with more than 20 hairs mm-1. Pod development therefore appears to be a transition stage for roots of Wasedairyu, and towards maturity (pod filling, 100 DAS) root hair density was uniformly reduced by almost 75% compared to the flowering stage.
Experiment III
The total P content did not differ significantly between genotypes if P was only supplied to roots growing inside the 1.0 l bottle (Fig. 3). When P was supplied to the peg zone as well, all genotypes showed a slight increase in P uptake, however, the difference between both treatments was only significant for Wasedairyu. The additional P uptake through pegs amounted to 19.2% of the total plant-P content in Wasedairyu, but only to 8.9% in ICGV 87358 and 5.3% in Kintoki. Effects of this additional uptake on plant biomass accumulation were not detected in any genotype. The only significant change brought about by supplying P to pegs was an increase in the shoot-P concentration of Wasedairyu from 0.48 mg P g-1 dry wt. to 0.69 mg P g-1 dry wt. This could indicate reduced retranslocation of P from shoot tissue to growing fruits, if fruits are able to participate in P uptake and thus partly satisfy their own demand. That Kintoki failed to show any significant P uptake through pegs despite the presence of hairs could be due partly to the shorter life span of its peg hairs and to the much shorter length of the hairy zone in its pegs (Table 2).
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When only roots were able to contribute to P acquisition because plants were harvested 50 DAS, prior to peg formation, the P uptake of genotypes varied by 25% in the plus-P treatment, with Wasedairyu showing the highest and Kintoki the lowest uptake (Table 3
). P uptake in the minus-P relative to the plus-P treatment decreased on average by 70% and this decrease was more pronounced for ICGV 87358 and Kintoki. With a root system that did not significantly differ in size from other genotypes, Wasedairyu had a 70% higher P uptake in the minus-P treatment compared to the other genotypes. As a consequence Wasedairyu showed the highest root efficiency (P uptake per root surface area). When plants were grown for 120 d with roots in soil and pegs in silica-sand without the addition of P to eliminate any P uptake through pegs, the average P uptake from the plus-P soil was 53.0 mg P plant-1 compared to 18.6 mg P plant-1 from the P-deficient soil (Table 3). Genotypic differences were not significant for P uptake, but highly significant for root surface area. ICGV 87358 had the largest and Wasedairyu the smallest root system, regardless of P supply. The root surface area of Kintoki and Wasedairyu decreased with decreasing soil-P status whereas ICGV 87358 maintained a large root system despite P deficiency. Genotypes did not differ in root fineness (data not shown). Genotypic differences for root efficiency were highly significant as a result of similar P uptake but large differences in root surface area. In the P-deficient soil the root efficiency of Wasedairyu was almost three times as high as for ICGV 87358. In the plus-P soil all genotypes showed an increase in root efficiency but differences between ICGV 87358 and Wasedairyu remained significant. To investigate potentially confounding effects of mycorrhiza association on P uptake, the infection rate of roots with VAM was measured. Roots of all genotypes were infected with VAM to a similar degree but the infection rate doubled in the plus-P relative to the minus-P treatment.
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Discussion |
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Screening a selection of 18 peanut genotypes revealed the presence of large variation for the ability to develop hairs on roots and pegs. Whereas several genotypes completely lacked hairs on both organs, others consistently developed hairs on roots and pegs. The conclusion drawn by several authors that peanuts lack the ability to form root hairs along the root axis (Pettit, 1985
; Chandler, 1978; Krishna and Bajyaraj, 1984) may thus have been caused by the choice of genotype. All studies used a single genotype rather than a more representative sample. In addition to qualitative differences (hairs yes or no), up to 10-fold differences in hair density were observed. As for qualitative differences, root and peg hairs concurred quantitatively (r=0.76). This suggests that the ability to form hairs on roots and pegs is a linked process, possibly under common genetic control. To date the genetics of hair formation has not been investigated in peanuts. Several hairless mutants have been characterized in Arabidopsis thaliana (Schiefelbein and Somerville, 1990; Gilroy and Jones, 2000), indicating the involvement of several genes in root hair formation. These Arabidopsis mutants could be rescued by the application of either auxin or ethylene to the growth medium (Masucci and Schiefelbein, 1994). That qualitative as well as quantitative variation in hair formation was observed could suggest the involvement of more than one gene in the expression of phenotypic differences among the peanut genotypes studied. To test if an exogenous supply of hormones could induce root hair growth in peanuts, auxin and/or ethylene were added to the growth medium of hairless peanut genotypes. This, however, had no effect (data not shown) indicating that the genetic cause of hairlessness in peanuts differs from that of the Arabidopsis mutants described previously (Schiefelbein and Somerville, 1990; Gilroy and Jones, 2000).
The more detailed study of the three genotypes ICGV 87358, Kintoki and Wasedairyu furthermore showed that hair density varied over time in a genotype-specific manner. Kintoki and Wasedairyu did not differ in peg hair density at 75 DAS, but at 100 DAS the peg hair density of Kintoki had slightly decreased. This was followed by a sharp drop towards maturity when the peg hair density of Kintoki was more than 75% lower compared to Wasedairyu. Similar differences had been observed in a previous study when pegs were analysed 115 DAS (Wissuwa and Ae, 1999). Root hair density followed an opposite pattern, with Wasedairyu showing a decrease in root hair density towards maturity. Since root hairs are found in highest density near the tip of developing lateral roots, a more rapid ageing process of the root system of Wasedairyu may have caused the decrease. In a previous study very little new root development was observed for Wasedairyu after flowering whereas Kintoki developed new roots until pod filling (Wissuwa and Ae, 1999). A similar trend was observed in this study. The root size of Wasedairyu was equal to Kintoki at the flowering stage but significantly smaller at maturity. Age differences of root tips analysed 75 DAS could therefore account for the high variation in hair density observed for Wasedairyu at that time. Roots had probably aged uniformly by 100 DAS because few additional roots are formed during pod filling and this would explain the low root hair density of Wasedairyu. The increase in root hair density during pod filling of Kintoki can not be explained at present. It is, however, interesting to note that this increase is accompanied by a decrease in hair density on pegs whereas that was not the case for Wasedairyu. It is possible that Wasedairyu relies more strongly on P uptake through pegs during later growth stages than does Kintoki. This hypothesis would offer an additional explanation for the dramatic increase in daily P uptake rates previously observed for Wasedairyu after 85 d despite stagnating root size (Wissuwa and Ae, 1999).
The importance of root hairs for the acquisition of phosphorus has been confirmed for a wide variety of plant species (Caradus, 1982; Itoh and Barber, 1983; Föhse et al., 1991). This poses the question how ICGV 87358 can maintain an equally high P uptake despite a complete lack of root hairs. One potential mechanism to compensate for the absence of hairs is to develop a larger root system and these data show that this was the case (Table 3). Wasedairyu and ICGV 87358 thus represent two different strategies for P acquisition: high root efficiency possibly due to the presence of root hairs in high density in the case of Wasedairyu compared to the production of a large but inefficient root system in the case of ICGV 87358. In addition, Wasedairyu can increase P uptake through direct P acquisition via pegs and pods. Furthermore these contrasting genotypes differ in their response to P deficiency in terms of assimilate distribution between plant organs. In the minus-P treatment Wasedairyu shows only a 10% reduction in total pod biomass but large reductions in root and shoot biomass whereas the opposite was observed for ICGV 87358. To maintain pod growth despite P deficiency Wasedairyu needs to retranslocate assimilates and P from vegetative tissue to developing pods at a much higher rate than ICGV 87358. As a consequence of the high demand exerted by developing pods of Wasedairyu, vegetative growth ceased at some point during the generative phase and, at harvest, shoots and to a lesser extend roots, showed strong signs of senescence. This was an adaptive rather than a general characteristic of Wasedairyu because vegetative growth in the plus-P treatment was observed until the final harvest. ICGV 87358 on the other hand, continuously developed new leaves and roots (visible by white coloration of fresh roots) regardless of P supply.
Genotypic differences in the presence of root hairs and in root growth both contributed to the higher root efficiency of Wasedairyu. The calculated difference increased from 50% at 50 DAS to 200% at 120 DAS. In a simulation study on P uptake, it was estimated that the positive effect of root hairs would increase over time due to a higher rate of P depletion from outside the root hair zone when hairs are present (Itoh and Barber, 1983). The 50% difference in root efficiency therefore can be expected to underestimate the real discrepancy, whereas the possible reduction in root size of Wasedairyu at 120 DAS due to senescence likely led to an overestimation.
In addition to differences in root hairs, high root efficiency could have been caused by other factors such as colonization with mycorrhiza or excretion of root exudates capable of solubilizing soil-bound P. However, it has been reported that peanuts did not excrete a substantial amount of exudates under P-deficiency (Otani et al., 1996). Roots of all three genotypes were colonized by VAM to a similar degree. Observed differences in root efficiency could therefore not have been due to the effect of mycorrhiza. That infection rates roughly doubled in the plus-P treatment could indicate that the beneficial effect of VAM would decrease under extreme P deficiency. Recently it has been suggested that the relatively high tolerance of peanuts to P deficiency is partly caused by an as yet unknown cell wall component that is able to solubilize Fe or Al-bound P (Ae and Otani, 1997). Initial results, however, indicate that this cell wall component is present in similar quantities in the roots of the three genotypes studied here (Shen, unpublished data). Since other potential mechanisms fail to explain the observed genotypic differences in root efficiency it is concluded that they were most likely caused by differences in root hair density and length.
Genotypic differences in root size and hairiness may have been the result of environmental conditions at the respective geographic origins. ICGV 87358 has been developed in the semi-arid tropics where unreliable rainfall patterns are likely to be the main yield-limiting factor. A large root system is a good strategy to overcome this limitation and at the same time maintains sufficient P uptake. The root-efficient genotype Wasedairyu on the other hand, was developed in Japan where rainfall is abundant but the typical soils used for peanut cultivation (volcanic ash soils, Andosol) are characterized by a high P-fixing capacity. In the pot experiment in this study both strategies resulted in similar P uptake. To increase the P-deficiency tolerance of peanuts further it would be desirable to combine both strategies in a single genotype. Additional studies on the genetics of hair formation may be necessary to provide peanut breeders with the essential information to initiate a breeding programme aimed at developing cultivars with a large root system that produce root hairs in high density.
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1 To whom correspondence should be addressed. Fax: +81 298 388315. E-mail: wissuwa{at}yahoo.de
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References |
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