Plant and Soil 248: 247–256, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Localized supply of phosphorus induces root morphological andarchitectural changes of rice in split and stratified soil cultures

Yong He, Hong Liao & Xiaolong Yan1

Laboratory of Plant Nutritional Genetics and Root Biology Center, South China Agricultural University,Guangzhou 510642, China. 1Corresponding author∗

Received 31 December 2001. Accepted in revised form 17 April 2002

Key words: localized phosphorus supply, rice, root architecture, root morphology

Abstract

A localized supply of phosphorus may affect root morphology and architecture, and thereby affect phosphorusuptake by rice plants. In the present study, we attempted to test this hypothesis using two rice cultivars representingupland and lowland ecotypes grown in specially designed split and stratified soil cultures with a low-phosphorusred soil. Our data indicate that a localized supply of phosphorus increased both total root length and root fineness,particularly in the high-phosphorus zone. In split culture, plants roots tended to preferentially grow on the high-phosphorus zone, with about 70–75% of the total root length allocated to the high-phosphorus compartment. Thetotal root length on the high-phosphorus side in the split-phosphorus treatment was significantly longer than thatin the homogenously high-phosphorus treatment, implying that a phosphorus-deficiency signal from the low-phosphorus side may stimulate the growth of the roots located in the high-phosphorus zone. In stratified soilculture, changes in root morphology and architecture were also observed as indicated by increased total root length,root fineness and relative root allocation in the high-phosphorus layers, again suggesting altered root morphologyand preferential root proliferation in the high-phosphorus regions. The induced changes in root morphology andarchitecture by localized phosphorus supply may have both physiological significance and practical implicationsin that plants can meet the demand for phosphorus with parts of the roots reaching the high-phosphorus zone,hence localized fertilization methods such as side dressing or banded application of phosphorus fertilizers mayboth minimize phosphorus fixation by the soil and increase phosphorus uptake efficiency from the fertilizers.

Introduction

Rice (Oryza sativa L.) is the main food for nearlyhalf of the world’s population, and it plays an import-ant role in human nutrition for people in developingcountries, especially in China, which accounts for22% of world’s rice cultivated area and 36% world’srice grain yield (IRRI, 2000). Traditionally, rice ismostly grown in flooding paddy fields where irrigationis available. As water resources are becoming moreand more limiting, upland rice provides an alternat-ive for rice cultivation in areas with water scarcity(Jin and Ouyang, 1999). However, as compared with

∗ FAX No: 86-20-85281829E-mail: xlyan@scau.edu.cn

traditional flooding cultivation, upland rice is morestrongly restricted by deficiency of nutrients, partic-ularly phosphorus, due to changes in physical andchemical properties of the soil (Sample et al., 1980).

Phosphorus is an essential nutrient for rice growth,but phosphorus is usually deficient in tropical soils,particularly in upland soils where phosphorus is com-monly bound to iron and/or aluminum oxides throughchemical precipitation or physical adsorption. In orderto minimize phosphorus fixation in the soil, localizedapplication methods (such as banded or side dressing)of phosphorus fertilizers are commonly recommendedin practice (Tisdale and Nelson, 1975). However, wedo not know if such application methods would affectphosphorus uptake and plant growth through inducingchanges in root growth and development.

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Previous studies have shown that plant roots havegreat plasticity in response to phosphorus status(Snapp et al., 1995). For example, phosphorus defi-ciency in the soil could induce various morphologicalchanges in plant roots, including formation of roothairs (Bates and Lynch, 1996; Föhse and Jungk,1983; Gahoonia and Nielsen, 1998) and cluster roots(Barum, 1995; Dinkelaker and Romheld, 1989; John-son et al., 1994), changes in root length and diameter(Föhse et al., 1991), and formation and growth of sec-ondary lateral roots (Drew and Saker, 1978; Sun andZhang, 2000). Adaptive changes in root architecture inresponse to phosphorus deficiency were also observed,including changes in basal root growth angle (Bonseret al., 1996; Liao et al., 2001) and relative distributionof roots in the topsoil (Liao et al., 2001). Neverthe-less, little information is available if localized supplyof phosphorus fertilizers would induce changes in rootmorphology and architecture and subsequently affectphosphorus uptake by rice.

In the present study, two rice cultivars representingupland and lowland ecotypes were grown in both splitand stratified soil cultures that resemble those uplandsoils with localized supply of phosphorus fertilizers insuch field practices as side dressing and banded ap-plications (Tisdale and Nelson, 1975). The objectiveof this study was to determine the possible effects oflocalized supply of phosphorus on root morphologyand architecture and subsequently phosphorus uptakeof rice grown under upland conditions.

Materials and methods

Plant and soil materials

Seeds of two varieties of rice (Oryza sativa L.), Azu-cena and IR1552, were provided by Dr. Ping Wufrom the College of Life Science, Zhejiang University,China. These two varieties represent different ecotyp-ical origins: Azucena is a tropical upland rice cultivarand IR1552 is a lowland rice cultivar. The soil wasa Ustisol collected from the Experimental Station atthe South China Agricultural University, Guangzhou,China (Latitude: 26◦ 06′ N, Longitude: 113◦ 15′ E).The available phosphorus content of the soil was 0.806mg/kg by Bray II extraction. The soil was mixed withcleaned quartz sand in a ratio of 2:1 to improve soiltexture and aeration.

Figure 1. Schematic representation of the split-root soil culture sys-tem. (a) the growth box with two separated compartments; (b) thethree phosphorus treatments in the split-root experiment: L-L, nophosphorus added in either side; L-H, one side without phosphorusaddition and the other side with phosphorus added as KH2PO4 at136.1 mg/kg soil; H-H, phosphorus was added as KH2PO4 at 136.1mg/kg soil in both sides.

Split-root soil culture

The set up of the split soil culture system isshown in Figure 1. Growth boxes were made oftransparent plexiglass at 30 cm×20 cm×30 cm(length×wide×height). The box was evenly dividedinto two independent compartments separated by a3-mm-thick plexiglass partition board with a sharpedge on the top for convenient splitting of roots. Thetwo boards on both sides were removable for finalsampling of roots. During rice growth, the boxes werecovered with opaque white board on the sides.

The split experiment included three treatments,which were low phosphorus-low phosphorus (nophosphorus added in either side, designated asL-L), low phosphorus-high phosphorus (one sidewithout phosphorus addition and the other sidewith phosphorus added as KH2PO4 at 136.1 mg/kgsoil, designated as L-H), and high phosphorus-high phosphorus (phosphorus was added as KH2PO4at 136.1 mg/kg soil in both sides, designated asH-H). The other nutrients were added as (mg/kgsoil): 236.2 Ca(NO3)2.4H2O, 34.3 NH4NO3, 15.2

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Fe-EDTA(Na), 0.64 (NH4)6Mo7O24.4H2O, 410.9MgSO4.7H2O, 1.20 H3BO3, 0.04 g ZnSO4.7H2O,0.004 CuSO4.5H2O and 1.54 MnSO4.H2O. In all low-phosphorus treatments, KH2PO4 was replaced by 89.4K2SO4 (g/kg soil).

Rice seeds were surface sterilized with 10%NaClO for 2 min, then geminated in quartz sand at30 ◦C for 5 days before transplanting. Rice seedlingswere transplanted with adventitious roots equally sep-arated into both sides of the growth boxes. Two riceseedlings were planted in each growth box. The plantswere grown in a greenhouse at the South China Agri-cultural University, Guangzhou, China, in completelyrandomized block design with three replicates. Plantswere irrigated with distill water to maintain the soilwater content at about 70% field capacity (previouslycalibrated for this soil as the best water content forboth rice varieties) by regular weighing of the soil box.

At 40 days after transplanting, plant shoots wereharvested and dry weights were taken after oven-drying for 3 days at 60 ◦C. Phosphorus was analyzedcolorimeterically as described by Murphy and Riley(1963). Meanwhile, plant roots were harvested fromboth sides of the growth box after removing the sideboards. Roots were carefully recovered from the soiland rinsed with water for root measurements. All theroots (not subsamples) from each layer were spreadout in a transparent plexiglass tray with a thin layerof water. Root morphological and architectural para-meters, such as root length, and average root diameterwere quantified with a computer image analysis soft-ware (WinRhizo Pro, Régent Instruments, Québec,Canada) after acquiring the root images from a scan-ner with a blue board as background. Total rootlength, root length per diameter class (an index toindicate fineness of the roots) and relative root alloca-tion (the percentage of root length over the total rootlength) in each compartment were calculated as rootmorphological and architectural parameters.

Stratified soil culture

The growth boxes for the stratified soil culture weresimilar to those of the split culture system except thatno partition board was installed in the middle (Figure2a). The soil profile was stratified into three layerswith fiberglass meshes (2 mm) installed at 10 cmintervals. Each layer was treated either with low phos-phorus (no phosphorus added) or high phosphorus(phosphorus added as KH2PO4 at 136.1 mg/kg soil).In this experiment, there were five treatments with

Figure 2. Schematic representation of the stratified soil culture. (a)growth box with three layers; (b) the five phosphorus treatmentsalong soil profile: L-L-L, with homogenously low phosphorus (nophosphorus addition) along the soil profile; L-L-H, with high phos-phorus in the bottom layer and low phosphorus in the top and middlelayers; L-H-L, with high phosphorus in the middle layer and lowphosphorus in the top and bottom layers; H-L-L, with high phos-phorus (phosphorus added as KH2PO4 at 136.1 mg/kg soil) in thetop layer (0–10 cm) and low phosphorus in the middle (10–20 cm)and bottom (20–30 cm) layers; and H-H-H, with homogenouslyhigh phosphorus along the soil profile.

different combinations of phosphorus levels in thethree layers (from top to bottom): (1) low phosphorus-low phosphorus-low phosphorus (designated as L-L-L, with homogenously low phosphorus along thesoil profile); (2) low phosphorus-low phosphorus-highphosphorus (designated as L-L-H, with high phos-phorus in the bottom layer and low phosphorus inthe top and middle layers); (3) low phosphorus-highphosphorus-low phosphorus (designated as L-H-L,with high phosphorus in the middle layer and lowphosphorus in the top and bottom layers); (4) highphosphorus-low phosphorus-low phosphorus (desig-nated as H-L-L, with high phosphorus in the top layer(0–10 cm) and low phosphorus in the middle (10–20 cm) and bottom (20–30 cm) layers); and (5) highphosphorus-high phosphorus-high phosphorus (desig-nated as H-H-H, with homogenously high phosphorusalong the soil profile) (Figure 2b).

Rice seeds were germinated and plant seed-lings were transplanted into the soil boxes with theabove-mentioned treatments. Two rice seedlings wereplanted in each growth box. Nutrient and water man-agements were the same as those previously described

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for the split experiment, with the exception that waterwas irrigated from below with a glass tube inserted tothe bottom of the box to avoid P movement among dif-ferent layers causing by watering. The experiment wasdone in the same greenhouse with three replicates asdescribed above. At 40 days after transplanting, plantshoots were harvested and dry weight and phosphoruscontent were taken as described above. Meanwhile,the roots from each stratified layer were carefully re-covered from the soil and roots were scanned for mor-phological and architectural parameters as previouslydescribed for the split experiment.

Data analysis

The data of shoot biomass, phosphorus content andall the root parameters were analyzed by two-wayANOVA (Excel 97, Microsoft Corporation, 1985–1997). The data of root length per diameter class wereanalyzed by three-way ANOVA with SAS program(SAS Institute, Cary, NC). The univariate F statisticspresented here were all in agreement with multivari-ate F statistics (Wilk’s Lambda, Pillai Trace, andHotelling-Lawly Trace).

Results

Split soil culture experiment

Plant growth and phosphorus uptakePhosphorus treatments had great effects on plantgrowth and shoot phosphorus content in the split soilculture experiment. At 40 days after transplanting,the L-H and H-H treatments increased shoot biomassby about 3–4 folds and shoot phosphorus content byabout 8–16 folds for both rice varieties, indicatingsevere phosphorus deficiency in this soil (Figure 3a,b). Interestingly, plant growth and phosphorus con-tent of the L-H treatment did not differ significantlyfrom that of the H-H treatment, suggesting that plantscan grow normally even with supply of phosphorus tohalf of the roots. The two rice varieties from differentecotypic origins did not differ significantly in shootbiomass or phosphorus content in the L-L treatment,although in the H-H treatment, the upland rice vari-ety, Azucena, seemed to have higher shoot biomassand phosphorus uptake than the lowland rice variety,IR1552 (Figure 3).

Figure 3. Shoot biomass and phosphorus content of two rice variet-ies in split-root soil culture. (a) shoot biomass; (b) shoot phosphoruscontent. L-L, no phosphorus added in either side; H-L, one sidewithout phosphorus addition and the other side with phosphorusadded as KH2PO4 at 136.1 mg/kg soil; H-H, phosphorus was addedas KH2PO4 at 136.1 mg/kg soil in both sides. Each bar is the meanof three replicates with standard error.

Total root length

Phosphorus significantly affected total root length ofboth rice varieties in the split soil culture experiment(Table 1). The total root length in both H-H and L-Htreatments more than doubled that in the L-L treat-ment. The total root length on the low-phosphorusside of the L-H treatment was similar to that of theL-L treatment (F=1.73, nonsignificant), but the totalroot length on the high-phosphorus side was signific-antly longer than that of the H-H treatment (F=8.83,P<0.01). Both varieties showed similar response pat-tern, although Azucena (upland cultivar) generally hadhigher root length than IR1552 (lowland cultivar).

Root length per diameter class

The major portion of the root length of the tworice varieties fell into the diameter class smaller than0.5 mm, and phosphorus treatments mainly affectedroot length in this class (Table 2). Root length inthe <0.5 mm diameter class of the low-phosphorustreatment (L-L) was about the same as that on thelow-phosphorus side of the split-phosphorus treat-

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Table 1. Total root length (m/plant) of two rice varieties in different phosphorus treatments of the split root experiment

Genotype L-La L-H b H-Hc F value

Low Pd High Pe Genotype P treatment G×P

IR1552 5.07 (0.42) 5.36 (1.08) 17.36 (3.61) 12.62 (1.56) 9.903 16.053 2.841

Azucena 8.50 (0.84) 12.34 (2.79) 26.54 (4.08) 13.46 (1.97) ∗∗ ∗∗∗ ns

Each value in the table is the mean of three replicates with standard error.∗0.05>P>0.01; ∗∗0.01>P> 0.001; ∗∗∗P<0.001; ns, nonsignifiant at the 0.05 level.aLow phosphorus-low phosphorus (no phosphorus added in either side); b low phosphorus-high phosphorus (one side without phosphorusaddition and the other side with phosphorus added as KH2PO4 at 136.1 mg/kg soil); chigh phosphorus-high phosphorus (phosphorus wasadded as KH2PO4 at 136.1 mg/kg soil in both sides); d root length calculated by multiplying a factor of 2 to have the same soil volume asL-L treatment; eroot length calculated by multiplying a factor of 2 to have the same soil volume as H-H treatment.

Table 2. Root length (m/plant) per diameter class and its relative root allocation (%) of two rice varieties in different phosphorus treatmentsof the split root experiment

P treatment IR1552 Azucena

<0.5 mm 0.5–1.0 mm >1.0 mm <0.5 mm 0.5–1.0 mm >1.0 mm

L-La 4.07±0.36 0.96±0.06 0.04±0.01 6.14±0.46 2.23±0.34 0.14±0.05

L-Hb L Pd 3.84±0.88 1.45±0.24 0.07±0.02 7.36±1.63 3.89±0.98 1.08±0.22

H Pe 13.62±2.95 3.07±0.53 0.67±0.22 20.52±2.83 4.64±1.00 1.38±0.41

H-Hc 9.59±1.20 2.66±0.37 0.37±0.04 10.13±1.36 2.77±0.57 0.55±0.11

Each value in the table is the mean of three replicates with standard error. Numbers in parenthesis are percentage of root length per diameterclass relative to total root length.aLow phosphorus-low phosphorus (no phosphorus added in either side); b low phosphorus-high phosphorus (one side without phosphorusaddition and the other side with phosphorus added as KH2PO4 at 136.1 mg/kg soil); chigh phosphorus-high phosphorus (phosphorus wasadded as KH2PO4 at 136.1 mg/kg soil in both sides); d root length calculated with the same soil volume as L-L treatment; eroot lengthcalculated with the same soil volume as H-H treatment.

ment (L-H ) (F=0.25, nonsignificant), but significantlyless than that on the high-phosphorus side of split-phosphorus treatment, indicating that more fine rootswere found in the high-phosphorus regions. Interest-ingly, the root length with diameter finer than 0.5 mmon the high-phosphorus side of the L-H treatment weresignificant longer that of the H-H treatment (F=10.44,P<0.01), indicating that the enhanced growth of fineroots in the high-phosphorus zone might be inducedby the low-phosphorus stress from the other half ofthe same root system.

Relative root allocation

Phosphorus allocation significantly affected relativeroot allocation in the split system. Relatively speak-ing, about 75% of the root length for IR1552 and70% for Azucena, respectively, were allocated on thehigh-phosphorus side of the L-H treatment, while ap-proximately same amount of roots were distributed onboth sides of the L-L and H-H treatments (Figure 4).

Figure 4. Relative root allocation (percentage of root length in eachside over total root length) in the split-root soil culture. L-L, nophosphorus added in either sides; L-H, one side without phosphorusaddition and the other side with phosphorus added as KH2PO4 at136.1 mg/kg soil; H-H, phosphorus was added as KH2PO4 at 136.1mg/kg soil in both sides. Each bar is the mean of three replicateswith standard error.

Stratified soil culture experiment

Plant growth and phosphorus uptakePhosphorus allocation in different layers along thesoil profile significantly affected plant growth andphosphorus uptake in stratified soil culture. Althoughno significant difference was found for shoot bio-

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Figure 5. Shoot biomass and shoot phosphorus content of two ricevarieties in the stratified soil culture. (a) shoot biomass; (b) phos-phorus content. L-L-L, with homogenously low phosphorus (nophosphorus addition) along the soil profile; L-L-H, with high phos-phorus in the bottom layer and low phosphorus in the top and middlelayers; L-H-L, with high phosphorus in the middle layer and lowphosphorus in the top and bottom layers; H-L-L, with high phos-phorus (phosphorus added as KH2PO4 at 136.1 mg/kg soil) in thetop layer (0–10 cm) and low phosphorus in the middle (10–20 cm)and bottom (20–30 cm) layers; and H-H-H, with homogenouslyhigh phosphorus along the soil profile. Each bar is the mean of threereplicates with standard error.

mass among the H-L-L, L-H-L and H-H-H treatments,shoot biomass of all these treatments was much higherthan that of the L-L-L and L-L-H treatments (Figure5a). Phosphorus content represented almost the samepattern as shoot biomass, namely, phosphorus con-tent was not significantly different among the H-L-L,L-H-L and H-H-H treatments, but much higher thanthat of the L-L-L and L-L-H treatments (Figure 5b).This indicates that phosphorus application in the top-soil (upper and middle layers, or 0–20 cm) had bettereffects on plant growth and phosphorus uptake than inthe subsoil (bottom layer, or below 20 cm).

Total root lengthPhosphorus allocation also significantly affected rootmorphological and architectural parameters in thestratified soil culture experiment. Phosphorus local-ized on the upper layer significantly increased totalroot length of both rice varieties, and no differencein total root length was found among H-L-L, L-H-L

Figure 6. Relative root allocation of two rice varieties in the strat-ified soil culture system. Relative root allocation is the percentageof the root length per layer relative to total root length. Sectionson which bars are divided mean relative root allocation of differentlayers. L-L-L, with homogenously low phosphorus (no phosphorusaddition) along the soil profile; L-L-H, with high phosphorus inthe bottom layer and low phosphorus in the top and middle layers;L-H-L, with high phosphorus in the middle layer and low phos-phorus in the top and bottom layers; H-L-L, with high phosphorus(phosphorus added as KH2PO4 at 136.1 mg/kg soil) in the toplayer (0–10 cm) and low phosphorus in the middle (10–20 cm) andbottom (20–30 cm) layers; and H-H-H, with homogenously highphosphorus along the soil profile. Each bar is the mean of threereplicates with standard error.

and H-H-H treatments (Table 3), suggesting that phos-phorus supply to the topsoil (upper and middle layers)is more effective to induce root growth.

Root length per diameter class

We also found that phosphorus allocation may changeroot fineness. Table 4 shows that the total root lengthwas always higher in the high-phosphorus zone, whichalso had relatively higher percentage of fine roots(diameter class less than 0.5 mm), suggesting thatphosphorus allocation increases root length mainlythrough increasing fine root length in the layer sup-plied with high phosphorus (Table 4). Interestingly,high phosphorus localized in the upper layer (0–10cm) enhanced root length in the middle layer (10–20cm), while high phosphorus localized in the middlelayer (10–20 cm) also enhanced root length in the up-per layer (0–10 cm), but phosphorus localized in thebottom layer (below 20 cm) did not change the rootlength in the middle and upper layers. This again in-dicates that phosphorus located in the topsoil (0–20cm) would better affect root growth than located in thesubsoil.

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Table 3. Total root length (m/plant) of two rice varieties in different phosphorus treatments of the stratified soil experiment

L-L-La L-L-Hb L-H-Lc H-L-Ld H-H-He F value

Genotype P treatment G×P

IR1552 4.69±0.42 8.06±0.13 12.64±2.12 14.79±0.95 16.87±0.78 14.70 10.97 0.90

Azucena 5.80±0.72 16.39±1.11 20.30±1.60 23.99±7.13 21.03±1.31 ∗∗∗ ∗∗∗ ns

Each value in the table is the mean of three replicates with standard error.∗0.05>P>0.01; ∗∗0.01>P>0.001; ∗∗∗P<0.001; ns, nonsignifiant at the 0.05 level.aWith homogenously low phosphorus (no phosphorus addition) along the soil profile; bwith high phosphorus (phosphorus added asKH2PO4 at 136.1 mg/kg soil) in the top layer (0–10 cm) and low phosphorus in the middle (10–20 cm) and bottom (20–30 cm) layers;cwith high phosphorus in the middle layer and low phosphorus in the top and bottom layers; dwith high phosphorus in the bottom layer andlow phosphorus in the top and middle layers; ewith homogenously high phosphorus along the soil profile.

Relative root allocationPhosphorus allocation changed rice root architecture.When phosphorous was homogenously low along thesoil profile, rice roots of both varieties became shal-lower (i.e. greater relative root allocation in 0–10 cmand less roots below 20 cm). The upland variety,Azucena, tended to have deeper roots than lowlandvariety, IR1552. On the other hand, when phosphoruswas supplied to a certain layer, the relative root al-location in that layer was dramatically increased. Forexample, as compared with that in the homogenoushigh-phosphorus treatment (H-H-H), the relative rootallocation in the high-phosphorus layer of the L-H-Ltreatment increased about 2 folds for both IR1552 andAzucena (Figure 6).

Discussion

Our results indicate that phosphorus deficiency is in-deed a major limiting factor for the red soil usedin the present experiment. Without addition of phos-phorus fertilizer, rice plants produced less than 30%of the shoot biomass with phosphorus addition inboth split-root and stratified soil cultures (Figures 3and 4). The growth of both rice varieties represent-ing different ecotypical origins was equally restric-ted by phosphorus deficiency, but the upland vari-ety, Azucena, grew better than the lowland variety,IR1552, under high phosphorus conditions, probablybecause the former in general had deeper root sys-tems and therefore is more adaptive to the uplandconditions provided phosphorus is no longer a limitingfactor. Generally speaking, the upland variety, Azu-cena, was more responsive to localized phosphorussupply than the lowland variety, IR1552, probablybecause the former was evolved under upland condi-tions where phosphorus is often patchy thus developed

more plasticity in root morphological and architecturaladaptation to ununiform phosphorus supply.

Phosphorus availability may affect root morpho-logy and architecture. Previous studies with commonbean indicated that phosphorus stress generally tendsto induce root shallowness while high phosphorus pro-motes root proliferation to the high-phosphorus zone(Liao et al., 2001). The effects of localized phos-phorus supply on root morphology and architecturewere also observed in our study with the stratified soilsystem. Phosphorus-rich layer generally had highertotal root length (Table 3), possibly through increas-ing the growth of fine roots (<0.5 mm in diameter) inthe layer supplied with high phosphorus (Table 4). Infact, visual observation of the root samples recoveredfrom the soil show that localized phosphorus supply(such as the L-H-L treatment) induced formation ofcluster-root-like fine roots (Figure 7). Furthermore,the relative root allocation is always higher in the soillayer with high-phosphorus supply, suggesting prefer-ential root proliferation to the high-phosphorus zone(Figure 6). Our results from the stratified root exper-iment are consistent with previous findings in otherspecies that nonuniform supply of nutrients (nitrogenand phosphorus) would induce preferential root pro-liferation and growth in nutrient-rich zones (Drew andSaker, 1975, 1978; Snapp et al., 1995; Zhang andForde, 1998). In the split soil culture, phosphorusside dressing significantly affected root distributionby preferential allocation of root length on the high-phosphorus side (Table 1). The increased root lengthwas probably attributed to the increased formationof fine roots (diameter class <0.5 mm) (Table 2). Itshould be noted that the total root length on the high-phosphorus side of the L-H treatment was even longerthan that of the H-H treatment. Apparently, phos-phorus deficiency on one side could stimulate rootgrowth on the other side with sufficient phosphorus

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Table 4. Root length (m/plant) per diameter class and its relative root allocation (%) of two rice varieties in the stratified root experiment

IR1552 Azucena

<0.5 mm 0.5–1.0 mm >1.0 mm <0.5 mm 0.5–1.0 mm >1.0 mm

Layer 1 (0–10 cm)

L-L-La 2.26±0.12 0.50±0.07 0.04±0.01 2.04±0.05 0.78±0.09 0.04±0.01

L-L-Hb 1.50±0.16 1.02±0.24 0.08±0.04 1.99±0.64 0.74±0.19 0.44±0.17

L-H-Lc 3.77±0.58 1.81±0.27 0.47±0.19 2.83±0.63 1.53±0.04 0.67±0.25

H-L-Ld 9.19±0.72 2.74±0.28 0.97±0.27 12.41±3.90 3.04±0.85 1.32±0.38

H-H-He 7.93±0.92 2.81±0.25 0.83±0.05 3.92±0.39 1.58±0.09 1.12±0.16

Layer 2 (10–20 cm)

L-L-L 0.96±0.17 0.19±0.07 0.01±0.00 1.17±0.05 0.39±0.04 0.03±0.00

L-L-H 0.53±0.08 0.24±0.06 0.01±0.00 1.07±0.59 0.36±0.16 0.25±0.09

L-H-L 4.36±0.84 1.16±0.06 0.38±0.10 7.53±0.23 2.83±0.10 1.74±0.22

H-L-L 0.95±0.11 0.34±0.05 0.04±0.01 2.52±1.29 1.00±0.42 0.44±0.23

H-H-H 2.87±0.77 0.90±0.08 0.14±0.02 4.42±0.60 1.54±0.38 0.61±0.24

Layer 3 (20–30 cm)

L-L-L 0.53±0.07 0.19±0.06 0.01±0.00 0.96±0.46 0.37±0.16 0.06±0.05

L-L-H 3.76±0.45 0.71±0.02 0.22±0.03 8.08±0.41 2.53±0.27 0.94±0.07

L-H-L 0.42±0.18 0.25±0.12 0.02±0.02 1.71±0.30 1.17±0.22 0.30±0.12

H-L-L 0.46±0.10 0.11±0.03 0.00±0.00 2.19±0.52 0.80±0.12 0.26±0.15

H-H-H 1.08±0.18 0.26±0.11 0.04±0.02 5.22±0.79 2.02±0.36 0.58±0.22

Each value of root length was the mean of three replicates with standard error and numbers in parenthesis are percentage of root length perdiameter class relative to total root length.aWith homogenously low phosphorus (no phosphorus addition) along the soil profile; bwith high phosphorus (phosphorus added as KH2PO4at 136.1 mg/kg soil) in the top layer (0–10 cm) and low phosphorus in the middle (10–20 cm) and bottom (20–30 cm) layers; cwith highphosphorus in the middle layer and low phosphorus in the top and bottom layers; dwith high phosphorus in the bottom layer and lowphosphorus in the top and middle layers; ewith homogenously high phosphorus along the soil profile.

supply. This may imply that the phosphorus deficiencysignal comes from the low-phosphorus side, while en-hanced root growth is realized on the high-phosphorusside where phosphate as a growth substrate is suffi-cient. However, in a hydroponic study with dividedroots of Hakea prostrata to investigate the effectsof external phosphorus supplies on cluster root ini-tiation and growth, Shane et al. (2002) found thatroot phosphorus concentration of root halves suppliedwith low phosphorus was not significantly differentfrom that with high-phosphorus treatments, suggest-ing that the signal for changes in cluster-root growthwas not a local one, but must have been a systemicsignal. Also, by using a similar split-root approach,Burleigh and Harrison (1999) showed that the expres-sion of a phosphorus starvation-inducible gene (Mt4)is repressed in the phosphorus-deficient side, implyingthat phosphorus fertilization on one-half of the rootresults in the systemic down-regulation of Mt4 expres-sion throughout the whole root system. Therefore, the

molecular aspects of rice roots in response to localizedphosphorus supply deserve further investigation.

The induced changes in root morphology and ar-chitecture by localized phosphorus supply may havephysiological significance that phosphorus uptake byplants can be achieved by allocating a portion of theroots to the high-phosphorus zone (Pang et al., 2000).Indeed, almost all localized phosphorus treatments(except the L-L-H treatment where phosphorus waslocated in the bottom layer) had similar amounts ofphosphorus uptake to that of the homogenously high-phosphorus treatment, indicating that plants can meetthe demand for phosphorus with parts of the rootsreaching the high-phosphorus zone (Figures 3 and5). In other words, a partial supply with 1/2 (split-root treatment) and 1/3 (stratified soil treatment) ofthe phosphorus in the homogenously high-phosphorustreatment could achieve similar biomass and phos-phorus uptake (Figures 3 and 5, except the L-L-Htreatment where phosphorus was applied in the bot-tom layer). This might mean that phosphorus uptake

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Figure 7. Representative root images selected from the stratified soil culture experiment with rice cultivar, Azucena. L-L-L, with homogenouslylow phosphorus (no phosphorus addition) along the soil profile; L-H-L, with high phosphorus in the middle layer and low phosphorus in thetop and bottom layers (image was from high phosphorus layer); and H-H-H, with homogenously high phosphorus along the soil profile.

efficiency from the fertilizers is increased. This haspractical implications that side dressing or bandedapplication of phosphorus fertilizers may not onlyminimize phosphorus fixation by the soil but also in-crease phosphorus uptake efficiency from fertilizersas compared with other application methods such asbroadcasting.

Acknowledgements

This research was supported by the National KeyBasic Research Special Funds of China grant(G1999011700), the Guangdong Natural ScienceFoundation and the National Natural Science Found-ation of China grants (39925025/30070441). The au-thors are grateful to Dr. Ping Wu for supply of the riceseeds and helpful discussion. We also thank XiurongWang, Shaoling Zheng, Ruibin Kuang, Lide Wang,Aiqin Cao, Xianglian Deng and Weifeng Zhang fortheir assistance.

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