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Environmental and Experimental Botany 64 (2008) 75–82

Root growth, aerenchyma development, and oxygen transportin rice genotypes subjected to drought and waterlogging

Roel R. Suralta, Akira Yamauchi ∗Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan

Received 23 May 2007; received in revised form 4 November 2007; accepted 24 January 2008

bstract

Soils under field conditions may experience fluctuating soil water regimes ranging from drought to waterlogging. The inability of roots tocclimate to such changes in soil water regimes may result in reduced growth and function thereby, dry matter production. This study comparedhe root and shoot growth, root aerenchyma development, and associated root oxygen transport of aerobic and irrigated lowland rice genotypesrown under well-watered (control), waterlogged, and droughted soil conditions for 30 days. The aerobic genotypes were as tolerant as the irrigatedowland genotypes under waterlogging because of their comparable abilities to enhance aerenchyma that effectively facilitated O2 diffusion tohe roots for maintaining root growth and dry matter production. Under drought, aerobic genotypes were more tolerant than the irrigated lowland

enotypes due to their higher ability to maintain nodal root production, elongation, and branching, thus, less reduction in dry matter production.erenchyma was also formed in droughted roots regardless of genotypes, but was resistant to internal O2 transport under O2 deficiency. The abilityf roots to resist temporal variations in drought and waterlogging stresses might have strong implications for the adaptation of rice growing innvironments with fluctuating soil water regimes.

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eywords: Root growth; Aerenchyma; Root oxygen transport; Waterlogging; D

. Introduction

Roots play important roles by exhibiting various adaptedesponses specific to the prevailing soil moisture stress condi-ions (Yamauchi et al., 1996). For instance, one of the adaptiveesponses of plants to drought conditions is the developmentf deep and extensive root systems (Fukai and Cooper, 1995;erraj et al., 2004), which include thick roots (Price et al., 2000)nd increased root length density (Siopongco et al., 2005) as aesult of the plasticity in lateral root development (Azhiri-Sigarit al., 2000; Banoc et al., 2000; Kamoshita et al., 2000). Thesedaptations are perceived to be associated with increased waterxtractions (Kamoshita et al., 2000, 2004; Kato et al., 2007;iopongco et al., 2005, 2006), increased nutrient uptake, escaperom root diseases and be competitive to weeds (Richards, 2008).ice roots are also known to produce signals in response to pro-

ressive drought, which regulates leaf stomatal conductance,ranspiration and shoot growth (Siopongco et al., 2008). It ismportant to understand the fact that many of these studies

∗ Corresponding author. Tel.: +81 52 789 4022; fax: +81 52 789 4022.E-mail address: ayama@agr.nagoya-u.ac.jp (A. Yamauchi).

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ht; Aerobic rice

ound the importance of such developmental and physiologi-al responses under drought particularly in the cycle of dryingnd wetting of soils.

Under waterlogged soil, on the other hand, plant roots accli-ate to soil oxygen (O2) deficiency by developing shallow root

ystems with enhanced aerenchyma that can provide less resis-ant pathway for internal atmospheric O2 diffusion to the rootips (Justin and Armstrong, 1987; Colmer, 2003a). The condi-ion will result in the maintenance of root aerobic respirationor continued energy production of the root, nutrient absorp-ions (Jackson and Armstrong, 1999) and rhizosphere activitiesVartapetian and Jackson, 1997; Wang and Yamauchi, 2006).

Under field conditions, however, it is unusual for soil mois-ure condition to remain constant throughout a cropping season;he soil normally experience fluctuating soil water regimes dueo the intermittent nature of rainfall patterns and irrigation sys-ems. Consequently, the soils are exposed to frequent episodesf alternate dry and wet conditions to various degrees. These areommon in environments especially in some tropical savannas

nd temperate sub-humid grasslands (Sarmiento, 1984; Soriano,992), rainfed lowlands (Zeigler and Puckridge, 1995; Wade etl., 1998), floodprone (Khush, 1997), and intermittently irri-ated rice fields (Lu et al., 2000). The desirable root traits for

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6 R.R. Suralta, A. Yamauchi / Environment

daptation to these fluctuating environments may be differentrom those under progressive drought or constant waterlogginglone. The quickness of response of root traits to these fluctu-ting soil water regimes was proposed as one of the importanthysiological traits for adaptation (Ingram et al., 1994).

Drought stress may be perceived as more severe to plantshen their roots previously experienced waterlogging stress.imilarly, waterlogging stress may also be perceived as severe

o plants when their roots were previously drought-stressed. Fornstance, deep and extensive roots, developed under drought,hould immediately enhance its aerenchyma to facilitate O2 dif-usion to the roots when confronted with sudden waterlogging.n the other hand, shallow roots with enhanced aerenchyma,eveloped under waterlogged condition, should rapidly expandnder progressive soil moisture deficit enough to meet the tran-piration demand for water. Hence, roots have to acquire bothxygen (O2) and water under such adverse conditions. Thenability to acclimate to such changes in soil water regimess assumed to affect root growth and functions, thus, biomassroduction.

Aerobic rice genotypes are new classes of upland adaptedenotypes with improved lodging resistance, harvest index,nput responsiveness and tolerance to occasional floodingBouman, 2001). These genotypes have characteristics with theombination of the drought-resistant characteristics of uplandarieties with the high-yielding characteristics of lowland vari-ties (Lafitte et al., 2002). Thus, they have a better yielderformance under both favorable and drought-stressed uplandonditions, outperforming traditional and improved upland, andigh-yielding irrigated lowland genotypes because of their abil-ty to retain both high biomass production and harvest indexhen grown under aerobic conditions (Atlin et al., 2006). The

erobic conditions are defined as a free draining, non-flooded,nd non-puddled soil with water content that is always below sat-ration (George et al., 2002). Aerobic conditions can be rainfedr irrigated. Belder et al. (2005) reported that aerobic rice fieldsith supplementary irrigations usually had soil water potential

hat fluctuates between 0 and −35 kPa.We hypothesized that the difference in potential biomass pro-

uction between aerobic and irrigated lowland genotypes undeructuating soil water regimes are partially due to their basicifferences in the ability of roots to adapt to either drought oraterlogging or both. Hence, in this study, we compared the

oot growth and aerenchyma development between two aerobicnd two irrigated lowland rice genotypes under soil moistureonditions limited only to constant drought and waterloggedonditions. The contribution of aerenchyma in roots grownnder different soil moisture conditions was also examined tossess its ability for atmospheric O2 transport in the root underow O2 deficiency (hypoxia).

. Materials and methods

.1. Plant cultivation and soil moisture treatments

Four rice genotypes were used in two simultaneous exper-ments. UPLRi7 and NSICRc9 (herein referred to as aerobic1

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Experimental Botany 64 (2008) 75–82

nd aerobic2, respectively) were identified as high-yielding riceenotypes (Cruz and Atlin, 2003) under aerobic condition. Aer-bic2 (an ‘Apo’ cultivar released in the Philippines) was alsodentified as high-yielding genotypes based on its consistentood performance under aerobic conditions and its respon-iveness to nutrients (George et al., 2002). Those two indicaerobic rice genotypes were identified to possess a high abil-ty to retain both high dry matter production and harvest indexnder both favorable and stressful upland conditions (Atlint al., 2006). PSBRc82 (herein referred to as lowland1) is aigh-yielding irrigated lowland genotype released in the Philip-ines. IR73888-1-2-7 (herein referred to as lowland2) is anrrigated lowland IR64-type genotype with resistance to ricepherical tungro virus (RSTV) inherited from wild speciesryza rufipogon (IRRI, 2000). Its recurrent parent, IR64, is aigh-yielding irrigated lowland genotype with excellent eatinguality, which makes it popular among farmers in many Asianountries.

The seeds were soaked in water and incubated in seederminator maintained at 28 ◦C for 24 h prior to sowing. Pre-erminated seed from each genotype was individually grown inoxes (25 cm × 2 cm × 40 cm, L × W × H) filled with air-driedlluvial loamy sand according to the method of Kono et al.1987a). The soil was pre-mixed with fertilizer containing 60 mgitrogen (N), 80 mg phosphorus (P), and 70 mg potassium (K).he plants were grown for 30 days.

In the waterlogged treatment, water level was maintained atcm above the soil surface throughout the experimental period.

n well-watered (control) treatment, the soil was first submergedn the water for 24 h followed by draining to maintain 24% soil

oisture content, SMC. The 24% SMC is close to field capacityf the loamy sand (26%), which therefore is favorable for plantrowth. In droughted treatment, the SMC was maintained at6% SMC for the first 2 weeks and then was allowed to reducend maintained at 8% for the final 2 weeks. The 8% SMC is theritical soil moisture that most of legumes and cereals we haveested so far start to show sign of wilting especially in the latefternoon but recover in the following morning. Furthermore,e set the 16% SMC as an intermediate between 24 and 8%MC, prior to further reduction to 8%.

We had carefully established the relationship between the soiloisture content and soil water potential in loamy sand we used

efore starting the experiment. Based on this relationship, the4% SMC had an equivalent soil potential at −0.006 MPa, 16%MC at −0.04 MPa, and 8% SMC at −0.28 MPa.

Watering was done every 2 days for all the treatments to sethe target SMC. The reduced amount of water was added to

aintain the target soil moisture. Based on our previous stud-es, the soil moisture conditions at the same depth (horizontalirection) of the rootbox were almost uniform whereas it dif-ered at vertical direction (Kono et al., 1987a,b). The topmostortion of the soil had 2% lower soil moisture content than theowermost one 24 h after submergence followed by draining.

ithholding watering for another 7 days, the difference in soiloisture increased up to 5.4% (Kono et al., 1987a). For lower

oil moisture conditions, the difference in soil moisture betweenhe topmost and lowermost portion when the soil was allowed

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o reach to 15 and 12% SMC after 7 and 14 days without water-ng, respectively, was only 1.3 and 0.3%, respectively (Kono etl., 1987b). Since the interval of our watering was 2 days in theresent experiment, such differences along vertical directions ofhe root box must have been much smaller.

The actual observed soil moisture content did not alwaysatch with the intended percentage given as treatment names.he range of SMC for given treatment were as follows: 24%MC, 22.2–24.0% range (−0.009 to −0.006 MPa), 16% SMC,4.8–16.0% range (−0.056 to −0.04 MPa), and 8% SMC,.8–8.0% range (−0.28 to −0.37 MPa).

A pot experiment with treatments and growth duration similaro box experiment was conducted to evaluate the contributionf root aerenchyma development for root O2 transport underypoxia. A single plant was grown in a small pot filled with.72 kg of alluvial loamy sand soil pre-mixed with 41 mg N,4 mg P, and 48 mg K. Three plants were used from each treat-ent for root O2 consumption and root porosity measurements.

.2. Shoot and root growth

The shoots were cut and the number of tillers was countedrior to oven drying and weighing. The root system was sam-led using a pinboard (Kono et al., 1987a). Thereafter, rootsere stored in FAA (formalin:acetic acid:70% ethanol = 1:1:18/v) solution for further measurements. The length of semi-al roots was measured using a ruler and the number of nodaloots was manually counted. For root length, each root sam-le in FAA was rinsed with water and spread on a transparentheet without overlap. The digitized images were taken usingscanner with a resolution of 300 dpi and an output format of56 grey scales. The total root length was determined using aacro-program developed by Kimura et al. (1999) and Kimura

nd Yamasaki (2001) on the NIH image software version 1.60public domain released by the National Institute of Health,SA).

.3. Root porosity

The amount of aerenchyma was estimated using the relativeolume of internal gas space (root porosity). The root porosityay not be identical with aerenchyma because rice roots grown

ven under well-drained conditions have porosity of 10–12%W. Armstrong, personal communication). This means that thestimates of aerenchyma based on porosity may possibly overes-imate the amount of aerenchyma. However, there is no effective

ethod to distinguish this aerenchyma from total porosity, andhus we used total porosity as an estimate for aerenchyma devel-pment in this study.

All nodal roots from each plant were collected for root poros-ty measurement following the microbalance method of Vissernd Bogenmann (2003). Briefly, the length of each nodal rootxis was divided into three major portions such as basal, middle,

nd apical portion. For the basal and apical root portions, 1 cmoot segment was collected at about 1 cm apart from the basal andpical end of the root axis, respectively. For the middle portion, acm segment was collected at the middle portion of the root axis.

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Experimental Botany 64 (2008) 75–82 77

he collected segments from each root portion were then pooled.he root segments were cut with a razor blade and gently blottedy rolling it with a small brush on tissue paper for about 2 s toemove adherent water. To prevent weight loss by evaporation,he segments were then transferred into a capsule with cover thatad been tared on a microbalance. After closing the capsule, theeight of the segments was measured (w1 in �g), transferred toholder with small vials filled with water, and stored for max-

mally 30 min. In this way, 36 samples were weighed beforehe samples were infiltrated with tap water under vacuum twiceor 30 min each. After water infiltration, the root segments werelotted again on tissue paper and weighed in a capsule (w2 ing). Using the specific weight (SW) obtained from larger sam-les (1.04 g ml−1) (Visser and Bogenmann, 2003), the porosityas calculated using the formula:

orosity (%; v/v) = 100(w2 − w1)SW/w2

ith the specific weight of water being 1.00 g ml−1.

.4. Root oxygen consumptions

The removal of shoots by scissors and sealing the cut por-ion prevent the entry of atmospheric oxygen into the plants’erenchyma systems (Teal and Kanwisher, 1966), allowingerenchyma function to be investigated; therefore, the dif-erence between these two flux rates equals the amount ofxygen transported internally through the plant (Lee, 2003;aricle and Lee, 2007). Intact plants with shoots and rootsere sampled from the pots and washed to remove soil andther dirt. Root O2 consumptions were determined by seal-ng an intact plant roots into a flask of nitrogen (N2)-flushedater with initial O2 concentration, of 16.6 �mol, within theask measured using dissolved O2 (DO) meter. After 2 h, theubsequent decrease in flask O2 concentration was recordedo assess O2 consumption by the roots. Background rates of

2 flux were also determined in flask with N2-flushed waterithout plant to adjust O2 measurements. There was a sig-ificant change in O2 concentration in the blank flask; hence,his was used to correct O2 consumption readings of flaskith suspended roots. Shoots were then removed by cutting

t the stem base, and the cut surface was immediately sealedith thick parafilm. O2 consumption from the flask was theneasured again after 2 h to determine the changes in root O2

onsumption. Differences of root O2 consumption betweenlants with and without shoot indicate the contribution oferenchyma to atmospheric O2 transport to the roots under O2eficiency.

.5. Statistical analyses

Shoot dry weight, number of nodal roots and tillers, and sem-nal and total root lengths were analyzed using two-way analysisf variance (ANOVA) to determine the main effects and inter-

ctions between genotype and soil moisture treatments on theariables used. Root O2 consumptions were analyzed in one-wayNOVA to determine the contribution of aerenchyma in facil-

tating atmospheric O2 diffusion to the roots when subjected

78 R.R. Suralta, A. Yamauchi / Environmental and Experimental Botany 64 (2008) 75–82

Fig. 1. Shoot dry weight of four rice genotypes grown under well-watered,waterlogged, and droughted soil conditions. Data shown are means ± S.D. oft1

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Table 1Effect of soil moisture treatments on the number of nodal roots and tillers, andseminal root length of different rice genotypes

Genotypes/growingconditions

Nodal roots(no. plant−1)

Tillers(no. plant−1)

Seminal rootlength (cm)

Aerobic1Well-watered 27.7 2.7 28.7Waterlogged 51.3* 3.7 ns 38.3 nsDroughted 11.3 ns 2.0 ns 25.5 ns

Aerobic2Well-watered 22.0 3.3 32.0Waterlogged 46.7* 2.7 ns 36.5 nsDroughted 22.3 ns 2.7 ns 34.2 ns

Lowland1Well-watered 65.0 3.3 35.2Waterlogged 112.3** 2.7 ns 30.0 nsDroughted 19.0** 2.3 ns 28.8 ns

Lowland2Well-watered 35.7 3.7 37.1Waterlogged 34.7 ns 2.3* 25.8 ns

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hree replicates. *,**Significantly different from well-watered (control) at 5 and% level LSD, respectively.

o low O2 deficiency (hypoxic) conditions. Means of shoot dryeight, number of tillers and nodal roots, and total root lengthere compared using the least significant difference (LSD) test

t the P = 0.05 level between the well-watered (control) and thewo stress treatments (waterlogging and drought) within eachenotype.

. Results

.1. Shoot and root growth under waterlogged androughted conditions

The response of shoot dry weight to different soil moisturereatments significantly differed among genotypes (Fig. 1). Rela-ive to plants in well-watered (control) conditions, waterloggingid not significantly affect shoot dry matter productions. Underroughted conditions, all genotypes had a tendency to decreasehoot dry matter with lowland1 and lowland2 showing signifi-ant 70–79% reductions.

For the nodal root productions, significant interactionsxisted between genotypes and soil moisture treatments. Water-ogging significantly increased the nodal root productions by5% in aerobic1, 54% in aerobic2 and 42% in lowland1, but didot affect nodal root production in lowland2 (Table 1). Droughttress significantly reduced nodal root productions by 70% inSBRc82 and 64% in lowland2, but did not affect nodal rootroductions in aerobic1 and aerobic2.

Treatment effects on tiller production are shown in Table 1.aterlogging significantly decreased the number of tillers in

owland2 by 37%, but it did not affect those of aerobic1, aerobic2nd lowland1. Drought also significantly reduced the number ofillers in lowland2 by 64%, but it did not affect those of aerobic1,erobic2 and lowland1.

There was a significant interaction between genotypes andoil moisture treatments on seminal root elongation (Table 1).

aterlogging did not significantly affect the seminal root lengths

f any of the genotypes. On the other hand, drought significantlyeduced the seminal root length of lowland2 by 65%, but it didot affect those of aerobic1, aerobic2 and lowland1.

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Droughted 12.7* 1.3* 10.7**

s: not significant. *,**Significant at 5 and 1% level LSD, respectively.

There was a significant interaction between genotypes andoil moisture treatments on total root length (Fig. 2). Waterlog-ing significantly increased total root length by 56% in aerobic1,ut did not affect those of aerobic2, lowland1, and lowland2.n the other hand, drought significantly decreased the total

oot length by 66% in aerobic1, 86% in lowland1, and 92%n lowland2, but it did not affect those of aerobic2.

.2. Root porosity under waterlogged and droughtedonditions

Waterlogging significantly increased the proportion of nodaloots with higher root porosity (Fig. 3). Aerobic1 and aerobic2esponded sharply to waterlogging by increasing the proportionf nodal roots with enhanced aerenchyma notably between 10

ig. 2. Total root length of four rice genotypes grown under well-watered, water-ogged, and droughted soil conditions. Data shown are means ± S.D. of threeeplicates. **Significantly different from well-watered (control) at 1% levelSD.

R.R. Suralta, A. Yamauchi / Environmental and Experimental Botany 64 (2008) 75–82 79

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ig. 3. Histogram showing the distribution of nodal roots at different root porosonditions. Data shown are means ± S.D. of three replicates.

roportion of nodal roots in any of the root porosity ranges underaterlogged conditions.On average, waterlogging increased root porosity by 28%

n aerobic1, 23% in aerobic2, 18% in lowland1, and 70%n lowland2 (Fig. 3). Aerenchyma also formed in drought-

tressed roots in all genotypes, but values were not significantlyifferent (Fig. 3). Furthermore, the average porosity of therought-stressed roots of each genotype was lower comparedith those of their well-watered counterparts (Fig. 3). The ratio

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ig. 4. Root oxygen consumption rates under low O2 deficiency (hypoxia) of fouronditions.At the end of the treatment, plants were extracted from the soil and transfonsumption with and without shoots. Data shown are means ± S.D. of three replicat

nges of rice plants grown under well-watered, waterlogged, and droughted soil

f aerenchyma formed in drought-stressed roots was 50–60%ower than the constitutive aerenchyma formed in well-wateredoots.

.3. Root O2 consumptions from the hypoxic solution

The removal of shoots from intact plants led to significantlyigher O2 consumptions from hypoxic medium of waterloggedoots of all genotypes (Fig. 4). With the exception of aerobic1,

rice genotypes grown under well-watered, waterlogged, and droughted soilerred to low O2 hydroponics medium for differential measurements of root O2

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Table 2Effect of soil moisture treatments on the contribution of aerenchyma to atmo-spheric O2 transport to the roots under low O2 deficiency conditions

Genotypes Soil moisture conditions

Well-watereda Waterlogged Droughted

Aerobic1 16.7 c 56.5 d −1.7 aAerobic2 39.4 b 95.8 a −3.5 aLowland1 46.3 a 96.8 b −4.5 aLowland2 48.9 a 78.6 c −4.0 a

The increases or decreases in respiratory oxygen consumption are expressed aspercentages of the total respiration measured with the roots excised. In a column,means followed by the same letters are not significantly different at 5% levelLSD.

0 R.R. Suralta, A. Yamauchi / Environment

he removal of shoots in well-watered plants of all genotypeslso showed significant increase in root O2 consumptions. Fur-hermore, drought-stressed roots regardless of genotypes did nothow significant changes in their root O2 consumptions afterhoot removal.

. Discussion

Results showed that the two aerobic genotypes had higherbility than the irrigated lowland genotypes in maintaining rootlongation under drought. Additionally, the genotypes have nor-al ability to adjust aerenchyma that can effectively facilitate2 diffusions to roots under waterlogged conditions. Thus, the

erobic genotypes were found to have higher stability in dryatter production than the lowland genotypes when both grown

nder drought-stressed or waterlogged soil conditions.Aerobic genotypes generally showed less reduction in shoot

ry matter than irrigated lowland genotypes by drought-stressedonditions. This indicates higher drought tolerance to limitedoil moisture. Such differences in the ability to produce shootry matter under drought between aerobic and irrigated lowlandenotypes were partially explained by their notable contrastsn maintaining nodal root production (Table 1) and total rootength (Fig. 2), which were significantly higher in aerobic geno-ypes, particularly, aerobic2. Hence, these facts suggest that thebility of the plant to maintain greater root length as a functionf maintained nodal root production and promoted lateral rootrowth (Banoc et al., 2000) can compensate for the lower watervailability in soil by extending the absorbing surface area ofhe root.

It has also been proven that the aerobic genotypes were ableo produce almost comparable dry matter with irrigated lowlandenotypes under the waterlogged conditions (Fig. 1). This maye partially attributed to their abilities to maintain tiller produc-ions (Table 1) and other root traits such as nodal root production,eminal root elongation (Table 1), and the entire root systemevelopment as expressed in total root lengths (Fig. 2) underaterlogging (O2 deficiency), which were almost equivalent to

he abilities of irrigated lowland genotypes. The fact that uplandice genotypes showed almost the same waterlogging toleranceith irrigated lowland genotypes reflects that episodes of water-

ogging are also common in many upland soils used to cultivateice, hence, the traits associated with waterlogging tolerance areetained in upland rice (Colmer, 2003b).

Enhanced aerenchyma formation is one of the most com-on adaptive responses of plants to soil hypoxia and anoxia

Vartapetian and Jackson, 1997; Jackson and Armstrong, 1999;olmer, 2003b; Wang and Yamauchi, 2006). Waterloggingnhanced the aerenchyma in roots of the aerobic genotypesFig. 3) in a similar manner with irrigated lowland genotypes.nhancements of aerenchyma under O2 deficient conditionsre common in rice (Colmer et al., 1998), but key differencesn aerenchyma formations were not confounded with ecotypes

i.e. upland, lowland or deepwater types) (Colmer, 2003b).urthermore, the enhanced aerenchyma in waterlogged rootsFig. 3) effectively facilitated atmospheric O2 diffusion to theoots (Fig. 4). The removal of shoots limited the function of

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Experimental Botany 64 (2008) 75–82

erenchyma for O2 diffusion to the roots and could be used asndicator for its functionality under O2 deficiency. For water-ogged plants, shoot removal led to higher root O2 consumptionrom the hypoxic medium indicating the presence of functionalerenchyma. The pattern of increase in root O2 consumptionsfter shoot removal, however, differed with genotypes (Fig. 4).oot aerenchyma in aerobic2 and lowland1 had higher ability

or facilitating O2 diffusions to the roots than those in aerobic1nd lowland2.

It is interesting to note, however, that the rates of O2 consump-ion of excised roots (plant without shoot) obtained in this studyconverted in fresh weight, FW, basis) were generally lower by5% or more than the 4 �mol g−1 root FW h−1 recently reportedn rice (Maricle and Lee, 2007). The lower values in this studyight have been partially caused by the unstirring of the hypoxicedium used during the measurements. Such unstirring might

ave caused large boundary layer effect that may result in unevenistribution of O2 throughout the root system and thus, likelyeduced the potential rates for root O2 consumptions.

The difference between the total O2 consumption rate of plantithout shoot and the consumption rate of the plant with shootas used to estimate the amount of O2 supplied to the rootsia the aerenchyma (Fig. 4). The estimated contributions oferenchyma for O2 transport in waterlogged roots were morehan 95% for aerobic2 and lowland1 while it was 56% forerobic1 and 79% for aerobic2 (Table 2). These contributionsf aerenchyma in facilitating O2 diffusion to the roots underow O2 deficiency partially explained their abilities to main-ain total root elongation under O2 deficient conditions. Exceptor aerobic1, higher contribution of aerenchyma for root O2ransport under low O2 deficiency (Table 2) was partially corre-ated with maintained root elongations (Fig. 2). Maintained totaloot elongations under waterlogged conditions relative to well-atered conditions (Fig. 2) may influence nutrient explorationnder reduced soil conditions, and may provide a competitive

a O2 consumption of plants without shoot gave a measure of total O2 demandue to respiration of the plant, i.e. consumption not supplemented by theerenchyma supply. The rate of oxygen transport through the plant’s aerenchymaystem was calculated from the difference between O2 concentrations underlants with and without shoots (see Fig. 2 for the values).

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dvantage of deeper waterlogged roots to adapt to drying soilonditions would also likely to depend on the plasticity in nodaloot production and lateral root growth (Banoc et al., 2000).

It was also observed that well-watered and droughted rootsad considerable proportion of nodal roots with aerenchymaFig. 3). The constitutive aerenchyma formed in well-wateredoots of all genotypes, with the exception of aerobic1, can alsoffectively transport atmospheric O2 to the roots when exposedo low O2 deficiency. This constitutive aerenchyma developednder well-watered conditions can account for an initial sup-ly of more than 40% of root O2 requirements under low O2eficiency (Table 2).

In rice, the notable increase in the size of root aerenchymas a plastic response to hypoxia began only at least 4 daysfter exposure (Insalud et al., 2006). Hence, the constitutiveerenchyma could provide initial O2 supply to the roots beforelastic response took place under sudden exposure to water-ogging, which then enabled rice plants to grow under diversenvironment that differ markedly in water availability such asrained upland and waterlogged paddy fields (Colmer, 2003b).owever, such ability to adapt to sudden waterlogging needs toe examined for drought-stressed roots. Our results also showedhat aerenchyma formed in drought-stressed roots (Fig. 3) can-ot transport atmospheric O2 to the roots when exposed to low2 deficiency (Fig. 4). This indicates that due to resistance to O2iffusion, further enhancement of root aerenchyma is needed tounction for effective internal O2 transport under sudden water-ogging.

Scarcity of water for rice production stimulated the devel-pment of production systems that use less water (Boumant al., 2002). Several water-saving systems such as saturatedoil culture, alternate submergence-nonsubmergence and aero-ic rice with supplementary irrigations (alternate wetting andrying) (Bouman et al., 2002) have been recommended. Theseater-saving systems have great potential to save water but withield penalty to various extents. Using aerobic rice, for exam-le, immediate yield reductions of 15–39% were observed onhe first season of growing (Belder et al., 2005; Bouman et al.,005) and yield continued to decline on the succeeding growingeasons with continuous cropping (Peng et al., 2006).

The causes of yield reduction and continuous decline remainnclear. We assume that such yield reduction especially thosebserved at least in the first growing season, may partially bettributed to the occurrence of alternating progressive droughtnd sudden waterlogging that would adversely affect root growthnd functions. In such cases, the results of our study have a strongelevance on the ability of rice plants to resist such temporal vari-tions in soil moisture for better adaptation under water savingystems and under those environments such as rainfed lowlandice (Zeigler and Puckridge, 1995) and submerged prone uplandreas (Khush, 1997).

The ability of waterlogged roots to enhance aerenchymappears to be important not only for adaptation under period of

2 deficiency to maintain root elongations but also under tran-

ient progressive drought conditions. Similarly, the ability ofrought-stressed roots to enhance its aerenchyma and facilitatetmospheric O2 diffusion to the roots under periods of low O2

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Experimental Botany 64 (2008) 75–82 81

eficiency appears to be important under the period of transientaterlogged conditions. The above mentioned root abilities, or

he lack of it, under transient moisture stresses could affect shootry matter productions.

Further studies are needed to determine the responses of aer-bic rice roots to soil moisture fluctuations specifically undererobic conditions with supplementary irrigations (i.e. anaero-ic to aerobic (mild drought) conditions and vice versa) and todentify desirable root traits that strongly contribute to dry mat-er production. Such soil moisture fluctuations are common inerobic conditions with supplementary irrigations.

cknowledgments

The seeds were provided by the International Rice Researchnstitute (IRRI). The authors would like to thank Prof. Williamrmstrong for critically reviewing the manuscript. This workas partially funded by the grant-in-aid from the Japan Society

or the Promotion of Science (No. 13575032 and 19380011).

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