Water-Saving and High-Yielding Irrigation for Lowland Rice by Controlling Limiting Values of Soil Wa

Joumal of Integrative Plant Biology 2007, 49 (10): 1445 -1454Water-Saving and High-Yielding Irrigation for LowlandRice by Controlling Limiting Values ofSoil Water PotentialJianchang Yang", Kai Liu1, Zhiqin Wang', Yong Du1 and Jianhua Zhang2"('Kay Laboralory o1 Crop Genetics and Physiology of Jiangsu Province, Yangzhou Univrsity, Yangzhou 225009. China;2Depertment of Biology, Hong Kong Baptst Universty, Hong Kong, China)AbstractThe present study investigated whether an irigation system could be established to save water and Increase grain yleld toenhance water productivity by proper water management at the fleld level In lrigated lowland rice (Oryza sativa L小Usingtwo fheld-grown rice cultlvars, two irigation systems; conventional irigation and water-savIng irigation, were conducted.In the water-saving imrigation system, limting values of soil water potential related to specific growth stages were proposedas irigation indices. Compared with conventional irigation where drainage was In mid-season and flooded at other times,the water-saving Irigation increased grain yield by 7.4% to 11.3%, reduced irigation water by 24.5% to 29.2%, and increasedwater productivity (grain yield per cubic meter of Irigation water) by 43.1% to 50.3%. The water-saving imrigation significantlyIncreased harvest Index, improved millig and appearance qualities, elevated zeatin + zatlIn rlboside concentrations inroot bleedings and enhanced activities of sucrose synthase, adenosine diphosphate glucose pyrophosphorylase, starchsynthase and starch branching enzyme in grains. Our results indicate that water-saving Irigation by controlling limitingvalues of soll water potential related to specific growth stages can enhance physiological activities of roots and grains,reduce water input, and increase grain yield.Key words: cyokinin; nice; soll water ptential; starch synthase; water proutily watersaving inigation.Yang J, Lu K, Wang z, Du Y, Zhang J (2007). Water-saeving and hg-yieldig imigation for lowiand rhce by contolling lming values of sol waterpotential. J. Integr. Plant Biol. 49(10), 1445- -1454.Available onine at www. backwell synergy. cmlnishoclipb, www jipb.netRice (Oryza sativa L.) is the most impotant staple in Asia,(Hossain 1997; Pingali et al. 1997). About 75% of total riceproviding on average 32% of total calorie intake (Maclean et al.production comes from imigated lowlands (Maclean et al.2002).2002; Belder etal.2004). To keep up with populatingrowthand In Asia, irigated rice accounts for about 50% of the total amountincome-induced demand for food in most Asian counties, riceof water diverted for irigation, which in itself accounts for 80%production must be increased by 56% over the next 30years of the amount of fresh water diverted (Guera et al. 1998). Freshwater, however, is becoming increasingly scarce (Bouman andTuong 2001) because of population growth, increasing urbanand industrial development, and decreasing availability resultingsupported by the National Natural Science Foundaion of China (30671225),form pllution and resource depletion (Bouman and Tuongthe State Key Project (2004-BA520A12-5), the Natural Science Foundetion2001). Decreasing water aiability for agriculture threatensof Jiangsu Province (BK2006069), and Hong Kong Research Grants Counailthe produtivity of the irigated ecosystem and ways must be(Projeat HKBU 2485/05M).sought to save water and increase the water productity of rice"Authors for corespondence.(Guerra et al. 1998; Belder et al. 2004).Te(Fax}: +86 (0)514 7979317;The high water demand of imigated lowland nice mainly arisesEmail: .from keeping the field continuously submerged as a result ofTol: +852 34117350;Fax; +852 3411 5995;high中国煤化工rcolation in the fields(GuemTo reduce water use inE-mait .imigateHC N M H Gate weting and drying◎2007 Instute of Botany, the Chinese Academry of Sciencesor altemate submergence and nonsubmergence have beendoi: 101.112.7207.05555developed (Bouman and Tuong 2001; Belder et al. 2005). It1446 Joumal of Integrative Plant Biology Vol, 49 No. 10 2007has been reported that altemate wetting and drying systemsend-iling stage (Figure 1). The decrease in grain yield undercan maintain or even increase grain yield (Mao 1993; Wusevere water deficits was mainly attributed to the reduction of1999; u 2001). However, experimental evidence is stil scarcelypanidle number at the active tlring and end-ileing stages,reported in intemational published reports (Belder et al. 2004).the reduction in spikelets per panicle at the panicle developmentFurthermore, there are reports that altemate weting and dryingand heading stage, and the reduction in grain weight at the grainsystems often reduce, rather than increase, grain yield whenfling stage (data not shown). Excluding the active tlering stage,compared with continuously submerged conditions (Mishraetal.the yield was the highest at al other growth stages when SWP1990; Yang et al. 1995; Tabbal et al. 2002; Belder et al. 2004).was -15kPa. The two cultivars behaved the same (Figure 1).Obviously, it remains a major challenge to reduce water inputIncrease in grain yild under such a SWP was mainly a result ofwithout compromising yield and to optimize scarce water inan increase in panicle number, more spikelets per panicle andrice production. The objective of the present study was tohigher grain weight, respectively, at the end-illering, at panicleestablish an imigation system to save water and increase graindevelopment and heading, and at grain fling stage (data notyield, therefore enhancing water productity by proper watershown). The results indicate that it is not necessary to keep themanagement at the field level in imigated lowland rice. Thefield continuousty submerged at most growing stages for highphysiological mechanisms involved were also investigated.yields of modem rice cultivars.ResultsEffect of water-saving rrigation on grain yield and qualityBased on our previous work (Yang and Ding 1992), a water-Sensitivity to water deficits at different growth stagessaving imigation system by cortrlling the limiting values of soilwater potential related to specific growth stages was conducted.Grain yield varied with soil water potential (SWP)treatments andCompared with conventional irigation (drainage in mid-seasongrowth stages at which water deficits were imposed (Figure 1).and fooded at other times), the water-saving imigation increasedSevere water deficits (SWP <- 45kPa) reduced the yieldgrain yield by 7.4% to 11.3% (Table 1). Increases in grainsignificanty. The reduction in grain yield was highest at theyield under the water-saving imigation were mainly atributedactive tlring stage, fllowed by panicle development, theto the signifcant increases in spikelets per panicle and grainheading stage, the grain flig stage, and being lowest at theweight. The water-saving imigation also significanty increased11002hendso 8100000 t8007006004005 11000YangdaoYngdsol00AT3FaFTotment stagosTru rtnunt stog畔Year: 2002Year 2003中国煤化工Figure 1. Grain yield of two nice cutvars of Zhendao 88 (japonica) (A, B) and YangdeCNMH(: potentals, respectively.Vertical bars represent士SE of the mean (n= 3) where these exceed the size of the symDol. AT, adive tlerin; ET, end-ilring; GF. grain fling:; PD,panidle development.Water- Saving and HighYielding lmigation in Rice 1447two imigation systemsYear CutivarsImigationPanicle numberSpikeletsRipenedGrain weightGrain yieldGrainHanvestsystemsPer m2per panicle grains (%) (mg per grain)(g/m2)plumpness (%)index2002 Zhendao 88 CI2841189.526.176990.70.47NSI282120*89.127.4**826**94.1*0.53**Yangdao6 Cl2431384.627.076488.90.46WSI244144*85.42B.3**B49**92.6*2003 ZhendaoB8 CI28611489.026.276092.50.49119*27.6**846**96.7*0.54**2411483.827.480290.50.45242150*28.4"*872"93.8*0.51**Grain plumpness was delermined as the weight retio of frilied spikeletes to fll spikeleles (10lzigrain wigh1000-pluoped-gainweighti under specic gavity≥1.10 x 10 Harvest index is the ratio of the total grain welght to total aboveground dry weight. .,P<0.5.**,P <0.01.The comparison was made within a column. the same utivar and the same year. CI, conventonal migation (contro); WsI, watersaving igatin.n2002B20032.5- CtD; 2.01.560.Zhendao8 Yangdno 6Zhondao88 Yangdao 6ClliarsCultivanFIgure 2 Amount of water imigated (A, B) and water podudctvity (C, D) of two nice cutivars of Zhendao 88 (aponica) and Yangdao 6 (ndica) grownin the field under two imigation systems.The water productvity was defined as the grain yield per cubic meter of imigation waler. Vertical bars represent+ SE ofthe mean(n=3)where theseexceed the size of the symbol. CI, conventional irigation; wsl, water-saving imigatlon.grain plumpness and harvest index (Table 1). indicating that thissignificant dfferences in other quality traits, such as gel con-technique enhances grain fling and transport of asimilates tosistency, amylose content and protein content, between thegrains.water-saving and conventional iriatins (Table 2).The water-saving irigation reduced imigation water by 24.5%to 29 2%, and increased water productivity (grain yield per cubicPhys中国煤化工vVng iriationmeter of irigation water, kg/m3) by 43.1% to 50.3% (Figure 2).Chalkiness was significanty reduced, wthereas rates of brown To ulMYHCNMHGnSWPandplantwaterrice, milled rice and head milled nice were very significantystatus, leaf water potentials were detemined at vanious sWPs.increased, under the water- saving imigation. There were noAs expected, midday (12 ,00hours) leaf water polentials were148 Joumnal of Integrative Piant Biology Vol.49 No. 10 20072002 .2003Zhendao 88Yangdao6Quality tats_CWSI_VSIcvsICWSIKarmel length (mm)5.085.136.356.384.926.396.41Lengthidth1.64r622.342.241.621.602.40Kemel weight (mg)21.122.2*22.323.8*21.322.523.6*Chlkiness (%)11.07.92**18.79.51**10.86.90*20.511.1**Brown rice (%)78.284.5*79.182.2*80.685.180.983.2"Milled nice (%)72.075.7*73.776.2*73.271.974.8* .Headnce (%)63.267.9458.461.7*62.865.5059.262.5*Gai consistency (mm)77.441.479.441.8Alkal spreading value6.316.643.733.696.536.743.833.81Amylose content (%)17.67.917.217.120.9Protein content (%)9.899.638.218.339.959.938.318.35The quality taits were measured according to nice qualtly measurement standards (Ministry of Agriculture. PR China 1988). *, P<0.05;", P<0.01. .The compenison was made within a row, the same culivar and the same year. CI, conventionel imigation (contro); WSI, water-saving irigation.reduced with the decrease in SWPs (Table 3). However, thethe index for rice imigation. There are several advantages in thedifferences in leaf water potentials were rather small amonguse of SWP in water-saving imigation. First, SWP can be rapidlythe growth stages when SWP was the same. No significantand instantaneously determined by insaling tension metersdiferences in predawn (06.00hours) leaf water potentials werein the field, and the accuracy and precision in monitoring soilobserved when SWP was ranged from 0 to - -30kPa (data notmoisture can be increased. Second, the avalbility for plantsshown).to use the water in soil is litle affected by soil textures whenThe dfferences in photosynthetic rates between the water-SWP is the same. We observed that, for example, when SWPsaving and conventional irigations were rather small at alofthewas - -20kPa, the soil water contents were 3.5%, 33% andmeasurement times (Figure 3A-D), whereas the water-saving47%, respectively, for soils of sand, loam and clay. Thoughinigation significanty reduced leaf conductance (Figure 3E H),water contents of the three types of soils were rather dferentleading to the increase in water use eficiency (defined as theat such an SWP, water status of nice plants were almost theratio of CO2 fixed in photosynthesis to H20 lostin transpiration)same, with leaf water potentials at 0.86土0.06, 0.87 +0.08 and(Figure 31HL).0.85 t 0.06, respectively, for the plants grown on sand, loam,Except those at the active tlring stage, concentrationsand clay, suggesting that the same a value of SWP shouldof zeatin (Z +zeatin riboside (ZR) in root bleedings at allbe used as an imigation index for dfferent types of soil. Third,other stages were greater under the watersaving irigationphysiological issues can be deeply investigated related t water-than under the conventional imigation (Figure 4). The water-saving imigatin and in the study of the soil plantatmospheresaving irigation significantly increased ativities of sucrosecontinum. .synthase (SuS), adenosine diphosphate glucose pyrophos-In previous paddy field water management of altemate wet-phorylase (AGP). starch synthase (StS) and starch branchingting and drying systems, grain yield was Increased in someenzyme (SBE) in grains during the grain fling period (Table 4).published reports (Mao 1993; Wu 1999; Li 2001) but reduced inothers (Mishra et al.1990; Yang et al.1995; Tabbai et al.2002).The discrepancies between researchers may be atributed toDiscussionmany reasons (Belder et al. 2004). Drying conditions, however,are considered to be very important in the increase or decreaseUsul, soil moisture content andlor percentage of field ca-in grain yield in the systems (Zhu et al. 1994; Yang et al.pacity are used as imigation indices in rice (OTools and Chang2001; Tao et al. 2006). In our water-saving imigation system,1979; Rahman and Yoshida 1985; Zhu 1998; Cheng et al.2003;limiting values of SWP were proposed的imigation indices.Tuong and Bouman 2003). These indices, however, are difcultTheswth stages, so that theto be determined rapidly and instantaneously. Physiologicalwettir中国煤化工协and delomen ofeffects on plants are varied with soil textures are even, and soilnice.十HC N M H Gle notonly sgicantmoisture content or percentage of field capacity are the samereduceo waler Input, DUI 21s0 increased grain yield, therefore(Zhu et al.1994; Zhu 1998) We have therefore applied SWP asenhancing water produtvity (Table 1, Figure 2). The techniqueWater-Saving and High-Yielding Irlgation in Rice 1449Table 3. Leaf water potentials of two rice cultivars of Zhendao 88 (iaponica) and Yangdao 6 (ndica) grown in the field at varous soil water potentialsYearCultivarsSoil water potential (-kPa)Leaf water polential (-MPa)TEG2002Zhendao 880.48士0.050.50+ 0.040.51土0.060.54土0.060.49 t 0.040.52 + 0.050.53土0.030.57 + 0.03100.62土0.060.63士0.050.64士0.070.65土0.06200.83土0.070.82士0.030.84土0.050.87 + 0.071.12士0.101.14土0.111.13士0.091.17士0.12Yangdao 60.48土0.030.50士0.050.53 + 0.030.51士0.060.52士0.050.52土0.070.54士0.040.64土0.060.62士0.060.61土0.040.65 + 0.0720.85 + 0.070.84土0.090.86士0.080.90士0.0731.18士0.121.19士0.131.21士0.111.26士0.1520030.49土0.040.53士0.040.51士0.050.53土0.070.56士0.060.64 + 0.070.67士0.072(0.85土0.060.84土0.060.82土0.070.B8士0.053C1.15士0.111.13土0.121.16士0.121.20士0.14.Yangdao600.48土0.050.49士0.060.52土0.060.53 +0.050.55土0.070.54士0.050.58土0.0810.68士0.070.66士0.050.69土0.060.87土0.060.88士0.070.87士0.050.92士0.08301.23土0.141.24士0.121.29土0.16Measurements were made on the top fl-expanded leaves at midday (12:00h) at the stages of active tilring (AT). end-tlering (ET), panicledevelopment and heading (PD), and graln fling (GF). Data are expressed as means士SE of six leaves.was demonstrated and applied in eight different ecologicalsaving irigation was much less than the extent to which leafrice-growing areas in China. Compared with the conventionalconductance decreased (Figure 3). We speculate that a mid-dayimigation that drainage was in mid-season and flooded at otherdecrease in leaf conductance under the water-saving irigationtimes, the water-saving imigation technique increased grainyieldwould reduce daily transpiration more than photosynthesis,by 6.1% to 14.2%, reduced irigation water by 25.4% to 38.2%,leading to increases in water use eficiency.and increased water productivity (grain yield per cubic meter ofWe observed that the water-saving irigation significantlyimigation watr) by 26% to 47%.enhanced activities of SuS, AGP, StS, and SBE in grains duringHow could this technique save irigation water as well asthe grain flig period (Table 4). The four enzymes are generallyincrease grain yield and water use eflciency? We observed thatconsidered as key enzymes involved in the sucrose-to-starchsensitivity of nice to water defcits varied with the growth stages.pathway in the grains (Hawker and Jenner 1993; Ahmadi andA mild soil-drying (SWP at - 15kPa) at most growth stagesBaker 2001; Hurkman et al. 2003), and signifcantly correlatedbenefited rice plants (Figure 1). The result indicates that keepingwith the grain fling rate or starch accumulation rate in rice andthe field continuously submerged would be not good for a highwheat (Tnicum aestivum L.) grains (Yang et al. 2003, 2004). Ityield and much water could be saved from the oonventionalwould be understandable that enhancement in activities oftheseirmigation.enzymes under the water-saving irigation may contribute to theIn the water-saving imigation, the water saved may be at-increase in grain plumpness and grain weight.tributed to the reduction in either transpiration rate and/orOur results also showed that water-saving irigation signif.soil surface evaporation. Usually, a decrease in transpirationicanty中国煤化工kinins (Z+ZR) in rootrate by reducing leaf conductance will result in the loss ofbleediE tating, heading, milk,photosynthesis (Wong et al.1979, 1985; Farquhar and Sharkeyand wHHC N M H Gourced homones are .1982; Weng et al. 2005). We observed, however, that the extentbelieved to play a major role in promoting cell division andto which the photosynthetic rate was reduced by the water- delaying senescence (Davies 2004; del Pozo et al. 2005).1450 Joumal of Integrative Plant Biology Vol. 49 No. 10 2007，230Zhendao 88:I[(Yangdao 6:°E 20815;1山E Zhendao 88「F Yangdao 6H Yangdao600 tJ Yangdao6[K Zhendao 8L Yangdao 6_ATSBHDMKWXATsBHDMKWXATsBHDMKwXGrowth stageYear: 2002Year: 2003Figure 3. Photosynthetic rate (A- D), leaf conductance (E- H), and water use ffciency (1-L) of two rice cutivars of Zhendao 88 (laponica)(A, C,E,G, , K) and Yangdao 6 (indica) (B, D, F, H, J, 4 grown in the fiold under two imigation systems.Measurements were made on the top fll expanded leaves. The water use eficiency was defined as the ratio of COz fixed in photosynthesis to H2Olost in transpiration. Vertical bars represent士SE of the mean (n= 6) where these exceed the size of the symbol. AT, active tlering; CI, conventionalimigation; HD, heading; MK, stages of milk; SB, secondary brancdhifferentiating; WsI, water-saving irigation; WX stages of waxy.Under the water-saving irigation, high cytokinin concentrationsrice growing season (May to October) of 2002, and repeated inin roots at the pai-diferentiating stage may account for the2003. Two high-yielding rice (Oryza sativa L.) cultivars currentlylarge panicle (more spikelets per panicle), and those duringused in local production, Zhendao 88 (aponica) and Yangdao 6grain setting and flig periods may contribute to better grain-(indica), were grown in the paddy field. Seedlings were raised inflig (nigh grain plumpness and grain weigh) by promotingthe field with sowing date on 10-11 May and transplantedon 10-endosperm cell division, delaying senescence, and/or regulating11 June at a hill spacing of 0.20 x 0.16 m with two seedlings perkey enzymes involved in the sucrose-to-starch pathway in thehil. The soil of the field was sandy loam (Typic fluvaquents, Eti-grains.sols [US taxonomy]) with 24.5 g/kg organic matter and availableIn conclusion, the water-saving irigation by controlling limitingN~-P-K at 105, 33.5 and 66.0mg/kg, respetively. N (60kg/ha asvalues of soil water potential related to specific growth stagesurea), P (30 kg/ha as single superphosphate) and K (40 kg/ha asnot only reduced water input, but also increased grain yield, KCI) were aplied and incorporated before transplanting. N astherefore enhancing water productivity. High harvest index,urea was also applied at mid-ilring (40 kg/ha) and at paniclemaintenance in photosynthetic rates, reduction in leaf con-initiation (25 kg/ha). Both cultivars (50% of plants) headed onductance, increases in cytokinin concentrations in roots and20 -22 August, and were harvested on 9 -10 October. The totalactivities of key enzymes involved in sucrose-to starch pathwayprecipitation during the growing season was 365.6mm in 2002in the grains may all contribute, at least partly, to high yield andand 348.7 mm in 2003, 75.2% to 76.5% of which was in Junehigh water use eficiency under the irigation system.and July. The mean solar radiation was 17 .6 MJ/m2 per day in2002 and 17.8 MJ/m2 per day in 2003.Materials and MethodsTreatmentsPlant materials and cultivationTwo experiments were conducted at a farm of Yangzhou Uni中国煤化工ere divided from therecove-vest: () active tlerinversity, Jiangsu Province, China (32*30'N, 119*25'E) during the stage,:TYHC N M H G ofproductive tlerinWater Saving and High-Yielding Irigation in Rice 1451Zhendao 88-mCI|Zhendao 82z WsI16 t2-Yangdao618上B下B HIK WXB HlMK WXGrowth utagoGrowth stageYear, 2002Year: 2003Figure 4. Concentrations of zeatin + zeatin riboside in root bleeding saps of two nice alivers of Zhendao 88 (jeponica) A, B) and Yangdao 6 (ndica)(C, D) grown In the field under two lmigation systems.'Vertical bars represent士 SE of the mean (n=6) where these exceed the size of the symbol. AT, active tleing; CI, conventional imigation; HD,heading; MK, stages of milk; SB, socondary brancdiifrentiatin; Wsl, water saving inigatin; WX stages of waxy. ,Tablo 4. Activtls of sucrose synthase (SuS), adenosine diphosphate gluC0s8 pyrophosphorylase (AGP), starch sythse (StS), and starch-branchingenzyme (SBE) in the grains of two nice cultivars of Zhendao 88 (iaponica) and Yangdao 6 (indica) grown in the feld under two imrigation systemsYearCultivarslmigationSusSAGPStSsystems(nmol grain-1 min-1)(Units grain-1 min-1)2002CI103.986.66.621141WsI134.5**103.5*7.89*1376*Yangdao 65.46983WSI123.2**92.7**6.87*1098*2003cI4.98119119.3**89.5**6.71*1332**Yangdao 8Cl104.588.6.3435137.8**102.5*"8.13*1018*The data were means of four determinations at B, 15, 25, and 40 days ater heading. , P< 0.05;”. P< 0.01. The comparison was made within acolumn, the same cultivar and the same year. CI, conventional imigation (control): WsI, water-saving imigation,(leaf age at N-n, N= total number of leaves on the main stem,CK], -15kPa, -30kPa, - 45kPa, and - 60kPa, respectively).n= number of elongated intermodes); i) endi-ilring stage,Each of the treatments had three plots as repetitions in afrom critical leaf age of productive tllring to secondary branch-complete randomized block design. The lot dimensions were indifrentiating stage; (间panicle development and heading中国煤化]-8 1-m wide aly usingstage, from secondary brandi-ferentiating stage to 10daysplastic=h of 50cm. SWP wasafter heading; and (iv) grain fling stage, from 11 days to 45daysmonitoMYHC N M H Gthe 15-cm soil depth.after heading. Five levels of SWP were designed at each stage,Four tension meters were Installed in each plot for monitoring.(.e. 0kPa [a thinlayer of water to 100% of field capacit,Tension meter readings were recorded every 4-h from 06.001452 Joumal of Itegrative Plant Biology Vol. 49 No. 10 2007to 18.00hours. When the reading reached the designed value,measure photosynthtic rate, leaf conductance and transpi-100-L, 90-L, 80-L and 70-L of tap water per plot were addedration rate. Measurements were made during 09:00-11:00hmanally t如o treatments of SWP at -15kPa, -30kPa, - 45kPa,when photosynthetic active radiation above the canopy wasand -60 kPa, respectively. A rain shelter consisting of a steel-1000 to 1100 umol/m per s. Six leaves were used for eachframe covered with a plastic sheet was used in each block totreatment.protect the plot during rain and during the treatment period.Root bleeding saps were collected and concentrations ofIn Experimnent 2, two inigation systems; water-saving imiga- Z+ ZRin the beeding saps were determined onthe above dates.tion and conventional imigation, were conducted. We observedMethods for root bleeding collection and Z+ZR measurementfrom our previous work (Yang and Ding 1992; Zhu et al.1994)were described previously (Yang et al.2002).that the effect of low SWP on grain yield of rioe varied withAt heading time, ffy panicles that headed on the same daygrowth stages and soil water potentials from -10 to - -30kPawere tagged in each plot. Ten tagged panicles from each plotduring drying periods in an alternate wetting and drying systemwere sampled at B, 15, 25, and 40days after heading, respe~would not seriously reduce the yield. Therefore a water-savingtively. AIl grains from each panicle were removed and used toimigation system by contolling limiting values of SWP related tomeasure activities of SuS (EC 2.4.1.13), AGP (EC 2.7.7.27),specifc growth stages was designed in the present study. ThisStS (EC 2.4.1.21) and SBE (EC 2.4.1.18). The measurementinvolved a water level of 3 4 cm being kept in the field duringmethod for enzymatic actvities was described elsewhere (Yangthe first week ater transplanting. The limiting value of SWP waset al.2004).- 5kPa at the active tlring stage, -30kPa at the end-tileingsate, -10kPa at panicle development and heading stage, and-20kPa at the grain fling stage. A thin-layer (1-2 cm) of waterFinal harvestwas applied to the plots when the SWP readings reached theAlI plants were harvested on 9-10 October. Grain yield wasImiting values. The irigation water was aplied via pipelines,determined from all plants from a 4 m2 site (except border ones)and the amount of imigation water was monitored with the water-in each plot and adjusted to a moisture content of 0.14g H2O/gmeter tables istalld on the pipelines. In conventional imigation,fresh weight. Aboveground biomass and yield components, (i.e.a water level of 35cm was kept in the field from transplanting tothe panicles per m2, the percentage of ripened grains and grainphysiological maturity, except for drainage at the iti jaintngweight) were determined from 50 hils (excluding the border(midseason drainage). Each of the treatments had three plotsones) sampled randomly from ach plot. The percentage ofas repetitinis in a complete randomized block design. Plotripened grains was defined as the ripened grains (specifcdimensions were in5 x4m and plots were separated by agravily21.00) as a percentage of total spikelets. The total1-m wide aley using plastic film inserted into the soil to a depthspikelets were calculated from the grain yield, grain weight (14%of 50 cm. SWP monitoring and rain prevention were the samemoisture content), and percentages of ripened grains, (i.e. to-as Experiment 1.tal spikelets = grain yeld/[grain weight x percentago of ripenedgrains], and spikelets per panicle = total spikelets/panicles).Grain plumpness was calculated using the fllowing formula:Physiological measurementsGrain plumpness(%) = 1000- frilized grain weight/1000Al physiological measurements were made in Experiment 2.- plumped grain weight (specific gravity = 1.10) x 100.Leaf water potential of the top fll-expanded leaves weremeasured at midday (12.00hours) at the stages of activeAbout one kilogram of grains harvested form each plottllering, end-tlring, panicle development and heading, andwas dried at 40。C in a forced-air oven for quality analysis.grain fling when SWPs wereO, -5, -10, -20, and -30kPa,Rice quality traits, including brown rice rate, head milled rate,respectively. Wel-lluminated leaves were chosen randomly forchalkiness, alkali spreading value, amylose content and proteinsuch measurements. A pressure chamber (Model 3000, Soilcontent, were measured according t如rice quality measurementMoisture Equipment Corporation, Santa Barbara, CA, USA)wasstandards (Ministry of Agriculture, China 1988).used for leaf water potential measurement with six leaves foreach treatmentThe photosynthetic rate, leaf conductance, and transpirationStatistical analysisrate of the top fll-expanded leaves were measured on 15, 40,72, 81, and 92 days ater transplanting, the growth stages core-The中国煤化工sing SAS/STAT satissponding above dates were active ilering, secondary branchtical as Istiute, Cary, NC,diferentiating, heading, mik, and waxy stage, respectively. 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