Using Chlorophyll (SPAD) Meter Reading and Shoot Fresh Weight for Recommending Nitrogen Topdressing Rate at Panicle Initiation Stage of Rice

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1 RESEARCH ARTICLES J. Crop Sci. Biotech. : 5 ~ 4 Using Chlorophyll (SPAD) Meter Reding nd Shoot Fresh Weight for Recommending Nitrogen Topdressing Rte t Pnicle Initition Stge of Rice Hung The Nguyen, Ln The Nguyen, Yong-Feng Yn, Kyu-Jong Lee nd Byun-Woo Lee * Fculty of Resource nd Environment Mngement, Thi Nguyen University of Agriculture nd Forestry, Thi Nguyen City, Vietnm; Fculty of Agronomy, Thi Nguyen University of Agriculture nd Forestry, Thi Nguyen City, Vietnm; Deprtment of Plnt Science, Seoul Ntionl University, Seoul 5-9, Kore Abstrct Nitrogen mngement t the pnicle initition stge (PI) should be fine-tuned for securing concurrent high yield nd high qulity rice production. For clibrtion nd testing of the recommendtion models of N topdressing rtes t PI for trget grin yield nd protein content of rice, three split-split-plot design experiments including five rice cultivrs nd vrious N rtes were conducted t the experimentl frm of Seoul Ntionl University, Kore from to 5. Dt from the first two yers of experiments were used to clibrte models to predict grin yield nd milled-rice protein content using shoot fresh weight (FW), chlorophyll meter vlue (SPAD), nd the N topdressing rte (Npi) t PI by stepwise multiple regression. The clibrted models explined 85 nd 87% of the vrition in grin yield nd protein content, respectively. The clibrted models were used to recommend Npi for the trget protein content of 6.8%, with FW nd SPAD mesured for ech plot in 5. The recommended N rte tretment ws chrcterized by n verge protein content of 6.74% (similr to the trget protein content), reduced the coefficient of vrition in protein content to.5% (compred to 4.6% of the fixed rte tretment), nd incresed grin yield. In the recommended N rte tretments for the trget protein content of 6.8%, grin yield ws highly dependent on FW nd SPAD t PI. In conclusion, the models for N topdressing rte recommendtion t PI were successful under present experimentl conditions. However, dditionl testing under more vrible environmentl conditions should be performed before universl ppliction of such models. Key words: grin yield, protein content, SPAD reding, fresh weight, nitrogen fertilizer. Introduction Nitrogen fertiliztion is crucil determinnt of grin yield nd qulity of rice. Bsed on the criticl stges of determintion of rice yield nd its components, the ppliction of N fertilizer hs been recommended in severl splits t trnsplnting for crop recovery t ctive stge for incresing tiller nd pnicle number, t PI for incresing number of spikelets per pnicle, nd t heding for incresing grin weight nd ripening percentge (De Dtt 98). The N split ppliction for ordinry rice cropping in Kore hs been recommended to be 5% t trnsplnting, % t stge, % t PI, nd % t full heding until it ws chnged recently to 5,,, nd %, respectively. N topdressing t full heding stge is not recommended becuse it increses the protein content of milled-rice, leding to poor eting qulity. Milled-rice protein content hs been reported to hve negtive correltion with the pltbility of cooked rice s high protein content in rice grin increses the hrdness nd * To whom correspondence should be ddressed B. W. Lee E-mil: leebw@snu.c.kr decreses the stickiness of cooked rice (Chikubu et l. 985; Ishim et l. 974; Tir 995). Presently, more emphsis is plced on improving rice qulity thn on incresing grin yield since Koren rice should compete with the imported rice in the domestic mrket. This sitution mkes it necessry to reevlute the existing nitrogen mngement prctices tht hve been pursued minly on incresing grin yield. Nitrogen ppliction t different growth stges exerts different effects on grin yield nd qulity of rice (Mtsushim 995; Nguyen et l. 6). Nguyen et l. (6) reported tht n increse of the N topdressing rte t PI incresed grin yield nd milled-rice protein content while n increse in the N topdressing rte t the stge incresed grin yield but not milled-rice protein content. Mtsushim (995) lso reported tht N bsorbed from lte N topdressing fter PI resulted in much higher grin protein content in comprison to tht from erlier N topdressing s the former could be trnslocted directly into the grin. Nguyen et l. (6) found tht the higher shoot N nd biomss ccumultion t PI resulted in the higher rice grin yield while resulted in the lower milled-rice protein Journl of Crop Science nd Biotechnology 5

2 Hung The Nguyen et l. content possibly due to the dilution effect. These results indicte tht for chieving high grin yield nd good qulity rice with low protein content it is necessry to fine-tune the N mngement t PI ccording to the dignosis of crop growth nd N nutritionl sttus. Different crop growth nd N nutritionl sttus indictors hve been used for recommending N topdressing mngement t PI (Dobermnn et l. ; Peng et l. 996). Of these indictors, shoot dry weight, shoot N concentrtion, nd shoot N ccumultion were most frequently used. However, chrcteriztion of these prmeters is lborious nd time consuming. Although prediction of shoot N density nd concentrtion by cnopy reflectnce hs been previously reported s relible, fst, nd non-destructive method for whet (Hnsen nd Schjoerring ) nd rice crops (Nguyen nd Lee 4), it is still difficult to introduce it into the ctul frming prctices. Alterntively to directly mesuring lef N concentrtion, the chlorophyll meter (SPAD) provides simple, quick, nd non-destructive method for estimting N concentrtion of rice leves (Chubchi et l. 986; Tkebe nd Yoneym 989; Tkebe et l. 99; Peng et l. 995, 996). Therefore, SPAD vlues hve been used to ssess crop N sttus nd to determine the plnt s need for dditionl N fertilizer by Turner nd Jund (99), Peng et l. (995, 996), nd Singh et l. () who used N mngement tctics to pply N topdressings only when SPAD vlues of the index leves fell below predetermined threshold. These SPAD-bsed N fertilizer mngements resulted in higher N use efficiency (NUE) nd gronomic NUE thn the fixed-rte tretment. However, SPAD vlues do not indicte how much N should be pplied. They only indicte the need for dditionl N (Turner nd Jund 994). SPAD vlues re needed to be used in combintion with other crop growth nd/or soil vribles to determine the N topdressing rte congruent to N demnd for trget grin yield nd/or grin qulity. The objectives of our studies were () to quntify the response models of grin yield nd milled rice protein content to the pplied N rte t PI in reltion to FW nd SPAD vlues, nd () to pply the models for N topdressing rte recommendtion t PI for trget grin yield nd milled-rice protein content. Mterils nd Methods Tble : Design of the experiment. Tretment 4 N rtes t trnsplnting Tble b: Design of the 4 experiment. Tretment 4 N rtes t trnsplnting Tble c: Design of the 5 experiment. Tretment 4 N rtes t trnsplnting N rtes t N rtes t N rtes t N rtes t PI N rtes t PI Vrible N rtes c Vrible N rtes N rtes t PI Vrible N rtes c Recommended N rte d Rice cultivr b, V, V, V4, V, V, V4, V, V, V4, V, V, V4 Applied N (kg h - ), b -V4 were Hwseongbyeo, SNU-SG, Junbyeo, nd Surbyeo, respectively. Applied N (kg h - ), b nd V5 were Hwseongbyeo nd Denbyeo, respectively. c N rtes t PI for tretments nd 4 ws 5.±6.7 nd 5. ±.8 kg h -, respectively. Rice cultivr b, V5, V5, V5, V5 Rice cultivr b Applied N (kg h - ), b nd V5 ws Hwseongbyeo nd Denbyeo, respectively. c N rtes t PI for tretment ws 4.±. kg h -, nd this tretment ws used only for model vldtion.. d Detils for recommended N rte t PI, SPAD, nd shoot fresh weight were presented in the next section.. Three experiments (, 4, nd 5) were conducted on rice field with n re of, m t the experimentl frm of Seoul Ntionl University, Suwon, Kore. The field hd sndy cly lom soil texture, CEC of.9 cmol( + ) kg - ; orgnic mtter of 4.4 mg g -, totl N of.75 mg g -, nd ph of 5.4. The experiment included ten nitrogen (N) tretments nd four rice cultivrs of medium mturity: Hwseongbyeo (), SNU-SG (V), Junbyeo (V), nd Surbyeo (V4). The 4 nd 5 experiments included nd nine N tretments, respectively, with two rice cultivrs: Hwseongbyeo () nd Denbyeo (V5). The detils of experiments re presented in Tbles -c. It is noted from Tble b tht the purpose of the vrible N-rte tretments (tretments nd 4) in the 4 experiment ws to widen nd increse the vribility of the N testing rte for the clibrting models to predict crop vribles for the 5 experiment. PI ws defined here s common dte (4 dys before heding) for N ppliction t pnicle initition stge in Kore. The experiments were conducted ccording to split-split-plot design, with the N rte t stge, PI, nd rice cultivrs rndomly ssigned into min plots, sub-plots nd sub-sub-plots, respectively. Sub-sub-plot size ws 4 m in, nd m in 4 nd 5. Rice ws trnsplnted mnully t spcing of.5 m x. m with three -dy-old seedlings per hill on My th in, My rd in 4, nd My th in 5. The whole experimentl field ws fertilized with the sme mount of 8 kg P O 5 + kg K O h - t trnsplnting nd 4 kg K O h - t PI. Other mngement techniques for the whole field such s lnd preprtion, weeding, wter supplies, etc., were pplied homogenously ccording to the stndrd cultivtion prctices in Kore. Plnt smpling nd mesurements On the dtes of N topdressing t PI of ech yer (July 9 th in, July nd in 4, nd July st in 5), five rice hills per plot were rndomly selected. From these five hills, chlorophyll meter (SPAD 5, Minolt Co. LTD, Jpn) reding (SPAD) ws tken for the five most fully-expnded leves per hill. Immeditely following SPAD mesurement, the five hills were smpled nd weighed for shoot fresh weight (FW). At hrvest (October th, October 6 th 4, nd October 9 th 5), hills for grin yield nd protein content mesurement were smpled from ech plot, nd whole-plnt smple of five rice hills per plot ws collected for plnt N concentrtion nlysis by Kejeltec Auto 5 System (Tector, Sweden).

3 Using SPAD Meter for Recommending Pnicle N Rte of Rice. Model clibrtion for N topdressing rte recommendtion Dt collected from the first two yers (-4) ws used to exmine the reltionship of grin yield nd protein content with FW, SPAD, nd pplied N rtes t PI (Npi). Models were clibrted by stepwise multiple liner regression (SMLR) in SAS 8. (SAS Inc., USA) to predict rice grin yield nd protein content using Npi, FW, nd SPAD (liner nd qudrtic) s independent vribles. Model vlidtion The clibrted models were used to clculte grin yield nd protein content using crop prmeters mesured t PI in 5. The performnce of the clibrted models ws evluted by the coefficient of determintion (R ) nd the root men squre error in prediction (RMSEP), n indictor of the verge error in the nlysis s expressed in the originl mesurement unit. RMSEP ws clculted using Eq. : ^ where y i nd y i were ctul nd clculted vlues of crop prmeters t hrvest (grin yield nd protein content, respectively), nd n is the number of smples in the dtset. Another prmeter used ws the reltive error of prediction (REP) which shows the predictive bility of the model s clculted by Eqution : ^ Similr to Eqution, y i nd y i were ctul nd clculted vlues of crop prmeters, n ws the number of smples in dtset, nd y ws men of the ctul vlues of crop vribles. Testing the recommended nitrogen rte In the 5 experiment, tretment (tretment 4) ws dded, mnged homogenously, nd dt for crop prmeters were collected t ll stges s in the other tretments described in the bove sections, with n exception tht Npi ws clculted by the model equtions clibrted in the previous step. Bsed on the SMLR equtions, which show the dependence nd interdependence of grin yield nd protein content on FW, SPAD, nd Npi, Npi for ech plot of tretment 4 in 5 ws clculted for the trget protein content of 6.8% with the mesured FW nd SPAD. Dt nlysis Grin yield nd protein content of the recommended N topdressing tretment were reported by tretment mens in comprison to tretments nd (with no N t PI s the control tretment) nd kg N h - pplied t PI (the commonly recommended N rte t PI in Kore), respectively. Results nd Discussion Model clibrtion For the N topdressing rte recommendtion bsed on FW nd SPAD, grin yield nd protein content response to Npi in combintion with FW nd SPAD requires quntifiction. To quntify the responses of grin yield nd protein content to Npi in the plots with () () different FW nd SPAD, stepwise multiple liner regression with forwrd selection t the probbility of <.5.ws used. The Input vribles included liner nd qudrtic terms of Npi, FW, nd SPAD mesured from the nd 4 experiments. The qudrtic effect of Npi, biomss, nd shoot N concentrtion on grin yield hs been previously reported by Kim (4). The stepwise multiple regression model to predict grin yield nd protein content using Npi, FW, nd SPAD (Tble ) indicted tht both grin yield nd protein content hve qudrtic reltionship with Npi nd FW, but liner reltionship with SPAD (Eq. nd 4 in Tble ). Tble : Clibrtion models for grin yield nd milled-rice protein content prediction. Model equtions (n = 66) Grin yield (kg h - ) = Npi.5Npi +.8FW 7.75 x-4fw R SPAD (Eq. ) Protein b = Npi +.5Npi -.FW +.9 x-7fw SPAD (Eq. 4) Npi, FW nd SPAD were pplied N rte (kg h - ), shoot fresh weight (g m - ), nd SPAD vlues t PI. b Protein ws milled-rice protein content (%). Eqution llows one to clculte the optimum vlues of Npi nd FW for the mximum grin yield. The clculted optimum vlues of the Npi nd FW for the mximum grin yield of rice were 56 kg N h - nd 8 g m -, respectively. The clculted 56 kg N h - ws lmost twice the commonly recommended Npi in Kore. The qudrtic effect of shoot biomss on grin yield my be result of the negtive effect of shoot biomss ccumulted during the vegettive growth stge on the rice hrvest index s discussed by Kumur (995). Similrly, ccording to Eqution 4, the optimum vlue of FW for minimum protein content ws 5 g m -, vlue quite similr to the optimum FW of 8 g m - for mximum grin yield. Vlidtion for the clibrted models The coefficient of determintion (R ), RMSEP, nd REP using the clculted nd ctul vlues of grin yield nd protein content in the clibrtion ( nd 4 experiments) nd vlidtion (5 experiment) dtsets re presented in Tble. The stepwise multiple regression models using the Npi, FW, nd SPAD explined 85 nd 87% of the vrition in grin yield nd protein content in the clibrtion dt set, respectively. In ddition, REPs were smll: 6. nd.% in grin yield nd protein content predictions, respectively. However, the R in the vlidtion dtset decresed significntly compred to the clibrtion dtset while the RMSEP nd REP incresed slightly. These results re indictive of the decrese in precision of the models for grin yield nd protein content prediction, lthough the ccurcy in the vlidtion set did not vry. The decrese in precision for the model my be prtilly due to the nrrow rnge of grin yield nd protein content in the vlidtion dtset (Tble nd Fig. ), s it included the tretment recommended for the trget protein content of 6.8%. Recommended N topdress rte t PI To determine the N topdressing rte recommendtion t PI in the 5 experiment using the models quntified from the -4 experiments, FW nd SPAD vlues for nine plots (three replictes of three levels of pplied N rtes:,, nd kg N h - ) were mesured. The ctul vlues of FW nd SPAD re presented in Tble 4. Journl of Crop Science nd Biotechnology 7

4 Hung The Nguyen et l. Tble. Performnce prmeters of the predictive models for grin yield nd milled-rice protein content. Crop vribles Descriptive sttistics Model performnce prmeters b Men CV (%) Min Mx R RMSEP REP (%) Clibrtion dtset (n = 8), nd 4 experiment Yield (kg h - ) Protein (%) Vlidtion dtset (n = 4), 5 experiment Yield (kg h - ) Protein (%) should be noted tht the kg N h - tretment hd higher vlues of FW nd SPAD but required less N t PI thn the kg N h - tretment. This reflects the qudrtic effect of FW on protein content in Eqution 4. Fig. indictes tht the N topdressing rte t PI required for protein content of trget vlue vries gretly depending on plnt biomss nd SPAD nd Npi should be djusted bsed on their vlues. For exmple, it is desirble to increse Npi with the incresing plnt biomss up to certin level in order to stisfy the trget protein content nd chieve higher grin yield s well, even though rice plnt t PI hs the sme N nutrition sttus (the sme SPAD vlue). Yield nd Protein were grin yield nd milled-rice protein content (%) t hrvest, respectively. b R, RMSEP nd REP were coefficient of determintion, root men squre error in prediction nd reltive error of prediction, respectively. The ppliction of or kg N h - incresed FW from, g m - (control) to,87 nd,988 g m -, respectively. The coefficients of vrition in the FW were 6.% in the control tretment (no N pplied t stge), nd 6.4 nd.% in the nd kg N h - pplictions t stge, respectively. On verge, the FW for the nine recommended N plots ws,698 g m - with coefficient of vrition of.7%. Similrly, the ppliction of N t stge incresed SPAD vlues from 8. (no N ppliction) to 9.8, nd.7 for the nd kg N h - tretments, respectively. The verge vlue of SPAD for the nine recommended N plots ws 9.9 with coefficient of vrition of 9.9%. A higher within-field sptil vrition of rice biomss thn SPAD nd shoot N concentrtion hs previously been reported (Nguyen et l, 6). Bsed on the mesured vlues of FW nd SPAD of the nine N topdressing recommendtion plots, it is possible to compute n N topdressing rte t PI for trget grin yield or protein content using Eqution or 4, respectively. However, Eqution 4 ws used to recommend N rte t PI for the trget protein content due to the recent emphsis on producing high qulity rice with lower protein content. Protein content hs been reported to hve significntly negtive correltion with rice grin qulity, especilly for tste of cooked rice (Ishim et l. 974; Chikubu et l. 985; Tir 995). Moreover, the result from the nlysis of vrince indicted tht yerly vrition significntly ffected grin yield, not protein content, nd tht both were significntly ffected by Npi (Nguyen 5). Therefore, the eqution for protein content prediction is more dptble for yer to yer vrition nd thus more relible for N topdressing recommendtion. The clculted Npi for the trget protein content of 6.8% (% moisture bsis) bsed on the mesured FW nd SPAD s clculted by Eqution 4 is presented in Tble 4. The Npi for the trget protein content of 6.8% ws 5.5, 4., nd 7. kg N h - for the tretments with,, nd kg N h - pplied t stge, respectively. It Fig.. The mesured versus clculted vlues of grin yield nd milled-rice protein content by Equtions nd, respectively. Solid line is : line. Tble 4. Prmeters of the recommended N rte tretment t PI for the trget milled-rice protein content of 6.8% in cv. Hwseongbyeo in the 5 experiment. N rte t (kg h - ) Averge Shoot fresh weight (g m - ) SPAD vlues Recommended N rte (kg h - ) Men CV (%) Min Mx Men CV (%) Min Mx Men CV (%) Min Mx Recommended N rte were clculted by Eq. 4 for trget milled-rice protein content of 6.8%. 8

5 Using SPAD Meter for Recommending Pnicle N Rte of Rice. Fig.. Isopleths of the recommended N rte (number plced on the line of grph; kg - ) t PI for the trget milled-rice protein content of 6.8% bsed on shoot fresh weight nd SPAD vlues mesured t PI. Grph ws drwn by Eqution 4 in Tble. According to the results shown in Tble 5, the men vlue of protein content in the ws 6.74%, similr to the trget protein content of 6.8%. The vrition in protein content decresed from 4.6% in the to.5% in the. The lower vrition nd nrrower rnge of protein content in the (compred to in ) indicted the potentil ppliction of the for trget protein content in rice production. Moreover, the hd higher rice grin yield thn observed in the ; grin yield incresed on verge from 656 kg h - in the to 6888 kg h - in the. The higher grin yield in the my hve resulted from the higher N ppliction rte (Tble 4) nd the incresed response of grin yield to the pplied N t PI of the, s the N rte clcultion reflected crop growth nd N nutritionl sttus prior to the N ppliction. Dependence of grin yield on shoot fresh weight nd SPAD vlue t PI The grin yield vlue mesured in the recommended N plots ws indictive of the high dependence of grin yield on FW nd SPAD in the for the trget protein content of 6.8% (s in Fig. ). These results imply tht the vegettive growth nd N nutrition sttus should be secured to certin level through crop mngement prior to ppliction of the recommended N topdressing t PI to ensure concurrent high yield nd qulity production of rice. The mximum yield ws ttined t 78 g fresh weight m - nd SPAD vlue of.9. This optimum vlue of FW ws consistent with the optimum vlue of 8 g fresh weight m - for mximum grin yield, s determined by Eqution, clibrted ccording to dt collected in nd 4. Test of N topdressing rte recommendtion The recommended N topdressing rtes presented in Tble 4 were pplied to nine plots for the 5 experiment. The mesured grin yield nd protein content of the recommended N rte tretment () compred to the fixed N rte tretment of kg N h - t PI () nd the control tretment with no N pplied t PI re presented in Tble 5. Tble 5. Descriptive sttistics of grin yield nd milled-rice protein content of the recommended N rte tretment (), fixed rte tretment (, kg h - ), nd no N tretment t PI of cv. Hwseongbyeo in the 5 experiment. N rte t (kg h - ) Pooled Tretment Grin yield (kg h - ) Milled-rice protein content (%) Men CV (%) Min Mx Men CV (%) Min Mx Fig.. Reltionship between crop prmeters mesured t PI nd grin yield of the recommended N rte tretment plots for the trget protein content of 6.8% in cv. Hwseongbyeo. Conclusions Stepwise multiple regression using FW, SPAD, nd Npi s predictor vribles explined 85 nd 87% (in the clibrtion dtset using dt from nd 4), nd 68 nd 4% (in the vlidtion dtset using dt from 5) of the vrition in grin yield nd protein content, respectively. Appliction of N topdressing s recommended by the clibrted models for trget protein content of 6.8% were chrcterized by n verge protein content of 6.74%, with rnge of 6.58 to 7.7%. The recommended N topdressing reduced the vrition of protein content nd significntly incresed grin yield s compred to the of kg h -, regrdless of the rte of N ppliction t the stge. In the for the trget protein content of 6.8%, grin yield ws highly dependent upon FW nd SPAD vlues t PI, with optiml FW nd SPAD vlues of pproximtely 8 g m - nd.9 for mximum yield, respectively. In conclusion, the model for N topdressing rte recommendtion t PI ws successful under the present experimentl conditions. However, dditionl testing under more vrible environmentl conditions should be performed before universl ppliction of these models. Journl of Crop Science nd Biotechnology 9

6 Hung The Nguyen et l. Acknowledgement This reserch ws supported by the project CATER 6-4 from the Center for Atmospheric nd Erthquke Reserch (CATER), Kore Meteorologicl Administrtion (KMA). References Chubchi T, Asno I, Oikw T The dignosis of nitrogen nutrition of rice plnts (Ssnishiki) using chlorophyll-meter. Jpn. J. Soil Sci. Plnt Nutr. 57: 9-9 Chikubu S, Wtnbe S, Sugimoto T, Mnbe N, Ski F, Tniguchi Y The estblishment of formul to estimte rice eting qulities through multple regression nlyses. J. Jpn Soc. Strch Sci. : 5-6 De Dtt SK. 98. Principles nd Prctices of Rice Production. JohnWiley nd Son Inc. Dobermnn A, Witt C, Dwe D, Abdulrchmn S, Gines HC.. Site-specific nutrient mngement for intensive rice cropping systems in Asi. Field Crops Res. 74: 7-66 Hnsen PM, Schjoerring JK.. Reflectnce mesurement of cnopy biomss nd nitrogen sttus in whet crops using normlized difference vegettion indices nd prtil lest squres regression. Remote Sens. Environ. 86: Ishim T, Tir H, Tir H, Mikshib K Effect of nitrogenous fertilizer nd protein content in milled-rice on orgnoleptic qulity of cooked rice. Rep. Nt. Food Res. Inst. 9: 9-5 Kim MH. 4. Pnicle nitrogen topdressing recommendtion bsed on nondestructive dignosis of growth nd nitrogen nutrition sttus t pnicle initition stge of rice. Ph. D. thesis. Seoul Ntionl University, Seoul, Kore. Kumur A Physiology of high-yielding rice plnts from the viewpoint of dry mtter production nd its prtitioning, In T. Mtsuo, K. Kumzw, R. Ishii, K. Ishihr, H. Hirt, Science of The Rice Plnt, Volume : Physiology. Food nd Agriculture Policy Reserch Center, Tokyo, Jpn. pp 74-7 Mtsushim S Physiology of high-yielding rice plnts from the view point of yield components, In T. Mtsuo, K. Kumzw, R. Ishii, K. Ishihr, H. Hirt, Science of The Rice Plnt, Volume : Physiology. Food nd Agriculture Policy Reserch Center, Tokyo, Jpn. pp Nguyen TH, Lee BW. 4. Selection of the most sensitive wvebnd reflectnce for normlized difference vegettion index clcultion to predict rice crop growth nd grin yield. Koren J. Crop Sci. 49: Nguyen TH, Kim MH, Nguyen LT, Lee BW. 6. Response of grin yield nd milled rice protein content to nitrogen rtes pplied t different growth stges of rice. Koren J. Crop Sci. 5: 4-5 Nguyen TA. 5. Sptil yield vribility nd site-specific nitrogen recom mendtion for the improved yield nd grin qulity of rice. Ph. D. thesis. Seoul Ntionl University, Seoul, Kore. Peng S, Lz RC, Grci FC, Cssmn KG Chlorophyll meter estimtes lef-re bsed N concentrtion of rice. Commun. Soil Sci. Plnt Anl. 6: Peng S, Grci FV, Lz RC, Snico AL, Vispers RM, Cssmn KG Incresed N-use efficiency using chlorophyll meter on high yielding irrigted rice. Field Crops Res. 47: 4 5 Peng S, Buresh R, Hung J, Yng J, Zou Y, Zhong X, Wng G, Zhng F. 6. Strtegies for overcoming low gronomic nitrogen use efficiency in irrigted rice systems in Chin. Field Crops Res. 96: -47 Singh B, Singh Y, Ldh JK, Bronson KF, Blsubrmnin V, Singh J, Khind CS.. Chlorophyll meter nd lef color chrt bsed nitrogen mngement for rice nd whet in Northwestern Indi. Agron. J. 94: 8-89 Tir H Physicochemicl properties nd qulity of rice grins, pp In: Science of The Rice Plnt, In T. Mtsuo, K. Kumzw, R. Ishii, K. Ishihr, H. Hirt, Volume : Physiology. Food nd Agriculture Policy Reserch Center, Tokyo, Jpn. Tkebe M, Yoneym T Mesurement of lef color scores nd its impliction to nitrogen nutrition of rice plnts. Jpn. Agric. Res. Q. : 86 9 Tkebe M, Yoneym T, Ind K, Murkm T. 99. Spectrl reflectnce rtio of rice cnopy for estimting crop nitrogen sttus. Plnt nd Soil. : Turner FT, Jund MF. 99. Chlorophyll meter to predict N topdress requirement of semidwrf rice. Agron. J. 8: Turner FT, Jund MF Assessing the nitrogen requirements of rice crops with chlorophyll meter. Aust. J. of Exp. Agric. 4(7):