QTL mapping for stay-green in maize (Zea mays)

Transcription

1 QTL mapping for stay-green in maize (Zea mays) Ai-yu Wang 1, Yan Li 1, and Chun-qing Zhang 2 State Key Laboratory of Crop Biology, Shandong Key Laboratory of Crop Biology, College of Agronomy, Shandong Agricultural University, Taian , China. Received 1 June 2011, accepted 11 October Can. J. Plant Sci. Downloaded from by on 04/09/18 Wang, A.-y., Li, Y. and Zhang, C.-q QTL mapping for stay-green in maize (Zea mays). Can. J. Plant Sci. 92: Stay-green is a desirable character for crop production. In order to explore the genetic basis for stay-green traits in maize, 112 polymorphic simple sequence repeat (SSR) markers were used to analyze 189 F 2 individuals derived from a single cross of inbred lines A (a stay-green inbred line) and Mo17 (a normal inbred line). A total of 14 quantitative trait loci (QTLs) were detected for three stay-green related traits, green leaf area per plant at 30 d after flowering (GLA2), green leaf area per plant at the grain-ripening stage (GLA3), and left green leaf number per plant at the grain-ripening stage (LLN). Single QTL explained from 3.16 to 12.50% of the phenotypic variance. Among them, three were major QTLs. In addition, we analyzed the other two traits, green leaf area per plant in the whole growing period (GLA1) and total leaf number per plant in the whole growing period (TLN), and detected eight QTLs for them. Our results will be helpful to the maize breeders for marker-assisted selection. Key words: Maize, stay-green, quantitative trait loci, simple sequence repeat Wang, A.-y., Li, Y. et Zhang, C.-q Cartographie des QTL du caracte` re stay-green chez le maïs (Zea mays). Can. J. Plant Sci. 92: Le caracte` re stay-green est souhaitable pour les cultures. Afin d en pre ciser l origine ge ne tique chez le maïs, les auteurs ont recouru à 112 marqueurs microsatellites (SSR) pour analyser 189 individus de la F 2 issus d un unique croisement des lignées endogames A (ligne e stay-green) et Mo17 (ligne e normale). En tout, ils ont de cele 14 locus quantitatifs (QTL) pour trois caracte` res associés au caractère stay-green, à savoir la superficie des feuilles vertes par plant 30 jours apre` s la floraison (GLA2), la superficie des feuilles vertes par plant à la maturation du grain (GLA3) et le nombre de feuilles vertes par plant à la maturation du grain (LLN). Des QTL simples expliquent de 3,16 % à 12,50 % de la variabilite phénotypique. Parmi eux figurent trois importants QTL. Les auteurs ont e galement analyse les deux autres caracte` res, soit la superficie des feuilles vertes par plant durant la pe riode ve ge tative (GLA1) et le nombre total de feuilles par plant durant la pe riode vége tative (TLN), et de cele huit QTL. Ces résultats aideront les améliorateurs à sélectionner des variétés a` l aide de marqueurs. Many researchers have demonstrated that a positive correlation exists between crop yield and duration of green leaf area (Heath and Gregory 1938; Tollenaar and Daynard 1978; Thomas and Smart 1993). Therefore, the stay-green trait strongly attracts crop breeders because the leaves remain green even after the grainripening stage. Jiang et al. (2004) classified stay-green into two groups: functional stay-green and nonfunctional stay-green. Functional stay-green genotypes have been drawing more and more attention due to increased photosynthetic capacity in the final stage of plant growth, and consequent high grain yield (Gentinetta et al. 1986; Thomas and Howarth 2000), increased nutritional quality of stems and leaves for herbivorous animals (Gentinetta et al. 1986; Van Oosterom et al. 1995), and increased tolerance to biotic and abiotic stress (Thomas and Smart 1993; Xu et al. 2000a). On the other hand, non-functional stay-green genotypes lack photosynthetic activity, although the 1 Ai-yu Wang and Yan Li contributed equally to this paper. 2 Corresponding author ( cqzhang@sdau.edu.cn). Mots clés: Maı s, stay-green, QTL, SSR leaves remain green in the final stage of plant development (Thomas and Howarth 2000; Cha et al. 2002). The genetic basis of functional stay-green traits is complicated. Thomas (1987) initially characterized a stay-green mutant sid in Festuca pratensis, which indicated it is a qualitative character. Thereafter, several researchers drew the same conclusion in soybean and rice (Guiame t et al. 1990; Cha et al. 2002). Furthermore, a cytoplasmic gene (cyt G) was identified as controlling the stay-green trait in soybean (Guiame t et al. 1990). Many more researchers, however, have demonstrated that functional stay-green is a largely polygeneregulated quantitative trait. Genetic mapping of the quantitative trait loci (QTLs) for stay-green has been Abbreviations: GLA1, green leaf area per plant in the whole growing period; GLA2, green leaf area per plant at 30 d after flowering; GLA3, green leaf area per plant at the grain-ripening stage; LLN, left green leaf number per plant at the grain-ripening stage; LOD, logarithm of odds; MCIM, mixed-model-based composite interval mapping; PCR, polymerase chain reaction; QTLs, quantitative trait loci; RFLP, restriction fragment length polymorphisms; SSR, simple sequence repeat; TLN, total leaf number per plant in the whole growing period Can. J. Plant Sci. (2012) 92: doi: /cjps

2 Can. J. Plant Sci. Downloaded from by on 04/09/ CANADIAN JOURNAL OF PLANT SCIENCE conducted in many crops such as sorghum (Crasta et al. 1999; Subudhi et al. 2000; Tao et al. 2000; Xu et al. 2000b; Kebede et al. 2001; Haussmann et al. 2002), rice (Jiang et al. 2004; Yoo et al. 2007), wheat (Kumar et al. 2010) and maize (Beavis et al. 1994; Zheng et al. 2009). Most studies have been performed in sorghum, a member of the tribe Andropogoneae, like maize, and have identified four major QTLs, named Stg1, Stg2, Stg3 and Stg4, and many additional minor QTLs (Xu et al. 2000b; Haussmann et al. 2002). However, until now, only two reports about QTL mapping of staygreen in maize have been available. Beavis et al. (1994), using restriction fragment length polymorphisms (RFLP) markers, identified three and five stay-green QTLs in an F 4 and topcrossed maize population generated from B73 Mo17, respectively, based on visual rating of health and vigor of plants at the time of harvest. Zheng et al. (2009) using simple sequence repeat (SSR) markers detect 14 QTLs in an F 2 population derived from a cross between a stay-green inbred line (Q319) and a normal inbred line (Mo17), with the ratio of green leaf area to total leaf area as the quantitative index for measurement. In the present study, we analyzed five traits, i.e., green leaf area per plant in the whole growing period (GLA1), green leaf area per plant at 30 d after flowering (GLA2), green leaf area per plant at the grain-ripening stage (GLA3), total leaf number per plant in the whole growing period (TLN) and left green leaf number per plant at the grain-ripening stage (LLN). Among them, GLA2, GLA3 and LLN are related to stay-green characters, and have not been investigated in maize by previous researchers. Fourteen QTLs for the three staygreen related traits and eight QTLs for the other two traits were identified from an F 2 population derived from a cross between a stay-green inbred line A and non-stay-green inbred line Mo17. Our results will be helpful to the maize breeders for marker-assisted selection. MATERIALS AND METHODS Plant Materials The 189 plants of the F 2 mapping population were derived from the cross A Mo17. A is a stay-green inbred line, which is bred in our laboratory and Mo17 is a non-stay-green inbred line. All the plants were sown in the farmland of Shandong Agricultural University on 2007 Apr. 26, with 30 cm between plants and a row space of 80 cm. The 189 F 2 plants, 30 F 1 plants, 30 male parent plants and 30 female parent plants were selected randomly for the present study. Trait Measurements GLA1 was measured as follows: fully expanded green leaves were marked at different stages during the whole growth period, leaf area was measured in time, and the measurements were added. Fully expanded green leaf area was measured for GLA2 30 d after initial flowering date. GLA3 was measured at grain-ripening stage. Leaves with a ratio of green leaf area to total leaf area over 2/3 were considered as green leaves. Leaf area was calculated using the formula: leaf area leaf length maximum leaf width 0.75 (Montgomery 1911; Birch et al. 1998). In addition, TLN was the number of total photosynthetic leaves during the whole growth phase. LLN was counted at grain-ripening stage. Construction of the SSR Linkage Map SSR primers were synthesized according to the sequences published on Maize GDB (Maize Genetics and Genomics Database, Five hundred and eighty-two pairs of SSR primers for the present study were evenly distributed on 10 chromosomes. Genomic DNA was extracted according to the procedure described by Murray and Thompson (1980). The amplifications were performed in 10 ml reaction mixture containing 2.0 mm Mg 2, 0.1 mm dntps, 0.25 mm of each primer, 0.5 U Taq DNA polymerase and 20 ng genomic DNA. The parameters for the reaction were as follows: pre-denaturation at 948C for 5 min, followed by 35 cycles of 948C for 1 min, annealing at the most appropriate temperature for each primer pair for 2 min, 728C for 2 min, and then a final extension at 728C for 5 min. PCR products were separated on 8% non-denaturing polyacrylamide gels and visualized by silver staining (Bassam et al. 1991). All markers were tested using the x 2 -test for the Mendel segregation ratio (1:2:1 or 3:1). Linkage analysis was conducted using MAPMAKER/EXP 3.0b (Lander et al. 1987). First, Group command was used to group markers in various linkage groups with a logarithm of odds (LOD) threshold of 4.0. Then Compare and Try commands were used to locate the SSR markers. Finally, the linkage groups were generated by the Map command. The Kosambi function was used to convert recombinant frequencies into map distances (cm) (Kosambi 1944). Final linkage maps were drawn by MapChart 2.1 (Voorrips 2002). QTL Mapping QTL mapping was done with QTLNetwork-2.0 ( ibi.zju.edu.cn/software/qtlnetwork), using the mixedmodel-based composite interval mapping (MCIM) with a 10 cm window size and a 1 cm walking speed. A 10 cm filtration window was used to distinguish two adjacent test statistic peaks whether or not they were two different QTLs. One thousand permutation tests were performed on every possible QTL to calculate the critical F value at a 5% probability level. The QTLs were named as described by McCouch et al. (McCouch et al. 1997).

3 WANG ET AL. * QTL MAPPING FOR STAY-GREEN IN MAIZE 251 Can. J. Plant Sci. Downloaded from by on 04/09/18 RESULTS Construction of SSR Linkage Map Out of 582 pairs of SSR primers, one hundred and twelve pairs showed polymorphism between two parental lines. Among them, 10 SSR primer pairs tested by the x 2 -test, showed segregation distortion. The map was cm with an average distance of cm between two adjacent markers. The locations of most SSRs were the same as those published in Maize GDB, except that the five markers namely, umc1824, bnlg1730, umc2191, bnlg2148 and phi072 were mapped on chromosome 2, 4, 9, 10 and 10 respectively. Phenotypic Variation of Stay-green Traits SPSS 13.0 software (SPSS, Chicago, IL) was used to test the distribution of the five traits. As described in Table 1, all the five traits showed nearly normal distribution (skew B1.00), and were therefore suited for QTL analysis. In addition, all the five traits displayed transgressive segregation phenomenon in the F 2 population (Table 1). QTL Analysis of Stay-green Traits The QTL mapping for all traits was conducted using QTLNetwork-2.0, and 22 QTLs were detected (Fig. 1, Table 2). Among them, five QTLs on chromosome 1, 1, 1, 4 and 9, respectively, were identified for GLA1 (Fig. 1, Table 2), which explained 26.43% of the total phenotypic variance, and single QTL accounted for from 2.47 to 9.30% of the phenotypic variance (Table 2). The five QTLs, qgla1-1-1, qgla1-1-2, qgla1-1-3, qgla1-4-1 and qgla1-9-1, were in marker intervals umc1703-umc2151, mmc0041-bnlg1556, bnlg1429- bnlg1178, umc2281-bnlg1265 and bnlg1129-bnlg1375, respectively. And the nearest marker to them was umc2151, mmc0041, bnlg1429, umc2281 and bnlg1129, respectively (Table 2). Among the five QTLs, qgla1-1- 2, qgla1-1-3 and qgla1-4-1 were closely linked to the corresponding nearest marker (B5 cm), with the respective genetic distance of 0.3, 2 and 3cM. QGLA1-1-1, qgla1-1-2 and qgla1-1-3 were characterized as partial dominance, and qgla1-4-1 and qgla1-9-1 showed an overdominant effect. Four QTLs for GLA2, accounting for 19.62% of the total phenotypic variance, were identified on chromosome 1, 4, 5 and 6, respectively (Fig. 1, Table 2). Single QTL explained from 3.62 to 6.34% of the phenotypic variance (Table 2). QGLA2-1-1, qgla2-4-1, qgla2-5-1 and qgla2-6-1 were within marker intervals umc1601-bnlg2238, bnlg1337-bnlg1890, umc1098- bnlg557 and umc1083-umc1229, and were nearest to umc1601, bnlg1337, bnlg557 and umc1083, respectively. Among them, qgla2-4-1, qgla2-5-1 and qgla2-6-1 were in close proximity to the corresponding nearest markers, with the respective genetic distance of 2, 1.8 and 0.3 cm. In addition, qgla2-1-1 was characterized as partial dominance, qgla and qgla2-6-1 were overdominant, and qgla2-5-1 showed an additive effect. As for GLA3, six QTLs explaining 43.37% of the total phenotypic variance were identified on chromosome 1 (qgla3-1-1, umc1601-bnlg2238), 4 (qgla3-4-1, bnlg1755-bnlg2291), 4 (qgla3-4-2, bnlg1890-phi019), 5 (qgla3-5-1, umc1852-umc1870), 5 (qgla3-5-2, umc1416-umc1253) and 9 (qgla3-9-1, phi065-bnlg127), respectively (Fig. 1, Table 2). QGLA3-1-1 was in close proximity to bnlg2238 (3.4 cm), and qgla3-5-1 and qgla3-5-2 was in close to umc1852 (4 cm) and umc1416 (4 cm), respectively. In addition, complete linkages were identified in the present study between qgla3-4-1 and bnlg1755, qgla3-4-2 and bnlg1890, and qgla3-9-1 and phi065. Furthermore, qgla3-4-1 and qgla3-5-1 explaining 11.72% and 11.02% of phenotypic variance respectively, were two major QTLs. Those two major QTLs showed an additive effect and were derived from the female parent A (the additive effect was positive, Table 2). Three QTLs for TLN were detected on chromosome 2 (qtln-2-1, umc2178-bnlg1018), 3 (qtln-3-1, bnlg1796- bnlg1951) and 5 (qtln-5-1, umc1098-bnlg557) respectively (Fig. 1, Table 2), explaining 25.47% of the total phenotypic variance, and single QTL accounted for from 4.40% to 11.50% of the phenotypic variance (Table 2). QTLN-3-1 and qtln-5-1 were identified complete linkage with bnlg1796 and umc1098 respectively. A major QTL qtln-5-1 could explain 11.50% of phenotypic variance, and was also from A QTLN-2-1 and qtln-3-1 showed overdominant effect, and qtln-5-1 was partially dominant. Four QTLs for LLN were detected on chromosome 4 (qlln-4-1, bnlg1755-bnlg2291), 5 (qlln-5-1, Table 1. Variation of five traits between two parents and the F 2 population Parents F 2 population Trait A Mo17 Range Mean Skewness Kurtosis GLA1 (cm 2 ) GLA2 (cm 2 ) GLA3 (cm 2 ) TLN LLN

4 Can. J. Plant Sci. Downloaded from by on 04/09/ CANADIAN JOURNAL OF PLANT SCIENCE Fig. 1. Frequency distribution of GLA1, GLA2, GLA3, TLN and LLN in F 2 population derived from A Mo17. umc1098-bnlg557), 5 (qlln-5-2, umc1416-umc1253) and 9 (qlln-9-1, phi065-bnlg127) respectively (Fig. 1, Table 2), explaining 31.29% of the total phenotypic variance, and single QTL could explain from 4.83 to 12.50% of the phenotypic variance (Table 2). QLLN-5-1 was in close proximity to bnlg557 (3.8 cm), and qlln-5-2 was in close proximity to umc1416 (4 cm). QLLN-4-1 and qlln-9-1 showed complete linkage with bnlg1755 and phi065, respectively. A major QTL qlln-4-1 from A accounted for 12.50% of phenotypic variance. QLLN-4-1 and qlln-9-1 were characterized as partial dominance, qlln-5-2 was overdominant, and qlln-5-1 showed an additive effect.

5 Can. J. Plant Sci. Downloaded from by on 04/09/18 Table 2. QTL analysis and genetic parameter estimates for the stay-green traits in the F 2 population QTL designation Chromosome Position (cm) Marker intervals Size of intervals (cm) The nearest marker to QTL Genetic effect z a d jd/aj Gene action z F value Explained variance (%) qgla umc1703-umc umc *** PD qgla mmc0041-bnlg mmc *** PD qgla bnlg1429-bnlg bnlg *** PD qgla umc2281-bnlg umc *** OD qgla bnlg1129-bnlg bnlg *** *** 2.47 OD qgla umc1601-bnlg umc *** PD qgla bnlg1337-bnlg bnlg * *** 1.99 OD qgla umc1098-bnlg bnlg *** A qgla umc1083-umc umc * ** 1.91 OD qgla umc1601-bnlg bnlg *** PD qgla bnlg1755-bnlg bnlg *** *** 1.49 OD qgla bnlg1890-phi bnlg * ** 1.56 OD qgla umc1852-umc umc *** A qgla umc1416-umc umc * *** 3.82 OD qgla phi065-bnlg phi *** * 0.52 PD qtln umc2178-bnlg bnlg *** 6.64 OD qtln bnlg1796-bnlg bnlg * * 1.57 OD qtln umc1098-bnlg umc *** *** 0.70 PD qlln bnlg1755-bnlg bnlg *** *** 0.72 PD qlln umc1098-bnlg bnlg *** A qlln umc1416-umc umc * * 3.77 OD qlln phi065-bnlg phi *** * 0.70 PD z a, additive effect; d, dominant effect; jd/aj, dominant degree; A, additive (dominant degree00.2); PD, partial dominant (dominant degree ); D, dominant (dominant degree ); OD, over dominant (dominant degree1.20). *, ** *** Significant levels at 5, 1 and 05, respectively. WANG ET AL. * QTL MAPPING FOR STAY-GREEN IN MAIZE 253

6 254 CANADIAN JOURNAL OF PLANT SCIENCE chr1 chr2 chr3 chr4 umc1703 phi127 bnlg1182 phi072 Can. J. Plant Sci. Downloaded from by on 04/09/ chr5 umc2151 umc1725 bnlg421 phi037 mmc0041 bnlg1556 umc1601 bnlg2238 bnlg182 bnlg147 umc2226 bnlg1429 bnlg1178 bnlg1112 umc2308 bnlg386 phi128 phi048 phi087 umc1629 umc1098 bnlg557 umc1852 umc1870 phi113 umc1416 umc1253 DISCUSSION The pursuit of high yield, good quality and stress resistance is eternal for crop breeders, so the desirable crop production characteristics of functional stay-green genotypes make them very attractive. In the present study, using 189 F 2 population derived from a cross between a stay-green inbred line A and non-stay-green inbred line Mo17, we identified 14 QTLs for three stay-green related traits and eight QTLs for the other two traits. The female parent inbred line A used in this study had better yield-related traits than normal inbred lines, with a higher photosynthetic rate, higher chlorophyll content and longer qgla1-1-1 qgla1-1-2 qgla1-1-3 qgla2-5-1q GLA3-5-2 qgla2-1-1 qgla3-5-1 qlln-5-2 qgla3-1-1 qtln qlln umc1042 umc2178 bnlg1018 bnlg2277 umc1824 bnlg1297 bnlg1017 umc1165 phi96100 chr6 qtln-2-1 bnlg1538 umc1083 umc1229 bnlg1867 bnlg249 bnlg1188 umc1887 bnlg1617 bnlg1922 bnlg1732 umc1859 bnlg1740 umc qgla bnlg1754 umc1641 bnlg1496 bnlg1796 bnlg1951 mmc0022 bnlg1601 phi073 bnlg1019 bnlg1452 bnlg1129 bnlg1375 umc2191 umc2343 qtln-3-1 phi065 bnlg127 umc1037 bnlg1583 bnlg umc2148 bnlg1730 umc2281 bnlg1265 bnlg1741 bnlg1755 bnlg2291 dupssr34 umc1532 bnlg1337 bnlg1890 phi019 Fig. 2. Location of the QTLs for three traits related to stay-green (GLA2, GLA3 and LLN) and two other ones (GLA1 and TLN). GLA1, green leaf area per plant in the whole growing period; GLA2, green leaf area per plant at 30 d after flowering; GLA3, green leaf area per plant at the grain-ripening stage; TLN, total leaf number per plant in the whole growing period; LLN, left green leaf number per plant at the grain-ripening stage. Nomenclature of QTLs was as described by McCouch et al. (1997) chr9 umc1279 qgla1-9-1 qgla3-9-1 qlln-9-1 growth stage, indicating the presence of a functional stay-green genotype. The SSR linkage map constructed in this study covered cm in total, with an average distance of cm between two adjacent markers. The relatively large gap suggested that some QTLs may remain undetected due to the shortage of available markers in the corresponding regions (Li et al. 2007). Green leaves with photosynthetic activity are responsible for assimilating carbohydrates and then remobilizing them to newly developing organs or storage parts. Therefore, it is very important for plant to maintain sufficient green leaf area until the final stage of plant qgla1-4-1 qgla2-4-1 qgla3-4-1 qgla3-4-2 qlln-4-1

7 WANG ET AL. * QTL MAPPING FOR STAY-GREEN IN MAIZE 255 Can. J. Plant Sci. Downloaded from by on 04/09/18 development in order to achieve higher yield (Borrell et al. 2000a). Furthermore, green leaf area at maturity has proved to be an excellent indicator of stay-green (Borrell et al. 2000b), and it has been used by researchers to select drought-resistant sorghum lines (Rosenow et al. 1983; Henzell et al. 1992). In the present study, we analyzed three green-leaf-area related traits, GLA1, GLA2 and GLA3, and detected 15 related QTLs. Interestingly, QTLs for GLA1, GLA2 and GLA3 were all identified on chromosome 1, which indicates that chromosome 1 was important in controlling green leaf area. In addition, two QTLs for stay-green traits, qgla2-1-1 and qgla3-1-1, were detected in the same marker interval, between umc1601 and bnlg2238 on chromosome 1, which indicates that this genomic region was very important for maintaining green leaf area at the final developmental stage of maize. In addition, we analyzed two leaf-number related traits TLN and LLN, and detected three and four QTLs, respectively. Among them, qtln-5-1 and qlln-5-1 were detected in the same genomic region between the markers umc1098 and bnlg557 on chromosome 5. In maize, Pelleschi et al. (2006) and Tang et al. (2007) identified two and eight QTLs related to leaf number. Among them, qln9a, detected by Tang et al. (2007), between bnlg127 and bnlg1208 on chromosome 9, was near the QTL qlln-9-1 identified by us, between phi065 and bnlg127, indicating that the adjacent genomic regions were important for green leaf number. GLA3 and LLN are two traits at the same developmental stage, grain-ripening stage. Interestingly, some of their QTLs were identified at the same marker intervals: qgla3-4-1 and qlln-4-1 were between bnlg1755 and bnlg2291 on chromosome 4; qgla3-5-2 and qlln-5-2 were between umc1416 and umc1253 on chromosome 5; and qgla3-9-1 and qlln-9-1 were in an interval flanked by phi065 and bnlg127 on chromosome 9. The phenomenon of clustering of functionally related genes has been identified in many species, including common bean (Koinange et al. 1996), maize (Khavkin and Coe 1997), and rice (Xiong et al. 1999; Bres-Patry et al. 2001). This cluster phenomenon is explained by multifactorial linkage followed by natural selection favoring co-adapted traits and pleiotropy of some unknown key factor(s) controlling various traits through diverse metabolic pathway (Cai and Morishima 2002). The adjacent genomic location of QTLs for the two different stay-green related traits would facilitate their utilization in selective breeding of maize through marker-assisted selection. ACKNOWLEDGMENTS This work was supported financially by Seed Improvement Project of Shandong Province, China (20077 and 20086). Bassam, B. J., Caetano-Anolles, G. and Gresshoff, P. M Fast and sensitive silver staining of DNA in polyacrylamide gels. Anal. Biochem. 196: Beavis, W. D., Smith, O. S., Grant, D. and Fincher, R Identification of quantitative trait loci using a small sample of topcrossed and F 4 progeny from maize. Crop Sci. 34: Birch, C. J., Hammer, G. L. and Rickert, K. G Improved methods for predicting individual leaf area and leaf senescence in maize (Zea mays). Aust. J. Agric. Res. 49: Borrell, A. K., Hammer, G. L. and Henzell, R. G. 2000a. Does maintaining green leaf area in sorghum improve yield under drought? II. Dry matter and yield. Crop Sci. 40: Borrell, A. K., Hammer, G. L. and Douglas, A. C. L. 2000b. Does maintaining green leaf area in sorghum improve yield under drought? I. Leaf growth and senescence. Crop Sci. 40: Bres-Party, C., Lorieux, M., Clement, G., Bangratz, M. and Ghesquiere, A Heredity and genetic mapping of domestication-related traits in a temperate japonica weedy rice. Theor. Appl. Genet. 102: Cai, H. M. and Morishima, H QTL clusters reflect character assoxiations in wild and cultivated rice. Theor. Appl. Genet. 104: Cha, K. W., Lee, Y. J., Koh, H. J., Lee, B. M., Nam, Y. W. and Paek, N. C Isolation, characterization, and mapping of the stay green mutant in rice. Theor. Appl. Genet. 104: Crasta, O. R., Xu, W. W., Rosenow, D. T., Mullet, J. and Nguyen, H. T Mapping of post-flowering drought resistance traits in grain sorghum: association between QTLs influencing premature senescence and maturity. Mol. Gen. Genet. 262: Gentinetta, E., Ceppl, D., Lepori, C., Perico, G., Motto, M. and Salamini, F A major gene for delayed senescence in Maize. Pattern of photosynthates accumulation and inheritance. Plant Breed. 97: Guiame t, J. J., Teeri, J. A. and Nooden, L. D Effects of nuclear and cytoplasmic genes altering chlorophyll loss on gas exchange during monocarpic senescence. Plant Cell Physiol. 31: Haussmann, B., Mahalakshmi, V., Reddy, B., Seetharama, N., Hash, C. and Geiger, H QTL mapping of stay-green in two sorghum recombinant inbred populations. Theor. Appl. Genet. 106: Heath, O. V. S. and Gregory, F. G The constancy of the mean net assimilation rate and its ecological importance. Ann. Bot. 2: Henzell, R. G., Brengman, R. L., Fletcher, D. S. and McCosker, A. N Relationships between yield and non-senescence (stay-green) in some grain sorghum hybrids grown under terminal drought stress. Pages in M. A. Foale, ed. Proceedings of the Second Australian Sorghum Conference. Gatton, Australia. Jiang, G. H., He, Y. Q., Xu, C. G., Li, X. H. and Zhang, Q The genetic basis of stay-green in rice analyzed in a population of doubled haploid lines derived from an indica by japonica cross. Theor. Appl. Genet. 108: Kebede, H., Subudhi, P. K., Rosenow, D. T. and Nguyen, H. T Quantitative trait loci influencing drought tolerance in grain sorghum (Sorghum bicolor L. Moench). Theor. Appl. Genet. 103:

8 Can. J. Plant Sci. Downloaded from by on 04/09/ CANADIAN JOURNAL OF PLANT SCIENCE Khavkin, E. and Coe, E Mapped genomic locations for developmental functions and QTLs reflect concerted groups in maize (Zea mays L.). Theor. Appl. Genet. 95: Koinange, E. M. K., Singh, S. P. and Gepts, P Genetic control of the domestication syndrome in the common bean. Crop Sci. 36: Kosambi, D. D The estimation of map distance from recombination values. Ann. Eugenics 12: Kumar, U., Joshi, A. K., Kumari, M., Paliwal, R., Kumar, S. and Ro der, M. S Identification of QTLs for stay green trait in wheat (Triticum aestivum L.) in the Chirya 3 Sonalika population. Euphytica 174: Lander, E. S., Green, P., Abrahamson, J., Barlow, A., Daly M. J., Lincoln, S. E. and Newburg, L MAPMAKER: an interactive computer package for constructing primary genetic linkage maps of experimental and natural populations. Genomics 1: Li, S., Jia, J., Wei, X., Zhang, X., Li, L., Chen, H., Fan, Y., Sun, H., Zhao, X., Lei, T., Xu, Y., Jiang, F., Wang, H. and Li, L A intervarietal genetic map and QTL analysis for yield traits in wheat. Mol. Breed. 20: McCouch, S. R., Cho, Y. G., Yano, M., Paul, E. and Blinstrub, M Report on QTL nomenclature. Rice Genet. Newsl. 14: Montgomery, E. G Correlation studies in corn. Pages in 24th Annual Report, Agricultural Experiment Station, Lincoln, NE. Murray, M. G. and Thompson, W. F Rapid isolation of high molecular weight plant DNA. Nucleic Acids Res. 8: Pelleschi, S., Leonardi, A., Rocher, J. P., Cornic, G., de Vienne, D., The venot, C. and Prioul, J. L Analysis of the relationships between growth, photosynthesis and carbohydrate metabolism using Quantitative trait loci (QTLs) in young maize plants subjected to water deprivation. Mol. Breed. 17: Rosenow, D. T., Quisenberry, J. E., Wendt, C. W. and Clark L. E Drought tolerant sorghum and cotton germplasm. Agric. Water Manage. 7: Subudhi, P. K., Rosenow, D. T. and Nguyen, H. T Quantitative trait loci for the stay green trait in sorghum (Sorghum bicolor L. Moench): consistency across genetic backgrounds and environments. Theor. Appl. Genet. 101: Tang, J., Teng, W., Yan, J. Ma, Meng, X. Y., Dai, J. and Li, J Genetic dissection of plant height by molecular markers using a population of recombinant inbred lines in maize. Euphytica 155: Tao, Y. Z., Henzell, R. G., Jordan, D. R., Butler, D. G., Kelly, A. M. and McIntyre, C. L Identification of genomic regions associated with stay green in sorghum by testing RILs in multiple environments. Theor. Appl. Genet. 100: Thomas, H Sid: a Mendelian locus controlling thylakoid membrane disassembly in senescing leaves of Festuca pratensis. Theor. Appl. Genet. 73: Thomas, H. and Howarth, C. J Five ways to stay green. J. Exp. Bot. 51: Thomas, H. and Smart, C. M Crops that stay green. Ann. Appl. Biol. 123: Tollenaar, M. and Daynard, T. B Leaf senescence in short-season maize hybrids. Can. J. Plant Sci. 58: Van Oosterom, E. J., Jayachandran, R. and Bidinger, F. R Diallel analysis of the stay-green trait and its components in Sorghum. Crop Sci. 36: Voorrips, R. E MapChart: software for the graphical presentation of linkage maps and QTLs. J. Hered. 93: Xiong, L. Z., Liu, K. D., Dai, X. K., Xu, C. G. and Zhang, Q Identification of genetic factors controlling domestication-related traits of rice using an F 2 population of a cross between Oryza sativa and O. rufipogon. Theor. Appl. Genet. 98: Xu, W., Rosenow, D. T. and Nguyen, H. T. 2000a. Stay green trait in grain sorghum: relationship between visual rating and leaf chlorophyll concentration. Plant Breed. 119: Xu, W., Subudhi, P. K., Crasta, O. R., Rosenow, D. T., Mullet, J. E. and Nguyen, H. T. 2000b. Molecular mapping of QTLs conferring stay-green in grain sorghum (Sorghum bicolor L. Moench). Genome 43: Yoo, S. C., Cho, S. H., Zhang, H., Paik, H. C., Lee, C. H., Li, J., Yoo, J. H., Lee, B. W., Koh, H. J., Seo, H. S. and Paek, N. C Quantitative trait loci associated with functional stay-green SNU-SG1 in rice. Mol. Cells 24: Zheng, H. J., Wu, A. Z., Zheng, C. C., Wang, Y. F., Cai, R., Shen, X. F., Xu, R. R., Liu, P., Kong, L. J. and Dong, S. T QTL mapping of maize (Zea mays) stay-green traits and their relationship to yield. Plant Breed. 128: 5462.