| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a IITA-Maize Breeding, c/o L.W. Lambourn & Co., Carolyn House, 26 Dingwall Rd., Croydon CR9 3EE, UK
b INRAB-Plant Breeding
* Corresponding author (b.badu-apraku{at}cgiar.org).
ABSTRACT
Two maize (Zea mays L.) populations, TZE-W Pop DT STR C4 (Reg. No. GP-574, PI 654407), white- grained, flint/dent, and TZE-Y Pop DT STR C4 (Reg. No. GP-575, PI 654408), yellow-grained, flint/dent with drought tolerance and moderate levels of resistance to the flowering parasitic witchweed, Striga hermonthica (Del.) Benth, were developed at the International Institute of Tropical Agriculture (IITA). The two breeding populations were released to national maize programs in west and central Africa as source germplasm in 2004 for the development of Striga-resistant synthetic varieties, parental inbred lines, and hybrids. The populations were released for their superior grain yield under Striga-infested and noninfested conditions. They also have good levels of resistance to maize streak virus, tropical lowland rust (incited by Puccinia polysora Underw.), and blight [caused by Bipolaris maydis (Nisikado & Miyake) Shoemaker]. In multilocation trials in Benin Republic and Nigeria, 2006 and 2007, TZE-W Pop STR C4 outyielded the Striga-susceptible check TZE Comp. 4 by 44% under Striga infestation and 12% when noninfested. Under the same conditions, the increased yield for TZE-Y Pop DT STR C4 was 42% (Striga infested) and 16% (noninfested). The check variety TZE Comp. 4 suffered the highest Striga damage and supported the highest number of emerged Striga plants when Striga infested.
Abbreviations: ASI, anthesis-to-silking interval • DAP, days after planting • EPP, number of ears per plant • EV, experimental variety • IITA, International Institute of Tropical Agriculture • RSVT-Early • Regional Striga Variety Trial-Early • WAP, weeks after planting • WCA, west and central Africa • WECAMAN, West and Central Africa Collaborative Maize Research Network
Drought and Striga are the two most important constraints to maize (Zea mays L.) production and productivity in the savanna ecologies of west and central Africa (WCA). Annual yield losses from Striga spp. are estimated to cost U.S. $7 billion and are detrimental to the lives of over 100 million African people (M'Boob, 1986). Annual yield loss from drought stress in the savannas of WCA is estimated at 15%, although localized losses may be much higher in the marginal areas where the annual rainfall is below 500 mm and soils are sandy or shallow (Edmeades et al., 1995).
TZE-W Pop DT STR C4 (Reg. No. GP-574, PI 654407) and TZE-Y Pop DT STR C4 (Reg. No. GP-575, PI 654408) Striga-resistant and drought-tolerant maize populations have outstanding yield potential under Striga-infested and noninfested conditions and were released in 2004 by the International Institute of Tropical Agriculture (IITA) to national maize programs in WCA as source germplasm for the development of Striga-resistant synthetic varieties, parental inbred lines, and hybrids. The two source populations, TZE-W Pop DT STR and TZE-Y Pop DT STR, were developed from local and improved drought-tolerant germplasm identified through several years of extensive testing in WCA. The populations and several of the derived varieties have shown superior performance under both Striga-infested and noninfested conditions and have proved to be invaluable sources of Striga-resistant synthetics and inbred lines. Several Striga-resistant and/or drought-tolerant varieties and inbred lines from the two source populations have been made available to the national maize programs and farmers of WCA (Badu-Apraku et al., 2006; Badu-Apraku and Lum, 2007).
Methods
Development of Source Populations
A breeding program was initiated in 1994 at Ferkéssedougou, (Ferké) (9°30' N 5°10' W), Côte d'Ivoire, to develop the drought-tolerant and Striga-resistant early populations TZE-W Pop DT STR (white) and TZE-Y Pop DT STR (yellow). Backcrossing, inbreeding, hybridization, and selection were all adopted in the program. The details of the procedure used for the development of the two populations were described in detail by Badu-Apraku and Fakorede (2001) and Badu-Apraku et al. (2008). TZE-Y Pop DT STR was developed from diallel crosses involving outstanding maize germplasm identified through several years of extensive testing in WCA. DR-Y Pool BC2F2, KU 1414, and Tzi 28 (9499) served as the sources of drought tolerance. These sources were recombined in a half-sib recombination block to form TZE-Y Pop. To upgrade the level of Striga resistance, the Striga-resistant, fixed, yellow-grained inbred line Tzi 25 was crossed to TZE-Y Pop. Tzi 25 was derived from temperate germplasm (Kim et al., 1984), and its tolerance is inherited quantitatively by a multigenic system. In addition to Striga resistance, Tzi 25 has good levels of resistance to the major maize diseases in WCA and, furthermore, supports fewer emerged Striga plants than susceptible genotypes. As Tzi 25 is of intermediate maturity and hence the F1s were later in maturity, the F1 progenies were backcrossed to TZE-Y Pop to recover earliness. This was followed by five cycles of recombination using the half-sib system and selecting alternately for Striga resistance under artificial Striga infestation and induced moisture stress in an effort to improve Striga resistance and drought tolerance. The resulting population after five cycles of random mating was designated TZE-Y Pop DT STR C0.
The sources of drought tolerance for TZE-W Pop DT STR were Pool 16 DT, Pool 16 Sequia, DR-W Pool BC1F2, Tzi 3, and the inbred line TZi 9 (5012). The source of Striga resistance was the white-grained, fixed inbred line TZi 3 (1368 STR), and TZE Comp 4 was the source of high grain yield. In addition to Striga resistance, Tzi 3 has good levels of resistance to the major maize diseases in WCA. Diallel crosses were made among Pool 16 DT, Pool 16 Sequia x Pool 16 DT, Tzi 3, TZi 9, DR-white Pool BC1 F1, and TZE Comp 4. The F1 crosses were recombined using the balanced composite formed by bulking equal amounts of seed from selected families of Pool 16 Sequia x Pool 16 DT and Pool 16 DT as the sole pollen source to obtain TZE-W Pop. In an effort to introgress Striga resistance into TZE-W Pop, the backcrosses (1368 STR x Pool 16 DT) x TZE-W Pop and (Pool 16 Sequia x 1368 STR) x TZE-W Pop were planted under artificial Striga infestation and the agronomically desirable plants were selfed to produce S1 lines. These were evaluated under artificial Striga infestation, and the resistant families were selected and incorporated into TZE-W Pop to upgrade the level of Striga resistance. Similarly, backcrosses involving TZE-W Pop and 5012 were evaluated under drought, and selected tolerant families were identified and incorporated into TZE-W Pop. TZE-W Pop was taken through five cycles of compositing while alternately screening for Striga resistance and drought tolerance under artificial Striga infestation and drought conditions. The resulting population was designated TZE-W Pop DT STR.
The screening method developed by IITA Maize Program (Kim, 1991; Kim and Winslow, 1991) was used to screen for Striga resistance in the two maize populations at Ferké and Sinematialli (Sine), both in Cote d'Ivoire, and Mokwa and Abuja, Nigeria. The Striga seeds used for artificial infestation were collected from fields of sorghum [Sorghum bicolor (L.) Moench] at the end of the previous growing season and were mixed with finely sieved sand in the ratio of 1:99 by weight. About 5000 germinable Striga seeds were placed in each planting hole made on ridges spaced 75 cm apart with 40 cm between the holes. The sand served as the carrier material and provided adequate volume for rapid and uniform infestation. Three maize seeds were placed with the Striga seeds in the same hole. Screening of segregating materials derived from the two source populations was done using 5-m rows with susceptible checks planted at regular intervals of 10 rows. Both the segregating materials and the susceptible checks were Striga infested.
Selections for drought tolerance were made under controlled conditions at Ferké and Sine, as well as at Kamboinse (Burkina Faso). At Ferké and Sine, the crop was grown under irrigation during the dry season, using an overhead sprinkler irrigation system that applied 12 mm of water per week. Irrigation water was withheld to induce drought stress from about 2 wk before anthesis to the end of the season. At Kamboinse in the Sudan savanna zone, tied and untied ridges (Badu-Apraku et al., 2008) were used to simulate different levels of drought stress.
Recurrent Selection in Source Populations
In each of the two populations, S1 family recurrent selection was initiated for the improvement of grain yield and Striga resistance in 1996. In 1998, the first cycle of improvement in each population was completed by intermating the top 25 to 30% families identified in each population through progeny yield trials conducted in 1997 in Ferké (under artificial Striga infestation), Sine (high-yield environment), and Kamboinse (drought-stress environment). In addition, the top 7 to 10% families identified in TZE-W Pop DT STR C0 and TZE-Y Pop DT STR C0 were recombined to form the varieties EV DT-W 98 and EV DT-Y 98, respectively. Since then, each population has gone through three more cycles of S1 recurrent selection. From each of the three cycles of improvement, F1 progenies were screened under artificial Striga infestation and noninfested conditions at Ferké and/or Abuja and Mokwa. The number of progenies screened in each cycle ranged from 196 to 280, with a selection intensity of 25 to 30%. Based on the across-location data for each cycle of selection, 25 to 30% of the top performers of each population were recombined to reconstitute the respective populations. In addition, the top 10% S1 families of each cycle were intermated to form Striga-resistant experimental varieties (EV) for each population. The varieties extracted from C2 to C4 are EVDT-W 98 C2, EVDT-Y 98 C2, EV DT-W 2000 STR, EV DT-Y 2000 STR, and TZE-W Pop x 1368 STR, EV DT-W 99 STR C0, 2004 TZE-W Pop STRC C4, and 2004 TZE-Y Pop STR C4.
Evaluation of Progress
The regional Striga variety trial-Early (RSVT-Early) was conducted in 2006 and 2007 at three locations in WCA (Abuja and Mokwa in Nigeria and Angaredebou in the Republic of Benin). Twenty-two varieties were evaluated for grain yield performance and Striga resistance. These were the two source populations, the derived varieties, elite Striga-resistant varieties derived from other sources, a local check (the best available early variety nominated by each national collaborator), and a reference entry (a leading early cultivar provided by IITA West and Central Africa Collaborative Maize Research Network [WECAMAN] that was not resistant to Striga). A randomized complete block design with four replications was used in the trials. Each plot consisted of four rows, each 5 m long, spaced 0.75 m apart, with 0.40 m spacing between plants within the row. Three maize seeds were planted per hole in each trial. Two of the four rows in each plot were infested artificially with about 5000 germinable S. hermonthica seeds per hole at planting using the Striga infestation method of IITA Maize Program (Kim, 1991; Kim and Winslow, 1991). Except for Striga seed infestation, all management practices for both Striga-infested and noninfested plots were the same. Fertilization of the artificially Striga infested maize field was delayed until about 30 d after planting (DAP). At this stage of plant growth, 30 to 50 kg N ha–1 was applied as 15–15–15 NPK, depending on the fertility of the soil. The amount of NPK applied was monitored so as to ensure that there were not too many escapes as a result of high levels of fertilizer. Weeds other than Striga were handpicked regularly. The maize plants were thinned to two plants per stand about 2 wk after emergence to give a final population density of 66,000 plants ha–1. Observations recorded included grain yield, days to 50% anthesis and silking, ear number, ear rot, husk cover, plant and ear heights, and percentage root and stalk lodging on both infested and noninfested plots. Anthesis-to-silking interval (ASI) was determined as the difference between 50% silking and anthesis. Number of ears per plant (EPP) was obtained by dividing the total number of ears per plot by the number of plants harvested. In addition, host plant damage syndrome rating (Kim, 1991) and emerged Striga counts were made at 8 and 10 wk after planting (WAP) (56 and 70 DAP) in the Striga-infested rows, as described by Kim (1991) and Badu-Apraku (2007). Analysis of variance was performed on plot means of the individual characters combined for the sites using PROC GLM (SAS Institute, 1987).
Characteristics
Agronomic Description
Under Striga infestation, the white-grained, flint/dent early-maturing population silks about 56 DAP and grows to a height of about 132 cm. The yellow-grained, flint/dent early-maturing population silks 56 DAP and has a plant height of 138 cm. Under Striga-free conditions, both populations silk in 55 DAP with the white population reaching a height of 158 cm and the yellow, 167 cm.
Field Performance
Combined ANOVA across environments showed significant mean squares for genotypes, locations, years, and location x year for most traits studied under Striga infestation and Striga-free conditions (Table 1
). By contrast, the interactions of genotype x year, genotype x location, and genotype x year x location which together constitute the genotype x environment interactions, were not significant for the most important traits associated with resistance to Striga (grain yield, host plant damage rating symptoms, emerged Striga plants, and other traits recorded) except for ASI, plant height, EPP, and Striga damage at 8 WAP under artificial infestation and percentage stalk lodging when noninfested. The lack of genotype x year x location interactions for grain yield and most other traits of the early-maturing varieties under Striga-infested and Striga-free conditions suggest that there are no different biotypes of S. hermonthica at the different locations used in the study. This implies that the tolerance of the varieties will be consistent in all the environments used in the evaluations and confirms the merits in using multilocations (Ferké, Mokwa, Abuja, and Angaredebou) in our screening and evaluation programs. Similar findings were reported by Badu-Apraku and Lum (2007). Mean grain yield of the varieties ranged from 1477 kg ha–1 for the Striga susceptible check, TZE Comp 4 to 2084 kg ha–1 for 2004 TZE-W Pop STR C4 under Striga infestation and from 2028 kg ha–1 for TZE Comp.5-Y C6S6 (Set B) to 3171 kg ha–1 for TZE-W Pop DT STR C4 under noninfested conditions (Table 1). Under Striga infestation, TZE-W Pop DT STR C4 and TZE-Y Pop DT STR C4 were among the most promising genotypes in terms of grain yield, Striga damage rating, and Striga emergence. It is interesting to note that the derived varieties of the white population, 2004 TZE-W Pop STR C4 (derived from C4 of TZE-W Pop DT STR), and EV DT-W 99STR C0 (derived from C2 of TZE-W Pop DT STR) along with Pool 15 SR/ACR94 TZE Comp5/ACR94 TZE Comp5-W and Early STR-Syn F2 (varieties not from the recurrent selection program) were among the top-ranking genotypes. These varieties and the source populations were also among the highest-yielding genotypes when noninfested. TZE-W Pop STR C4 outyielded TZE Comp 4 by 44% under Striga infestation and 12% when Striga free. Under the same conditions, the increase for TZE-Y Pop DT STR C4 was 42% (Striga infested) and 16% (Striga free). Under Striga infestation, TZE Comp 4 sustained about 60% yield loss, suffered the highest Striga damage, and supported the highest number of emerged Striga plants, suggesting that the level of infestation was adequate for the trials. In comparison, TZE-W Pop DT STR C4 suffered yield losses of 42% and TZE-Y Pop DT STR C4 37%, while 2004 TZE-W Pop DT STR C4 had the least reduction in grain yield (31%) under Striga infestation. Overall, the most promising Striga resistant variety, 2004 TZE-W Pop DT STR C4, outyielded TZE Comp 4 by 49% under Striga infestation. Even though the susceptible check, TZE Comp 4, was the lowest-yielding entry under Striga infestation, it was one of the highest yielding when Striga free. The second highest yielding group under Striga infestation had yields ranging from 1527 to 1706 kg ha–1 and contained five genotypes, one of which (EV DT-W 99 STR) was a derived variety from the white population. The two Striga-resistant commercial checks, Acr 94 TZE Comp. 5-w and Acr 94 TZE Comp. 5-Y, were also in this group. There were six low-yielding genotypes in the third group. One of these, EV DT-W 2000 STR, is a derived variety from C3 of TZE-W Pop DT STR. TZE Comp.5-Y C6S6 (Set B), a synthetic formed from inbred lines extracted from TZE Comp 5 population, was outstanding in terms of number of emerged Striga plants at 8 and 10 WAP, but this was not translated into higher grain yield. The variety presumably has a low yield potential, as grain yield was also low when Striga free. The Striga damage rating of the white population was 4 at 8 and 10 WAP, while the yellow population had a rating of 3.6 and 4.1 at 8 and 10 WAP on a scale of 1 to 9, where 1 = little or no damage and 9 = severe damage. The mean numbers of emerged Striga plants per plot of the source populations and the derived varieties were generally high under artificial Striga infestation. For example, the white population had mean Striga emergence of 65 and 94 plants plot–1 at 8 and 10 WAP. The yellow population had 83 and 111 plants plot–1 at 8 and 10 WAP. It was striking that the susceptible check, TZE Comp 4, had high numbers of emerged Striga plants at 8 and 10 WAP (109 and 115 plants plot–1) and sustained severe Striga damage at 10 WAP. Under Striga infestation, the genotypes silked between 54 and 62 DAP, with plant height ranging from 119 to 141 cm. The white population silked in 56 DAP with plant height of 132 cm; the yellow population silked in 56 DAP with a plant height of 138 cm under Striga infestation. On the other hand, the days to silking of the yellow population was 55 and the plant height 167 cm, while the days to silking of the white population was 55 and plant height 158 cm when Striga free.
|
Badu-Apraku et al. (2008) reported broad-sense heritability estimates that were low to moderately high as well as high means and ranges of traits for TZE-W Pop DT STR C4 and TZE-Y Pop DT STR C4. This indicates that sufficient residual genetic variability still exists in each population to allow continued gain from selection for high grain yield and Striga resistance, especially in TZE-W Pop DT STR C4. Furthermore, the national maize programs of Côte d'Ivoire, Ghana, and Burkina Faso are already making use of the populations and their derived inbreds in breeding for resistance to S. hermonthica and drought stress. The national maize program of Burkina Faso has introgressed genes from selected drought-tolerant and Striga-resistant inbreds derived from the source populations into other breeding populations (Badu-Apraku et al., 2008).
Availability
Small quantities of seed (1 kg) may be made available to crop researchers for at least 5 yr on request to the Maize Breeding Unit, IITA, PMB 5320, Ibadan, Nigeria, for research purposes, including development and commercialization of new cultivars. In the United States, small quantities of seed may be obtained from the National Plant Germplasm System. We request that appropriate recognition of the source be given when the source populations contribute to an improved cultivar, inbred line, or hybrid. No application will be made for U.S. plant variety protection for the two source populations.
Acknowledgments
The authors are grateful to Dr A. Menkir for contributing some Striga-resistant entries evaluated in this study. We are also grateful to the staff of the IITA Maize Program in Ibadan for technical assistance, the national maize collaborators of Benin Republic for excellent management of trials, and Mrs. Rose Umelo for editing the manuscript. We also acknowledge the financial assistance of USAID.
Footnotes
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication June 22, 2008.
References
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Soil Science Society of America Journal | Journal of Environmental Quality |
The Plant Genome | |||
conditions at Abuja, Mokwa, and Angaredebou in 2006 and 2007.