| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
IITA, Oyo Road, PMB 5320, Ibadan, Nigeria
* Corresponding author (a.menkir{at}cgiar.org)
The International Institute of Tropical Agriculture (IITA) has developed maize (Zea mays L.) inbred lines (Reg. no. GP-428 through GP-453 and PI 639925 through PI 639950) with resistance to a parasitic weed, Striga hermonthica (Del.) Benth. adapted to the lowlands (TZSTRI101 to TZSTRI117) and midaltidues (TZSTRI118 to TZSTRI126). Striga hermonthica infests millions of hectares of arable land in sub-Saharan Africa and limits cereal crop production including maize (Lagoke et al., 1991). These Striga resistant lowland tropical inbred lines also possess resistance to southern corn leaf blight [caused by Bipolaris maydis (Nisikado & Miyake) Shoemaker], southern corn rust (caused by Puccinia polysora Underw.), and Maize streak virus, while the Striga resistant midaltitude inbred lines possess resistance to northern leaf blight [caused by Exserohilum turcicum (Pass.) K.J. Leonard & E.G. Suggs.], common corn rust (caused by Puccinia sorghi Schwein.), and Maize streak virus. These lines are presently at the S6 to S8 stages of inbreeding and were released as Striga resistance source germplasm through regional trials in and outside of west and central Africa.
The lowland Striga resistant inbred lines were derived from Striga resistant genetically broad-based populations (TZL COMP.1-W, TZE COMP.5, and Zea diploperennis Iltis, Doebley and Guzman BC4) and synthetics (STR SYN-W and STR SYN-Y) that also possess resistance to Maize streak virus. TZSTRI101 to TZSTRI103 were extracted from the fourth cycle of selection of TZL COMP.1-W formed by crossing TZB-SR with seven Striga resistant maize inbred lines from IITA (Kling et al., 2000). This composite has undergone six cycles of recurrent selection, using S1 selection in C0 and C1, full-sib family selection in C2, S2 selection in C3, full-sib family selection in C4, and S1 selection in C5, under artificial infestation with S. hermonthica in the field at Abuja (9°16' N, 7°20' E, altitude 490 m) and Mokwa (9°18' N, 5°04' E, altitude 210 m) in Nigeria. TZSTRI104 to TZSTRI108 were extracted from a BC4 population involving a selected accession of teosinte (Zea diploperennis) that supported little or no S. hermonthica emergence as a donor parent (Kling et al., 2000). The wild accession was crossed to an adapted maize germplasm and the resulting F1 was backcrossed four times to four adapted maize genotypes (9022–13, TZL COMP.1-W, SUWAN 1-SR, and STR SYN-W) under artificial infestation with S. hermonthica in the screen house at Ibadan, Nigeria, to form a Z. diploperennis BC4 population. TZSTRI109 to TZSTRI111 were developed from two synthetics (STR SYN-W and STR SYN-Y) developed by intercrossing four white-endosperm and four yellow-endosperm Striga resistant maize inbred lines, respectively (Kim et al., 1998). These synthetics were improved for resistance to Striga under artificial infestation with S. hermonthica at Abuja and Mokwa. TZSTRI112 to TZSTRI117 were developed from an early-maturing composite (TZE COMP.5) formed by crossing TZESR-W C3 with 10 Striga resistant inbred lines (Kling et al., 2000). The composite has undergone seven cycles of recurrent selection using the S1 and full-sib family selection schemes under artificial infestation with S. hermonthica in the field at Mokwa and Abuja in Nigeria.
S1 lines extracted from each source population (TZL COMP.1-W, TZE COMP.5, Z. diploperennis BC4 and STR SYN-W and STR SYN-Y) were evaluated in replicated trials in single-row plots under artificial infestation with S. hermonthica in the field at Abuja and Mokwa. To ensure uniform infection with the parasite, single plants in each S1 line were infested with approximately 3000 germinable Striga seeds mixed with fine sand. The S. hermonthica seeds were collected from sorghum [Sorghum bicolor (L.) Moench] and millet [Pennisetum glaucum (L.) R. Br.] fields in Nigeria. At each location, Striga-induced damage symptom was visually rated at 10 wk after planting on a scale of 1 to 9, where 1 = no visible Striga damage symptom and 9 = all leaves completely scorched resulting in premature death (Kim, 1994). Numbers of emerged Striga plants were counted 10 wk after planting; grain yield and other agronomic traits were also recorded under Striga infestation.
At the S1 to S3 stages of inbreeding, rows that exhibited Striga damage symptom ratings of 5 or less, supported the least number of Striga plants and produced high grain yields with good ear aspect scores (<5 scores) under Striga infestation in a trial conducted at two locations, were selected. At the same time the S1 to S3 lines were planted at Ibadan (7°26' N, 3°54' E, altitude 150 m) and Striga resistant or tolerant lines selected based on their reaction to S. hermonthica in the field were selfed. Single plants were selected from each selected row based on visual assessment for bigger plants with healthy leaves, synchrony between pollen shed and silking, low ear placement, resistance to lodging, and well-filled ears. The selected S2 or S3 plants from each source population were grown ear-to-row in a replicated trial at Abuja and Mokwa under artificial infestation with S. hermonthica and the same selection criteria described above were used to advance resistant lines.
The S4 lines selected for resistance to S. hermonthica were evaluated in replicated trials as lines per se and in hybrid combinations with a resistant or tolerant inbred tester at Abuja and Mokwa with and without Striga infestation. S4 lines that showed lower Striga damage symptoms (
5 score), supported fewer Striga plants, and produced higher yields, were advanced to S5 and subsequent stages of inbreeding with repeated evaluation for resistance to S. hermonthica. Inbred lines that formed hybrids in combination with tester inbred lines, which supported fewer emerged Striga plants, sustained lower Striga damage symptoms and produced as high as or higher yields under Striga infestation in comparison with a known Striga tolerant hybrid check in field trials (Kling et al., 2000; Menkir et al., 2004), were selected. These lines were released to the national agricultural research systems and seed companies in and outside of west and central Africa.
In the context of Striga research, Ejeta et al. (1991) defined resistance as the ability of a genotype to support significantly fewer Striga plants and produce higher yield than a susceptible genotype under Striga infestation. On the other hand, tolerant genotypes germinate and support as many Striga plants as do susceptible genotypes without sustaining a corresponding reduction in grain yield and overall plant productivity. To obtain a precise estimate of yield loss from Striga, each line was planted in one infested and one noninfested rows of 5-m length spaced 0.75 m apart with 0.25-m spacing between plants within each row (Kling et al., 2000). For the same genotype, the infested row was planted directly opposite to the noninfested one separated by 1.5-m alleys. To differentiate between resistant and susceptible maize genotypes under Striga infestation, only half of the recommended rate of nitrogen (60 kg ha–1) was applied in both the inbred and testcross trials. In field trials involving the lowland Striga resistant maize inbred lines conducted at Abuja and Mokwa with and without Striga infestation for 2 yr, a susceptible inbred line (5057) exhibited about 90% yield loss under Striga infestation, sustained the highest Striga induced damage symptom and supported the largest number of emerged Striga plants (Table 1 and 2). In the first trial (Table 1), TZSTRI101 to TZSTRI108 had yield losses ranging from 7 to 26%, which were lower than that of the tolerant inbred line, 1368STR (42%). Also, these lines sustained lower Striga-induced damage symptom and supported fewer Striga plants than 1368STR. Several of these lines were found to be as high yielding as or higher yielding than the tolerant inbred line under no Striga infestation. These inbred lines silked between 62 and 67 d and had plant heights varying from 123 to 157 cm under Striga infestation. To provide an indication of the yield levels of these lines in hybrid combinations, grain yields of their testcrosses recorded under Striga infestation in different trials are presented in Table 1. The testcrosses produced grain yields varying from 3144.4 to 4419.3 kg ha–1, which were similar to or higher than that of a tolerant hybrid check (2196.4 kg ha–1). All the lines are white, except TZSTRI106, with most of them having flint or semiflint kernel texture (Table 1). In the second trial (Table 2), TZSTRI109 to TZSTRI117 sustained yield losses varying from 6 to 27%, which were similar to or lower than that of the resistant check inbred line, 9450 (30%). These lines showed lower Striga-induced damage symptoms and supported as many as or more Striga plants than 9450. The lines also yielded as much as or more than the resistant inbred check under no Striga infestation. These inbred lines silked between 56 and 64 d and had plant heights ranging from 98 to 138 cm under Striga infestation. Testcross grain yields of these inbred lines under Striga infestation (Table 2) varied from 3071.9 to 3950.5 kg ha–1, which were similar to or higher than that of a tolerant hybrid check (2681.8 kg ha–1). The lines are yellow and have predominantly flint or semiflint kernel texture (Table 2).
|
|
|
Small quantities of seed (30 kernels) are available to crop researchers on written request to the leader of the maize breeding unit at IITA, PMB 5320, Ibadan, Nigeria. In the USA small quantities of seed may be obtained from the National Plant Germplasm System (NPGS). Recipients of seed are requested to make appropriate recognition of the original seed source of the Striga resistant maize inbred lines if used for the development of new lines, hybrids or synthetics.
ACKNOWLEDGMENTS
This research was conducted at the International Institute of Tropical Agriculture (MS no. IITA 05/29/JA) and financed by IITA. The authors express their appreciation to all staff members that participated in planting, data recording, harvesting, and management of the trials at two locations.
NOTES
Accepted for publication October 31, 2005.
REFERENCES
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Vadose Zone Journal | |||
| Journal of Natural Resources and Life Sciences Education |
Soil Science Society of America Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
