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Contributors to this page: CIMMYT, Mexico (Suketoshi Taba, Bonnie J. Furman), with inputs also received from IITA, Nigeria (Dominique Dumet), EMBRAPA (maize and sorghum genebank), Brazil (Flavia Teixeira), USDA (ARS/NC7, ISU), USA (Mark Millard).
Planting and cultural practices for characterization
A genebank is often constrained by local environments that may be differ from ‘best practice’ defined environments.
- If possible choose an environment corresponding to the original collection site conditions.
Maize needs a large amount of macronutrients: nitrogen, potassium, and phosphorus. Maize often suffers of iron and zinc micronutrient deficiency.
- Maize genotypes can differ in adaptation in the different soils and nutrient conditions.
- Normal maize growth should be supported by the best possible soil conditions for cultivation to support accurate maize germplasm characterization
For example, CIMMYT Tlatizapan station has black vertisols that often limit iron availability for maize growth due to the alkaline conditions of the soil. Some inbred lines and landrace accessions present a retarded growth and even fail to flower due to these conditions.
- 500-700 mm is the optimal range in non-irrigated maize growing.
Planting seasons may have a substantial effect on maize growth patterns. Even in the same locality, a winter season has slower plant growth and more days to flower than a summer season. Rain fall and temperature regimes significantly influence maize growth pattern. In addition, data on leaf diseases, ear quality, ear rots and plant lodging can be affected by wind, edaphic and biotic environmental factors in the different growing seasons.
- Characterization data should be associated with the season in which the maize germplasm accessions are characterized.
- When characterization is performed at the same time of regeneration, use 16 rows, 5 m long, with 256 plants per plot for landrace accessions.
- If an evaluation trial is conducted for characterization purposes only, plant one accession in two 5 m long rows and in 2 replications.
- Half-size plots can be used for inbred lines.
- The distance between rows should be 75 cm.
- Adjust the plot size according to maturity and plant height, to facilitate normal plant growth.
Sampling area/border area
- Use at least one border row encircling characterization trials, to avoid border effects.
- For tall and late tropical maize landraces, plant two seeds per hill to establish 16 plants per row after thinning.
- Adjust density according to maturity and plant height.
- Aged seeds from the genebank storage may have a reduced rate of field germination.
- Single seeds per hill may be planted if seed accessions have good seed germination.
- Normally un-replicated entries are planted for the regeneration of accessions.
- For characterization trials of accessions, two replications are often used for a more rigorous statistical analysis.
Standard check cultivars
- In replicated characterization trials, standard check cultivars can be landrace accessions and single cross hybrids or commercial hybrids.
- Several common check cultivars should be included across different characterization trials.
Frequency of standard check cultivars
- Usually 4-5 check cultivars can be used across evaluation trials.
Time of day for data collection
- Field data is usually best collected in the morning.
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Morphological descriptors for characterization
- See list for maize descriptors developed by IBPGR (now Bioversity International) and CIMMYT, 1991.
- Key access and utilization descriptors for maize, developed by Bioversity International and an international advisory group (2009).
Pictures for characterization
Sufficient detail should be captured in images to taxonomically identify the plant and demonstrate the traits that show variation.
- Take images for character(s) which may be difficult to describe verbally.
- Ear type, transversal ear dissection and kernel type.
Herbarium samples for characterization
- Some genebanks may keep single ears from regeneration and conserve them in the active collection.
- Ear and kernel photos can however replace the herbarium samples.
Molecular descriptors for characterization
- Not yet incorporated, but possible in the future when molecular breeding databases are developed for genebank accessions (another dataset will be needed).
- Not yet incorporated in the characterization database (another dataset will be needed).
Nutritional traits for characterization
- Not yet incorporated in the characterization dataset (another dataset will be needed).
- Core subsets should be chosen based on various criteria, using the evaluation and characterization data (these can be efficient selection tools for the genetic variation in the genebank collection).
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Recording information during characterization
Below is an example of a maize descriptor used in the CIMMYT maize bank regeneration/characterization field book descriptors:
- Planting date (date).
- Field germination (%).
- Number of plants per plot (number).
- Number of tillers per plant (number).
- Days to silking (50% of the plants per plot) (number of days).
- Days to pollen shedding (50% of the plants per plot) (number of days).
- Tassel type (primary or secondary or tertiary branches).
- Pubescence of culm (presence or absence).
- Plant color (stalk) (green, purple, sun red, red).
- Leaf disease (see section on Plant Health).
- Insect damage (see section on Plant Health).
- Plant height (cm).
- Ear height (cm).
- Leaf number above the fist ear (number).
- Husk cover (rating 1-5 scale: 1 is good and 5 is poor).
- Harvest date (date).
- Field weight/plot (kg).
- Ear length (cm).
- Ear diameter (cm).
- Kernel length (mm).
- Kernel width (mm).
- Kernel row number (number).
- Grain type [floury, morocho, dent, flint, pop, sugary, waxy, white cap, QPM (modified oqaque-2)].
- Grain colour (white, yellow, purple, orange, variegated, brown, red, sun red, mottled).
- Ear rot (%) (see section on Plant Health).
- Stalk rot (%) (see section on Plant Health).
- Stalk lodging (%).
- Root lodging (%).
- Plant senescence [days from silking (50%) to ear leaf senescence].
- Shelling (%).
- Harvest grain moisture (%).
- Grain yield (ton/ha, based on 13.5% grain moisture).
- Race class: primary, secondary (maize race name, local name).
- Adaptation zone (tropical lowland, tropical highland, subtropical, and temperate zone).
- Agronomic scale (rating of 1-5 scale: 1 is good and 5 is poor).
References and further reading
Bioversity International, CIMMYT. 2009. Key access and utilization descriptors for maize genetic resources. Bioversity International, Rome, Italy; International Maize and Wheat Improvement Center, Mexico. Available here.
IBPGR and CIMMYT. 1991. Descriptors for Maize. International Maize and Wheat Improvement Center, Mexico City; International Board for Plant Genetic Resources, Rome. Available here.
Taba S, Diaz J, Franco J, Crossa J. 1998. Evaluation of Caribbean maize accessions to develop a core subset. Crop Science. 38:1378-1386.
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