Intro to Wheat (Triticum aestivum L.) Genetics and Production Technology

BIOLOGICAL CLASSIFICATION

Kingdom Plantae

Division Mangoliophyta

Class Liliopsida

Order Cyperales

Family Poaceae

Genus Triticum

Species T. aestivum L.

Intro to Wheat Genetics and Production Technology (Cover)

WHEAT TAXONOMY

In the 10,000 years of cultivation and selection process by man, wheat has evolved into numerous forms depending on morphological and genetic characteristics.

Common Name Traditional Classification (Dorofeev et al. 1979)

(Diploid (2x) Wild, Hulled)
i) Wild einkorn Triticum boeoticum Boiss.

(Diploid (2x) Domesticated, Hulled)

ii) Einkorn Triticum Monococcum L.

(Tetraploid (4x) Wild, Hulled)

iii) Wild Emmer Triticum Dicoccoides (K�rn. ex Asch. & Graebner) Schweinf.

(Tetraploid (4x) Domesticated, Hulled)

iv) Emmer Triticum dicoccum Scrank ex. Schubler.

(Tetraploid (4x) Domesticated, Free-threshing)

v) Durum or macaroni wheat Triticum durum Desf.

vi) Rivet or cone wheat Triticum turgidum L.

vii) Polish wheat Triticum polonicum L.

viii) Khorasan wheat Triticum turanicum Jacubz.

ix) Persian wheat Triticum carthlicum Nevski in Kom.

(Hexaploid (6x) Domesticated, Hulled)

x) Spelt wheat Triticum spelta L.

(Hexaploid (6x) Domesticated, Free-threshing)

xi) Common or bread wheat Triticum aestivum L.

xii) Club wheat Triticum compactum Host.

xiii) Indian dwarf or shot wheat Triticum sphaerococcum Percival.

MAJOR CULTIVATED WHEATS:

The major cultivated wheats are;

Durum Wheat

Common or bread wheat - most widely cultivated wheat in the world.
Durum wheat - the only tetraploid wheat which is cultivated and the second most cultivated wheat of world.
Einkorn wheat - A diploid species with wild and cultivated variants. Domesticated at the same time as emmer wheat, but never reached the same importance.
Emmer wheat - A tetraploid species, cultivated in ancient times but no longer in widespread use.
Spelt wheat - hexaploid species cultivated in limited quantities.

Club Wheat

Einkorn Wheat

Emmer Wheat

NAMING WITHIN CULTIVARS:

Wheat cultivars are classified by breeders and farmers on the following basis;

a) Growing Season - for example, winter or spring wheat

Spelt Wheat

b) Gluten Content - Hard wheat (high protein contents) and soft wheat (high starch contents)

c) Quality of wheat protein gluten - this protein determines the suitability of wheat for a particular dish. For example, strong and elastic gluten of bread wheat is used to make dough (traping carbon dioxide during dough making) and for making hard rolls for pasta durum wheat is used because of strong gluten which is not elastic.

d) Grain colour - red, white, amber, reddish-brown etc.

e) Protein Contents - bread wheat ranges from 10% in soft wheat and 15% in hard wheat.

Concept of Polyploidy

CHARACTERS OF WHEAT:

1. POLYPLOIDY:

Most of wild species of wheat are diploid or tetraploid. T. aestivum L. is hexaploid. Cells of diploid wheats each contains 2 complement of 7 chromosomes, one from mother and the other one from father (2n = 2x = 14, where 2n is number of chromosomes in somatic cells, and x is the basic chromosome number).

2. GENOME:

For Triticum aestivum, diploid number of chromosomes 2n = 6x = 42, since it is a Hexaploid species. For Durum (Triticum turgidum var. durum) 2n = 4x = 28.

3. HULLED AND FREE THRESHING WHEATS:

HULLED WHEAT: All wild wheat are hulled because their glumes are tightly enclose the grains. Each package of glume, lemma and palae is known as Spikelet.
At maturity the rachis disarticulates, allowing spikelets to disperse.
In semi-domesticated wheats, there was a semi-brittle rachis that breaks easily by threshing. The result is that when wheat is threshed the ear breaks into spikelets.
To obtain the grain, further processing such as milling and pounding is needed to remove the hulls or husks.
FREE THRESHING WHEAT: Free-threshing forms (or naked forms - such as durum wheat and common wheat) the glumes are fragile and the rachis tough. On threshing the chaff breaks up, releasing the grain.

Hulled wheats are often stored as spikelets because tough glumes give good protection against pests of stored grains.

HISTORY:

Wheat is considered to be the first of the cereals cultivated. The earliest cultivations were probably in the Fertile Crescent (The crescent shape fertile region in the Western Asia - it is believed that the earliest civilizations started here) and the Nile Delta. This includes Turkey, Lebanon, Syria, Israel and Egypt. Recent findings have narrowed this region to region of first domestication of wheat to the Southeastern Turkey and domesticate Einkorn wheat to Nevali Cori (64 km Northwest of Gobekli Tepe in Turkey) - has been dated to 9000 B.C. However, the evidences of exploitation of wild barley have been dated back to 23000 B.C and some consider same is true for pre-domesticated wheat.

ORIGION:

The archaeological records suggest that wild emmer was cultivated in Southern Levant (a geographical region including Lebanon, Israel, Syria, Jordan and Palestinian territories and parts of Turkey and Iraq) in 9600 BC.
Carbon-14 dating of einkorn remains suggest that it was cultivated in Abu Hureyra (Syria) in 7800-7500 BC.
The cultivation of wheat spread from this Fertile Crescent in 8000 BC. The spread of emmer wheat is traced from Fetile Crescent starting in 8600 BC and it reached Greece, India and Cyprus by 6500 BC, Egyptians were the first to make bread and use ovens for baking and developed food baking as the first large-scale food industry. By 3000 BC wheat reached England and Scandinavia. A millennium later it reached China.

DESCRIPTION AND GROWTH STAGES OF WHEAT PLANT:

DESCRIPTION

Plant Height:

Wheat is typically from 0.7 to 1.2 m tall.

Roots:

Wheat produces both seminal and nodal (or crown or adventitious) roots. The seminal roots form from the seed. The nodal roots form from the lower nodes, are associated with tillers and become increasingly important as the plant grows.

Stem and Tillers:

Wheat has a single main stem plus typically 2-3 tillers per plant. The number of tillers tends to increase with better growing conditions and with a lower crop density. Tillering starts at the 3-4 leaf stage; approximately when the first nodal roots can be seen.

Leaves:

Wheat leaves form at each node and include a leaf sheath that wraps around the stem and a leaf blade. Wheat has small auricles. These wrap around the stem at the point where the leaf sheath meets the leaf blade.

Spike:

The spike (also called the ear or head) forms at the top of the plant. A spike usually has 35-50 grains (or kernels). Most cultivars have awns, which are the appendages attached to each spikelet (unit of the inflorescence), giving the heads a bearded look.

Grain:

Wheat grain typically weighs 30-60 mg (i.e., 30-60 g/1000 grains) depending on variety and growing conditions. Reduced grain size often indicates moisture stress during grain filling.

STAGES OF GROWTH

At least five scales commonly are used worldwide to describe stages of growth of wheat and other small grains. The most widely used scale in the United States is the Feekes scale, although the Zadoks scale has more detail. Careful study of the developing crop and knowledge of factors that affect grain yield potential can enhance management decisions. These decisions can make wheat production more profitable.

Stages Name of Stage

Feekes 1.0: Emergence

Feekes 2.0: Beginning of Tillering

Feekes 3.0: Tillers Formed

Feekes 4.0: Beginning of Erect Growth

Feekes 5.0: Leaf Sheaths Strongly Erect

Feekes 6.0: First Node Visible

Feekes 7.0: Second Node Becomes Visible

Feekes 8.0: Flag Leaf Visible

Feekes 9.0: Ligule of Flag Leaf Visible

Feekes 10.0: Boot Stage

Feekes 11.0: Ripening

FARMING SYSTEMS:

In the Punjab region of India and Pakistan, as well as North China, irrigation has been a major contributor to increased grain output.
More widely over the last 40 years, a massive increase in fertilizer use together with the increased availability of semi-dwarf varieties in developing countries, has greatly increased yields per hectare.
The Australian wheat growing in the southern winter cropping zone, despite low rainfall (300 mm), is successful and this is achieved by 'rotation cropping' (traditionally called the ley system) with leguminous pastures and, in the last decade, including a canola crop in the rotations has boosted wheat yields by a further 25% .
In these low rainfall areas, better use of available soil-water (and better control of soil erosion) is achieved by retaining the stubble after harvesting and by minimizing tillage.

GLOBAL PRODUCTION:

According to FAO data in 2009, the most productive farms for wheat were in France producing 7.45 metric tonnes per hectare. The five largest producers of wheat in 2009 were China (115 million metric tonnes), India (81 MMT), Russian Federation (62 MMT), United States (60 MMT) and France (38 MMT). Pakistan is at 12th Stage with 21 MMT production.

AGRONOMY OF WHEAT CROP:

Wheat normally needs between 110 and 130 days between planting and harvest, depending upon climate, seed type, and soil conditions (winter wheat lies dormant during a winter freeze). Optimal crop management requires that the farmer have a detailed understanding of each stage of development in the growing plants.

LOCALITY AND SOIL:

Wheat is adapted to temperate regions from 30^o to 60^o N and 27^o to 40^oS latitude which recieves annual rainfall of about 500 - 1200mm.
WATER REQUIREMENTS: Higher precipitations causes lodging and diseases and interferes with field operation of the planting and harvesting.
Wheat is adapted to regions with a cool season followed by a bright, dry warm season. The best quality wheat is produced in areas having a cold winter and comparatively warm spring or summer with moderate rainfall.
It is a long-day plant; shorter days stimulate tillering and delay flowering.
SOIL: Wheat doesnot grow well on either poorly-drained or sandy soil.
It can be best grown on sandy loam to clay soils; however, silt to clay loam soils produce the best yields.

In 2009-10, wheat was cultivated on an area of 4092 thousand hectares in Pakistan producing 23864 million tonnes grain.

CULTURAL PRACTICES:

Seedbed Preparation and Manuring:

The land should be ploughed as soon as the previous crop is harvested.
In rainfed areas, fields should be ploughed deep at the onset of monsoon, and the soil stirred with a cultivator after heavy rainfall or at suitable intervals to control weeds and conserve moisture.
In irrigated areas, the land should be cultivated or ploughed to control weeds and dry the soil.
Fields should be irrigated at 7-15 days before planting wheat and be ploughed at a time when the field is neither so wet that structure is destroyed nor so dry that big clods are produced. A wooden plank should be dragged over the soil to break the clods and make a fine, smooth seedbed.
Two or three ploughing/cultivations are necessary to make a desirable seedbed for planting wheat with a drill.
Organic manures should be applied 40-50 days before sowing and incorporated into the soil by ploughing.
Commercial chemical fertilizers should be used to obtain higher yields.
All phosphatic and potassic fertilizers and half of the nitrogenous fertilizers should be applied before seedbed preparation and the remaining half of N with the first and second irrigation. In rainfed areas, all fertilizer should be applied before sowing.
In areas with zinc deficiency, zinc sulfate should be used to increase the yield. Fertilizer recommends of the agriculture department should be followed.

Time of Planting:

An average diurnal temperature of 22-23^oC have been found optimum for germination and emergence of wheat.
In the first to third week of November, temperature reaches optimum range for wheat sowing in different regions of Pakistan.
In rainfed areas, if rainfall occurs early, wheat can be planted in the last fortnight of October.
Late planting can be done up to the mid of December after that further delay results in yield reduction drastically.

Seed Rate:

Depending upon cultivar, time of sowing, method of sowing and availability of water, seed rate varies from 100-150 kg/ha.
In Sindh, 150 kg/ha is recommended for late sowing.
In rest of the provinces, 115-125 kg/ha are recommended for December sowing.

Method of Sowing:

In irrigated areas; a drill is best method, if drill is not available then use Kera method or seeds can be broadcasted. In case of broadcasting method, 5% more seeds are used.
In rainfed areas, the Pora method and drill are best for placing seeds in the moist soil.
In case of late sowing or sowing in saline soils, shallow sowing followed by irrigation has given better results than other methods.

Interculture and Weeding:

Usually no interculturing is done in wheat fields.
The bar harrow can be used in the early stages of a crop to uproot weeds.
If weeds are abundant, hand weeding can be done.
Interculture with three-tined hoe or rotary hoe before and after first irrigation will considerably reduce the weed population.
Rotation with winter forage legumes will also help to minimize the weed problems.
The agriculture department or agrochemical industries should be consulted about the use of weedicides or herbicides in the field. But they should be used as last option.

Rotation and Intercropping:

The following rotation schemes are commonly practiced;

Punjab I) wheat - rice

II) wheat - cotton

Sindh I) wehat - rice

KPK I) wheat - maize - covers

II) wheat - sugarcane

III) wheat - gram

Balochistan I) wheat - rice

II) wheat - sorghum - fallow - pulses - fallow

III) rapeseed - mustard - fallow - wheat

Irrigation:

There are two critical periods during which water stress reduces the yield greatly:

the period from the development of adventitious roots to the start of tillering and
the period from anthesis to the milk stage

Wheat usually needs 4 - 6 irrigations depending upon latitude, soil type, cultivar, rainfall and other factors affecting evapotranspiration.
Priority in watering should be assigned as follows: (1) adventitious root development, (2) anthesis, (3) milk stage, (4) tillering, (5) spike emergence, (6) dough stage.
The first irrigation should be given 12-18 days after emergence and the subsequent irrigations should be done according to the need of crop and the availability of water.

Harvesting:

When no more assimilates are partitioned to the grains in the wheat spike, the dry matter accumulation in the grain stops and wheat is physiologically mature.
Though the crop at physiological maturity contains more mositure than at harvest maturity, the crop can be harvested after physiological maturity with no loss in yield.
The best indicator of physiological maturity is complete loss of green colour from the glumes.
Complete loss of green colour from kernel is also a good indicator of physiological maturity.
Some of the new cultivars shatter their seeds if harvesting is delayed, so it is better to harvest at physical maturity, at least a few days earlier than traditional cultivars.
Tractor-driven threshers, combine harvestors can be used for mechanical harvesting.
Combine harvestors are growing popular due to ease of use and packing of grain at the field saves a lot of energy and labour.

Storage:

The grain must be dry enough for storage.
Wheat should be stored in clean, thoroughly dried and fumigated bags and bins and/or clean, fumigated stores.
The stores and bins should be inspected periodically and if there is any attack by rats or grain store pests, they should be eradicated with suitable bait or by fumigation.

[NOTE: Processing, Economic importance, Nutritional value and other uses, Diseases and Pests, Common cultivars and health concerns will be discussed in further series of this blog.]

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