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Growth & Development Of Wheat // The growth cycle of wheat
The growth cycle of wheat has the following divisions: germination, seedling establishment and leaf production, tillering and head differentiation, stem and head growth, head emergence and flowering, and grain filling and maturity
Figure 1. Spring wheat growth and development in Minnesota with approximate time to various growth stages. Zadoks code for each stage shown in parentheses.
Germination, seedling establishment, and leaf production
When a kernel is sown, the germination process begins. The radicle and seminal roots first extend, followed by the coleoptile ( figure 2 ). The crown is usually separated from the seed by a sub-crown internode. The length of this internode is greater as the depth of planting increases. As the coleoptile emerges from the soil, its growth stops and the first true leaf pushes through the tip ( figure 3 ).
Figure 2 . Germinating wheat kernels showing radicle, seminal roots, and coleoptile. Kernel on left is at Zadoks stage 04 and kernel on right is at stage 07.
After seedling emergence, leaves are produced at a rate of about one every 4 to 5 days. Figure 4 shows a young seedling at the two-leaf stage. A total of eight or nine leaves are usually produced
Figure 3 . Wheat seedlings at Zadoks stage 10. First leaf emerging from tip of coleoptile.
Figure 4 . Seedling at two-leaf stage (Zadoks stage 12). The third leaf is not yet 50 percent emerged.
Tillering and head differentiation
Tillering is an important development stage that allows plants to compensate for low plant populations or take advantage of good growing conditions. Tiller appearance is closely coordinated with the appearance of leaves on the main shoot. Tillers can form at the points of attachment of the coleoptile and the lower leaves on the main shoot.
During the time that tillering occurs, another less obvious but extremely important event occurs: the initiation of heads on the main shoot and tillers. Although the head at this time is microscopic, the parts that will become the floral structures and kernels are already being formed. When head formation is complete, the stem begins elongating. This corresponds to the "jointing" stage ( figure 5 ). A plant usually has about five leaves at this time.
Stem and head growth
Lower stem internodes on the plant remain short throughout development. The fourth internode is usually the first to elongate in a plant with nine total leaves. This is followed in sequence by the internodes above it ( figure 6 ). Each stem internode up the plant becomes progressively longer, and the last stem segment to elongate, the peduncle, accounts for a considerable proportion of the total stem length.
Stem elongation coincides with the period of rapid head growth in which the individual florets become prepared to pollinate and be fertilized. Throughout the preheading period, differences in the duration of the various developmental phases among shoots on the same plant help synchronize development.
Figure 5 . Wheat plant just after jointing (Zadoks stage 31).
Figure 6 . Base of wheat plant during early stem elongation. Individual stem internodes are elongating in sequence. Note the developing head.
Head emergence and flowering
As the stem continues to elongate, the head is pushed out of the flag leaf sheath, a stage referred to as "heading." Within a few days after heading, flowering (pollination) begins in the head, starting first with the florets in the central spikelets. Within the next few days flowering progresses both up and down the spike. Flowering is usually noted by extrusion of the anthers from each floret ( figure 8 ), although this can change depending on the variety and weather conditions. If the anthers within a floret are yellow or gray rather than green.
Figure 7 . The upper half of a plant at boot stage (Zadoks stage 45). Note swelling of sheath indicating the position of the developing head.
Figure 8 . Head at flowering (Zadoks stage 68) showing extruded anthers along most of head's length.
Kernel growth and maturity
Figure 9 shows the growth pattern for an individual wheat kernel. Growth progresses in three distinct phases spanning about four weeks under usual conditions. In the first phase, the "watery ripe" and "milk" stages, the number of cells in the endosperm (the major starch and protein storage portion of the kernel) is established.
Figure 9 . Kernel growth for a typical medium maturity spring wheat variety based on actual field observations at St. Paul, Minnesota. Conditions of the specific crop season will modify this pattern somewhat as well as the final weight attained. Note the water percentage in the kernel and its decline at the approximate time of physiological maturity.
Finally, growth of the kernel declines about three weeks into grain filling and its weight approaches a maximum attained at physiological maturity. As the kernel approaches maturity, its consistency becomes "hard dough." Figure 10 illustrates the appearance of wheat kernels during this developmental sequence.
Figure 10 . Kernels at various times during grain filling: a) kernel at watery ripe (Zadoks stage 71), b) kernel at late milk (Zadoks stage 77), c) kernel at soft dough (Zadoks stage 85), d) kernel at hard dough (Zadoks stage 87) showing loss of green color, and e) kernel ripe for harvest (Zadoks stage 92).
Kernel moisture does not always determine when physiological maturity occurs. A better indicator of maturity is when the head and the peduncle lose their green color ( figure 11 ). The green color is lost from the flag leaf blade when the kernel has attained about 95 percent of its final dry weight.
Figure 11 . Head at approximately physiological maturity when the kernels have attained maximum dry weight (Zadoks stage 89). Note the green color is gone from the peduncle and head parts.
   The part of the flower that produces the pollen.

   The sheath that encloses the first main shoot leaf and provides protection as it emerges from the soil.

Flag leaf:
   The leaf immediately below the head.

   An individual flower within the head.

   The pair of bracts located at the base of a spikelet in the head.

   The part of a stem between two nodes.

    Stage of wheat development when stem nodes are first detected above the soil: Zadoks stage 31.

Leaf blade:
   The flattened portion of a leaf above the sheath.

Leaf sheath:
   The lower part of a leaf enclosing the stem.

Main shoot:
    The primary shoot that emerges first from the soil and from which tillers originate.

Node (Joint):
   A region on the stem where leaves are attached.

   The top section of the stem between the flag leaf and the head.

Penultimate leaf:
   First leaf below the flag leaf.

Seminal roots:
   Roots arising at the level of the seed.

   The flower of a grass consisting of a pair of glumes and one or more enclosed florets.

   A shoot originating from the base of the plant.
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