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Plant interactions with temperature and day length prompt the transition from vegetative to reproductive stages. Reproductive development in true winter varieties is initiated by vernalization during exposure to cool temperatures for a required length of time. Temperatures below 50°F induce cold hardening and satisfy vernalization requirements. The required period of low temperature exposure varies with variety and decreases with lower temperatures and advancing plant development. In addition to vernalization, exposure to progressively longer day length periods is necessary to initiate reproductive development.

Spring varieties do not possess an absolute vernalization requirement. Reproductive development in most spring varieties is triggered by light and accumulative heat units (growing degree days)

The head or spike is initiated on each tiller during the fourth-leaf stage and before stem elongation begins. The maximum number of kernels that may mature on each head is determined by the number of florets that are initiated. Florets are first initiated in the middle portion of the microscopic head and then outward toward the ends (Fig. 28). Stress conditions may cause florets to abort in the reverse order in which they were initiated, resulting in empty or sterile florets at the ends of the head. Once head formation is complete, stem elongation elevates the terminal growing point of each tiller upwards within the leaf sheaths.

Fig. 28. Magnified head of six-leaf wheat plant.

Fig. 33. Cross section of wheat stem base during early stem elongation.
 

Fig. 34. First detectable node on the main stem of a wheat plant.

Fig. 29. Six-leaf wheat plant with four tillers and one node. (Z=16,24,31, F=6, H=6.3)

Fig. 30. Dissected six-leaf wheat plant with four tillers.

Stem elongation or jointing occurs as a result of internode elongation. Usually a plant has about five to six leaves on the main shoot when internode elongation first begins (Fig. 29-32). Throughout development, the lower four nodes remain in the crown. The fifth node may remain in the crown or be elevated slightly and nodes six and seven are generally elevated above the soil. The elongating internode is hollow between the crown and the elevated growing point, except in solid stem varieties. Rapid stem or internode elongation brings the developing head above the soil surface. Each elongating internode becomes progressively longer and eventually leads to head emergence.

Stems can be split with a knife to determine if a plant is in early stem elongation stage (Fig. 33). As the internodes elongate, the nodes become visually detectable on the stem and are easily counted (Fig. 34). The mature stem of most wheat and barley varieties has from three to four visible nodes.

The peduncle, the last elongated internode which supports the head, accounts for a good proportion of the overall stem length. Height is genetically determined but is subject to environmental influence. Certain growth regulators reduce plant height and increase lodging resistance. Their application is timed to inhibit peduncle elongation.

The period of rapid head growth in which individual florets become ready for pollination and fertilization parallels stem elongation (Fig. 35). Tiller development is in synchronization with the main stem so that tillers flower soon after the main stem.

Fig. 35. Wheat head development from six-leaf (left) to flag-leaf emergence.

Fig. 31. Seven-leaf wheat plant with four tillers and two detectable nodes. 
( Z=18,24,37, F=8, H=7.6)

Fig. 32. Dissected seven-leaf wheat plant with two tillers and two detectable nodes.

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 Date page last edited 09/19/2001