"By the time heading occurs, the development of all shoots (main stem and tillers) on the same plant is in synchronization even though there were large differences as to when the initiation of the various shoots occurred (i.e. tiller initiation occurs later than the main stem). However, throughout the pre-heading period, differences also occur in the duration of the various developmental phases among the shoots (i.e. developmental phases for tillers are shortened), which serves to synchronize tiller development with the main stem so that tiller head emergence and flowering occurs soon after the main stem has headed and flowered.
The heading stage begins when the tip of the spike (head) can be seen emerging from the flag leaf sheath (Feekes 10.1; Zadoks 50), and emergence continues until the head is completely emerged (Feekes 10.5; Zadoks 58). The heading date in most wheat varieties is determined by temperature (accumulation of heat units). In some varieties, a combination of heat accumulation and day length determines heading date.
Shortly after the wheat head has fully emerged, flowering (anthesis) occurs. However, flowering and pollination in cereals may occur either before or after head emergence, depending on plant species and variety. Thus, cereals are classified as either open-flowering or closed-flowering types. Flowering occurs in open-flowering types shortly after head emergence. Most varieties of wheat are of the open-flowering type. Generally, flowering in wheat begins within three or four days after head emergence. Open flowering is characterized by extrusion of the anther (reproductive portion of the flower which produces pollen) from each floret on the head. In contrast, closed-flowering types of varieties or cereals (i.e. barley) flower prior to head emergence and the anthers remain inside each floret.
Flowering and pollination of wheat normally begins in the center of the head and progresses to the top and bottom of the head. Pollination is normally very quick, lasting only about three to five days. Pollination occurs slightly later on tillers than on the main stem, but all heads on a plant pollinate within a few days of each other. Wheat is largely self-pollinated, and pollination and fertilization has already occurred before the pollen-bearing anthers are extruded from the florets. Kernels per head are determined by the number of flowers that are pollinated. Pollen formation and pollination are very sensitive to environmental conditions. High temperatures and drought stress during heading and flowering can reduce pollen viability and thus reduce kernel numbers.
Flowering is the transition between two broadly categorized growth stages in wheat. In the first stage, vegetative growth, reproductive initiation, and reproductive development occur and determine the final yield potential of the crop and also provide the photosynthetic factory necessary for maximum yield. The second stage is the grain-filling period in which the potential yield created in the first stage is realized. The extent to which the potential yield is realized will depend on the environment and on management inputs prior to and after anthesis.