Growth and Development of Horticultural Crops (1+1)

Initiation of flower primordial

• The initiation of flower primordia is a major event in the life cycle of a plant in that it involves a shift in the phase of development from vegetative to reproductive processes. The significance of flower initiation has been recognized by botanists for many years. In 1918, Klebs suggested that during the life cycle of plant it passes through several phases of development. Before floral primordia can be initiated, the plant must complete a period of vegetative growth or attain some minimal leaf number. When this condition is attained, the plant is said to be ripe to flower. Ripeness-to-flower is not recognized by any external characteristics, but it can be determined empirically by subjecting plants of varying age (From seedling emergence) to environmental conditions known to induce flowering. In most plants ripeness – to- flower is attained after the plant has produced several leaves. In some cereal grasses a minimum of seven leaves must be developed before the plant is ripe to flower. On the other hand, a plant like Pharbitis nil, the Japanese morning glory, is ripe to flower within a day after the cotyledons have emerged. Cotyledons presumably contain enough stored food to support subsequent reproductive development. Most common cultivated plants and weedy annuals, however, attain the ripe-to-flower condition 2 to 3 weeks after seedling emergence when a few leaves have fully developed.
  
• Attainment of the ripe-to-flower condition does not automatically lead to the initiation of flower primordial. Certain environmental conditions must follow. These same environmental conditions, if presented to a plant that is not ripe to flower, elicit no flowering response. The importance of temperature and the promotion of reproductive development, a phenomenon referred to as vernalization, were described by Gassner in 1918. About the same time W.W Garner and H. A. Allard (1920) two plant physiologists with the U.S. Department of Agriculture, found that day length, or the duration of light and dark periods within a 24 hour cycle, also influenced the initiation of flowering.
  
• Since the early work of Klebs, Gassner, and Garner and Allard, plant physiologists have carried out detailed studies of the anatomical, chemical, and biochemical processes that accompany the shift from vegetative to reproductive growth. Although much is still not known about floral initiation, several different kinds of processes are thought the choice of appropriate plant material and experimental techniques, to isolate individual reactions for detailed study. The transport of the flower stimulus or the transformation of the shoot apex can be studied, for example. In certain plants it has been possible to determine the length of time necessary to complete some of the partial reaction.