The sequence in time of the three developmental phases results in a spatial gradient of juvenility along the shoot axis. Because growth in height is restricted to the apical meristem, the juvenile tissues and organs, which form first, are located at the base of the shoot. In rapidly flowering herbaceous species, the juvenile phase may last only a few days, and few juvenile structures are produced. In contrast, woody species have a more prolonged juvenile phase, in some cases lasting 30 to 40 years (Table 24.1). In these cases the juvenile structures can account for a significant portion of the mature plant.
(A) Vegetative young adult plant
□ Vegetative adult
□ Reproductive O Flower
Processes required at all phases
□ Vegetative adult
□ Reproductive O Flower
Once the meristem has switched over to the adult phase, only adult vegetative structures are produced, culminating in floral evocation. The adult and reproductive phases are therefore located in the upper and peripheral regions of the shoot.
Attainment of a sufficiently large size appears to be more important than the plant's chronological age in determining the transition to the adult phase. Conditions that retard growth, such as mineral deficiencies, low light, water stress, defoliation, and low temperature tend to prolong the juvenile phase or even cause rejuvenation (reversion to juvenility) of adult shoots. In contrast, conditions that promote vigorous growth accelerate the transition to the adult phase. When growth is accelerated, exposure to the correct flower-inducing treatment can result in flowering.
Although plant size seems to be the most important factor, it is not always clear which specific component associated with size is critical. In some Nicotiana species, it appears that plants must produce a certain number of leaves to transmit a sufficient amount of the floral stimulus to the apex.
FIGURE 24.11 Schematic representation of the combinatorial model of shoot development in maize. Overlapping gradients of expression of the juvenile, vegetative adult, and reproductive phases are indicated along the length of the main axis and branches. The continuous black line represents processes that are required during all phases of development. Each of the three phases may be regulated by separated developmental programs, with intermediate phases arising when the programs overlap. (A) Vegetative young adult plant. (B) Flowering plant. (After Poethig 1990.)
Length of juvenile period in some woody plant species
Length of juvenile period
Rose (Rosa [hybrid tea]) 20-30 days
Grape (Vitis spp.) 1 year
Apple (Malus spp.) 4-8 years
Citrus spp. 5-8 years
English ivy (Hedera helix) 5-10 years
Redwood (Sequoia sempervirens) 5-15 years
Sycamore maple (Acer pseudoplatanus) 15-20 years
English oak (Quercus robur) 25-30 years
European beech (Fagus sylvatica) 30-40 years
Source: Clark 1983.
Once the adult phase has been attained, it is relatively stable, and it is maintained during vegetative propagation or grafting. For example, in mature plants of English ivy (Hedera helix), cuttings taken from the basal region develop into juvenile plants, while those from the tip develop into adult plants. When scions were taken from the base of the flowering tree silver birch (Betula verrucosa) and grafted onto seedling rootstocks, there were no flowers on the grafts within the first 2 years. In contrast, the grafts flowered freely when scions were taken from the top of the flowering tree.
In some species, the juvenile meristem appears to be capable of flowering but does not receive sufficient floral stimulus until the plant becomes large enough. In mango (Mangifera indica), for example, juvenile seedlings can be induced to flower when grafted to a mature tree. In many other woody species, however, grafting to an adult flowering plant does not induce flowering.
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