Flower Induction, Initiation and Development

Flower induction refers to the change from vegetative to reproductive phase and can be likened to a switch. However, no visible macroscopic or microscopic changes occur in the bud. Most flower induction occurs in early summer, but it can extend into early autumn under some conditions. Initiation begins when the meristem flattens – visible microscopically – and continues as primor-dial sepals, petals, stamens and pistils form centripetally on the apex and grow into fully formed appendages (Fig. 7.1). Most of the flower parts are present by early autumn, but continue to develop in temperate cli-mates until low temperatures prevent fur-ther growth. In such climates, meiosis begins in the anthers in late winter.

a b

c d

Fig. 7.1. Stages in the initiation of an apple flower at Long Ashton, Bristol, UK, 1974 (Abbott, 1977).

Scanning electron-microscope views. (a) 1 August – vegetative apex with five leaf primordia; (b) 15 August – apex becoming domed, ridges (floral initials) arising in leaf axil/bract primordia; (c) 21 August – longitudinal section showing ‘king’ flower at apex; (d) 19 September – five sepals forming on the uppermost lateral flower initial (subtending leaves and ‘king’ flower removed).

In most fruit-growing areas buds become dormant in late summer/early autumn and winter chilling is necessary to permit renewed growth the following year (see Chapter 10). Weinberger (1950), working in California, used the cumulative number of hours at or below 45°F (= 7.2°C) as an indica-tion of the amount of chilling received by peach trees. Although this model has been used at higher latitudes, temperatures below freezing have very little or no effect in break-ing dormancy. A model developed by researchers at Utah State University (the

‘Utah model’) can be used to predict response in areas with colder winters (Richardson et al., 1974). Temperatures between 0 and 15°C are effective, with maxi-mum response at 6–7°C, where 1 h of chill-ing equals 1 ‘chill unit’ (CU). Temperatures

 16°C have detrimental effects, reducing the response to previous chilling. This model works well in the temperate zone, but is less useful in the subtropics. In these warmer areas the ‘dynamic’ model (Erez et al., 1988) is a better predictor of response. This model assumes that chilling is accumulated in units and, once such a unit is acquired, high tem-perature cannot nullify its effect.

Cultivars differ in the amount of chilling necessary to break dormancy. Some (e.g.

‘Anna’, ‘Dorsett Golden’) require as little as 250–300 CU and can be grown in the sub-tropics, whereas others (e.g. ‘McIntosh’) require much more chilling (1000–1600 CU) and can only be grown at higher latitudes of the temperate zone. If chilling is insuffi-cient, both vegetative and flower buds are retarded in development and cropping is reduced. Exceptions to the requirement for chilling occur in some regions of the tropics, as in Indonesia, where defoliation soon after harvest induces bud break, resulting in two crops per year (Edwards and Notodimedjo, 1987).

Abbott (1977) reported that 20–24 nodes must develop before apple flowers can be initiated. Although studies with potted trees had suggested that flower ‘quality’ (= ability to set fruit) was optimum when initiation occurred at a specific time, Abbott observed little difference in time of initiation under field conditions. Others have suggested that,

if vigour is excessive, the optimum number is exceeded; if vigour is too low, too few nodes are formed and, in either case, shoots fail to form flower buds.

Flower induction can be inhibited by heavy cropping, some cultivars (e.g. ‘Yellow Newtown’, ‘Paulared’, ‘Fuji’) being notori-ous for their ‘biennial bearing’ habit, although all cultivars exhibit some degree of response to heavy cropping. This can be con-trolled in most cultivars by early fruit removal (‘thinning’), either by hand or with chemicals (see also Chapter 16). In general, fruit must be removed within the first 3 or 4 weeks following bloom for thinning to be effective, response declining as thinning is delayed (Fig. 7.2). The physiological basis for this effect of fruits on flowering has been the subject of much research, but remains to be determined. Apple seeds are rich sources of GAs. These compounds can inhibit flower-ing when applied to limbs or whole trees, and most of the theories proposed include a role for them in inhibiting flowering. In the facultatively parthenocarpic apple cultivar

‘Spencer Seedless’, seeded fruits inhibit flow-ering, whereas seedless fruits do not, sug-gesting that seeds do play a crucial role in flowering (Chan and Cain, 1967; Neilsen and Dennis, 2000). However, few such cultivars exist, and the evidence for the role of seeds in related species, such as pear (Pyrus com-munis L.), is more controversial (see Dennis and Neilsen, 1999; Weinbaum et al., 2001).

50 40 30 20 10 Spurs forming blossoms (%) 0

20 30 40 50 60 70 80

Days after full bloom

Fig. 7.2. Effect of time of thinning ‘Yellow Newtown’

apple fruits, to leave one fruit per 70 leaves, on flowering the following year (Harley et al., 1942).

Environmental factors also affect induc-tion and initiainduc-tion. Although photoperiod plays little or no role in flowering of apple under field conditions, solar radiation is important. Flowering is heavier in well-exposed sections of the tree and in trees in areas with high solar radiation, such as Washington State and California.

Experiments with artificial shading have indicated that flowering is reduced when-ever the light level is reduced below 30% of full sun (Fig. 7.3). Pruning to open the tree to sunlight is therefore encouraged. Young trees

of some cultivars (e.g. ‘Empire’) have wide crotch angles, which permit better light pene-tration; others (e.g. ‘Delicious’) have narrow crotch angles. Leaves, of course, are important for capturing sunlight, and defoliation reduces flower induction/initiation. Leaf injury from insects and disease can therefore reduce flowering. In controlled environments, high temperature can inhibit flowering in some cultivars (Tromp, 1976), but evidence that this occurs under field conditions in the temperate zone appears to be lacking.

Many cultural practices, including ring-ing or scorring-ing the trunk or scaffold limbs and bending of limbs, favour flower induction.

Ringing/scoring is recommended for young trees only when flowering is delayed consid-erably, as in ‘Northern Spy’, but bending or spreading limbs to increase exposure to light is a common practice in young orchards (see Chapter 14). Drought conditions tend to favour flower-bud formation, although this is not well documented and some researchers have questioned the relationship. Heavy applications of nitrogen stimulate growth and tend to reduce flower-bud formation.

However, limited evidence exists that appro-priate timing in applying summer nitrogen can improve flower quality, leading to better fruit set (Williams, 1965; Hill-Cottingham and Williams, 1967).

Some plant growth regulators affect flow-ering. Tri-iodobenzoic acid can promote flowering in ‘Delicious’. Several other compounds, including ethephon (2-chloroethylphosphonic acid) and the growth retardants butanedioic acid mono-(2,2-dimethylhydrazide) (Alar^®), 2-chloroethyl trimethyl ammonium chloride (CCC), and paclobutrazol ((1RS,3RS)-1–1-(4-chlorophenyl)- 4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-pentan-3-ol), are effective on numerous cultivars, although neither Alar^®nor paclobutrazol can be used commercially in the USA. In con-trast, as noted above, GAs inhibit flowering.

Plant responses to GAs differ with species and with the particular GA used; for exam-ple, GA₄₊₇is more inhibitory to flowering of apple than is GA₃or GA₄alone (Marino and Greene, 1981; Tromp, 1982), and some evi-dence suggests that other GAs can actually promote flowering (Looney et al., 1985).

80

70

60

50

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30

Fruit retained (%)

25 50 75 100

25 50 75 100

180

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140

Mean fruit weight (g)

Light transmitted by shades (%) Fig. 7.3. Effects of shading on percentage of fruits retained (top) and on mean fruit weight (bottom) (Jackson, 1975).

7.3 Flowering Habit, Flower Structure