Pineapple Inflorescence Open Heart

The reproductive phase of the pineapple begins in response to natural or plant-growth-regulator-forced induction of reproductive development (natural induction and forcing). Because the inflorescence of pineapple is terminal, when reproductive development begins, formation of new leaves ceases. Expansion of previously initiated leaves continues, but not all of these expand fully. Some are found on the fruit peduncle and their size is much reduced relative to older fully expanded leaves.

Once reproductive development is initiated, inflorescence and fruit development continue without interruption until the fruit matures, although development may be interrupted by disease or slowed by water or temperature stress. Fruit growth ceases at air temperatures below about 10°C and very high air temperatures - certainly above 35°C - can also retard growth (Malezieux et al., 1994). The effect of cold is especially pronounced. In winter in cool subtropical regions, such as southern Queensland, Australia, forcing of 'Smooth Cayenne' initiates reproductive development even though temperatures are too cold to allow for signifi-

© CAB International 2003. The Pineapple: Botany, Production and Uses (eds D.P. Bartholomew, R.E. Paull and K.G. Rohrbach)

cant fruit development. The delay in development collapses the harvest dates of plants forced up to 13 weeks apart into a period of no more than 3 weeks (Sinclair, 1992a). Thus, on average, fruit from plants forced on about 1 June are harvested on 2 March, a fruit development period of 274 days, while fruit from plants forced on about 1 September are harvested on 24 March, a fruit development period of only 204 days.

Inflorescence and Fruit Morphology and Growth

Early development

The meristematic area of the pineapple stem is small relative to the diameter of the stem near its apex. The vegetative apical dome is very small, but it broadens considerably after inflorescence induction has occurred. At the onset of reproductive development, leaf primordia width decreases and the number of primordia bordering the dome increases considerably (Kerns et al., 1936; Bartholomew, 1977; compare Fig. 8.1A and B). At this early stage, the first structures of the inflorescence to develop are bracts, and

Fig. 8.1. Scanning electron photomicrographs of stem apical development of 'Smooth Cayenne' pineapple after forced induction of inflorescence development with ethephon. A, Vegetative apex at approximately 185 X showing two leaf primordia surrounding the apical dome. B, Apex at approximately 133 X 8 days after induction showing additional primordia surrounding the apical dome as a result of induction. C, Apex at approximately 84 X 11 days after induction, showing increasing numbers of primordia. The bract at the lower left of the photograph has been removed to show the first floret primordium (F). D, Apex at approximately 40 X, with floret bract primordium removed to show the developing sepal (S) and petal (P) primordia. (From Bartholomew, 1977. © The University of Chicago, all rights reserved.)

Fig. 8.1. Scanning electron photomicrographs of stem apical development of 'Smooth Cayenne' pineapple after forced induction of inflorescence development with ethephon. A, Vegetative apex at approximately 185 X showing two leaf primordia surrounding the apical dome. B, Apex at approximately 133 X 8 days after induction showing additional primordia surrounding the apical dome as a result of induction. C, Apex at approximately 84 X 11 days after induction, showing increasing numbers of primordia. The bract at the lower left of the photograph has been removed to show the first floret primordium (F). D, Apex at approximately 40 X, with floret bract primordium removed to show the developing sepal (S) and petal (P) primordia. (From Bartholomew, 1977. © The University of Chicago, all rights reserved.)

each fruitlet forms in the axil of these bracts (Fig. 8.1C). Shortly after the bracts have been initiated, three sepal and petal primordia can be seen developing (Fig. 8.1D) and, soon-thereafter, six stamen primordia develop. The flower parts all originate at the same level and grow away from their point of origin. As they grow upward, a cavity is formed, which eventually closes over and three carpels are formed within it. The apex of the vegetative plant stem is broad and flat, but, as the first flower structures begin to develop, the stem apex narrows and the fruit stem or peduncle begins to elongate (Fig. 8.2).

Fig. 8.2. Vegetative (left) and reproductive stems of 'Smooth Cayenne' plants after induction with ethephon. Note the progressive elongation of both the upper portion of the stem and the fruit stem (peduncle).

The elongation of the stem tip in a cross-section taken through the centre of the plant is diagnostic of natural or growth-regulator-induced inflorescence development and can usually be seen within 2 weeks after induction has occurred. Development of the florets at the base of the young inflorescence continues at the same time that new bract and flower primordia are being produced higher on the young inflorescence (Fig. 8.3A). Eventually all flower parts become enclosed by the developing sepal primordia and are no longer visible without dissection (Fig. 8.3B). The number of florets that develop on a fruit varies considerably with the variety of pineapple, the size of the plant at induction, plant population density, the quality of forcing and other factors that have not been adequately characterized. For 'Smooth Cayenne' pineapple, at between 30 and 40 days after induction, the diameter of the apical dome decreases, the initiation of reproductive structures ceases and leaf bracts (Fig. 8.4) and, later, crown leaves begin to develop (Kerns et al., 1936; Bartholomew, 1977). By the time crown leaves are visible, there has been considerable elongation of the peduncle and the leaves (bracts) in the centre of the leaf whorl have turned red to red-orange.

The rate of early development of the

'Smooth Cayenne' inflorescence and fruit is determined almost entirely by the prevailing temperature of the environment where the crop is grown (Fleisch and Bartholomew, 1987; Malézieux et al, 1994). In the warm subtropical environment of Hawaii, fruitlet initiation ceased about 34 days after forcing with ethephon in midsummer (Bartholomew, 1977) and after 37 days in December (Kerns et al., 1936). In the somewhat warmer Côte d'Ivoire, when plant leaves were removed every few days after forcing, fruitlet number was reduced up to the 30th day after forcing (E. Malézieux, 1992, unpublished results), indicating that fruitlet number was fixed by 30 days after forcing in that environment. Beyond that time, leaf removal had no effect on fruitlet numbers. Cell division continues in the fruit until after anthesis and further development thereafter is primarily the result of cell enlargement (Okimoto, 1948).

The stages of fruit development after induction that can be defined on the basis of developmental morphology include the beginning and end of floret (fruitlet) initiation, the beginning and end of anthesis (flowering) and maturation. Other 'stages' have been defined for convenience in estimating progress towards maturation or to

Fig. 8.3. A, Young developing pineapple inflorescence (20X) with the bracts subtending the florets removed (bract scars) at 25 days after induction in the summer in Hawaii (from Bartholomew, 1977. © The University of Chicago, all rights reserved). B, Section through two 'Smooth Cayenne' florets at anthesis; S, sepals; P, petals; St, stamen; Pi, pistil; O, locule of ovary with ovules.

Fig. 8.3. A, Young developing pineapple inflorescence (20X) with the bracts subtending the florets removed (bract scars) at 25 days after induction in the summer in Hawaii (from Bartholomew, 1977. © The University of Chicago, all rights reserved). B, Section through two 'Smooth Cayenne' florets at anthesis; S, sepals; P, petals; St, stamen; Pi, pistil; O, locule of ovary with ovules.

Fig. 8.4. Stem apex (138X) at 47 days after induction in the summer in Hawaii. Note the small apical dome with a reduced number of primordia surrounding it when compared with Fig. 8.3A. (From Bartholomew, 1977. © The University of Chicago, all rights reserved.)

help in determining when disease organisms might enter the fruit (Rohrbach and Taniguchi, 1984). The earliest stage that can be seen in the centre of the leaf whorl as the peduncle elongates without sacrificing the plant is commonly referred to as 'open heart'

and the width of the opening is commonly estimated for crop-logging purposes. At the time the 'open heart' stage is reached, inflorescence bracts have turned bright red to red-orange.

The extent of open heart is defined by the diameter of the opening - nominally 1.0 or 2.0 cm (1/2 and 1 in. are used in Hawaii) (see Fig. 9.30) - and days from forcing to one of the open-heart stages is a valuable criterion for assessing the quality and effectiveness of forcing. These 'stages' are more quantitative than the sometimes used red-bud stage, which is less sharply defined.

The developing inflorescence is typically completely visible within 7-10 days after the 2.0 cm open-heart stage has been reached, and this has been referred to as the 'early cone' stage (Rohrbach and Taniguchi, 1984). By this stage, the sepals and bracts of the fruit floret are visible and the number of fruit florets can be counted. The stages 'mid-cone' and 'late cone' follow at approximately weekly intervals in summer in Hawaii and anthesis begins approximately 1 week after late cone. The tip of the corolla, typically blue, can usually be seen by the late-cone stage. Anthesis, which occurs progressively from the base upward, usually takes from 15

to 25 days, depending on the floret number and the average temperature. As is typical of the growth of most structures, the increase in fruit fresh and dry weight from the time of induction to maturity is sigmoid (Sideris and Krauss, 1938). Growth rate, discussed in more detail later, is primarily dependent on temperature.

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