Читать книгу Biotechnology of Fruit and Nut Crops - Группа авторов - Страница 106

The coconut inflorescence is enclosed in a sheath or spathe known as a ‘spadix’ and borne singly in the axil of each frond. Both male and female flowers produce nectar, encouraging pollination by bees and other insects, but the open inflorescence display also allows wind-borne pollination. A medium-sized fruit (a drupe) from a Tall type (180 mm in length, 150 mm in diameter) comprises a low-density husk which contains a spherical to ovoid nut of c.120 mm diameter. The shell of the nut is 3 mm thick, and its inner surface is occupied by a uniform layer of kernel (solid endosperm) 11–15 mm thick. The inner vacuole or cavity when free of kernel has a volume of c.890 ml and when mature, with a 12 mm kernel layer, an internal diameter of 90 mm and volume of 380 ml. It remains filled with water (liquid endosperm) until maturity. At this stage, following abscission of the vascular link to the palm, the water begins to evaporate, allowing air to enter the cavity. The embryo is embedded in the kernel directly beneath the ‘soft eye’ or operculum which covers the aperture in the shell through which the embryo expands at germination.

As the shoot extends through the husk, a spongy ‘haustorium’ expands from the inner end of the embryo into the cavity, absorbing the water and secreting enzymes which hydrolyse the kernel to supply energy-rich molecules and nutrients to the developing seedling. Within 2 months, the shoot and roots emerge through the husk, and, following one or two ‘leafless’ bracts, the first leaf lamina unfolds 2–3 months later. The seedling continues to draw upon the energy reserve of the kernel until it is exhausted at c.15 months.

The interval between the emergence of successive fronds is c.6 weeks at first, shortening gradually to 22 days, under ideal conditions, when the palm reaches 5–10 years of age, and then remains constant for the next 40 years. From 6–12 months of seedling age, the initial unsplit fronds become pinnate progressively from the proximal end. Varieties from South Asia are slower to develop the pinnate form than those from the Pacific region. The frond length reaches up to 6 m by year 10, with more than 100 leaflets on each side, while the Dwarf frond may reach 4 m in length. Successive fronds are displayed at 140° horizontal intervals around the vertical axis of the palm, forming a three-dimensional hemispherical crown as the frond number reaches up to 30, and the palm achieves maximum productivity. A productive palm >15 years of age displays an almost spherical crown as many lower fronds droop increasingly below the horizontal.

The trunk is formed at its full diameter beneath the apical meristem. Unlike dicotyledonous plants, there is no cambium to expand the trunk and therefore no bark layer forms on the outer surface. The trunk becomes visible at 4 years, with a diameter of c.500 mm in the Tall and forming a noticeable bulge 1 m above the ground. It then gradually narrows to 400 mm as the palm matures and bears fruit. The Dwarf trunk does not develop a bulge at its base as it flowers much earlier than the Tall, and so it diverts most assimilated biomass to support fruit development and only achieves a trunk diameter of 300 mm. Within 1–2 years, roots extend from beneath the perimeter of the trunk base. Prior to this, the palm is vulnerable to strong winds as its root system is restricted to just a small central cluster. The smaller circumference of the Dwarf trunk results in less grip on the soil and lower stability than the Tall. Both Dwarf and Tall types continue to be vulnerable to being blown over by strong wind up to 10 years of age and more in the case of the Dwarf. Size and strength of the trunk have been observed to be greater when soil chlorine is readily available (Ochs et al., 1991).

The assembly of fronds on a mature palm is known as the crown. Prior to c.20 years of age, the Tall palm has a hemispherical crown comprising 25–32 fronds, all held at angles above the horizontal. As the gap on the trunk between the base of successive fronds becomes shorter with age, the thick base of the frond results in mutual compression between neighbouring fronds. The base of older fronds is then forced outwards, hinging at the point of attachment, so that about half the fronds droop down at different angles below the horizontal, transforming the crown to a spherical shape (Foale and Ashburner, 2005).

The Tall palm sustains peak productivity from 15 years until c.35 years, although it is quite common that there will be alternate years of high and low productivity. Older palms exhibit a gradual reduction in the length of the frond and the diameter of the crown. The consequent reduction in light interception brings about a steady downward trend in biomass production, causing trunk extension to diminish after 60 years. This smaller demand provides sufficient biomass to support a small number of fruits even at 90 or more years of age, although such longevity is rare in cyclone-prone regions (Foale and Ashburner, 2005).

Following pollination, the female flower develops into a fruit that grows steadily, reaching its full volume in 6 months. The bunch of fruit can comprise about ten fruits of equal size, shape and colour. Bronze and green of varying intensity are the most common colours of the immature Tall fruit, while recessive yellow and orange and non-recessive green and brown are found in the self-pollinating Dwarf. The yellow colour occasionally appears in Tall palms in the Pacific region. At 6 months of age, the fruit comprises a soft husk and shell and a large cavity filled with liquid endosperm. The fibres of the husk are conducting channels, connected to apertures in the shell through which photo-assimilate can be delivered to support respiration and to form the kernel. The young kernel on the inside of the shell is very soft and gelatinous at first, but by 10 months it becomes firm, and by 12 months it is solid and fibrous and contains 50% water, 33% oil, 5% protein and the remainder as carbohydrate, mostly fibre. It is this ‘kernel’ that becomes the source of highly profitable commodities. In the dried form (copra), it has been traded to industrialized countries for >140 years, while many derivatives are critical components of the local diet as well as becoming commercial products.

The apical meristem of the palm comprises a broad cone-shaped mass of soft undifferentiated tissue on the ‘pedestal’ of the extending trunk. Around the periphery of the meristem, frond and inflorescence primordia form and expand while growth in the central part extends the trunk.

The coconut has an adventitious root system comprising hundreds of identical components growing from the inverted hemispherical base of the trunk. Each primary root is 7–10 mm in diameter and can extend up to 7 m horizontally and 2 m or more vertically. There are three orders of branching arising from these primary roots. Extending a few millimetres vertically from the upper surface of shallow primary roots are small whitish ‘pneumatophores’, being entry points for oxygen when the deeper roots are inundated by a rise in the water table.