Leaves

The sessile leaves enclose the stem on two-thirds of its circumference. The phyllotaxy varies, being 5/13 in large-fruited cultivated pineapples and 3/8 in small-fruited wild pineapples (Kerns et al., 1936). Leaf number is variable between cultivars but generally around 40-80. The lower ones, originating from the planting material or produced soon after planting, are smaller (5-20 cm) compared with the younger ones, which can reach more than 1.6 m in length and 7 cm in

width, depending on the cultivar and ecological conditions. The apical ones are short and erect. The leaves are ensiform and, except for the young apical ones, broader at their base, which forms a non-chlorophyllous sheath around the stem. The blades then taper progressively to a sharply pointed indurated tip. The constriction between the sheath and the blade is more marked in certain wild pineapples. Temporary stress during leaf growth may cause variations in width or spininess, or both, along the blade. Leaves are semirigid, thanks to their crescent-shaped section. As in other bromeliads, this allows the plant to collect water in the rosette, where it can be absorbed by the aerial roots present along the stem or through the epidermis of the sheath. The concave adaxial face is green or dark green, with some anthocyanins, to dark red or purple, according to the cultivar and conditions. The abaxial side is convex, with a surface corrugated by longitudinal grooves. Both sides are covered by peltate trichomes,

Fig. 2.2. A. Central region of the 'Smooth Cayenne' pineapple stem cut to show transverse, radial and tangential views of gross external and internal structure. Tangential section cut at boundary between cortex and stele. iar, internal, and ear, external portions of adventitious roots; cvt, circumferential band of vascular tissue; vn, vascular network at the boundary between cortex and stele; lg, leaf gap; ltb, leaf-trace bundle; ls, leaf scar; lbb, lateral branch bud; rb, tip of emerging root, commonly called a root 'bud'. B. Diagrammatic representation, showing portion of the cortex cut away to expose the vascular network at the boundary between cortex and stele, and xylem elements from roots. arb, adventitious root base; cvt, circumferential band of vascular tissue; lb, leaf base; lg, leaf gap; ltb, leaf-trace bundle. (After Krauss, 1948. © The University of Chicago, all rights reserved.)

particularly the abaxial one, which is densely furfuraceous and silvery (see Fig. 5.7). The leaf margins are usually thorny; however, certain cultivars are partially or totally iner-mous. In some smooth cultivars, the lower epidermis is folded over the leaf edge and extended over the upper surface, so producing a narrow silvery stripe, a trait called 'piping' by Collins (1960).

A section across the leaf (Fig. 2.3) shows successively a thick and smooth cuticle; a particular upper epidermis, which consists of a single layer of cells, each containing a silica body, orientated perpendicularly to the leaf axis, and rigidified by thick and undulated lateral and inner cell walls; the hypo-dermis; the water-storage tissue, consisting of various layers of thin-walled cells, which accounts for a quarter to half of the leaf thickness, depending on the water status of the plant; the chloroplast-rich mesophyll, with the vascular bundles, fibre strands and aerating canals; and the lower hypodermis and epidermis, with the stomata arranged longitudinally along the characteristic grooves of the abaxial leaf side, covered by numerous trichomes, giving it a silvery appearance and increasing reflectance. The fibre strands confer a high tear resistance to the pineapple leaves. Stomatal density is rather low, about 80 stomata mm-2. The trichomes, present in almost all the known bromeliads, are flat and shield-shaped, parallel to the leaf surface. They consist of a central disc of live cells, an outer ring of dead cells and a pluricellular stalk arising from the epidermis and subepi-dermis. Like the stomata, they are nested in small cavities along the bottom of the underside furrows, with their broad heads spreading out to virtually cover the entire leaf surface. In many bromeliad species, the dead cells can absorb water and nutrients and the stalk can carry them into the inner leaf tissues. The shield also acts as a plug, closing the concavity where the stalk is inserted and protecting it from drying out. These tri-chomes function as one-way valves, playing an important role in the capacity of bromeli-ads to improve and maintain their water status (Benzing, 1980). However, the pineapple trichomes appear to be hydrophobic and do not absorb water (Krauss, 1948, 1949a,b). The presence of large numbers of mitochondria in the stalk cells give indirect evidence for an important role in uptake of dissolved nutrients (Sakai and Sanford, 1980). Another important role is to protect the plant from excessive transpiration and intense sunlight. More generally, the thick cuticle, the water-storage tissue, the disposition of the stomata, the trichomes and the crassulacean acid metabolism (CAM) all contribute to the remarkable water economy of pineapple.

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