The Physiology of Flowering Plants - KHAM PHA MOI

gives the ‘bloom’ to glaucous leaves and fruits; some dark plums andso-called black grapes look almost sky-blue when untouched, owing tothe wax, but much of that gets rubbed off when the fruit is handled.The sheen and texture of many floral parts result from their wax layer.The wax is not a pure chemical, but a mixture of long-chain hydrocarbons,long-chain fatty acids, long-chain hydroxy acids, esters, alcohols,aldehydes and terpenoids. Each species produces its ownmixture; about 50 different chemicals have been detected in applewax. Even as for the cuticle, the thickness varies from a fraction of amm upwards. The wax palm (Klopstockia cerifera) native to the Andes, has5 mm of wax on its leaves; carnauba wax, from the leaves of Coperniciacerifera, is harvested commercially in Brazil.The thickness and chemical composition of the cuticle and waxdetermine the extent of transpiration with closed stomata, the ‘cuticulartranspiration’ discussed in Section 3.6.3. The more impermeablethese layers, the smaller is the cuticular transpiration (and themore complete is the control exerted by the stomata).The cuticle and wax also prevent the entry of water. It would not bea healthy situation if rain penetrated plants freely, flooded the airspaces, soaked the cell walls and leached out solutes from the apoplast.Raindrops largely roll off owing to the hydrophobic nature ofthe surface. The wettability is determined mainly by the epicuticularwax, its thickness and even more by its structure. To the eye a waxyleaf or fruit looks smooth. Microscopy, especially scanning electronmicroscopy, reveals that the wax is present as plates, rods, granulesor tufts (Fig. 3.15, 3.16). These formations increase the hydrophobicity;even smooth wax is water-repellent, but a surface bristling withsmall wax projections is almost unwettable. Agricultural and horticulturalsprays are mixed with detergent to lower the surface tensionand enable the fluid to wet plant surfaces. The cuticle and wax alsogive some protection against the penetration of pathogens, andagainst ultraviolet radiation.The mode of production of these extracellular layers by the plantis something of a puzzle. The components are generally assumed tobe secreted as liquid precursors by the epidermal cells and to solidifyon the outside, otherwise continuous layers cannot be produced, norcan solid wax rods be transported through a cell wall. The mostdifficult problem is the wax, which must move not only throughthe cell wall, but through the cuticle as well. It might be suggestedthat the wax layer is formed first and then the cuticle under it; butwhen the wax is wiped off, leaving the cuticle intact, a new layer ofwax is formed by young leaves (and in some species by mature leavesalso). Searches for channels in the epidermal cell walls have not givenclear-cut results. There have been reports of channel-like structuresin cell walls seen by light microscopy, but these have not beenconfirmed by electron microscopy. Another idea is that the waxmolecules are carried along with water vapour molecules duringcuticular transpiration, a process that has been compared to steamdistillation.WATER UPTAKE AND LOSS 93