Transpiration and the Ascent

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218 7. Transpiration Leal Orientation. Leaves with surfaces perpendicular to the sun's rays are warmer than those more Or less parallel, and are likely to have a higher rate of transpiration. Leaves of many plants tend to be more or less perpendicular to midday radiation, but there are exceptions such as some species of Lactuca and Silphium, turkey oak (Q. laevis), and jojoba (Simmondsia chinensis). The leaves of Stylosanthes humilis normally are perpendicular to the sun's rays but when the pulvini lose turgor they become parallel. Pine needles that OCCurin bundles, such as those of the three and five needle pines, shade one another and reduce transpiration and photosynthesis per unit of surface (Kramer and Clark, 1947). Leaf orientation may be less important in stands of plants than in individual plants. Leaf Surface Characteristics. The outer surfaces of leaves, flower petals, stems, and fruits often are covered by a relatively waterproof layer, the cuticle. This is composed of cutin and wax, anchored to the epidermis by a layer of pectin (Fig. 7.7). When dry, the cuticle is relatively impermeable to water, as shown by its high resistance (Table 7.4), but when wet it is fairly permeable to water vapor and a variety of solutes (Sch6nherr, 1976). The cuticle generally is thinner on shade than on sun leaves, and thinner on leaves grown with adequate water than on leaves subjected to water deficits (Bengston et at., 1978). Cuticle mutants have been found in sorghum, some of which displayed thin cuticles and high water loss through the cuticle (Jenks et at., 1994). Permeability to water must depend more on the amount of wax than on the thickness of the cuticle because cuticular transpiration often increases severalfold when the wax is re- cuticle birefringent wax -embedded in cutin Figure 7.7 Diagram of an upper surface of an epidermal cell of pear leaf, showing cuticle, composed of cutin and wax" The cutin is anchored to the cellulose wall by a layer of pectin" From Kramer (1983), after Norris and Bukovac (1968),
Other Factors AffectingTranspiration 219 Figure 7.8 Variations in patterns of wax deposited on leaves of four tree species: (A) American elm, (B)white ash, (C) sugar maple, and (D) redbud Also note differences in appearance of stomata, Photographs by w., J. Davies, from Kramer (1983)" moved (Radler, 1965; Jeffree et ai., 1971). However, Charnel et ai. (1991) claimed that the extraction of soluble lipids did not increase permeability. Examples of wax deposits on leaves are shown in Fig. 7.8. The early work on cuticle was reviewed by Stalfelt (1956b), and Kolattukudy (1981) reviewed the synthesis of cutin and suberin. Leaves of some plants are covered by thick layers of trichomes or epidermal hairs that reflect light, decrease the driving force by decreasing the leaf tempera-
Page 1 and 2: Transpiration and the Ascent ·ofSa
Page 3 and 4: Introduction 203 Table 7.3 Water Re
Page 5 and 6: ,iving Forces and Resistances The P
Page 7 and 8: The Processof Transpiration 207 sur
Page 9 and 10: The Process of Transpiration 209 .v
Page 11 and 12: Effect of Increasing Temperature of
Page 13 and 14: L6 L5 ~ L4 E-i •.... E;" o L3 +'
Page 15 and 16: Other FactorsAffectingTranspiration
Page 17: Other FactorsAffectingTranspiration
Page 21 and 22: Other Factors AffectingTranspiratio
Page 23 and 24: MeasurementofTranspirationand Evapo
Page 25 and 26: Measurement of Transpiration and Ev
Page 27 and 28: Measurementof Transpirationand Evap
Page 29 and 30: 00 :J: 25 20 15 E 10 E 5 0 8 6 4 2
Page 31 and 32: Measurementof Transpirationand Evap
Page 33 and 34: MeasurementofTranspirationand Evapo
Page 35 and 36: The Ascentof Sap 235 Root-Shoot Tra
Page 37 and 38: [; ,625 \547 !911 893 The Ascent of
Page 39 and 40: The Ascentof Sap 239 .vessels of vi
Page 41 and 42: A The Ascent of Sap 241 f,igure 7.1
Page 43 and 44: The Ascent of Sap 243 water in the
Page 45 and 46: 0' J The Ascent of Sap 245 7.18 Vel
Page 47 and 48: 12 The Ascentof Sap 247 and thickne
Page 49 and 50: The Ascentof Sap 249 Sirnonneau et
Page 51 and 52: Conductionin Leaves 251 and Sperry
Page 53 and 54: Summary 253 symplastic movement of
Page 55 and 56: SupplementatyReading 255 There has

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