New Phytol. (1985) 99, 407-416 407

DIRECT FOLIAR EFFECTS OF SIMULATED

ACID RAIN

II. LEAF SURFACE CHARACTERISTICS

BY B. L. HAINES1, J. A. JERNSTEDT AND H. S. NEUFELD2

Botany Department, University of Georgia, Athens, Georgia, 30602, USA

(Accepted 13 November 1984)

SUMMAR Y

Surface characteristics and wettability of the leaves of six plant species have been examined in

relation to their susceptibility to damage from simulated acid rain. The species examined differed

in the type and extent of epicuticular wax deposits, surface topography of the cuticle, trichome

type and distribution, and epidermal cell shape. Leaf wettability, as measured by either

water-holding capacity or droplet contact angle, was significantly different among species, and

is highly correlated with previous reports of damage from simulated acid rain. The leaves of

Platanus occidentalis L. were the most wettable of the species examined and are reported to be

damaged the most by repeated applications of simulated acid rain. Mature leaves of Liriodendron

tulipifera L., with high contact angles and very low water-holding capacity, are the least damaged

according to published reports. Leaf water-holding capacity and surface-droplet contact angle

may be useful first indicators of resistance to acid rain, and should be included in future studies

of foliar effects of acid rain.

Key words: Acid rain, cuticle, leaf surfaces, wax.

INTRODUCTIO N

Surface characteristics are important in determining the wettability of leaves, foliar

permeability and penetration, water retention, and rates of exchange of water and

dissolved substances between plant and atmosphere (Fogg, 1947; Martin &

Juniper, 1970; Hallam & Juniper, 1971; Holloway, 1971; Norris, 1974; Baker &

Hunt, 1981; Juniper & JefFree, 1983). The increasing use in recent years of foliar

applied herbicides, fungicides and specialized growth regulators has resulted in

renewed interest in leaf surface characteristics such as trichome distribution,

cuticle thickness and wax deposition and their influences on efficacy of foliar

applied compounds of various types (Norris, 1974; Bukovac, Flore & Baker, 1979;

Baker & Hunt, 1981).

An additional area of recent research in which leaf surface characteristics may

be of particular importance concerns the effects of acid rain on plant growth and

survival. Effects of simulated acid rain on leaf surface perturbations (Evans, Gmur

& DaCosta, 1977; Evans, Gmur & Kelsch, 1977; Paparazzi & Tukey, 1983), foliar

leaching (Evans, Curry & Lewin, 1981; Haines, Chapman & Monk, unpublished)

and photosynthetic rates (Ferenbaugh, 1976; Neufeld, Jernstedt & Haines, 1985)

have been reported. Sensitivity to damage by simulated acid rain has been reported

to vary with species (Evans & Curry, 1979; Haines, Stefani & Hendrix, 1980;

Evans, Gmur & Mancini, 1982), growth conditions (Evans et al., 1982) and with

1

 To whom correspondence should be addressed.

'- Biology Department, New Mexico State University, Las Cruces, New Mexico, 88003, USA.

0028-646X/85/030407 + 10 $03.00/0 © 1985 The New Phytologist
408 B. L. HAINES et al.

age of leaves (Evans & Lewin, 1981), all of which are factors influencing leaf surface

features (Juniper, 1959; Hull, Morton & Wharrie, 1975; Cutter, 1978).

A little-studied aspect of acid rain effects on crop and forest species is that of

leaf wettability. The work of Keever & Jacobson (1983a, b) with Glycine max

indicates that the greater leaf wettability of a glabrous soybean isoline may be a

factor in its increased susceptibility to foliar injury and leaching by simulated acid

rain compared with plants of a pubescent isoline. However, it appears that cuticle

thickness and form and epicuticular wax distribution have not yet been considered

as factors influencing the magnitude of effects of acid rain.

Because of the possibility that leaf wettability may be related to damage by acid

rain in other species, and because surface features such as trichome number and

distribution, cuticle thickness and pattern and wax distribution determine, at least

in part, leaf wettability, these aspects of leaf structure and function were investigated. The objectives of the present study were to compare (1) leaf surface

characteristics, especially epicuticular wax deposits, (2) leaf water holding capacity,

and (3) leaf surface-water droplet contact angles of several species which have been

subjects of previous studies of effects of simulated acid rain. The results are

discussed in terms of species susceptibility to and extent of damage resulting from

simulated acid rain.

MATERIAL S AND METHODS

Plant material

Plants of Liquidambar styraciflua L. (sweetgum), Platanus occidentalis L.

(sycamore) and Robinia pseudo-acacia L. (black locust) were grown from seed in

4 litre pots in a greenhouse. When the seedlings were 3 to 6 cm tall they were

thinned to one per pot for a total of 60 seedlings per species. Individuals of

Liriodendron tulipifera L. (tulip poplar) and Quercus prinus L. (chestnut oak) were

obtained as established seedlings from field populations in northeastern Georgia

and southern North Carolina, respectively, transplanted into 4 litre pots, and

maintained in a greenhouse, under which conditions most of the foliage and

approx. 90% of the total biomass developed. Stratified seeds of Erechtites

hieracifolia Raf. (fire weed) were germinated and grown to maturity in a greenhouse.

A loamy sand mixture was used for the potting soil. Plants were kept well-watered

and fertilized weekly with half-strength Hoagland's nutrient solution.

Leaf surface structure

Samples of fresh leaf material of all six species were mounted on aluminum

stubs with either silver conductive paint or double-sided tape. Specimens were

examined directly with a Cambridge Stereoscan (Cambridge Instruments Ltd.,

Cambridge, England) scanning electron microscope or after sputter-coating with

gold-palladium.

Water-holding capacity

Water holding capacities were determined in the laboratory for five tree

species, Liquidambar, Liriodendron, Platanus, Quercus, and Robinia, and the herb,

Erechtites. Individuals of each species were selected and the fourth or fifth leaf from

the apex sprayed from above with distilled water for 20 s from a distance of 30 cm,

using a hand sprayer. Droplet size frequency distribution was determined by the

flour method of Laws & Parsons (1943). The sprayer produced droplets such that
Foliar effects of simulated acid rain II. 4°9

68 % (so 8-54, n = 3) of the mass was between 0-59 and 1-0 mm diameter and the

remainder of the mass was between 1-0 and 2-0 mm diameter. For Robinia, the

compound leaf was detached first and placed on a horizontal surface because of

difficulty in spraying when still attached to the stem. Leaves of all other species

remained attached to the plant, in their natural orientations. Spraying took place

in the laboratory, in the absence of air currents which would cause leaf motion.

Spraying was sufficient to saturate the leaves. Water which remained on the adaxial

surface was soaked up with absorbent tissue paper and weighed. Leaf areas were

determined for each leaf with a Li-Cor (Li-Cor Inc., Lincoln, Nebraska) area

meter. Water holding capacity was expressed as mg H2O cm~2 projected leaf area.

Leaf contact angles

Contact angles were measured on adaxial surfaces of detached leaves of all six

species. Angles were characterized on both rapidly expanding and recently

matured leaves. To determine first if pH influenced the contact angle, 10 /A drops

of either deionized (pH = 5-8) or acidified water (pH = 2-0) were placed on

horizontal pieces of commercial waxed paper using an autopipette. Drop outlines

were projected onto a wall using a modified 35-mm slide projector and traced on

paper. Contact angles were calculated from the tracings as described by Fogg

(1947). Drops of deionized water (10/tl) were then placed on adaxial surfaces of

freshly detached leaf pieces, the outlines projected and traced, and contact angles

calculated according to Fogg (1947).

Data were analyzed using ANOVA programs in SAS (SAS, 1982) after, where

appropriate, transformation (Zar, 1974). Additional statistical calculations were

performed on an HP-1000 mini-computer following procedures in Zar (1974).

RESULTS

Leaf fine structure

Scanning electron micrographs of the leaf surfaces are shown in Figures 1

and 2. The species examined differed in type and extent of epicuticular wax deposits, cuticle surface topography, trichome type and distribution, and epidermal

cell shape. Adaxial leaf surfaces of Robinia pseudo-acacia were characterized by the

presence of uniseriate covering trichomes, flattened epidermal cells and a dense

covering of crystalline surface wax deposits overlaying an apparently smooth

cuticle [Fig. l(a), (b)]. Leaves of Liriodendron tulipifera were glabrous. Cells of the

adaxial epidermis were only slightly convex, with a dense covering of granular

surface wax deposited on an unornamented cuticle [Fig. l(c), (d)]. Leaves of

Platanus occidentalis bore branched covering trichomes on both surfaces. The

trichomes were located predominantly over the veins and occurred in extremely

low frequency on the adaxial surface. Adaxial epidermal cells were convex and

marked with distinct cuticular striations. Epicuticular wax was absent from the

adaxial leaf surface [Fig. l(e), (f)]. Leaves of Liquidambar styraciflua were glabrous.

Cells of the adaxial epidermis were convex with a smooth cuticle and devoid of

visible (crystalline) surface wax deposits [Fig. 2(a)]. Adaxial surfaces of Erechtites

hieracifolia leaves were glabrous and waxless; epidermal cells were flat with a

smooth cuticle [Fig. 2(b)]. Quercusprinus leaves bore uniseriate covering trichomes,

usually along the veins. Adaxial epidermal cells were convex with a smooth cuticle,

and covered by a relatively sparse layer of granular surface wax [Fig. 2(c), (d)].
410 B. L. HAINES et al.

(a

Fig. 1. Scanning electron micrographs of adaxial leaf surfaces, (a) Uniseriate trichome and

wax-covered epidermal cells of Robinia pseudo-acacia', scale bar — 20 //m. (b) Crystalline surface

wax deposits covering R. pseudo-acacia epidermis; scale bar = 2 /*m. (c) Slightly convex epidermal

cells of Liriodendron tulipifera covered with granular wax deposit; scale bar = 20 /<om. (d) Granular

surface wax on L. tulipifera leaf; scale bar — 2 //.m. (e) Convex epidermal cells and sunken

stoma of Platanus occidentalism scale bar — 20//m. (f) Cuticular striations on epidermal cells of

P. occidentalism scale bar — 5 //m.

Water-holding capacity

Leaf water-holding capacity, in mg H2O cm^2 projected leaf area (Table 1)

showed significant differences among species. Water-holding capacity was greatest

in Platanus, Quercus, and Erechtites, and least in Liriodendron, with Liquidambar

and Robinia intermediate in wettability as determined by this method.
Foliar effects of simulated acid rain II. 411

Fig. 2. Scanning electron micrographs of adaxial leaf surfaces, (a) Convex epidermal cells with

smooth cuticle and no surface wax, Liquidambar styraciftua; scale bar — 20//m. (b) Flattened,

waxless epidermal cells of Erechtites hieracifolia; scale bar = 20 /(.m. (c) Uniseriate trichome and

sparse wax covering of Quercus prinus leaf; scale bar = 50 /;m. (d) Sparse layer of granular surfacewax of O. prinus leaf; scale bar — 20//m.

Table 1. Mean contact angles and mean water-holding capacities (mg H2O cm '')

of adaxial leaf surface^

Species

Robinia pseudo-acacia

Liriodendron tulipifera*

Quercus prinus*

Liquidambar styraciflua

Platanus occidentatis

Erechtites hieracifolia

Wax paper pH 2J

(acidified H..O)

Wax paper pH 5'8

(deionized H.,O)

Contact angle

ff+SE

131-6 + 2-2a

124-8±l-6a

10O9 + 6-2b

97-8±4-7b

80-7 + 8-5c

70-l+2-7c

1 1 9-7 + 2-6a

116-() + 2-2a

n

5

5

3

3

3

5

3

3

Water holding

X±SE

15-4 + 3-2b

3-9 + (>8a

29-5 + 3-7f

19-7±(>8b,c

25-0 + 1 -9c

24-9 + 3 -6c —

capacity

n

5

5

3

3

3

5

* Transplanted as seedlings to the greenhouse, under which conditions all the foliage used in this study

and approx. 90 % of the total biomass developed.

f Not included in Duncan's multiple range test because of insufficient plants.

% Means for the wax paper comparison were analyzed using a Student's /-test (Sokal & Rohlf, 1981).

§ Means not followed by the same letter differ at the P < O'OS level, as determined by a Duncan's

multiple range test (SAS, 1982).
412 B. L. HAINES et al.

Fig. 3. Side views of 10 /i.l droplets of deionized water placed on adaxial leaf surfaces; contact angles

increasing from (a) to (d). (a) P/atanus occidental!*, 0 = 82-2°. (b) Liquidambar styraciflua, 0 = 88-9°.

(c) Liriodendron tu/ipifera, 0 — 118'6°. (d) Robinia pseudo-acacia, 0 = 126-3°.

Leaf contact angles

Leaf-water droplet contact angle measurements and the effect of pH on

contact angle are shown in Table 1. The mean contact angles of droplets of

deionized water (pH 5-8) and acidified water (pH 2-0) placed on commercial wax

paper were not significantly different (P < 0-05). Mean contact angles of deionized

water on young and mature leaves of all species examined were not significantly

different and were pooled for analysis. Mean contact angles for adaxial surfaces

of Platanus and Erechtites leaves were less than 90° and the low spreading droplets

covered a comparatively large area of the leaf surface [Fig. 3(a)]. Mean contact

angles for Robinia and Liriodendron exceeded 120° and only a small area of the

adaxial surface was covered by these rounded droplets [Fig. 3(c), (d)]. Quercus and

Liquidambar leaf contact angles were of intermediate magnitudes; droplet shape

and coverage were also intermediate [Fig. 3(b)].

DISCUSSIO N

The degree of damage which trees and herbs suffer when exposed to acid rain varies

widely among species (Wood & Bormann, 1974, 1975 ; Evans & Curry, 1979; Lee

& Weber, 1979; Haines et al., 1980; Neufeld et al., 1985), but the underlying

mechanisms are poorly understood at present. Leaf wettability may be an

important determinant of foliar damage by acid rain, and physical and chemical

characteristics of the leaf surface determine the wettability of most leaves (Martin

& Juniper, 1970; Evans, 1982; Shriner, 1983). An estimate of the potential

wettability of a leaf surface can be obtained by measuring the angle of contact

between a water drop and the surface on which it rests (Fogg, 1947). The higher

the contact angle, the more hydrophobic the leaf, while the lower the angle, the

more wettable it is. Water holding capacity of leaves, while influenced by such

factors as leaf motion and orientation, is also generally indicative of wettability,

with more wettable leaves retaining more moisture than more hydrophobic leaves.

In the present study the contact angles of leaf surface-water droplets and

laboratory estimates of water-holding capacity were somewhat positively correlated.

Of the tree species examined, mature leaves of Platanus retained the most water

and had the lowest contact angles. At the other extreme, the leaves of Liriodendron

had the lowest water-holding capacity and high contact angles. Although Robinia

leaves had the highest contact angles, they were intermediate in water-holding

capacity, while leaves of Liquidambar were intermediate in both measures of

wettability. The relatively high water-holding capacity of Robinia may have been
Foliar effects of simulated acid rain II. 413

because these leaves were not sprayed in situ. Differences in contact angles among

tree species could be due to the distribution and nature of waxes overlying the

cuticle. Both the chemical composition and orientation and pattern of surface

waxes on the cuticle are important determinants of leaf wettability (Holloway,

1969a, b; Leece, 1978).

In addition to the effects of superficial waxes, contact angles may be altered by

surface roughness (Wenzel, 1936; Gray, 1965). The presence of hairs may decrease

contact angles (Boize, Gudin & Purdue, 1976), but this can be dependent on the

basic contact angle. If the angle is originally less than 90°, increased surface

roughness further lowers the angle, but if it is greater than 90°, then it raises the

angle (Wenzel, 1936; Fogg, 1947). Surface roughness may partially explain the low

contact angles observed for Platanus. Numerous cuticular striations covered the

adaxial surface of Platanus leaves, and branched covering trichomes and sunken

stomata occurred as well, all of which might act collectively to lower the contact

angle. Robinia leaves showed the highest contact angles, as might be expected from

the presence of trichomes and a dense covering of crystalline wax deposits on

adaxial surfaces, although the flattened epidermal cells and abundant trichomes

may have also contributed to the intermediate water-holding capacity. No attempt

was made in the present study to estimate the relative contributions to the

determination of leaf contact angles made by epicuticular wax chemistry and

distribution and leaf surface features such as trichome type, number and position

or stomatal position. Nonetheless, it is probable that there are primary and

secondary effects of each on leaf contact angles, and that the relative importance

of the various features varies with the species.

Four of the tree species of this study are subjects of a previous report on the

effects of simulated acid rain on leaf damage, growth and gas exchange (Neufeld

et al., 1985). Although sample size is small, percent damage among species is highly

correlated with both the water-holding capacity of the leaves and with contact

angles. Platanus leaves, the most wettable by our present determinations, were

damaged the most by repeated applications of simulated acid rain. Mature leaves of

Liriodendron, with high contact angles and very low water-holding capacity, were

the least damaged, in terms of percent necrotic leaf area, wrinkling and abscission

(Neufeld et al., 1985). Liquidambar, which showed intermediate wettability in the

present study, also showed intermediate degrees of damage. Robinia shows a

somewhat anomalous response. Although it had the highest contact angles, in order

of damage it was second among these species (Neufeld et al., 1985). This may have

been due to the method by which damage was assessed. For Robinia, entire rachises

and leaflets often abscise after exposure to the acid rains, even though leaflets

themselves appeared green and healthy. Loss of a rachis was scored as 100% leaf

loss. Actual damage to leaflet laminae was quite low, which would correlate with

the high contact angles measured for this species.

It does not appear that leaf wettability is correlated with the susceptibility

threshold of tree leaves to acid rain as much as with percent damage that occurs.

According to Neufeld et al. (1985), the four hardwood species exhibited damage

at approximately the same pH level (less than pH 3-0). In Keever & Jacobson's

studies (1983a, b) contact angles were also more closely related to the degree of

damage than to the threshold for damage. It must therefore be concluded that the

susceptibility threshold and resistance to damage are not necessarily linked.

In the present study, contact angles of young and mature leaves of Platanus,

Liquidambar, Liriodendron and Robinia were not significantly different and conse-
4

J

4 B. L. HAINES et al.

quently were pooled for subsequent analysis. However, in other studies, species

specific differences in leaf wettability occurred during maturation, resulting from

changes in wax and cuticle development (Juniper, 1959; Martin & Juniper, 1970;

Baker & Hunt, 1981). For example, young leaves of one isoline of Glycine max had

lower contact angles than older leaves, and this was correlated with greater damage

to younger leaves by acid rain (Keever & Jacobson, 1983b). As emphasized by

Keever & Jacobson (1983b), rapidly expanding leaves may have low rates of wax

production and thereby develop discontinuities in the coverage of waxes over the

leaf surface (Baker & Hunt, 1981). Since the cuticle is more hydrophilic than the

surface waxes, this results in lower contact angles (Holloway, 1969a). Furthermore,

rainfall acidity is often higher in spring and summer (Hornbeck, Likens & Eaton,

1977), a period coinciding with rapid leaf expansion, and the potential for foliar

damage would therefore be greatest at this time of the year.

The leaching of nutrients and other substances from leaves in response to acid

rain has received attention recently (Evans et al., 1981; Keever & Jacobson, 1983).

Because the wax component of the cuticle is thought to be responsible for reduction

of nutrient losses by leaching (Martin & Juniper, 1970), leaf surfaces and

wettability were examined on two species for which rates of mineral element

leaching in response to simulated acid rain have been documented. Haines,

Chapman & Monk (unpublished) showed that the highest leaching rates occurred

in the herbs Erechtites hieracifolia and Erigeron canadensis. Our results show that

Erecht.it.es leaves had the lowest mean contact angle (0 = 70-1°) of the six species

examined. Adaxial leaf surfaces of Erechtites are glabrous and waxless, and

composed of flattened epidermal cells with a smooth cuticle. The resulting highly

wettable leaves of Erechtites seem to be more subject to leaching than another

species examined, Ouercus pnnus. Haines et. al. (unpublished) reported 10 to

100-fold differences in leaching rates between Erechtites and Quercus, which is

consistent with expectations based on leaf surface characteristics. Leaves of

Ouercus had a mean contact angle of 101-5°, and adaxial surfaces are covered with

a sparse but distinct layer of wax granules, indicating a lower degree of wettability

than Erechtites leaves. Keever & Jacobson (1983a) were able to correlate foliar

damage of soybeans from simulated acid rain with foliar leaching and leaf contact

angles. They observed that their glabrous isoline of Glycine max, which had more

wettable leaves than the pubescent isoline, showed more damage to acid rain and

was more prone to foliar leaching of

 8"Rb. They suggest that the greater wettability

of the glabrous isoline, as indicated by lower mean contact angles, may have been

a factor in the observed differences in foliar sensitivity.

Besides our own research and that of Keever & Jacobson (1983a, b), we know

of no other studies relating either leaf water-holding capacity or contact angle to

damage by acid rain. The effects of increased retention time of acid precipitation

on wettable leaves (i.e. water-holding capacity) and extent of surface area covered

by rain droplets (i.e. leaf contact angle) on susceptibility to damage by acid rain

may provide insight into the mechanism of foliar damage by acid rain. One or both

of these characteristics should be quantified in future studies because of the

possibility that these simple measurements may be useful first indicators of the

resistance of plants to acid rain.

ACKNOWLEDGEMENT S

Support for this research was provided in part by a University of Georgia Faculty

Research grant to B.L. H.
Foliar effects of simulated acid rain II. 415

REFERENCE S

BAKER, E. A. & HUNT, G. M. (1981). Developmental changes in leaf epicuticular waxes in relation to foliar

penetration. The New Phytologist, 88, 731-747.

BOIZE, L., GUDIN, C. & PURDUE, G. (1976). The influence of leaf surface roughness on the spreading of

oil spray drops. Annals of Applied Biology, 84, 205-211.

BUKOVAC, M. ]., FI.ORE, J. A. & BAKER, E. A. (1979). Peach leaf surfaces: changes in wettability, retention,

cuticular permeability, and epicuticular wax chemistry during expansion with special reference to spray

application. Journal of the American Society for Horticultural Science, 104, 611-617.

CUTTER, E. G. (1978). Plant Anatomy: Experiment and Interpretation, Part 2. Organs. Arnold, London.

EVANS, L. S. (1982). Biological effects of acidity in precipitation on vegetation: A review. Environmental

and Experimental Botany, 22, 155-169.

EVANS, L. S. & CURRY, T. M. (1979). Differential responses of plant foliage to simulated acid rain. American

Journal of Botany, 66, 953-962.

EVANS, L. S., CURRY, T. M. & LEWIN, K. F. (1981). Responses of leaves of Phaseolusvulgaris L. to simulated

acidic rain. The New Phytologist, 88, 403-420.

EVANS, L. S., GMUR, N. F. & DACOSTA, F. (1977). Leaf surface and histological perturbations of leaves of

Phaseolus vulgaris and Helianthus annuiis after exposure to simulated acid rain. American Journal of

Botany, 64, 903-913.

EVANS, L. S., GMUR, N. F. & KEI.SCH, J. J. (1977). Perturbation of upper leaf surface structures by simulated

acid rain. Environmental and Experimental Botany, 17, 145-149.

EVANS, L. S., GMUR, N. F. & MANCINI, D. (1982). Effects of simulated acidic rain on yields of Raphanus

sativus, Lactuca saliva, Triticum aestivum and Medicago sativa. Environmental and Experimental Botany,

22, 445-453.

EVANS, L. S. & LEWIN, K. F. (1981). Growth, development, and yield responses of pinto beans and soybeans

to hydrogen ion concentrations of simulated acidic ram. Environmental and Experimental Botany, 21,

103-113.

FERENBAUGH, R. W. (1976). Effects of simulated acid rain on Phaseolus vulgaris L. (Fabaceae). American

Journal of Botany, 63, 283 288.

FOGG, G. E. (1947). Quantitative studies on the wetting of leaves by water. Proceedings of the Royal Society

of London (Series B), 134, 503-522.

GRAY, V. R. (1965). Surface aspects of wetting and adhesion. Chemistry and Industry, No. 23 (June 6, 1965),

969-978.

HAINES, B., STEFANI, M. & HENDRIX, F. (1980). Acid rain: threshold of leaf damage in eight plant species

from a southern Appalachian forest succession. Water, Air and Soil Pollution, 14, 403 407.

HAI.LAM, N. D. & JUNIPER, B. E. (1971). The anatomy of the leaf surface. In: Ecology of Leaf Surface

Micro-organisms (Ed. by T. F. Preece & C. H. Dickinson), pp. 3-37. Academic Press, New York &

London.

HOLI.OWAY, P. J. (1969a). The effects of superficial wax on leaf wettability. Annals of Applied Biology, 63,

145-153.

HOLLOWAY, P. J. (1969b). Chemistry of leaf waxes in relation to wetting. Journal of the Science of Food and

Agriculture, 20, 124-127.

HOLLOWAY, P. J. (1971). The chemical and physical characteristics of leaf surfaces. In: Ecology of Leaf

Surface Micro-organisms (Ed. by T. F. Preece & C. H. Dickinson), pp. 39-53. Academic Press, NewYork & London.

HORNBECK, J. W., LIKENS, G. E. & EATON, J. S. (1977). Seasonal patterns in acidity of precipitation and

their implications for forest stream ecosystems. Water, Air and Soil Pollution, 7, 355-365.

HULL, H. M., MORTON, H. L. & WHARRIE, J. R. (1975). Environmental influences on cuticle development

and resultant foliar penetration. Botanical Review, 41, 421-452.

JUNIPER, B. E. (1959). Growth, development, and effect of the environment on the ultra-structure of plant

surfaces. Journal of the Linnean Society (Botany), 56, 413-419.

JUNIPER, B. E. & JEFFREE, C. E. (1983). Plant Surfaces. Arnold, London.

KEEVER, G. J. & JACOBSON, J. S. (1983a). Response of Glycine max (L.) Merrill to simulated acid rain. I.

Environmental and morphological influences on the foliar leaching of

 H6Rb. Field Crops Research, 6,

241-250.

KEEVER, G. J. & JACOBSON, J. S. (1983b). Response of Glycine max (L.) Merrill to simulated acid rain. II.

Localization of foliar injury and growth response. Field Crops Research, 6, 251-259.

LAWS, J. O. & PARSONS, D. A. (1943). The relation of raindrop-size to intensity. Transactions, American

Geophysical Union, 24, 452-460.

LEE, J. J. & WEBER, D. E. (1979). The effect of simulated acid rain on seedling emergence and growth of

eleven woody species. Forest Science, 25, 393—398.

LEECE, D. R. (1978). Foliar absorption in Primus domestica L. I. Nature and development of the surface

wax barrier. Australian Journal of Plant Physiology, 5, 749—766.

MARTIN, J. T. & JUNIPER, B. E. (1970). The Cuticles of Plants. St. Martin's Press, New York.
B. L. HAINES et al.

NEUFELD, H. S., JERNSTEDT, J. A. & HAINES, B. L. (1985). Direct foliar effects of simulated acid rain. I.

Damage, growth and gas exchange. The New Phytologist, 99, 389-405.

NORRIS, R. F. (1974). Penetration of 2,4-D in relation to cuticle thickness. American Journal of Botany, 61,

74-79.

PAPAROZZI, E. T. & TUKEY, H. B. JR. (1983). Developmental and anatomical changes in leaves of yellow

birch and red kidney bean exposed to simulated acid precipitation. Journal of the American Society for

Horticultural Science, 108, 890-898.

SAS INSTITUTE INC. (1982). SAS Users Guide: Statistics, 1982 Edn. Gary, North Carolina.

SHRINER, D. S. (1983). Leaf response to acidic deposition. In: The Acidic Deposition Phenomenon and Its

Effects: Critical Assessment Review Papers. Volume II. Effects Sciences. Public Review Draft (Ed. by

A. P. Altshuller & R. A. Linthurst), pp. 3-5 - 3-10. United States Environmental Protection Agency,

Washington, D.C.

SOKAL, R. R. &ROHLF, F. J. (1981). Biometry: The Principles and Practice of Statistics in Biological Research,

2nd Edn. W. H. Freeman and Company, San Francisco.

WENZEL, R. N. (1936). Resistance of solid surfaces to wetting by water. Industrial and Engineering Chemistry,

28, 988-994.

WOOD, T. & BORMANN, F. H. (1974). The effects of an artificial acid mist upon the growth of Betula

alleghaniensis Britt. Environmental Pollution, 7, 259-268.

WOOD, T. & BORMANN, F. H. (1975). Increases in foliar leaching caused by acidification of an artificial mist.

Ambio, 4, 169-171.

ZAR, J. H. (1974). Biostatistical Analysis. Prentice-Hall, Englewood Cliffs, New Jersey.