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Optics of sunlit water drops on leaves: Conditions under which sunburn is possible

Authors:
  • Eötvös Loránd University, Danube Research Institute, Centre for Ecological Research, Hungarian Academy of Sciences

Abstract and Figures

It is a widespread belief that plants must not be watered in the midday sunshine, because water drops adhering to leaves can cause leaf burn as a result of the intense focused sunlight. The problem of light focusing by water drops on plants has never been thoroughly investigated. Here, we conducted both computational and experimental studies of this phyto-optical phenomenon in order to clarify the specific environmental conditions under which sunlit water drops can cause leaf burn. We found that a spheroid drop at solar elevation angle θ ≈ 23°, corresponding to early morning or late afternoon, produces a maximum intensity of focused sunlight on the leaf outside the drop's imprint. Our experiments demonstrated that sunlit glass spheres placed on horizontal smooth Acer platanoides (maple) leaves can cause serious leaf burn on sunny summer days. By contrast, sunlit water drops, ranging from spheroid to flat lens-shaped, on horizontal hairless leaves of Ginkgo biloba and Acer platanoides did not cause burn damage. However, we showed that highly refractive spheroid water drops held 'in focus' by hydrophobic wax hairs on leaves of Salvinia natans (floating fern) can indeed cause sunburn because of the extremely high light intensity in the focal regions, and the loss of water cooling as a result of the lack of intimate contact between drops and the leaf tissue. © The Authors (2010)
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Cover: A water droplet on a water-repelling hairy leaf of
the floating fern,
Salvinia natans.
Courtesy of György
Kriska. (Egri
et al.
pp. 979–987)
March 2010
Vol. 185
No. 4
ISSN 0028-646X Forum
Commentary
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878 Not every fungus is everywhere: scaling to the biogeography of
fungal–plant interactions across roots, shoots and ecosystems
882 Moving from pattern to process in fungal symbioses: linking
functional traits, community ecology and phylogenetics
886 The Sphagnum air-gun mechanism resurrected
889 The Sphagnum air-gun mechanism resurrected? Not with a
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Tansley reviews
893 Induced resistance to pests and pathogens in trees
A. Eyles, P. Bonello, R. Ganley & C. Mohammed
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Rapid reports
909 Phosphatidic acid formation is required for extracellular
ATP-mediated nitric oxide production in suspension-cultured
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979 Optics of sunlit water drops on leaves: conditions under which
sunburn is possible
Á. Egri, Á. Horváth, G. Kriska & G. Horváth
• Commentary p 865
988 In folio isotopic tracing demonstrates that nitrogen assimilation
into glutamate is mostly independent from current CO2
assimilation in illuminated leaves of Brassica napus
P. P. G. Gauthier, R. Bligny, E. Gout, A. Mahé, S. Nogués,
M. Hodges & G. G. B. Tcherkez
1000 Modeling whole-tree carbon assimilation rate using observed
transpiration rates and needle sugar carbon isotope ratios
J. Hu, D. J. P. Moore, D. A. Riveros-Iregui, S. P. Burns
& R. K. Monson
1016 Freeze–thaw-induced embolism in Pinus contorta: centrifuge
experiments validate the ‘thaw-expansion hypothesis’ but
conflict with ultrasonic emission data
S. Mayr & J. S. Sperry
1025 Seasonal water relations of Lyginia barbata (Southern rush) in
relation to root xylem development and summer dormancy of
root apices
M. W. Shane, M. E. McCully, M. J. Canny, J. S. Pate,
C. Huang, H. Ngo & H. Lambers
1038 An experimental test of well-described vegetation patterns
across slope aspects using woodland herb transplants and
manipulated abiotic drivers
R. J. Warren II
1050 Underground friends or enemies: model plants help to unravel
direct and indirect effects of arbuscular mycorrhizal fungi on
plant competition
E. Facelli, S. E. Smith, J. M. Facelli, H. M. Christophersen &
F. Andrew Smith
1062 Ethylene signalling and ethylene-targeted transcription factors
are required to balance beneficial and nonbeneficial traits in the
symbiosis between the endophytic fungus Piriformospora indica
and Arabidopsis thaliana
I. Camehl, I. Sherameti, Y. Venus, G. Bethke, A. Varma,
J. Lee & R. Oelmüller
• Commentary p 868
1074 Suppression of hypernodulation in soybean by a leaf-extracted,
NARK- and Nod factor-dependent, low molecular mass
fraction
Y.-H. Lin, B. J. Ferguson, A. Kereszt & P. M. Gresshoff
1087 A multi-species experiment in their native range indicates pre-
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1100 Gene flow and population admixture as the primary
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1108 Stress-induced DNA methylation changes and their heritability
in asexual dandelions
K. J. F. Verhoeven, J. J. Jansen, P. J. van Dijk & A. Biere
• Commentary p 867
1119 Corrigendum
CONTENTS
March 2010 Vol. 185 No. 4 pp. 865–1120
www.newphytologist.org
March 2010
Vol. 185
No. 4
ISSN 0028-646X
Can water droplets on
leaves cause sunburn?
Dandelions remember stress
Ethylene signalling
Plant–fungal symbioses:
bridging scales from local
to global
Tansley reviews
Induced resistance
to pests and pathogens
in trees
nph_185_4_oc.qxp 2/4/2010 11:43 AM Page 1
Optics of sunlit water drops on leaves: conditions under
which sunburn is possible
A
´da
´m Egri
1
,A
´kos Horva
´th
2
, Gyo
¨rgy Kriska
3
and Ga
´bor Horva
´th
1
1
Environmental Optics Laboratory, Department of Biological Physics, Physical Institute, Eo
¨tvo
¨s University, H-1117 Budapest, Pa
´zma
´ny se
´ta
´ny 1, Hungary;
2
Max Planck Institute for Meteorology, D-20146 Hamburg, Bundesstrasse 53, Germany;
3
Group for Methodology in Biology Teaching, Biological
Institute, Eo
¨tvo
¨s University, H-1117 Budapest, Pa
´zma
´ny se
´ta
´ny 1, Hungary
Author for correspondence:
Ga
´bor Horva
´th
Tel: +36 30 64 64 371
Email: gh@arago.elte.hu
Received: 19 September 2009
Accepted: 5 November 2009
New Phytologist (2010) 185: 979–987
doi: 10.1111/j.1469-8137.2009.03150.x
Key words: environmental optics, leaf burn,
phyto-optics, plant leaf, ray tracing, solar
radiation, sunburn, water drop.
Summary
It is a widespread belief that plants must not be watered in the midday sunshine,
because water drops adhering to leaves can cause leaf burn as a result of the
intense focused sunlight. The problem of light focusing by water drops on plants
has never been thoroughly investigated.
Here, we conducted both computational and experimental studies of this
phyto-optical phenomenon in order to clarify the specific environmental conditions
under which sunlit water drops can cause leaf burn.
We found that a spheroid drop at solar elevation angle h23, corresponding
to early morning or late afternoon, produces a maximum intensity of focused sun-
light on the leaf outside the drop’s imprint. Our experiments demonstrated that
sunlit glass spheres placed on horizontal smooth Acer platanoides (maple) leaves
can cause serious leaf burn on sunny summer days.
By contrast, sunlit water drops, ranging from spheroid to flat lens-shaped, on
horizontal hairless leaves of Ginkgo biloba and Acer platanoides did not cause
burn damage. However, we showed that highly refractive spheroid water drops
held ‘in focus’ by hydrophobic wax hairs on leaves of Salvinia natans (floating
fern) can indeed cause sunburn because of the extremely high light intensity in the
focal regions, and the loss of water cooling as a result of the lack of intimate con-
tact between drops and the leaf tissue.
Introduction
It is a widely held belief in horticulture that plants must not
be watered in the midday sunshine. The most frequent
explanation for this is that in direct sunshine water drops
adhered to plants can scorch the leaves as a result of the
intense light focused on to the leaf tissue. Seventy-eight per
cent of the relevant topical websites surveyed by us (Sup-
porting Information, Table S1) answered the question ‘Do
sunlit water drops burn leaves?’ in the affirmative. This
attests to the fact that laymen and professionals alike com-
monly believe water drops on plants after rain or watering
can cause leaf burn in sunshine. (We add that morning dew
on plants can also persist into the daylight hours and might
thus cause leaf burn.) This is a long-standing environmental
optical problem, the solution of which is not trivial at all.
An analogous issue is whether or not human skin covered
by water drops can be damaged by focused sunlight during
sunbathing. Eighty-nine per cent of the surveyed dermato-
logical and cosmetics websites (Table S2) answered the ques-
tion ‘Can sunlit water drops burn the human skin?’ in the
affirmative. Similarly, in the forestry literature the prevailing
opinion is that forest fires can be sparked by intense sunlight
focused by water drops on dried-out vegetation (Table S3).
The closest atmospheric optics problem is the refraction
of sunlight by falling raindrops, which produces a rainbow.
Although the literature of rainbow optics is extensive (Des-
Cartes, 1637; Airy, 1838; Nussenzweig, 1977; Ko
¨nnen &
de Boer, 1979; Lee, 1998), these studies were all limited to
the spherical or semi-spherical shapes of falling water drops.
The problem of light focusing by water drops adhered to
plants has never been thoroughly investigated, either theo-
retically or experimentally. In order to fill this gap and
determine the specific conditions under which sunlit water
drops can cause leaf burn, we conducted both experimental
and computational studies. First, we exposed horizontal
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New Phytologist (2010) 185: 979–987 979
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leaves covered with glass spheres or water drops to sunlight,
considering plants of different wettabilities and surface
properties (smooth or hairy). Next, we performed an optical
modeling study of realistic water drops derived from digital
photographs. Using computer ray tracing, we calculated the
spatial distribution of sunlight intensity focused by a rota-
tion-symmetric water drop onto a horizontal leaf. This, in
turn, allowed us to determine the location and magnitude
of maximum light intensity on a leaf as a function of drop
shape and solar elevation angle h. Accounting for the h-
dependent solar spectrum and the absorption spectrum of
green plant tissue, we could finally determine the particular
drop shape and solar elevation that maximized the probabil-
ity of leaf burn by focused sunlight.
Materials and Methods
We performed three experiments in sunshine with various
leaves covered by glass spheres or water drops.
Experiment 1
We tested whether maple (Acer platanoides) leaves covered
with glass spheres of different diameters (ranging from 2 to
10 mm) can suffer sunburn when exposed to direct sunlight
for various periods. The experiment was performed on three
sunny, cloudless, warm, and calm days in a garden in Go
¨d
(4743¢N, 1909¢E), Hungary. Ten circular grey plastic
trays were put on a table (Fig. S1), each containing a single
fresh-cut maple (A. platanoides) leaf with a smooth hairless
surface. In trays 1–9, the leaves were covered with a single
layer of tightly packed glass spheres (index of refraction
n
glass
= 1.50), the diameter of which systematically
increased from 2 to 10 mm from trays 1 to 9. In tray 10 the
leaf remained free of glass spheres, functioning as a control.
The glass sphere-covered leaves were then exposed to direct
sunlight for three different time periods: long (from 08:00
to 17:00 h local summer time = UTC + 2 h, on 8 July
2007), medium (from 10:30 to 13:30 h on 14 July 2007),
and short (from 16:00 to 17:00 h on 17 July 2007). During
exposure the trays were continuously sunlit encountering
no shade. After the experiment, the maple leaves were
scanned by a Canon Arcus 1200 scanner in the laboratory
to document their sunburn (Fig. 1).
Experiment 2
Experiment 2 was devoted to determining whether or not
sunlit water drops on horizontal Ginkgo biloba and A. plat-
anoides leaves with smooth hairless surfaces can cause sun-
burn in the leaf tissue at any solar elevation angles
(Table 1). It was performed on 26 July 2007 at the same
place as Expt 1. The weather was sunny, cloudless, warm,
and calm. One pair of fresh-cut G. biloba and one pair of A.
platanoides (maple) leaves with smooth hairless surfaces
were fixed flat on to rectangular (10 cm ·10 cm) glass
panes with colourless transparent tape at a few points along
the leaf edges. In each pair of leaves, one leaf had its upper
surface and the other its lower surface facing upward. The
glass panes with the flat leaves were placed horizontally on a
table at a height of 10 cm above the tabletop (Fig. 2a). The
table was covered with a bright green matte cloth. Several
large drops (7–8 mm in diameter) of clean tap water were
placed on to all four leaves (Fig. 2). To produce these large
drops, four smaller droplets of the same size were dripped
from an eye-dropper on to a given location on a leaf blade.
Thus, the sizes of the large aggregate drops were practically
the same on each leaf. The function of the glass panes was
to ensure the orientation of the flat leaf blades was horizon-
tal, and to allow the prepared leaves to be illuminated from
below by the diffuse light reflected from the underlying
green matte surface, thereby imitating the ground-reflected
light received by vegetation under natural conditions.
The leaves holding numerous water drops were then
exposed to direct sunlight in three different sessions. The
first exposure began at low solar elevations at 07:55 h
(Table 1) and lasted until all water drops evaporated from
the leaves. Then, two more exposures were performed using
newly cut pairs of Ginkgo and Acer leaves prepared as
described earlier, with the third and last exposure finishing
in the early afternoon. The experiments were not continued
into the late afternoon because of cloudy weather, which is
typical of the Hungarian summer. Under cloudy conditions
the exposure by continuous sunlight could not be ensured.
After the experiment the leaves were scanned in the labora-
tory to determine their possible sunburn.
The time, solar elevation angle, and air temperature at the
start and end of a given exposure, as well as the number of
water drops on the leaves, are summarized in Table 1, sepa-
rately for each exposure. The start time of a given exposure
was the same for all four leaves but the end times were deter-
mined by the duration of evaporation of the water drops,
which, in turn, depended on leaf albedo and the shape of
water drops, both varying with leaf type (see Figs 4a, 5a, 6a).
Experiment 3
Experiment 3 was conducted to test whether or not sunlit
water drops held by hydrophobic wax hairs of floating fern
leaves (Salvinia natans) can induce sunburn. It was per-
formed in the Botanic Garden of Eo
¨tvo
¨s Lora
´nd University
in Budapest (4728¢N, 1905¢E) on 30 July 2007 on a
sunny, cloudless, warm, and calm day. Floating ferns (S. na-
tans) with a large number (120) of leaves were put into two
small water-filled containers (Fig. 3a), which were then
exposed to direct sunlight for 2 h from 13:00 to 15:00 h
local solar time (UTC + 2 h). Before exposure, numerous
(five to 20) water drops of varying sizes (0.5–8 mm in
980 Research
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New Phytologist (2010) 185: 979–987
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diameter) were dripped sprayed on to the hairy Salvinia
leaves (Fig. 3b,c) with an eye-dropper water-sprayer.
Throughout the experiment the positions and orientations
of the Salvinia leaves floating on the water surface did not
change. After the 2 h exposure, smaller water droplets com-
pletely evaporated, while larger drops did not. The experi-
ment was concluded by cutting and scanning several
Salvinia leaves – still holding water drops – in the labora-
tory in order to document their sunburn (Fig. 3d–i).
Computer ray tracing
To better understand sunlight focusing by water drops, we
also performed a computer ray tracing. First, we determined
the rotational-symmetric shape of water drops of different
sizes on various horizontal leaves (rowan, Sorbus aucuparia;
plane tree, Platanus hybrida; maple, A. platanoides) with
zdifferent water repellencies. Then, we computed the paths
of rays refracted by these water drops when they were illu-
minated by parallel rays of sunlight at different solar eleva-
tion angles, h. These calculations enabled us to determine
the two-dimensional distribution of the light-collecting effi-
ciency of water drops along horizontal leaf blades as func-
tions of drop shape and h. Finally, we calculated the
intensity of focused sunlight absorbed by the leaf tissue,
which is the parameter that ultimately determines leaf-dam-
age potential. Further details of these computations can be
found in the online Supporting Information (Notes S1A
and S1D, Figs. S2, S3, S4).
Results
In Expt 1 using glass spheres, all maple (A. platanoides)
leaves suffered serious sunburn (Fig. 1), regardless of the
MediumLong
2 mm
10 mm
10 mm 2 mm
Short
Duration of exposure
(b)
(h)
(e)
(k)
(c)
(i)
(f)
(l)
(a)
(g)
(d)
(j)
Diameter of glass spheres
Fig. 1 (a–f) Sunburnt maple (Acer platano-
ides) leaves exposed to direct sunlight for
long (left column), medium (middle column),
and short (right column) periods covered by
glass spheres of 2 and 10 mm diameter in
Expt 1. The grid pattern of sunburnt brown
patches caused by intense focused sunlight is
clearly visible on the green leaves. (g–l) As
(a–f) but with a fourfold enlargement.
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length of sunlight exposure (long, medium, or short) and
diameter of the spheres (2–10 mm). From this we con-
cluded that sunlight focused by glass spheres (refractive
index n
glass
= 1.5) can burn the leaf tissue any time of day
(morning, midday, or afternoon).
However, the effect of water drops may be different from
that of glass spheres because the drop shape is usually not
spherical; the refractive index of water (n
water
= 1.33) is
smaller than that of glass (n
glass
= 1.50); and water cools the
leaf tissue, if there is an intimate contact between a water
drop and the leaf. Thus, we performed a second, more real-
istic experiment, in which horizontal and smooth G. biloba
and A. platanoides leaves covered with water drops (c.5
7 mm in diameter) were exposed to sunlight (Table 1,
Fig. 2). In contrast to glass spheres, water drops did not
cause any visible sunburn (browning).
In Expt 2, water drops were sitting directly on the leaf
surface, and consequently their focal regions fell far below
the leaf blade, explaining the lack of sunburn together with
water cooling. If a water drop were further away from the
leaf blade, it could not cool the leaf, and its focal region
may fall on to the leaf surface, thus possibly inducing sun-
burn (browning) in the leaf tissue. This could be the situa-
tion for hairy leaves, where hydrophobic wax hairs can hold
a water drop at a distance from the leaf surface. In order to
study such a situation, we conducted Expt 3, in which
highly water-repellent hairy leaves of floating fern (S. na-
tans) holding water drops were exposed to sunlight
(Fig. 3a–c). The hairs of S. natans are composed of bundles
of thin, hydrophobic wax fibres, which can hold even large
water drops. Fig. 3d–i shows pictures of Salvinia leaves after
exposure, where brown sunburnt patches are clearly visible.
Table 1 Local solar time (tUTC + 2 h), solar
elevation angle h, and air temperature T(C)
at the start and end of the three exposures of
Acer platanoides and Ginkgo biloba leaves in
Expt 2 (Fig. 2), and the number Nof water
drops on the sun-facing lower or upper leaf
surfaces
Exposure
Leaf (exposed
surface)
Start End T(
o
C)
Nt h()th() Start End
First Acer (lower) 7:55 27.5 9:40 44.9 24.0 27.0 25
Acer (upper) 7:55 27.5 9:35 44.1 24.0 27.0 21
Ginkgo (lower) 7:55 27.5 10:30 52.6 24.0 29.0 8
Ginkgo (upper) 7:55 27.5 10:00 48.1 24.0 28.0 11
Second Acer (lower) 10:30 52.6 11:28 60.1 29.0 31.0 24
Acer (upper) 10:30 52.6 11:20 59.2 29.0 30.5 29
Ginkgo (lower) 10:30 52.6 12:00 63.0 29.0 32.0 18
Ginkgo (upper) 10:30 52.6 11:31 60.4 29.0 31.0 21
Third Acer (lower) 12:00 63.0 13:11 64.4 32.0 34.0 29
Acer (upper) 12:00 63.0 13:01 64.7 32.0 33.5 31
Ginkgo (lower) 12:00 63.0 13:45 62.3 32.0 34.5 19
Ginkgo (upper) 12:00 63.0 13:17 64.1 32.0 34.0 21
Ginkgo biloba
(a) (b) Acer platanoides
Upper Ginkgo
Ginkgo
Acer
Acer
Upper
Lower
Lower Fig. 2 (a) Setup of Expt 2 with two water-
covered maple (Acer platanoides) leaves
(bottom) and two Ginkgo biloba leaves (top)
on glass panes. In each case, both upper and
lower leaf surfaces were exposed to sunlight.
(b) Close-up photographs of water drops on
Ginkgo (left column) and Acer (right column)
leaves.
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In conclusion, hairy leaves can suffer sunburn after rain-
ing watering because the focal region of water drops held
by water-repellent wax fibres falls right on to the leaf sur-
face, which is not cooled by water because of the lack of
intimate contact between drops and leaf.
Let us now turn to our optical modeling results. Fig. 4a
shows a typical water drop on a horizontal rowan (S. aucu-
paria) leaf, which is approximately spheroid as a result of
the large contact angle, v145, between water and leaf
blade. Fig. 4b represents the ray tracing through this spher-
oid water drop along its vertical main cross-section for vari-
ous solar elevation angles, h, while Fig. 4c shows the
resulting two-dimensional distributions of the light-collect-
ing efficiency Qvs h. According to Fig. 4c, the focal region
is mostly within the drop’s imprint and is thus cooled by
water for h>50. For h<40, however, the focal region is
not cooled by water as it falls outside the drop’s imprint,
increasing the probability of tissue damage by sunburn.
Figs 5 and 6 show the same result for a flat water drop on a
maple leaf (A. platanoides) and a hemispherical drop on a
plane tree leaf (P. hybrida).
Fig. 7 shows the intensity I(h) of drop-focused sunlight
that is actually absorbed by the green tissue of maple, plane
tree, and rowan leaves (Figs 4–6) at a given solar elevation
h. Apparently, the h-dependence of I(h) is driven by that of
the light-collecting efficiency Q(n
water
,h) (see Fig. S5).
Therefore, I(h) monotonically increases with decreasing
solar elevation for both the flat drop on the maple leaf and
the hemispherical drop on the plane tree leaf (Fig. 7a,b).
For these two plants, absorbed sunlight intensity Iis largest
at sunrise sunset but with quite different peak intensities:
log
10
Ireaches 4.37 for plane tree and 2.85 for maple at
h=5, corresponding to a factor of 10
4.37)2.85
=
10
1.52
33 difference (Fig. 7a,b).
Fig. 7c and Fig. S5c show that both the light-collecting
efficiency Q(n
water
,h) and the intensity I(h) of absorbed sun-
light have two maxima in the case of a spheroid water drop
on a rowan leaf. This is because such a nonspherical drop
has two different focal regions as a result of astigmatism:
one at h
1
=13(further from the drop) with log
10
I= 4.7,
and one at h
2
=23(nearer to the drop) with log
10
I= 5.1.
These are formed by refracted rays crossing the drop along
its horizontal and vertical main cross-sections, respectively.
As a result, the first focal region is elongated parallel to the
antisolar meridian, while the second is perpendicular to the
antisolar meridian (see Fig. 4c, panel 6 for h
1
=13, and
Fig. 4c, panel 4 for h
2
=23), and they show a factor of
10
5.1)2.7
=10
2.4
251 and 10
4.7)2.8
=10
1.9
79
increase in absorbed intensity compared with a flat drop.
Therefore, the chance of leaf tissue sunburn caused by the
spheroid water drop in Fig. 7c is significantly higher than
that for either the flat drop in Fig. 7a or the hemispherical
drop in Fig. 7b. However, as we showed in Expt 2, even
these most intensely focusing spheroid water drops fail to
cause sunburn (at least on Ginkgo leaves). Further results
can be found in the online Supporting Information (Note
S1B, Figs. S5–S8).
Discussion
Leaf surface characteristics are important in determining
wettability, foliar permeability penetration, water retention,
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
Fig. 3 (a) Setup of Expt 3 with leaves of floating fern (Salvinia natans) in two water-filled containers. (b, c) Water drops on green water-
repelling hairy Salvinia leaves. (d–i) Brown sunburnt patches on Salvinia leaves after 2 h exposure to sunlight.
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as well as exchange rates of gas, water and dissolved
substances between plant and atmosphere (Fogg, 1947;
Holloway, 1969; Martin & Juniper, 1970; Juniper & Jef-
free, 1983). If, after rain, leaf blades were covered by a water
film, they could not breathe, because gas exchange through
the stomata would be blocked. To avoid this, plants evolved
efficient water-repelling and water-channeling structures
which build up and roll off rain drops (de Gennes et al.,
2004). A general rule is that the more hydrophobic the leaf
surface (i.e. the greater the leaf-water contact angle), the
smaller is its water-holding capacity. For example, water
drops easily roll off the highly hydrophobic leaves of lotus,
Ginkgo (Fig. 2b), and floating fern (Fig. 3b,c) if leaves are
tilted or shaken.
log
10
Q
+2.50
(a)
(b) (c)
+2.00
+1.50
+1.00
+0.50
0.00
–0.50
< –1.00
1
2
3
4
5
6
7
8
θ = 90°
θ = 50°
θ = 30°
θ = 23°
θ = 16°
θ = 13°
θ = 5°
θ = 2°
Fig. 4 (a) Side-view photograph of a spheroidal water drop on a
horizontal rowan (Sorbus aucuparia) leaf. (b) Ray tracing through
the vertical main cross-section of the water drop (the contour of
which is shown by the red curve) vs solar elevation angle h. (c) Two-
dimensional distribution of the light-collecting efficiency Qof the
water drop on the leaf. The area where the water drop contacts the
leaf is shown by the inner circle, while the contour of the drop as
seen from above is indicated by the outer circle.
θ = 90°
(a)
(b) (c)
θ = 70°
θ = 50°
θ = 30°
θ = 20°
θ = 15°
θ = 10°
θ = 5°
log10Q
+0.75
+0.50
+0.25
0.00
–0.25
–0.50
< –1.00
–0.75
1
2
3
4
5
6
7
8
Fig. 5 As Fig. 4 but for a flat water drop on a maple (Acer platano-
ides) leaf. In panel (c), the circles mark the water drop’s imprint on
the leaf.
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In Expt 1, we showed that glass spheres on smooth hori-
zontal leaves can cause sunburn (Fig. 1), because their focal
region falls on or near the underlying leaf surface for a wide
range of solar elevation h. Water drops, however, have a
smaller index of refraction than glass spheres (n
water
= 1.33
vs n
glass
= 1.50) and thus have smaller refractive power as
well. In addition, the shape of water drops on leaves is usu-
ally ellipsoidal (Figs 2, 3b,c, 4a, 5a, 6a), which further
decreases their refractive power. As a result, the focal region
of water drops falls far below the leaf at higher solar eleva-
tions and can fall on to the leaf only at lower solar eleva-
tions, when the intensity of light from the setting sun is
generally too small to cause sunburn. Furthermore, water
drops, especially flatter ones, contact the leaf surface over
much larger areas than glass spheres and can thus produce
significant water cooling. The intimate contact between a
water droplet and a hairless leaf considerably increases the
thermal mass in the contact region, which inevitably has the
effect of greatly reducing any damaging temperature rise of
the underlying leaf surface, and thus preventing sunburn.
These factors explain our experimental finding that sunlit
water drops on horizontal leaves without waxy hairs (Fig. 2)
cannot cause sunburn regardless of solar elevation and drop
shape.
In Expt 3 with Salvinia, however, the water drops were
not residing on but were held above the leaves by hydropho-
bic wax hairs (Fig. 3b,c). This separation resulted in the
focal region of drops falling on the leaf surface, and the loss
of cooling by contact. In addition, the water drops had a
high refractive power because of their spheroidal shape. As a
consequence of these factors, the Salvinia leaves did get
burnt in sunshine (Fig. 3d–i).
A more water-repellent leaf surface entails a greater con-
tact angle between water and leaf cuticle, a lower water-
holding capacity, and a more spheroid water drop. Because
of their greater curvature, spheroid water drops have larger
refractive power and, thus, greater light-focusing ability
than flat ellipsoid drops; therefore, they are more likely to
cause thermal damage in the leaf tissue. However, spheroid
water drops (Fig. 4a) easily roll off highly hydrophobic
leaves which are tilted or shaken by wind, practically elimi-
nating the possibility of sunburn. By contrast, water drops
can stick to wettable leaves, such as those of Acer, for an
+2.25
+1.50
+2.00
<–1.00
+1.00
+0.50
0.00
–0.50
log
10
Q
θ = 90°
θ = 70°
θ = 50°
θ = 30°
θ = 20°
θ = 15°
θ = 10°
θ = 5°
1
2
3
4
5
6
7
8
(a)
(b ) (c )
Fig. 6 As Fig. 5 but for a hemispherical water drop on a plane tree
(Platanus hybrida) leaf.
5.0
4.5
4.0
3.5
3.0
2.5
10° 20° 30° 40° 50° 60° 70°
12
3
4
4
4
5
5
5
6
6
6
7
7
7
8
8
8
3
3
2
2
1
1
80° 90°
log10I
Solar elevation angle θ
Horizon Zenith
(c)
(b)
(a)
Fig. 7 Log
10
Ivs solar elevation angle h, where I=Q(n
water
= 1.33,
h)Æa(h) is the maximum sunlight intensity absorbed by green leaf tis-
sue in the focal region of a water drop on a horizontal maple (a),
plane tree (b) and rowan (c) leaf with decreasing wettabilities from
(a) to (c). Here, Q(n
water
= 1.33,h) is the maximum light-collecting
efficiency of water drops in the focal region (Fig. S5), and a(h) is the
solar absorption factor of leaves (Fig. S4c). Insets show side-view
photographs of water drops. Data corresponding to panels 1, 2,
7, 8 in Figs 4–6 are marked by filled circles, triangles, and squares,
respectively.
New
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extended period of time (Fig. 5a). However, this is counter-
balanced by the weak light-focusing ability (refractive
power) of drops, which tend to be rather flat on a wettable
leaf because of the small contact angle of water. In conclu-
sion, water drops cannot cause sunburn, either on water-
repellent or on wettable leaves with smooth, hairless sur-
faces, as clearly corroborated by Expt 2.
As shown in Expt 3, sunlit water drops can cause sunburn
(Fig. 3d–i) but only if they are held at appropriate heights
above the leaf surface. This is the case for the superhydro-
phobic leaves of lotus (Cheng & Rodak, 2005) and floating
fern (Fig. 3b,c), where wax hairs can hold highly refractive
spheroid drops ‘in focus’. Note that sunburn potential is the
net result of competing factors, the relative strengths of
which depend on drop size. Although smaller water droplets
have greater curvature and thus larger focusing capability
than larger drops, they also evaporate more rapidly, reduc-
ing the exposure to focused sunlight and the chance of sun-
burn. Furthermore, the amount of light focused by smaller
droplets is also smaller than that collected by larger drops,
because light-collecting efficiency is proportional to effec-
tive cross-section perpendicular to incident sunlight. As a
result, sunburn tends to favor larger drops over smaller
droplets. However, sunburn caused by tiny droplets could
potentially be more injurious than that caused by a few large
drops, as they cover a leaf more extensively.
At the start of this paper, we quoted the oft-stated advice
that plants must not be watered in the midday sunshine in
order to avoid tissue damage as a result of intense sunlight
focused by water drops on leaves (Table S1). We have
shown that this widely held belief is only correct for hairy
leaves. Based on the computed light-collecting efficiency of
water drops (Figs 4–7), we have also determined that the
risk of sunburn of smooth, hairless leaves is theoretically the
highest at a solar elevation of h23, corresponding to
early morning or late afternoon. In practice, however, the
light intensity from such an oblique position of the sun is
too weak to be a factor, as was confirmed by Expt 2.
We believe that completely unrelated types of leaf damage
might be partly responsible for the widespread belief about
sunburn caused by water drops. For example, drops of acid
rain, salty sea tap water, chlorinated water, and concen-
trated solutions of nutrition, fertilizers, and chemicals can
all cause sunburn-like brown patches via osmotic dehydra-
tion of leaf tissue (Boize et al., 1976; Haines et al., 1980,
1985; Appleton et al., 2002). In addition, guttation and
chlorophyll deficiency can also be associated with a brown-
ish pale appearance of leaves. Finally, spraying cold water
on to plants in hot, sunny weather might also induce physi-
ological stresses with damaging consequences, such as with-
ering or browning of leaves, to name but a few possibilities.
These results also enable us to comment on the analogous
issue of whether human skin covered with water drops can
be damaged by focused sunlight during sunbathing, as is
often claimed on dermatological and cosmetics websites
(Table S2). If the skin is not oily, the contact angle of water
relative to the skin surface is small, and consequently water
drops are flat and their focal region falls far below the skin
surface, eliminating the possibility of sunburn. Neverthe-
less, such drops may still increase the risk of skin cancer by
focusing and enhancing ultraviolet (UV) radiation in deeper
skin layers, but this effect will also depend on the UV
absorption properties of the drops. However, a situation
similar to floating fern (S. natans) leaves is plausible for
humans as well. Most of us have tiny hairs covering our skin
(except on our lips and the undersides of our hands and
feet), and tiny water droplets with greater curvature and lar-
ger focusing capability could rest on this layer of fluffy hairs,
potentially causing skin burn. Hence, this problem appears
rather complex and should be investigated for a definitive
answer. By contrast, if the skin is oily (e.g. because of sun-
screen), the water drops formed are spheroidal and can eas-
ily roll off the water-repelling skin, thereby minimizing the
possibility of both sunburn and skin cancer. Further discus-
sion can be found in the online Supporting Information
(Note S1C).
Conclusions
In sunshine, water drops residing on smooth, hairless plant
leaves are unlikely to damage the underlying leaf tissue,
while water drops held above leaves by plant hairs can
indeed cause sunburn, if their focal regions fall on to the
leaf blade. The same phenomenon can occur when water
drops are held above human skin by body hair. However,
sustained exposure of a given patch of skin to intense
focused sunlight would require that a sunbather’s position
remained constant relative to the sun; otherwise, the water
drops receive sunlight from a continuously changing direc-
tion, and therefore focus it on to different skin areas. There-
fore, we treat claims of sunburn resulting from water
droplets on the skin with a healthy dose of skepticism.
Lastly, a similar phenomenon might occur when water
droplets accumulate on dry vegetation (e.g. straw, hay,
fallen leaves, parched grass, brush-wood) after rain. If the
focal region of drops falls exactly on the dry plant surface,
the intensely focused sunlight could theoretically spark a
fire. However, the likelihood of this is considerably reduced
by the fact that after rain the originally dry vegetation
becomes wet, and as it dries water drops also evaporate.
Thus, claims of fires induced by sunlit water drops on vege-
tation should also be treated with a grain of salt.
Acknowledgements
The equipment donation from the Alexander von Hum-
boldt Foundation (Germany) received by GH is gratefully
acknowledged. The financial support of the Max Planck
986 Research
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Phytologist
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New Phytologist (2010) 185: 979–987
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Institute for Meteorology (Hamburg, Germany) to A
´His
also highly appreciated. Many thanks to Dr La
´szlo
´Orlo
´ci
(director, Botanic Garden of Eo
¨tvo
¨s Lora
´nd University,
Budapest, Hungary) for allowing us to perform Expt 3 in
the botanic garden. The constructive comments of two
anonymous reviewers greatly improved the quality of the
paper.
References
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Supporting Information
Additional supporting information may be found in the
online version of this article.
Notes S1 (a) Computer ray tracing, (b) supplementary
results, (c) supplementary discussion, (d) supplementary
references.
Fig. S1 The 10 circular grey plastic trays with maple (Acer
platanoides) leaves covered by glass spheres used in our first
experiment.
Fig. S2 Ray-tracing geometry of a ray of light incident on,
passing through, and leaving a water drop above a horizon-
tal surface.
Fig. S3 Angles of incidence (a,d) and of refraction (b,c),
and unit direction vectors (e
0
, e
1
, e
2
) of incident and
refracted rays of light at the air–water interface.
Fig. S4 (a) Relative irradiance of unpolarized direct sun-
light for solar elevation angles h= 60, 40, 20, 10, 5, 4, 3, 2,
1 and 0, computed using the 1976 US Standard Atmo-
sphere; (b) absorption spectrum A(k) of green plant leaves
averaged for bean, spinach, Swiss chard and tobacco; and
(c) the solar absorption factor of plant leaves vs h.
Fig. S5 Log
10
Qvs solar elevation angle hcomputed for a
water drop on a horizontal maple (a), plane tree (b) and
rowan (c) leaf with decreasing wettabilities.
Fig. S6 As Fig. 4 but for a glass sphere with a refractive
index of n
glass
=1.5.
Fig. S7 (a) Log
10
Qvs solar elevation angle hcomputed for
a glass sphere contacting a horizontal surface; (b) logarithm
of the maximum intensity I(h)=Q(n
glass
= 1.5,h)Æa(h)of
sunlight absorbed by a green leaf tissue in the focal region
of a glass sphere on a horizontal leaf surface.
Fig. S8 Logarithm of light intensity Iabsorbed by the leaf
tissue as a function of the ratio HR, computed for incident
angles h=60and 90in the focal region of a spherical
water drop with radius Rplaced at distance Hfrom the leaf
surface.
Table S1 Survey of horticultural websites discussing the
possibility of leaf damage caused by sunlight focused by
water drops
Table S2 Survey of dermatological and cosmetics websites
considering the possibility of sunburn of human skin caused
by sunlight focused by water drops during sunbathing
Table S3 Survey of websites discussing the possibility of
forest fires caused by sunlight focused by water drops
Please note: Wiley-Blackwell are not responsible for the
content or functionality of any supporting information sup-
plied by the authors. Any queries (other than missing mate-
rial) should be directed to the New Phytologist Central
Office.
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... Horváth Gábor és munkatársainak a Fizikai Szemlé ben megjelent egyik cikkét [1,2] és a hozzá tartozó egyéb szakirodalmat [3,4] elolvasva, valamint az errôl szóló egyik elôadását meghallgatva (amirôl a Magyar Televízió Delta mûsorában is sugároztak egy rövid filmet [5]) ismertem fel, milyen izgalmas lehetne gimnazistákkal is elvégeztetni a vízcseppes levelek napégésével kapcsolatos terepkísérleteket. Akadt még néhány ötletem, amelyek segítségével más megközelítésben is vizsgálható az adott probléma. ...
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