AnInexpensive Setup for Assessing the Impactof Ambient
Solar Ultraviolet Radiationon Seedlings
Paulien Adamse, Herbert E. Reed, Donald T. Krizek,* Steven J. Britz,
ABSTRACT
Becauseof reductionsin stratosphericozonelevels due to
chlorofluoromethanes
and other trace gases, there has been
growingconcernabout the impactof possible increases in
ultraviolet-B (UV-B)radiation. Until recently, moststudies
havefocusedon the effects of enhanced
UV-Blevels, however,
these haveinherenttechnicaldifficulties. Ultraviolet-Bexclusion studies afford the investigatora rapidmeansof assessing
the effects of present levels of solar UV-Bradiation. Unlike
UV-Benhancement,
UV-Bexclusion studies use the sun as the
sourceof UV-B
radiationandselective filters to transmitor
absorbthis portionof sunlight.Thisarticle describesa simple,
inexpensivesystemthat wasused overa 3-yr periodto determineseedling responseof cucumber
(Cucumis
sativus L.), soybean [Glycine max(L.) Merr.], and NewZealandspinach
[(Tetragoniatetragonoides(Pallas) Kuntze]to UV-B
exclusion.
Plants of all three species grownoutdoorsunderUV-Babsorbing polyestershowedan increase in leaf enlargement
andbiomassaccumulationin comparisonto those grownunderUV-B
transmittingcellulose acetatefilters. Thebasic materialsused
consist of plastic window
boxes,plastic filters that transmitor
absorbin the UV-Bregion, wire supports,andbinder clips.
Thissetup canbe usedto demonstrate
basic principlesof photobiologyandstress physiology.
It is ideal for studentsinterested in conductingshort-termscience projectson the effects of
solar UVradiation.
S
INCEthe early 1970s, there has been worldwideattention
given to studying the impact of stratospheric ozone(03)
reduction and the projected biological effects of increased
UV-B(280-320 nm) radiation (Caldwell et al., 1994;
Madronichet al., 1994; Tevini, 1993). Ozoneabsorption of
UVradiation is relatively weakat 320 nmand above, but
increases toward shorter wavelengths leading to a steep
decline in UVirradiance reaching the ground (Stamnes,
1993). A reduction in total colunm03 will cause an increase
in short waveUVradiation reaching the earth. The UV-B
region is most affected (Caldwell and Flint, 1994;
Madronichet al., 1994). The amountof solar UVirradiance
reaching the earth’s surface depends on manyatmospheric
and geometric factors (Caldwell and Flint, 1994; Madronich
et al., 1994). Natural UVlevels at the groundvary with time
on a scale of minutes to hours in response to changinglocal
atmospheric conditions (e.g., cloudiness, tropospheric
ozonelevels, and amountof particulates) or up to decades in
response to changesin the stratospheric ozonelayer.
and RomanM. Mirecki
The most significant geometric factor that dictates the
groundlevel UVirradiance is the solar elevation that varies
greatly with latitude, season, and time of day (Frederick,
1997). The coupling between changes in the stratospheric
ozone layer and the amount of biologically effective UV
radiation (UV-BBE)(or weighted UV-B)at the ground
well understood and has been examined using numerical
models(Frederick, 1997; Stamnes, 1993).
There has beenconsiderable effort to determinethe influence of elevated UV-Bradiation on higher plants; but the
response of plants to ambient levels of UVradiation needs
to be assessed as well. A simple UVexclusion approach for
investigating this problemthat is amenableto student and
teacher use is describedin this paper.
BIOLOGICAL IMPACT OF INCREASED
ULTRAVIOLET-B
Numerousstudies have been conducted on the biological
effects of elevated UV-Bradiation on higher plants
(Bommanand Teramura, 1993; Caldwell and Flint, 1994;
Caldwell et al., 1994; Tevini, 1993). Manystudies have
shownthere is a wide range in UV-Bsensitivity between
and within plant species (Caldwell and Flint, 1994;
Caldwellet al., 1994; Krizek, 1978). Studyingthese differences can give moreinsight into the effects of UV-Bradiation on plants and the natural mechanisms
of adaptation, and
can provide a basis for selection of UV-Btolerant
germplasm.
ULTRAVIOLET PROTECTIVE MECHANISMS
Nucleic acids and photosynthetic proteins in the leaf
mesophyll are the primary targets or chromophoresfor UVB radiation damage(Day, 1993). Plant tolerance to high
UV-Bradiation levels thus depends, in part, on howmuch
UV-Breaches the leaf mesophyll containing these chromophores(Day, 1993).
The amountof incident UV-Bthat reaches the leaf mesophyll varies greatly betweenplant species and is affected
by several factors. Leaf surface reflectance provides a first
line of defense against UV-B;but usually <10%of incident
UV-Bis reflected from leaves in most species (Day, 1993).
Other factors include epidermaltransmittance and thickness
of the epidermis (Day, 1993). Day et al. (1992) reported
complete attenuation of all incident UV-Bin the epidermis
of conifer needles, and up to 60%attenuation in the epidermis of herbaceous dicots. Plants originating from sites
receiving naturally high UV-Bfluxes tend to be moreeffecClimate Stress Laboratory,Natural ResourcesInst., USDA-ARS, tive in UV-Babsorption than those from low UV-Bsites
Beltsville, MD
20705.Currentaddressfor P. Adamse
andH.E.Reedare
(van de Staaij et al., 1995).
Asterstmat343, 6708DV,Wageningen,
the Netherlands
and the Prince
FrederickCooperative
Extension
Service,PrinceFrederick,MD,
respecCA,celluloseacetate;PAR,
photosynthetically
activeraditively. Received
11Oct. 1996.*Corresponding
author(dkrizek@asrr. Abbreviations:
ation(400-700
run);PE,polyester;SLW,
specificleafweight;T, transmitarsusda.gov).
tance;U-V-A,
ultraviolet-Aradiation(320-400
nm);U-V-B,
ulWaviolet-B
radiation(280-320
nm);UV-BBE
Published
in J. Nat.Resour.LifeSci. Edue.26:139-144
(1997).
, biologicallyeffectiveU-V-B.
J. Nat. Resour.Life Sci. Educ.,Vol. 26, no. 2, 1997¯ 139
The concentration of UV-Babsorbing compoundsin the
epidermis has been shownto be an important factor contributing to UVprotection (Caldwellet al., 1983;Day, 1993;
Dayet al., 1994;Flint et al., 1985;van de Staaij et al., 1995).
Flavonoids and nonflavonoid phenylpropanoid compounds
in the epidermis absorb strongly in the UVregion serving as
optical screens against potentially damagingUVradiation
(Caldwellet al., 1983). Mutantsdefective in the synthesis
these secondary metabolites are knownto be moresensitive
to UV-Bradiation than wild types (Reuber et al., 1996).
Elevated UV-Birradiation has been shown to induce an
increase in the concentration of UVabsorbing compounds
(Flint et al., 1985;vande Staaij et al., 1995).
DIFFICULTIES
UV-B.In manyspecies, growth is inhibited; in a few,
growth is stimulated, while in others, it is unaffected by
ambient UV-B.Growthof somespecies is also inhibited by
ambient UV-A(Krizek et al., 1997).
For the teacher or student interested in studying UV
exclusion effects, we describe here a simple "mini-greenhouse"designed to test the sensitivity of seedlings to incident UV-Bradiation. Wealso present typical results that
were obtained with several crop species during a 3-yr period (1992-1994). The experimental setup requires little
investment, is simple to construct, and can be used to rapidly comparethe sensitivity of a wide range of plants to ambient UV-B.The basic materials are plastic windowboxes and
plastic filters that transmit or absorb in the UV-Bregion.
OF SIMULATING THE SUN
Most UV-Benhancement studies are performed under
supplemental UV-Blamps in controlled-environment chambers or greenhouses. Results from greenhouse or growth
chamberstudies and field studies on UV-Beffects are often
conflicting (Caldwell and Flint, 1994). There are several
possible explanations for these differences. In growthchambers and greenhouses, visible (400-700 nm) and UV-Aradiation (320-400nm)is usually muchless than in full sunlight
(Caldwell and Flint, 1994). The spectrum of the UVlamps
and backgroundlighting generally do not simulate sunlight
precisely, so that the ratios of UV-Ato UV-B,or blue
(400-500 nm) to UV-Bwavelengths are often relatively low
(Middleton and Teramura, 1993).
ULTRAVIOLET ATTENUATION AND EXCLUSION
STUDIES: AN ALTERNATIVE APPROACH
To avoid manyof the problems inherent in assessing
plant response to UV-Bradiation under controlled-environmentconditions, several investigators have filtered existing
solar radiation to modify the UV-Bcomponentor the UV-A
and UV-Bcomponents(Krizek et al., 1997 and references
therein; Searleset al., 1995).
One methodof UV-Battenuation that has been used suecessfully is to construct an O3-containingenvelope of U-V
transparent plexiglas to reduce the amountof UV~B.These
"03 cuvettes" are placed over growth chambers outdoors
(Tevini et al., 1990). They have been used in Portugal
simulate ambient and elevated UV-B conditions in
Germany.This system is rather expensive for most investigators, however, and has to be monitored for possible 03
leaks.
Anotherapproachfor studying effects of UVexclusion is
to use greenhouses completely enclosed by materials having
different spectral cutoff properties in the UVregion. A
microenvironmentalcontrol is normally established using a
UV-transparent filter material (Caldwell and Flint, 1994;
Krizek et al., 1997). A less expensive approachhas been to
use small enclosures onto whichvarious glass or plastic filters are mountedto exclude or transmit selected regions of
the solar UVspectrum. These two approaches have been
used successfully by several investigators in India, Japan,
Panama,the Philippines, and the USA(Krizek et al., 1997
and references therein; Searles et al., 1995).
The results of UVexclusion studies conducted to date
indicate that plants vary greatly in their responseto ambient
140¯ J. Nat. Resour.Life Sci. Educ.,Vol. 26, no. 2, 1997
MATERIALS AND METHODS
Plant Material and Cultural Procedures. Experimental
plants studied during 1992 to 1994 included cucumber
(Cucumissativus L.), soybean[Glycine max(L.) Merr.], and
NewZealand spinach [(Tetragonia tetragonoides (Pallas)
Kuntze]. Other crops used in experiments at Beltsville
included cotton (Gossypiumhirsutum L.), lettuce (Lactuca
sativa L.), and broccoli (Brassica oleracea). Contrasting
cultivars of cucumberand soybean were chosen because of
their previously determinedsensitivity to supplementalUVB (Adamseet al., 1994; Krizek 1978; Reed et al., 1992).
Poinsett cucumberand CNSsoybean were highly sensitive
and Ashley cucumberand Richland soybean were relatively
insensitive.
Seeds were planted in 16 plastic pots (9 cmdiam.) filled
with vermiculite (Terralite no. 3, W.R. Grace&Co.) and the
pots placed in the windowboxes covered with plastic UV
filters. Generally, three seeds were planted in each pot.
Uponseedling emergence,the plants were selected for uniformity and thinned to a single plant. Plants weresubirrigated daily with a complete nutrient solution as described by
Silvius et al. (1978).
Experimental System. Plastic windowboxes (20 by 90
by 12 cm) were obtained from a local garden supply house
and covered with either UV-Btransparent cellulose diacetate (CA, 0.08 mmthick) or UV-Babsorbing polyester
(PE, 0.13 mmthick) (Cadillac Plastics, Baltimore, MD).
FromFig. 1, one can clearly see the difference in cutoff
between CAand PE and how closely the solar spectrum
through CAmatches that of the unfiltered sun. Under PE,
there is 90% less unweighted and 99.99% less weighted
UV-B(data not shown) irradiance transmitted than under
CA. In terms of relative amountsof UV-Btransmitted, we
have therefore chosen to label the CAtreatment as +UV-B
and the PE treatment as -UV-B.
Aclar (Allied-Signal
Inc. Specialty Film Dep.,
Morristown,N J), or Teflon or Tefzel (DuPont, Circleville,
OH)maybe used in place of CA, since they are even more
transparent to UV-Bradiation. Information as to the availability of these films from local plastics suppliers should be
obtained directly from the manufacturers.
The transmission properties of these films are as follows:
CA(10%Tat 291 nm); PE (10%Tat 319 nm); Aclar (10%T
at 208 nm) and Teflon and Tefzel (10%Tat 245 nm).
UVcutoff filters were placed onto 24 cm high, arched
frames madeout of heavy duty wire and clipped to the sides
of the windowboxes with binder clips (Fig. 2). In some
experiments, blowers with a rubber hose inserted into one
end of the tunnel were used to improvethe air circulation
abovethe canopy(air speed 6 ms-l).
If interested in examining possible "cuvette" effects
caused by covering the plants, the teacher or student may
use additional windowboxes containing sets of plants that
are left uncovered. In subsequentstudies that we conducted
in the summerof 1995, we observed no significant differences in leaf area, stem or petiole elongation, or dry weight
of tops of cucumberseedlings grownunder UV-Btransparent CAor teflon and those left uncovered(D.T. Krizek and
R. Sandhu,1995, unpublishedresults).
For those interested in determiningthe additional effects
of solar UV-Aradiation, extra setups that are covered with
Llumar(Courtaulds PerformanceFilms, Martinsville, VA),
a film that absorbs both UV-Aand UV-Bradiation, maybe
used. The cutoff of this film (at 10%T)is about 401
(Krizeket al., 1997).
In using clear UV-Btransmitting plastic filters, such as
CAor Teflon, one should be aware there is approximatelya
10%loss in visible radiation becauseof reflection and scattering of light (D.T. Krizek and R.M.Mirecki, 1996, unpublished results). This is illustrated in the longer wavelength
region of Fig. 1.
The windowboxes were placed outdoors and oriented in
a north-south direction. To exclude both direct and most of
the diffuse UVradiation, it is importantto place experimental plants in the center portion of the window
box and to use
the outer plants as border plants.
Ambient temperatures were measured with thermistors
(Campbell Scientific, Logan, UT). Leaf temperatures were
measuredusing a copper constantan thermocoupleplaced in
10000
UV-B I
UV-A
PAR
1oo
contact with the underside of the leaf. Total radiation measurements from 400 to 1100 nmwere made with a pyranometer (LI-CORInc., Lincoln, NE, Model LI-200SZ) and
photosynthetically active radiation (PAR, 400-700 rim)
were made with a quantum flux sensor (LI-COR Inc.
Lincoln, NE, Model LI-190SZ). Occasional broad band U-V
measurements under the setups were made with a YMT
meter (see Sullivan et al., 1994). Measurements
unweighted solar irradiance were made with a UV-VIS
spectroradiometer (Model 752, Optronics Laboratories,
Orlando, FL). Students and teachers interested in building
their ownUVradiometer should examinea recent article by
Mims(1990). The cost of the componentslisted are slightly
over $100.
GrowthMeasurements. Cucumber and soybean plants
were grown for 3 wk from seeding under +UV-Bor -UV-B
conditions; NewZealand spinach was grownfor 31 d. At the
end of each experiment, the plants were scored for visual
damage, leaf area measured (LICOR, Model LI-3000,
Lincoln, NE, USA), and biomass measurements taken. An
alternative methodof determining leaf area that might be
used is to photocopythe leaves and comparethe weights of
the cut out images against "calibrated" pieces of graph
paper. Dry weights were determined after drying the samples in a forced draft ovenat 70°Cfor at least 48 h.
Since experiments were conducted by different investigators during the summersof 1992 to 1994, there was no
attemptto collect identical data for all species. For example,
specific leaf weights (SLW)were determined for cucumber
and NewZealand spinach, but not for soybean, while leaf
area ratio (LAR),or the ratio of the total leaf area over the
total plant dry weight was determined for soybean. The
growthresponses reported are representative of the kinds of
information that a student might collect using the setup
described. The experiments were repeated five times
(cucumber), three times (soybean), or two times
Zealand spinach).
Ultraviolet-Absorbing Compounds.Although high
performanceliquid chromatographyor other analytical techniques are required for quantification and identification of
the precise compounds involved, measurement of the
absorption of a UV-absorbingextract provides a rapid and
convenient method by which the student can assess the
¯
i~
- : -- - Sunthrough 0.08 mmCA
-- -- Sun through 0.13 mmPE
metal wire
0.01
280 300 320 340 360 380 400 420 440
Wavelength(nm)
Fig. 1. Unweightedsolar irradiance alone and through UV-Btransmitring cellulose acetate (CA)or UV-Babsorbing polyester (PE) a
clear June day at Beltsville, MD.Data plotted on a logarithmic
basis. Vertical lines delineate the UV-A,UV-B,and PARregions of
the spectrum.
windowbox
Fig. 2. Window
box "mini-greenhouse"covered with plastic filter for
UV-Bexclusion study.
Nat. Resour.Life ScLEduc.,VoL26, no. 2, 1997¯ 141
potential effects of ambient UVto induce these UV
absorbers.
To determine the absorption of UVabsorbing compounds in the leaves, leaf discs were punched out with a
cork borer (1 cm diam.) and extracted in glass centrifuge
tubes containing 5 cm3 ethanol acidified with glacial acetic
acid (ethanol/acetic acid, 99:1, v/v) as described by Flint
al. (1985). The sampleswereboiled gently in a water bath
80°C for 10 min, allowed to cool, and then brought to volume.
Making measurements anywhere in the broad band
region of 300 to 330 nmis suitable for characterizing UV
absorption since these wavelengthsare in the region of maximal UVabsorbance observed in manyspecies and are commonlyreportedin the literature (Dayet al., 1994;Flint et al.,
1985). Typically, we use a scanning spectrophotometer
(Shimadzu, Columbia, MD)and obtain measurements
UVabsorbance every nmfrom 250 to 360 nm, but report
only selected wavelengths. Typical UV-Bresponses are
shownfor NewZealand spinach (Fig. 4).
Other solvent systems, for example, acidified methanol,
mayalso be used, but these are not as safe as ethanol. Since
most schools maynot have a recording spectrophotometer,
we recommendthat measurements be taken simply with a
colorimeter or standard spectrophotometerat a single wavelength (e.g., 300 or 330 nm).
Analysis of Data. The statistical design used in each UV
study was a complete randomized block design, replicated
in time or space; in the case of cucumberand soybean, cultivars were handled as a split plot. Data for each species
wereanalyzed separately. Differences referred to as significant were at P < 0.05 as determinedby analysis of variance.
0.030
~E
+UV-B
"-" 0.024 W7--/I
f I---I -UV-B
a)
RESULTS AND DISCUSSION
Experimental System. The average daily maximumand
minimumtemperatures during the experimental period
(summers of 1992-1994) at Beltsville, MDwere 26.1 and
15.2°C, respectively. The average air temperatures (as well
as the leaf temperatures) inside the ’tunnel’ during the day
were generally 2 to 3°C higher than the outside temperatures, but no significant differences in air or leaf temperature
were observed between the CAand PE filters. The average
total quanta of PARduring the study was 31.1 mol m-2 -l
d
and the average total radiation was 15 798 kJ m-2 -l.
d
Oneshould be aware whenusing CAfilters that there is
approximately 20 to 25%less UV-Bradiation under a CA
filter than under sunlight alone. On an unweightedbasis,
spectroradiometric measurementsof irradiation from 280 to
320 nmtaken on a clear day in June (Fig. 1) indicated the
following values in mWm-2: Sun: 1414; Sun through CA:
1100; and Sun through PE: 112. This translates into a 22%
difference between Sun and Sun through CA.
With a broad-band YMT
meter used to obtain UV-Birradiance we obtained similar differences: 1.50 outside the
boxes; 1.25 underneath the CAfilter; and 0.25 underneath
the PE filter. Typical mid-daylevels of biologically effective
UV-Bradiation 0JV-BBE)
at Beltsville on a clear day in July
1993 were 400 mW
m-2 (Sullivan et al., 1994). This translates to approximately 330 mWm-2 UV-BBE
for +UV-Band
70 mWm-2 for -UV-Btreatments in the windowboxes.
Thus, it should be recognized that CAtreatments do not
duplicate "ambient" UV-Blevels exactly but only provide a
reasonable approximationof incident UV-B.Despite differences in weighted or unweighted UV-Birradiance between
true "ambient" controls vs. CAcovered controls, we have
0.040
0.020
0.032
0.015 -
0.024
’~ 0.012
~ 0.006
//1
0.000
"-’12
d)
0.008
0.005 -
0.000
0.000
2.0
e)
1.5
0.9
//1
0.6
//I
//1
0.3
0.0
0.010 -
0.016
Ashley Poinsett
Cucuml~r
1.00
0.75
7-71
1.0
0.5
0.50
0.25
0.0
0.00
Richland
CNS
Soybean
MainShootLat. Shoots
NewZealandSpinach
Fig. 3. Total leaf area (a, b, c) and total shoot dry weight (d, e,.f) of Ashley and Poinsett cucumber, Richland and CNSsoybean, and New Zealand
spinach under UV-B transmitting
cellulose acetate (+UV-B) or UV-B absorbing polyester (-UV-B). *Indicates significant
differences (P < 0.05)
between means within a cultivar or plant structure.
142 ¯ J. Nat. Resour.Life ScLEduc.,VoL26, no. 2, 1997
Table 1. Selected growth parameters of Ashley and Poinsett cucumber,
Richland and CNS soybean, and New Zealand spinach grown in sunlight under UV-B transmitting
cellulose
acetate (+UV-B) or UV-B
absorbing polyester (-UV-B).
Species
Parameter
Cucumber
Ashley
Poinsett
Ashley
Poinsett
Petiole length, m
Petiole length, rn
-2
SLW~’,g m
-2
SLW, g m
Soybean
Richland
CNS
LAP,, m2 g-~
LAR, m2 g-~
NewZealand spinach
Leaf number
FWof tops, g
Height, m
-2
SLW,g m
+UV-B
-UV-B
0.052
0.040
0.069
0.059
28.80
29.30
29.10
30.80
0.0197
0.0220
13.2
14.02
0.07
40.30
0.0198
0.0212
16.0
20.66
0.10
44.00
Difference
0.017"
0.019"
-0.3 NS
-1.5*
0.0001 NS
-0.0008 NS
2.8*
6.64*
0.02 NS
3.70 NS
* Significant at P < 0.05.
.
~" SLW:specific leaf weight; LAR:leaf area ratio; Difference: -UV-Bminus+UV-B
values. NS= nonsignificant.
been unable to detect any growth differences between the
two. In terms of separating +UV-Band -UV-Btreatments,
however,there is no question that CAand PEfilters provide
a clear-cut difference in UV-Birradiance (Fig. l) which
reflected in the growthdifferences described below.
Leaf Damage.No visual leaf damage was observed in
soybeanor NewZealand spinach in either treatment. In one
cucumber experiment, "cupping" of the cotyledons was
observed. Ultraviolet-B induced cupping of cotyledons has
also been observed in cucumberby Takeuchi et al. 0989).
Growth Responses. Excluding UV-Bradiation under PE
caused varying degrees of growth stimulation in all three
species, but the particular growth parameter affected was
highly species and/or cultivar dependent. Leaf area development of plants grown under PE was increased in all three
species compared to their +UV-Bcounterparts under CA
(Fig. 3a,b,c). Accumulationof shoot dry weight was also
greater in all three species in the -UV-Bthan in the +UV-B
treatment, but differences were not alwayssignificant (Fig.
3d,e,f) and maybe just a matter of replicates.
Petiole elongation in both cultivars of cucumberwas also
greater under -UV-B than under +UV-B(Table 1).
Excluding incident UV-Binduced a decrease in SLWin
Poinsett cucumber but had no effect on SLWin Ashley
cucumber. Excluding incident UV-Balso had no effect on
LARin soybean or on height and SLWof NewZealand
spinach, but -UV-Bdid cause an increase in leaf number
and fresh weight of NewZealand spinach (Table 1).
Our results on cucumberand soybeanare consistent with
those obtained in greenhouse and growth chamber experiments on plants exposed to supplemental UV-B, where
increased levels of UV-Breduce plant growth (Adamseet
al., 1994; Kdzek,1978; IO’izek et al., 1993, 1994; Reedet
al., 1992). There are no previous reports of ambient UV-B
effects on NewZealand spinach, but Hashimoto(1992, personal communication) reported that spinach grew better
without solar UV-B.
Ultraviolet-Absorbing Compounds.In many plant
species, the concentration of UV-absorbing compounds
increases in responseto increased UV-B
levels (Flint et al.,
1985; van de Staaij et al., 1995). This same pattern was
observed in NewZealand spinach in the +UV-Btreatment
comparedwith those in the -UV-Btreatment. Filtering UVB from sunlight decreased UVabsorbance of extracts
expressed on a leaf area basis in a broad UVband;
absorbance at 330 nmwas greater than at 300 nm. At both
wavelengths, UVabsorbance was nearly 18%greater in
extracts for +UV-B
comparedto -UV-Bplants (Fig. 4).
have observed that other plant species and cultivars may
respond differently to ambient UV-B.For example, extracts
from -UV-Bcucumber showed only a small decrease in UV
absorbance, while those from -UV-B soybean showed
greater UVabsorbance compared to extracts from +UV-B
plants (data not shown).
A direct relationship betweenextract UVabsorbance and
leaf UVabsorbancehas been reported by several investigators (Caldwellet al., 1983;Day,1993;Dayet al., 1994;Flint
et al., 1985; van de Staaij et al., 1995). For example, in
1993, Day found that UVabsorbances of the extracts taken
from leaves of several species were inversely correlated
with epidermal transmittance and depth of penetration of
UV-Binto the leaf, using a fiber-optic microprobesystem.
In the UV-Btolerant perennial herb, Silene vulgaris
(Moench)Garcke, van de Staaij et al. (1995) found a clear
effect of UV-Bradiation on the absorbing capacity of theextract; the higher the UV-Bflux applied during seedling
growth, the greater the absorbing capacity of the leaf
extracts. The impact of direct UV-Bradiation on absorbance
levels of leaf extracts was further demonstratedby the lower
UV-Babsorbance in Silene seedlings grown under shaded
conditions than in full sunlight.
CONCLUSIONS
Our studies demonstrate that windowboxes covered with
selective UVcutofffilters to exclude or transmit UV-Bradi0.05
i-----] -UV-B
~ +UV-B
0.04 -
0.03 -
0.02 -
0.01 -
0.00
I///1
////1
////1
///A
///A
I//A
~///A
///A
///A
///A
I//A
IlIA
33Ohm
30Ohm
NewZealandSpinach
Fig. 4. Ultraviolet absorbance (on an area basis) at 300 and 330 nm of
extracts obtained from leaves of New Zealand spinach. Plants
grown under UV-B transmitting
cellulose
acetate (+UV-B) or UVB absorbing polyester (-IJV-B). * Indicates significant
differences
(P < 0.05) between means at one wavelength.
J. Nat. Resour.Life ScLEduc.,VoL26, no. 2, 1997¯ 143
ation provide a rapid, convenient, and inexpensive "minigreenhouse" for studying the effects of solar UV-B radiation
on early seedling growth. This setup should be useful for
teachers to demonstrate basic principles of photobiology
and stress physiology, to evaluate plant sensitivity to current
levels of UV-B, and to illustrate how changes in ambient
UV-B with elevation may impact plant growth. It is also
suitable for students interested in conducting short-term science projects on the effects of solar UV radiation without
having to be concerned about the safety hazards of working
with artificial UV sources.
The simple UV exclusion setup described can also be
used to simulate different levels of stratospheric ozone
reduction by using cellulose acetate filters of different thickness, or to study the interactions of UV-B and water stress.
By using more elaborate setups, for example, plexiglas
exclusion chambers with special blowers, one could also
study interactions of UV-B and air pollutants, UV-B and
CO2, or UV-B and temperature stress under ambient UV
radiation. Since relatively few UV exclusion studies have
been published thus far, this area of research is wide open to
the imaginative investigator. The possibility of using ambient UV-B to harden plants against elevated UV-B, elevated
temperature, or other stresses is an area deserving of further
study, since few experiments have been done on these problems (He et al., 1994).
The major limitation of UV exclusion studies is that they
do not enable one to predict the effects of elevated UV-B
radiation. Because of the limited number of species studied
to date, it is also not possible to draw generalizations as to
the genetic diversity of plant response to ambient solar UV
radiation.
ACKNOWLEDGMENTS
We thank Marc Svendsen for his help in preparing the
graphics. This work was supported hi part by the U. S.
Department of Energy, under the auspices of a Global
Change Fellowship awarded to Herbert E. Reed and by U.
S. Department of Agriculture grant CSRS 89-37280-4799
awarded to Steven J. Britz. Paulien Adamse was supported
in part by a U. S. Department of Agriculture, Post-Doctoral
Fellowship.
144 • J. Nat. Resour. Life Sci. Educ., Vol. 26, no. 2, 1997