Environmental and Experimental Botany 43 2000 111 – 119
The effect of exposure to NO
2
and SO
2
on frost hardiness in Calluna 6ulgaris
S.J.M. Caporn
a,
, T.W. Ashenden
b
, J.A. Lee
c
a
Department of En6ironmental and Leisure Studies, Crewe and Alsager Faculty, Manchester Metropolitan Uni6ersity, Crewe Green Road, Crewe CW
1 5
DU, UK
b
Institute of Terrestrial Ecology, Bangor Research Station, Deiniol Road, Bangor, Gwynedd LL
57 2
UP, UK
c
Department of Animal and Plant Sciences, Uni6ersity of Sheffield, Sheffield S
10 2
UQ, UK Received 22 February 1999; received in revised form 7 October 1999; accepted 7 October 1999
Abstract
Heather Calluna 6ulgaris was grown in pots in a natural heathland soil and exposed in outdoor fumigation chambers ‘solardomes’ to 40 nl l
− 1
of both nitrogen dioxide NO
2
and sulphur dioxide SO
2
simultaneously. Plants exposed to NO
2
and SO
2
for 8 months over a growing season February – November showed increases in the growth of shoots + 37 and the whole plant + 15 and a raised ratio of root to shoot dry matter in comparison
with control charcoal-filtered air plants. Fumigation raised the average foliar concentrations of nitrogen + 34 and sulphur + 173. The improvements in growth due to pollutant exposure were countered by reductions in
tolerance to experimental acute frost. Cellular damage of heather shoots was assessed using measurements of electrolyte leakage from cut shoots following controlled over-night frosts. The rates of leakage were consistently
increased in those plants that had been exposed to NO
2
and SO
2
for 5 months or more in comparison with non-fumigated controls. In some cases a greater leakage rate was recorded in fumigated plants than in controls even
in the absence of freezing temperatures. The pollutants caused a similar reduction in frost tolerance whether exposure was given during the hardening period August – January or the de-hardening stage November – April. These results
support the hypothesis that low concentrations of air pollutants can reduce the tolerance of plants to freezing stress. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Air pollution; Calluna 6ulgaris; frost tolerance; heathlands; NO
2
and SO
2
pollution; semi-natural vegetation www.elsevier.comlocateenvexpbot
1. Introduction
Experimental studies suggest that some of the major gaseous pollutants may cause changes in
vegetation by altering tolerance to natural stress conditions such as frost Barnes and Davison,
1988, drought Neighbour et al., 1988 and pests Dohmen et al., 1985. These effects may be evi-
dent at lower concentrations of pollutants than are required to cause direct injury by inhibition of
net photosynthesis, growth reduction or result in acute visible damage to tissues reviewed by Dar-
Corresponding author. Tel.: + 44-161-2475157; fax: + 44- 161-2476372.
E-mail address
:
s.j.m.capornmmu.ac.uk S.J.M. Caporn S0098-847200 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 9 8 - 8 4 7 2 9 9 0 0 0 5 0 - 7
rall 1989. In recent years research on pollution- induced changes in cold sensitivity has focused
mainly on conifer trees, notably Picea sitchensis Lucas et al., 1988, Picea rubra De Hayes et al.,
1989; Sheppard et al., 1993, Pinus syl6estris Dueck et al., 1990 or the agricultural grass
Lolium perenne Davison and Bailey, 1982; Davi- son et al., 1988 while less is known of responses
of other semi-natural vegetation. An example of this type of vegetation which forms a large part of
non-agricultural land in Britain is Calluna 6ulgaris heather — dominated heathland. C. 6ulgaris
and related species in the Ericaceae and Empe- traceae are the principal dwarf shrub species of
the small, highly valued areas of lowland heath in north-western Europe. These species also cover
extensive tracts of the upland regions of Europe. In some areas, for example parts of upland
Britain, C. 6ulgaris is an important food source for grazers including sheep and game birds such
as the red grouse Lagopus lagopus scoticus. The predominantly maritime distribution of C. 6ul-
garis Gimingham, 1972 suggests that low tem- perature is one important determinant of its
distribution.
Increased incidence of frost damage, as a result of atmospheric pollutant nitrogen deposition to
C. 6ulgaris, may be one cause of the recent decline in the abundance and health of this species in the
Netherlands Van der Eerden et al., 1991; Power et al., 1998, and could be of widespread signifi-
cance in natural vegetation INDITE, 1994. The aim of this study was to examine the hypothesis
that the frost tolerance of C. 6ulgaris is reduced after long-term exposure to a combination of two
of the most important gaseous pollutants, sulphur dioxide and nitrogen dioxide. The experiments
were designed to answer the following specific questions regarding possible changes to frost tol-
erance as a result of exposure to NO
2
and SO
2
. Firstly, how does the duration of exposure to
NO
2
and SO
2
affect the degree of frost tolerance? Secondly, are plants only affected if exposure to
the pollutants is during the growing season or could they also be damaged if exposure is in the
winter months? Experimental research using both gases simultaneously is justified because these pol-
lutants often occur together in rural and suburban atmospheres and many studies have shown that
NO
2
and SO
2
often exert greater-than-additive effects on various plant growth and physiological
processes Ashenden and Mansfield, 1978. The concentrations used in these experiments 40 nl
l
− 1
are higher than rural levels and were used in order to address the hypothesis. Concentrations
of NO
2
and SO
2
similar to those used in the experiments may be measured in some urban and
suburban regions of Britain Broughton et al., 1997. While these are not the main locations of
Calluna there do exist important pockets of low- land heath close to built-up areas in many parts
of western Europe. The experiments reported here form part of a programme of research into envi-
ronmental influences on the cold hardiness of C.
6 ulgaris Caporn et al., 1994; Foot et al., 1996,
1997; Lee and Caporn, 1998; Carroll et al., 1999.
2. Materials and methods
