NY523
Varietal improvement of indoor plants by
selection to withstand stresses of the
indoor environment
Margaret Burchett
University of Technology, Sydney
NY523
This report is published by the Horticultural Research and
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horticultural research and development undertaken for the
nursery industry.
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INDUSTRY
SUMMARY
In an increasingly urbanised society, most of us are spending more of our lives indoors, at
work and at home. But the indoor environment may be more polluted than the outdoors, with
over 250 volatile organic compounds (VOCs) having been identified in indoor atmospheres.
Although these are present only in trace amounts, it now seems that 'sick-building syndrome'
is more the result of exposure to cocktails of chemicals, than to micro-organisms circulating in
the air conditioning.
If it can be shown that plants make a real contribution to pollution reduction in the indoor
environment, and can be bred for this purpose, while thriving in the arid, often ill-lit
surroundings (for plants), the horticultural industry will be able to promote the maximum use
of indoor plants in every building, from factories to schools and hospitals, as well as in
commercial buildings and homes.
This project has investigated the ability of some common 'international' species to absorb
VOCs found in indoor environments in Australia, and assessed the stress responses of the
plants following exposure to the hazardous chemicals. This information is needed for plant
varietal improvement in absorptive capacity, and better survival in the indoor environment.
In an initial series of experiments Kentia palm (Howeaforsteriana,) Peace Lily(Spathiphyllum
'Petite' )and Weeping fig, Ficus benjamina , in standard potting mixtures, were evaluated for
their capacity to remove the VOCs benzene and n-hexane. These chemicals are toxic at low
concentrations, and benzene is an established human carcinogen and depressant of the central
nervous system. Both central and peripheral nervous systems are damaged by n-hexane, and
its break-down products are even more toxic. Both are found in paints, plastics, room
deodorisers, adhesives, cleaning compounds, personal deodorants, and other products.
All three species caused complete disappearance of the chemicals, at concentrations from 2 to 5
times higher than the Worksafe Australia Time-Weighted Average exposure standards (TWA),
over several days. The results clearly showed the capacity of the pot-plant system to absorb
VOCs.
Further experiments were carried out using Kentia as the model. It was found that potted
Kentia showed an 'induction' phase over the first two days, after which, if the concentrations
were topped up to original levels, it was able to reduce them to low levels in 24 hours. This
capacity was unaltered under light or dark conditions, indicating that it was the microorganisms of the potting mixture that were directly responsible for the reduction. When the
plant was removed and the soil returned to the test chamber, it continued to be effective over at
least a week, a further indication of the role of the soil micro-organisms. Also, these results
mean that the system (plant-soil, combination), gets better at removal once it is exposed to the
chemical, presumably because appropriate microbial enzymes are 'switched on' to break it
down.
Leaf stress parameters were also measured, before and after exposure to benzene. These
included chlorophyll and carotenoid (orange pigment) levels and peroxidase activity, all of
which show changes with plant stress, and much earlier than any visible symptoms. Changes
were found on exposure in the test chambers, suggesting that indoor plants have potential as
early-warning monitors of indoor air quality for human occupants, and as a early measure of
plant health, before visible disease symptoms appear.
In summary, the pot-plants tested can remove up to five times the occupational air levels of
benzene and hexane over 24 hours, once they have been stimulated by the presence of the
chemical. The micro-organisms of the potting mixture appear to be the direct agents of
removal. Future work is needed to extend the range of plants, potting mixtures and chemicals
tested, in the quest for varietal improvement to withstand the stresses of the indoor
environment, while acting to cleanse its atmosphere by supporting an appropriate soil flora.
TECHNICAL
Keywords:
SUMMARY
Indoor plants, stress tolerance, pollution reduction, varietal improvement.
If it can be shown that indoor foliage pot-plants make a real contribution to pollution reduction
in the indoor environment, and can be bred for this purpose, while also withstanding better the
stresses of that unnatural environment, the horticultural industry will be able to promote the
maximum use of indoor plants in every building, from factories to schools and hospitals, as
well as in commercial buildings and homes. Substantial progress has been made in this project
towards these goals.
The experimental aims of this project have been to investigate the ability of some common
'international' species to absorb VOCs found in indoor environments in Australia, and to
assess the stress responses of the plants following exposure to the hazardous chemicals, as a
basis for breeding more stress tolerant plants, with improved pollution-reducing capacities.
Kentia palm, Howea forsteriana, Spathiphyllum 'Petite' (Peace lily) and Ficus benjamina
(Weeping fig), were first evaluated for their capacity to remove the VOCs benzene and nhexane. These chemicals are toxic at low concentrations; both are depressant of the central
nervous system, and benzene is an established human carcinogen. The leaf stress parameters
measures included photosynthetic pigment levels and peroxidase activity, before and after
exposure to benzene, and significant differences were found. Changes in leaf biochemistry
occur much earlier than any visible symptoms of harm, and so can act as an early-warning
indicator of stress and hazard.
The preliminary results clearly showed the capacity of the pot-plant system to absorb VOCs.
Further experiments were conducted using Kentia as a model. It was found that plants under
light or dark conditions, in potting mixture or hydroponic medium, were effective in
eliminating the toxicant within 24 hours (after an induction phase). The used potting medium
and hydroponic solution, from which plants had been removed, were equally effective,
indicating that it is the soil micro-organisms which are the primary removers of the VOCs.
The results indicate that:
(a) Indoor pot-plants can reduce air pollution, the micro-organisms of the growth medium
apparently being the direct agents of removal;
(b) There is potential for plant breeding and growth-medium culture to enhance the plant-soil
capacity for pollution reduction, and
(c ) The plants also have potential to be used both as early-warning monitors of indoor air
quality for human occupants, and of plant health and stress resistance.
More work is needed, however, to show:
(i) quantitative dose-response relationships between the pot-plant system for a range of
species, including the role of the plant and growth medium in providing the habitat for
particular microbial development
(ii) responses to a range of pollutants, and
(iii) the micro-organisms responsible for the metabolic responses, so that they too, or the
growth media containing or favouring them, can be developed also.
INTRODUCTION
The aims of this project have been to investigate the genetic improvement of indoor plant
varieties, for enhanced stress tolerance characteristics and the ability to improve indoor air
quality. The project has therefore had two parts, (a) to study the capacity of the selected species
to remove volatile organic contaminants from the indoor atmosphere, and (b) to measure leaf
stress tolerance parameters before and after exposure to the contaminants, and before and after
hire to offices.
In an increasingly urbanised society, most of us are spending more of our lives indoors, at
work and at home. But the indoor environment may be more polluted than the outdoors, with
over 250 volatile organic compounds (VOCs) having been identified in indoor atmospheres.
Although these are present only in trace amounts, it now seems that 'sick-building syndrome'
is more the result of exposure to cocktails of chemicals, than to micro-organisms circulating in
the air conditioning.
Indoor plants are often used to beautify the basic dryness and hard edges of commercial
buildings, and to add a touch of living tranquility at home. Overseas studies have indicated
that indoor plants also have the potential to reduce air pollution, by absorption and breakdown
of the VOCs( Wolverton et al, 1989; Wolverton, 1991; Wolverton and Wolverton, 1993;
Giese et al, 1994). That is, it seems that plants not only look good, they can improve the
indoor environment as well.
If it can be shown that plants make a real contribution to pollution reduction in the indoor
environment, and can be bred for this purpose while thriving in the arid, often ill-lit
surroundings (for plants), the horticultural industry will be able to promote the maximum use
of indoor plants in every building, from factories to schools and hospitals, as well as in
commercial buildings and homes.
This project has investigated the ability of some common 'international' species to absorb VOCs
found in indoor environments in Australia, and assessed the stress responses of the plants
following exposure to the hazardous chemicals. This information is needed for plant varietal
improvement in absorptive capacity, and better survival in the indoor environment. The VOCs
used in this project were benzene and n-hexane. Benzene is an established human carcinogen and
depressant of the central nervous system (Leslie and Lunau, 1992). Both central and peripheral
nervous systems are damaged by n-hexane, and its breakdown products in humans are even more
toxic. Both compounds are found in indoor air, being components of paints, plastics, room
deodorisers, adhesives, cleaning compounds, and personal care products as well as many other
applications.
The plant varieties initially selected were Spathiphyllum hybrida (Peace lily), Howeaforsteriana
(Kentia palm) and Ficus benjamina. Trials with Ficus were discontinued fairly early, because of
the plant's lower tolerance to stress (which indicates the need for a later study on this species).
Work continued with the two monocot species, which are probably the most widely used of all
the 'international' indoor species. These two offer contrasts in culture and morphology, since S.
hybrida is herbaceous, and clonally produced and hence genetically homogeneous, whereas the
Kentia is a 'woodier' palm, and is produced from seed, and hence genetically diverse. The
results reported here are mainly from experiments using Kentia as the model plant. Experiments
are still in progress with the Spathiphyllum.
Leaf stress parameters measured in this project included chlorophyll a, chlorophyll b, the chl a I b
ratio, total carotenoids, total pigments, and peroxidase activity, all of which are known from
other studies to respond to physiological stressors Singh et al, 1991; Pandey and Agrawal, 1994;
Roy et al, 1991; Nast et al, 1993).
A - REMOVAL OF VOCs
A.l
A. 1.1
METHODOLOGY
Plant materials
Commercially grown plants were supplied from the Interior Plantscapers Association of NSW,
Mountain Range Nursery, Wollongong, and the Lord Howe Island Board. They were grown in
140 mm (1.51) pots in a composted pine bark and sand mixture. They were held under simulated
normal office conditions until required for testing.
A. 1.2
Test chambers
Four clear perspex chambers with internal volume 0.216 m 3 (60 cm cubes) were used, with
removable tops and silicone rubber septa on either side of the chamber for the injection of
VOCs and withdrawal of gas samples. An individual plant was placed in each. Condensation
was minimised by a cooling system with coolant circulated through a copper coil in each
chamber, assisted by small fans (2.4W), which also mixed the chamber air. Temperatures
were maintained at approximately 23°C. Removable light boxes maintained light levels at 120
umol"2 s"1 during lighted periods. The chambers were covered with black plastic sheeting
during dark periods.
A. 1.3
Test procedure
After placing the plants in the chambers, sealing lids and adjusting light boxes (when used),
the required volume of benzene or n-hexane was injected from a 50uL syringe onto a paper
tissue suspended inside the chamber. It was found that the concentrations in the chamber took
2 - 3 hours to equilibrate. Successive samples were then withdrawn at intervals for gas-liquid
chromatographic analysis (GLC), over a 96-hour period (sometimes extended for a further
period). Rates of VOC removal were compared using Student's paired t-test. Benzene was
introduced at a nominal concentration of 25 ppm, and n-hexane at 100 ppm, these being
respectively 2 and 5 times the Worksafe Australia Time-Weighted Average exposure standards
(TWA) over varying period for these compounds.
A.2 RESULTS
A.2.1
VOC removal in three species - initial trials
In the initial experiments, which were carried out under a continuous light regime, it was found
that all three species removed all detectable levels of the test chemical over the 96-hour test
period.
The results indicate general capacity among indoor plants to remove VOCs under the test
conditions. It was then decided to continue trials with Kentia, to obtain a profile of responses
for that species, and then to repeat the series of trials with Spathiphyllum. The results below,
therefore, have concentrated on Kentia, with further tests on Spathiphyllum still in progress.
A.2.2
VOC removal using potted Kentia palms
A.2.2.1.
Induction of VOC removal
It was found that, using Kentia plants with either chemical (Fig 1, 2), the initial rates of
removal over the first 24 hours were low, but that they increased over the following few days.
That is, there was an 'induction period' in which exposure of the plant to the chemical
stimulated the removal of the chemical from the air in the chamber. This is apparent in Fig. 1,
for hexane, over days 1 - 4; and in Fig. 2, for benzene, over days 1-2. It is interesting to
note the differences in induction period, the hexane response taking longer to develop than for
benzene.
The occurrence of an induction period indicates that the pot-plant system is stimulated by the
presence of the contaminant, to initiate and/or speed up removal activity.
A.2.2.2
Sustained VOC removal in the light
After the initial induction period, the Kentias were challenged with fresh additions of the VOC
at daily intervals. The amount added on each occasion restored the nominal concentration of
contaminant to its original level (100 ppm hexane, or 25 ppm benzene). For both chemicals, it
can be seen that rapid removal of the VOC was sustained for a further two days under the
lighted conditions (Fig. 1, days 5 - 6; Fig. 2, days 5 - 7).
Thus it appears that the VOC removal observed was not simply due to adsorption onto the
surfaces of plant, soil or chamber, which would have tended to come to equilibrium, but was
due to sustained biological processes, presumably enzymic metabolic breakdown reactions.
A.2.2.3
VOC removal activity is independent of light
If the rapid removal of the VOC were due to the leaf metabolism of the plant, having absorbed
the contaminant through the open stomates, then dark conditions would be expected to slow
the removal process down, or perhaps to reduce it to zero. However, the results show (Fig 1,
from day 7; Fig. 2 from day 8), that there was no drop in activity when the chambers with the
plants were placed in the dark (covered with black plastic). The VOC removal continued at its
high, induced rates, for a further 3 days, during which, again, concentrations were topped up
daily to original levels (Fig. 1, days 8-10; Fig. 2, days 9 - 11).
The results strongly suggest that the bacteria in the growth medium play a major role in the
removal of the VOCs from the system.
230
210
Light
Dark
190
Plants removed and soil
replaced in test chambers
170
150
130
Hexane
conc'n, 90
ppm
70
50
30
10
-10
I = hexane addition to
achieve 150 ppm
| = hexane addition to
achieve 100 ppm
-30
-50
I
-70
-2
0
I I I
I I I I I I
2
4
6
8
10
12
14
Time, days
I
16
18
20
22
Fig.1. Hexane levels in test chambers containing Howea forsteriana (Kentia palm)
Each point is the mean of 4 experiments (Mean + SEM).
60
Dark
50
Plants removed and soil
returned to test chambers
40
Benzene 30
conc'n,
ppm
20 10 -
|
= benzene addition
to achieve 25 ppm
-10
I
-20
-2
"T
0
I
•
I
2
I
•
I
4
Day 14 only: benzene
addition to achieve SO ppm
I
I I I I I
I
I I II
I
' — I — > — I — • — I — • — I — • — I — • — I — i — I — • — I — • — | — i — I — i — | — i — | — i
6
8
10
12
14 16 18 20
Time, days
22
24
26 28
Fig.2. Benzene levels in test chambers containing Howea forsteriana (Kentia palm).
Each point is the mean of 4 experiments (Mean ± SEM).
30
A.2.2.4
Saturation
test
To test whether the 'VOC-removal enzyme system' were saturated by the initial concentrations
of chemicals used above, the plants were then challenged with substantially higher
concentrations of each VOC (150 ppm hexane, and 50 ppm benzene). From Figs. 1 and 2
(days 10 and 14 respectively), it can be seen that the rates of removal of VOC rose
substantially over those previously observed. The changed rate was close to significant with
hexane, where only a 50% increase in concentration was introduced (p = 0.07), and highly
significant for benzene, where the increase was 2 1/2 times the original.
It can be concluded that the enzyme systems were not saturated at the nominal hexane or
benzene concentrations used, and that the plant-soil system has the capacity to handle
substantially higher concentrations of the test VOCs, if called upon to do so.
A.2.2.5.
Activity after removing the plant
To determine the relative contributions to VOC removal of the plant and the bacteria of the
growth medium (potting mixture), plants were then removed from the pots, after which the
pot-mix was carefully separated from the roots, replaced in the pots, watered, and returned to
the chambers (Fig. 1, day 12; Fig. 2, day 21). The 'pot-mix' only system was then challenged
with VOC (over 3 consecutive days with hexane; over 5 days with benzene) (Fig. 1, days 13 15; Fig.2, days 22 - 27). It can be seen that the rates of removal remained similar to those
observed with the plants (p < 0.05).
The results further indicate the primary role of soil bacteria in the removal of the VOCs,
although their population levels would presumably over the longer term be dependent on
nutrition from the plant.
A.2,.2.6
Complete removal of VOCs
It was considered important to ascertain whether the plant-pot combinations tested were
capable of reducing VOC levels to values well below the TWA exposure standards for the test
chemicals. In both experiments, reduction of concentrations to 'undetectable' levels (on the
GLC used) were achieved on a number of occasions (for hexane, Fig.l, days 5,14, 17, 20;
and for benzene, Fig.2, days 5, 12, 13, 17, 19 and 20).
The results confirm that the plant-soil combination can remove virtually all of the test VOCs in
relatively short times, a day or less. This capacity was demonstrated both in both 'plant-andsoil' and 'soil-only' situations.
A.2.2.
7 Persistence
of VOC removal
activity
Since an induction period was required before the pot-plant system achieved highest rates of
hexane and benzene removal, it was of interest to see whether a period without the VOC would
lead to a loss of removal capability. For both chemicals, therefore, the concentration was
allowed to decay to an undetectable level, and was then held there for 4 days, after which it
was challenged with more VOC. For hexane, this was performed in the 'soil-only' situation
(Fig. 1, days 17 - 20), while for benzene it was tested in the 'plant-and-soil' combination (Fig.
2, days 16 - 20). In both cases the absence of VOC had no effect on the rates of removal when
it was reintroduced.
The experiment showed that, once induced, the capacity for rapid VOC removal persists for at
least 4 days in the absence of the chemical.
3 . 2 Removal of benzene using hydroponic Kentias
In a further attempt to estimate the plant's contribution to the removal of VOCs from the
atmosphere, Kentia specimens were transferred to a hydroponic medium and tested for their
ability to remove benzene. The roots could not of course be sterilised before transfer to the
hydroponic medium, but they were washed several times in sterile, highly purified (reverse
osmosis) water. The growth medium was also made up in the same water, though not
otherwise sterilised. A nominal concentration of benzene was again used as the test VOC. The
results (Fig. 3) show many similarities with the plant-and-soil system.
There was an induction phase (Fig. 3, days 1 - 2), followed by sustained, increased rates of
removal over a further 3 days (days 2 - 4). In addition, the rates of removal increased slightly
(by 12.5%, p < 0.005) when the chambers were placed in the dark (day 3).When rewashed
and with new medium (day 5), the rates were about 50% lower than before removal, but the
benzene was still rapidly removed. When the medium-only was returned to the test chambers,
the rates of removal increased significantly (by 43%; p < 0.02; Fig. 3, day 7).
These results again indicate that bacterial growth in the medium were the main agents for
removing the VOC from the atmosphere of the chambers.
A.2.3.
Summary of VOC removal results
This work has shown that potted indoor plants show a general capacity to remove VOCs from
the atmosphere, that potted Kentia palm in particular can remove several times the occupational
levels of benzene and hexane, and that its ability to do so is increased by exposure to the VOCs
concerned. In this capacity, it appears to be the potting mixture which is the main direct
contributor to the removal. However, the micro-organisms depend in the long run on the plant
which supports them, and may be triggered to respond by chemical signals from the plant
through the roots, since as will be seen in the next section, the plants show stress responses
when after exposure to the VOCs. In addition, the study by Giese et al (1994) showed that
isolated plant leaf cells in suspension and away soil bacteria, were capable of VOC removal,
the test compound (formaldehyde) being metabolised, presumably via a respiratory pathway,
to carbon dioxide and water.
35
r™
Light
Dark
30
Plants removed and used
medium returned to chambers
25
20
Benzene
Conc'n,
15
ppm
10
5
0
| = benzene added to
achieve 25 ppm
I
-5
-1
)
1
I
I
2
3
W = Plant roots washed
in sterile water and
fresh medium supplied
W
I
I
I
I
4
5
6
7
8
9
10
Time, days
Fig.3. Benzene levels in test chambers containing Howea forsteriana
(Kentia palm): Bare plants under hydroponic conditions.
Each point is the mean of 4 experiments (Mean ± SEM).
B - PLANT STRESS RESPONSES
B.l METHODOLOGY
For the last 20 years a great deal of information has been obtained on the effects of various
pollutants on plants, including plant responses at physiological and biochemical levels. On the
basis of this information, the initial parameters chosen for this study included chlorophyll and
carotenoid contents, and peroxidase activity.
B.l.l.
Photosynthetic pigment contents
Changes in plant pigments were among the first injuries in plants to be directly associated with
the effects of air pollution (Koziol and Whatley,1984). Changes in chlorophyll content
(generally, though not always, a decrease), are now often used as an index of plant injury in
response to environmental pollutants. Carotenoid pigments may increase as chlorophylls
decline in concentrations under stress conditions. In addition, it has been found that
sometimes chlorophyll a levels decreased more rapidly than chlorophyll b , and hence that the
Chla / Chl£> ratios could be used as a biomonitoring index (Lauenroth and Dodd, 1981).
However, in some species both pigments declined in concentration, so that the chla / ch\b
ratios remain unchanged (Rabe and Kreeb, 1980; Mousine 1993; Mousine and Aliev, 1994).
Pigments were determined using extraction by N, N, Dimethylformamide (DMF) and further
spectrophotometric analysis at 647 and 664 nm.
B.l.2.
Peroxidase activity
Active oxygen radicals of several types are produced in response to pollutants, and these
radicals are very toxic. To avoid the production of these intermediates (hydrogen peroxide,
superoxide radical and hydroxyl radical), all biological tissues have a series of antioxidant detoxifying mechanisms (often called scavengers). The main plant defences against the
superoxide and hydrogen peroxide include various antioxidants. These include ascorbic acid,
and a range of enzyme pathways including peroxidase (POD), which detoxify the radicals .
Such compounds can be used as indicators of plant stress in general, including pollutant stress.
In particular, these changes can be used as early-warning symptoms of stress, since plants are
known to undergo such biochemical changes before any visible damage can be detected
(Kangasjarvi et al, 1994). The peroxidase preparation (donor: H202-oxidoreductase, EC
1.11.1.7) which was measured in our experiments, in fact includes a group of non-specific
enzymes from different sources, which is often referred to as non-specific or guaiacol
peroxidase. The determination was made following the method of Putter (1974) (guiacol
assay), the final reading again being made spectrophotometrically, at 470 nm.
B.2 RESULTS
Levels of the various leaf parameters for Kentia plants, before and after exposure to benzene in
the test chambers, are shown in Table 1. It can be seen that there was a consistent increase in
all parameters measured. Changes in these leaf parameters are general stress parameters, and
cannot be taken as specific to the particular contaminant present. In addition, the stress of the
test chambers themselves, in the absence of a VOC, which remain to be tested, may well result
in stress changes over a 96 hour period. If that were the case, and experiments are now being
planned to answer that question, then they will elucidate more about the stresses of airconditioned office environments with restricted air flows and low water supplies (though in
this case, fairly high humidities, because all water evaporated from the plants accumulated in
the bottom of the tanks). Nevertheless, although any changes may be non-specific, they serve
to indicate something of range of tolerance in the species tested, for the level of contaminant
present.
Table 1. Concentrations of chlorophyll a, chlorophyll b ,chl a I chl. b, total
carotenoids ( mg / g fwt); total pigments, and peroxidase activity (units / g
fwt), in Kentia, before and after exposure to a nominal daily initial
concentration of 5 times the TWA levels of benzene, replenished daily, over
96 hours.
Item
chl. a
chl. b
chl. a / chl. b
Tot. carotenoids
Tot. pigments
Peroxidase
B.2.1
Before
2.7 (+ 0.01)
1.1 (+0.05)
2.45
0.45 (+ 0.02)
4.25 (+0.06)
4000 (+ 150)
After
3.1 (+0.02)
1.7 (+0.08)
1.82
0.55 (+ 0.03)
5.35 (+ 0.06)
5000 (+130)
Chlorophylls
Plant species differ in their chlorophyll responses to stress. In some other species studied in
this laboratory (Burchett et al, 1997), and from other reported studies (REF), it has been found
that under some circumstances there can be an initial increase in chlorophyll levels in response
to stress, which is interpreted as an adaptive response to the change, that is, within the range of
tolerance or adjustment for that species, to that stressor. However, for all species, there is a
dose of any toxicant beyond which the chlorophyll concentrations will fall, and the leaf will
then develop symptoms of chlorosis or necrosis.
In this case, the chlorophyll levels rose in the presence of five times the occupational levels of
benzene, daily restored over a 96 hour period. The results indicate, first that the plants show
stress responses to the treatment, but secondly, that the stress is within the range of
adaptability in terms of chlorophyll production. However, stress is further indicated by the
change in the chl a / chl b value, which rose over the period. Such a change in the ratio is
specifically indicative of a stress response.
B.2.2
Carotenoids
A further common plant pigment response to stress is an increase in the carotenoid pigments,
which was observed here. With the increase in chlorophyll levels in this case also, the total
pigment concentration at the end of the experimental period was elevated over the initial levels.
B.2.3
Peroxidase activity
In contrast to the situation with chlorophyll levels, an increase in peroxidase activity is always
a clear indicator of stress, and in this case the experimental treatment resulted in an increase of
activity of 25 per cent, indicating a significant stress response in the plants.
B . 3 DISCUSSION OF STRESS RESPONSES
We have reported for an earlier project the results of the development of an Air Pollution
Tolerance Index (APTI) for indoor plants, using leaf chlorophylls, ascorbic acid content, leaf
pH and relative water content, as parameters. Although further investigation is clearly needed
in this area, the results of the current and previous experiments demonstrate that there is
potential for indoor plants to be used as an early-warning monitoring system of indoor
environmental quality for building occupants, and that the same tests could also be used as
early warning monitors of plant health, should that become economically desirable as well.
Apart from reacting to the stresses of the indoor environment, which have included the
presence of relatively high airborne levels of a particular toxicant, this species has shown an
adaptive response to stress, in terms of elevation of chlorophyll levels, which can be
considered as one of the bases for breeding for stress-resistant characteristics, which is the
longer term goal of the program.
In a parallel program in this laboratory (REFS), in co-operation with the EPA NSW, an
investigation is in progress on the feasibility of developing the use of leaf parameters as a lowcost air pollution monitoring system in the Sydney region. The experimental objectives of the
program have been first the development an appropriate eco-epidemiological methodology for
the purpose, and secondly the conducting of preliminary comparative sampling of selected
species across the Sydney region, in areas of known air pollution characteristics (from data
supplied by EPA NSW). The results are relevant to the planning of future work on indoor
plants and the improvement of indoor air quality. They include the following:
(i) Significant and consistent differences in a number of biochemical and physiological
parameters were found in the four species selected, among a range of sites across Sydney with
different air pollution patterns
(ii) Strong correlations were found between some of the plant parameters and combinations
of air pollutants (and indoor air is known to contain small concentrations of up to 250 VOCs as
contaminants), showing that air quality characteristics were a major factor in the induction of
stress responses among plants at different sites
(iii) Maximum 1-hour concentrations were found to be important in the onset of plant
responses, so that maximum concentrations, as well as average ones, may be important in the
indoor environment as well for plant (and people?) responses
(iiv) Preliminary statistical model testing gave promising results, indicating that there is every
possibility that reliable methods for plant biomonitoring air quality can be established, although
laboratory tests are needed to complement the field results.
The results of the current project similarly indicate the potential for these parameters to be used
both as an early-warning indicator of environmental quality for building occupants, and of the
condition of the plants themselves.
C
PROJECT OUTCOMES
C.l
Extension / Adoption
The goal of the project has been to investigate what is needed for the improvement of local plant
varieties, (a) in their ability to withstand the conditions of the indoor environment into which they
are sold or hired out, from private homes to air-conditioned offices, and (b) their beneficial
effects in improving indoor air quality.
The results of the project show clearly the capacity of indoor pot-plants to reduce concentrations
of VOCs, the direct agent being the soil flora, paving the way for focussed development, for a
wide range of species, of the plant-pot-mix complex for this capacity. The project has also
shown that leaf stress responses can be reliably evidenced and measured following exposure to
the indoor environment containing fluctuating levels of air contaminant.
These experiments need to be extended to a wider range of species and growth media and
contaminants. However the results reported here show sufficient promise to be adopted by the
industry in their marketing and promotional activities.
C.2
Directions for future research
As outlined above, the work needs to be extended to a wider range of plant species, growth
media and VOCs tested. Also needed is an investigation of the micro-organisms that may be
involved in the metabolic responses. Bioremediation efforts depend on micro-organisms with
particular capacities for degradation of pollutants. Some of the species involved can be expected
to be the same as those in indoor pot-plant mixtures. Earlier work on indoor plants and air
pollution (Wolverton, 1989; 1991) showed that for the same contaminant, different plant species
differed in the removal percentages achieved. This indicates, again, that the contribution of the
plant to the soil habitat may help determine microbial effectiveness; that is, that plant-microbe
interactions are important in indoor pot-plants, as well as for those outdoors.
C.3
Financial / commercial benefits
These results show that the indoor pot-plants studied are effective in removing from two to five
times the occupational levels of VOCs from the indoor environment, and that the system
improves in efficiency with exposure to the contaminant. This fact has immediate marketing
potential for the indoor plantscape industry, and longer term implications as more efficient
combinations of plants and micro-organism combinations are developed.
The findings on the stress responses of the plants concerned also shows promise for the use of
indoor plants as early warning indicators of deteriorating conditions of indoor quality, both for
building occupants and the plants themselves.
In both aspects of the project, more work is needed to develop breeding and growth-medium
manipulations. However, the results presented have implications for immediate commercial
benefit.
Publication schedule
The Kentia results of this study will be presented as a paper at the International Healthy
Buildings '97 meeting in Washington, DC, 28 September - 4th October. In addition, a fuller
paper is being prepared for publication in a journal of environmental pollution (still to be
finalised). The work is also being written up for trade and professional journals in Australia,
which should be submitted by November, 1997.
Acknowledgements
The funding from the Horticultural Stock and Nurseries Act and HRDC for this project is great
appreciated. We should also like to thank the Interior Plantscapers Association of NSW,
HousePlants Australia, and the Lord Howe Island Board, for assistance in the supply of plant
materials.
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