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Pot size matters: a meta-analysis of the effects of
rootingvolume on plant growth
Hendrik PoorterA,C, Jonas BhlerA, Dagmar van DusschotenA, Jos
ClimentB
and Johannes A. PostmaA
AIBG-2 Plant Sciences, Forschungszentrum Jlich, D-52425,
Germany.BINIA, Forest Research Centre, Department of Forest Ecology
and Genetics, Avda A Corua Km 7.5.,28040 Madrid, Spain.
CCorresponding author. Email: [email protected]
Abstract. Themajority of experiments in plant biology use plants
grown in some kind of container or pot.We conducted ameta-analysis
on 65 studies that analysed the effect of pot size on growth and
underlying variables.On average, a doubling ofthe pot size
increased biomass production by 43%. Further analysis of pot size
effects on the underlying components ofgrowth suggests that reduced
growth in smaller pots is caused mainly by a reduction in
photosynthesis per unit leaf area,rather than by changes in
leafmorphology or biomass allocation. The appropriate pot sizewill
logically depend on the size oftheplants growing in
them.Basedonvarious lines of evidencewe suggest that an appropriate
pot size is one inwhich theplantbiomass does not exceed 1 g L1. In
current research practice ~65% of the experiments exceed that
threshold. We suggestthat researchers need to carefully consider
thepot size in their experiments, as small potsmaychange
experimental results anddefy the purpose of the experiment.
Additional keywords: container volume, experimental setup,
meta-analysis, pot size, plant growth, rooting volume.
Received 16 February 2012, accepted 11 May 2012, published
online 15 June 2012
Introduction
A large number of studies in plant biology focus on
geneexpression, physiology or biomass production of
individually-grown plants. To this end, experiments are often
conducted onplants grown in somekindof container, fromhere on
referred to aspots. Pot size has received special attention in
forestry (Carlsonand Endean 1976) and horticulture (Kharkina et al.
1999), wherecommercial companies can prot from choosing the
smallest potvolume that still delivers a product with an
appropriate quality.Occasionally, the issue of pot size has
received attention in otherelds of plant biology. Arp (1991), for
example, emphasisedthat pot size might be an important issue in
experiments thatconsidered the effect of elevated CO2 on plants;
and recently, adiscussion in the eld of ecophysiology has emerged,
wherestudies on the recognition of roots of neighbours are thought
tobe confounded by pot size (Hess and De Kroon 2007).
Apart from the elds mentioned above, pot size seems tohave
received little consideration in the scientic literature andis
regulary not reported in the materials and methods section
ofpublications. Nonetheless, it is an important issue. In
mostlaboratories there is large demand for growth chamber
facilitiesand the use of small pots generally implies more
experiments orincreased replication. Space is probably less of an
issue in mostglasshouses. However, the plant biology community
currentlymakes a great effort to develop automated systems for
plant
phenotyping (Granier et al. 2006; Nagel et al. 2012).
Thesehigh-throughput systems allow for the handling of many
plants,which automatically implies that spatial demands on
growthfacilities will increase. The use of small pots has the
additionaladvantage that it does not exceed the capabilities of the
transportrobots to move the weight of pot plus plant.
The use of small pots for research purposes may also
havedisadvantages, which are more related to biological
constraints.A small container implies a small quantity of soil or
othersubstrate and thereby, almost invariably, a reduction in
theavailability of water and nutrients to the plant. In addition
toreduced resource availability, pots generally impede root
growth.Many species easily produce roots of more than 1m in
length(Jackson et al. 1996), even at a relative young age (Drew
1975;Fusseder 1987), thereby exceeding the dimensions of
mostcontainers used. Large plants in small pots may have a
largefraction of roots pot-bound, with all kind of
secondaryconsequences (Herold and McNeil 1979). In this paper
weexplore the consequences of the choice of pot size for
plantsstudied for experimental purposes. We rst analysed in
aquantitative way to what extent pot size affects plant growth
inthe studies that explicitly considered this factor. We did so by
ameta-analysis of 65 studies reported in the literature. Second,we
reported on some of the morphological and physiologicalcomponents
that might explain the observed growth patterns.
CSIRO PUBLISHINGFunctional Plant Biology
Reviewhttp://dx.doi.org/10.1071/FP12049
Journal compilation CSIRO 2012
www.publish.csiro.au/journals/fpb
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Third, we analysed whether there is a threshold in the plantsize
: pot size relationship above which growth is affected andcompared
the data for the current pot-size experiments with amore general
database on plant research and to plants grown inelds. Based on
these comparisons we suggest what might be anappropriate pot size
for a given experiment.
1. Effect of pot size on plant biomass
To analyse the effect of rooting volume on plant growth,
wescreened the literature of the last 100 years and arrived at a
totalof 65 publications where this factor was studied in pots
and~10 where rooting volume was constrained in hydroponically-grown
plants (see Appendix 1). For this analysis we will focusmainly on
the results for the pot-grown plants. A description ofthe
methodological approach is given in Appendix 2. The effectof pot
size has been studied in a wide range of pot volumes, withvalues
ranging from 5mL for a herbaceous greenhouse crop(Bar-Tal and
Pressman 1996), to 1700L for trees growing over aperiod of several
years (Hsu et al. 1996). The range of pot sizesused within the
various experiments was also large, varying by afactor 2 at least
and a factor 35 at most. A clear example in ourcompilation is the
experiment by Endean and Carlson(1975), who followed the growth of
Pinus contorta Douglasover time. In the very beginning of the
experiment, plants grewpresumably well in all pot sizes, but after
4 weeks of growthbiomass was already reduced in the smallest pot
volume(Fig. 1a), and by the last harvest, none of the pots
seemedlarge enough to ensure unrestricted growth.
In the full compiled data set the picture is similar: in
almostall of the experiments considered plants increase in weight
withevery larger pot that is used (Fig. 1b). Most experimentsfall
between the dotted lines, which indicate a xed ratio ofbiomass to
pot volume of 2 and 100 gL1, respectively, for thelower and higher
line. Only a few experiments with trees in largecontainers exceed
the 100 gL1. As all experiments start withsmall seedlings, plants
move over time from the bottom part ofthe graph upwards (cf. Fig.
1a). Clearly, the experiments at theupper right end of Fig. 1b are
also the ones that were plannedto last for the longest time, 2
years in case of the experiment withthe largest pots (Hsu et al.
1996), 5 years in case of the experimentwith the highest ratio
(Bar-Yosef et al. 1988).
Theoretically, doseresponse curves should level off athigher pot
sizes, as in Fig. 1a. However, this is not the casefor many of the
studies plotted in Fig. 1b, which mostly showlinear responses. We
discuss this further in section 4. To scale alldata to an equal
change in pot size, we calculated for eachspecies in each of the
experiments the average slope of thelog-transformed mass versus pot
size relationship and derivedfrom those values an easy to
understand expression that indicatesthe percentage increase in
biomass for a 100% increase in potsize (Appendix 3). On average,
plants increased 43% in mass forevery doubling in pot size (Fig. 2,
P < 0.001), with no signicantdifferences in response between
herbaceous and woody species.These response values are substantial
and imply that plantsare likely to be over 3 times larger in 2 L
pots than in pots of0.2 L. Given these differences, it is clear
that pot size shouldbe given careful consideration during the
planning phase ofan experiment.
2. Effect on components of the carbon-budget
What is the cause of the growth retardation in smaller pot
sizes?As clearly shown by Endean and Carlson (1975), the effect of
potsize on biomass gradually increases throughout (the later part
of)the experimental period. The rate bywhich biomass of
individualplants is accumulated is proportional to the size of the
plant andconveniently described by the relative growth rate (RGR,
therate of increase in biomass per unit biomass present;
Evans1972). The differences in RGR of plants growing in
differentpot sizes are always smaller than the differences in
biomass at theend of the experiment. This implies that the
physiological andmorphological factors that underlie the variation
in biomass willalso be affected to a smaller extent than biomass
itself (Poorteret al. 2012a). We analysed growth in terms of the
plants carboneconomy, using a top-down approach where the RGR
isfactorised into three components:
RGR SLA LMF ULR; 1
0.01 0.10 1.000.001
0.01
0.1
1
10
0.01 0.10 1.00 10.00 100.00 1000.000.01
0.1
1
10
100
1000
10 000
100 000
Tota
l dry
mas
s (g)
4
8
14
20
0
Pot size (L)
Herb.Woody
(a)
(b)
Fig. 1. (a) Doseresponse curves of total plant dry mass of Pinus
contortaas dependent on pot size. Plants were grown in pots with
six different sizesand harvested at several times during a 20-week
period. Numbers indicatethe time of harvest after sowing, in weeks.
Data from Endean and Carlson(1975), except for seedmass (week 0),
which was taken fromMcGinley et al.(1990). (b) Summary of total
biomass data of plants grown in pots ofvarious sizes, as reported
in the literature (65 experiments on 69 species,see Appendix 1).
Each line connects the observations of one species orgenotype in
one experiment. Values in red indicate woody species, in
blueherbaceous species. Dotted lines indicate a total plant biomass
per unit potvolume of 2 (lower line) and 100 (upper one) g L1.
Additional, unpublisheddata were obtained for Barrett and Gifford
(1995) and Liu and Latimer(1995), as well as from L. Mommer and H.
de Kroon, and R. Pierik.
B Functional Plant Biology H. Poorter et al.
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according to Evans (1972). Here, SLA denotes specic leaf
area(leaf area per unit leaf dry mass; m2 kg1), LMF the
relativeallocationof biomass to leaves (leafmass fraction, g leaf
g1plant)and ULR is a parameter that indicates the growth rate per
unitleaf area (unit leaf rate, gm2 day1). ULR is basically the
netresult of carbon gain through photosynthesis, corrected for
therate of respiration in the whole plant and the C-content of
thenewly added biomass. ULR and photosynthesis per unit leafarea
are often strongly positively correlated (Poorter 2002).
A characteristic of RGR is that an absolute difference in
RGRbetween treatments causes a relative difference in biomass
over
time (Poorter and Navas 2003). However, in the case where wewant
to understand the reason for the difference in growth andonly have
fragmented information, it ismore amenable to analysethe
proportional differences in RGR relative to that of the threegrowth
parameters that underlie RGR (Eqn 1). Only a fewexperiments report
data on these underlying components, butthe information we could
gather points into the followingdirection (see Fig. 2 and Fig. S1,
available as SupplementaryMaterial to this paper): a doubling in
pot size increases RGR by~5%. Consequently, each of the growth
parameters at the right-hand side of Eqn 1 may change by a very
small proportion, orone of them by a somewhat larger proportion in
the range of5%. At the nal harvest, SLA increased somewhat in
someexperiments and decreased in others. We could
quantitativelycompare the various experiments by calculating the
percentageincrease in SLA with a 100% increase in pot size and
found thattaken over all experiments, this variable did not
deviatesignicantly from zero (Fig. 2). Most, but not all
experimentswith root restriction in hydroponics conrm this response
(e.g.Carmi et al. 1983; Kharkina et al. 1999; but see Tschaplinski
andBlake 1985).
Allocation patterns are more frequently reported, generallyas
the biomass ratio between shoot and root. We prefer aclassication
in at least three plant organs (Poorter et al.2012b) and therefore
use LMF, SMF and RMF, which are thefractions of total vegetative
biomass invested to leaves, stemsand roots respectively.The
relatively scarce information indicatesthat LMF is not affected
(Fig. 2a), whereas SMF increasedslightly but non-signicantly
(0.05
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limited and fragmentary evidence yet available,
netphotosynthesis is likely to be the process that is
strongestaffected by pot size and may explain best the observed
potsize effect on biomass (Fig. 1b). Additional support comesfrom
experiments where photosynthesis recovered quicklyafter plants were
repotted in larger rooting volumes (Heroldand McNeil 1979).
3. What mechanism could explain a reducedphotosynthesis in
smaller pots?
Several factors could explain the reduced rate of photosynthesis
and thereby growth in smaller pots. A rst possible explanationis
that containers of smaller dimensions can be placed at a
higherdensity, with less light available for each shoot and hence,
alower rate of photosynthesis. Although this could be the case
insome of the compiled experiments, strong pot size effects
onbiomass and photosynthesis are generally also observed
whendensity is specically controlled for (e.g. Endean and
Carlson1975; Robbins and Pharr 1988; NeSmith et al. 1992; Climentet
al. 2011).
In the type of experiments that we included in our
meta-analysis, a smaller pot size will inadvertently decrease the
totalnutrient content in the pot. Low nitrogen and
phosphorusavailability are known to decrease photosynthesis
(e.g.,Sinclair and Horie 1989; Lynch et al. 1991) and growth
andincrease the root mass fraction (Poorter et al. 2012b).
Thus,lower resource supply could form a plausible explanation.
Wecalculated the response of leaf nitrogen concentrations to
changesin pot size, expecting to see an increase if nutrient
availabilitywould explain the pot size effect. On average, there
was a slight,but non-signicant increase in leaf nitrogen
concentrations withpot size (Fig. 2), suggesting that this factor
cannot completelyexplain the observed differences in photosynthesis
or growth.Similar results were found for phosphorus (Krizek et al.
1985).This conclusion is to a certain extent supported by
observationson hydroponically-grown plants, which have
decreasedphotosynthesis and growth (Fig. 2b) when the root
volumewas restricted, despite a continuous high supply of
nutrients.However, unlike plants grown in pots,
hydroponically-grownplants do not show an increased RMF when
restricted (Fig. 2b).As increased RMF is a good indicator for
nutrient stress, wepresume that nutrient limitation in small pots
is still a factor,although we cannot exclude possible allometric
effects whichcould explain a largerRMF in smaller plants aswell
(Poorter et al.2012b).
Water is the other commodity that may be in short supply.Small
pots could negatively impact the water status of plantsas they have
a reduced total water holding capacity andwill, therefore, dry out
more quickly (Tschaplinski and Blake1985) and at severe stress
levels increase RMF (Poorter et al.2012b). Ray and Sinclair (1998)
demonstrated with theirdrought experiment that soil in small pots
dries out faster andthereby caused more severe drought stress in
plants. However,pot size does not necessarily affect stomatal
conductance orleaf water potential (Ronchi et al. 2006) and as
fornutrients with plants growing in hydroponics there is still
aclear effect of root connement, even though water availability
isnot restricted.
Besides resource availability, the temperature of therooting
volume could be affected by pot size (de Vries 1980).Pots can
intercept a substantial amount of solar radiationespecially in
experimental gardens and glasshouses, whichmay increase the soil
temperature at the edge and eventuallyin the middle of the pot if
no precautions are made (Martiniet al. 1991; Xu et al. 2001). Small
pots have greater surfaceareas relative to their volume and thereby
heat up morequickly. Townend and Dickinson (1995) measured 5Chigher
day temperatures in 0.19 L pots compared with 1.9 Lpots. Keever et
al. (1986) suggest that the greater temperatureuctuations in small
pots may explain the reduced growth ofthe plants. High temperatures
in the pot may have several direct(respiration, root growth) and
indirect (through increasedmicrobial activity) effects on plant
growth. Pot temperaturesare rarely reported so it is difcult to
evaluate how oftentemperature differences between pots contribute
to reducedgrowth. However, given that growth reductions also have
beenobserved in hydroponically-grown plants suggests
thattemperature differences alone cannot explain the observed
poteffects either.
If neither nutrient orwater availability nor higher
temperaturescan (fully) explain the pot size effects on
photosynthesis andgrowth, it could be that root connement per semay
cause growthretardation, with reduced photosynthesis as a
consequence.Root growth is known to respond directly to
impedance.Impeded roots stay shorter whereas the initiation and
growthof side branches increases (Bengough and Mullins 1991;
Faliket al. 2005). Furthermore, Young et al. (1997) showed that
within10minof increasing the impedance to root growth, leaf
expansionrate is reduced. This suggests that some kind of signal
mayregulate shoot growth when a large proportion of the roots
areimpeded. The actual signal for such a response remains as
yetunknown. Possibly a reduced sink strength of the root
systemcould cause a direct negative feedback on photosynthesis
(Pauland Pellny 2003). Alternatively, a specic rootshoot signal
isinvolved (Jackson 1993).
A crucial point in the evaluation of the lastly discussedoption
is knowledge on the actual distribution of roots withinthe pots.
Although the vertical root distribution is relativelyeasily
measured (Price et al. 2002; Suriyagoda et al. 2010),analysis of
the horizontal distributions is more complicated.Using
non-destructive magnetic resonance imaging (MRI), wefollowed the
root development of Hordeum vulgare L. andBeta vulgaris L. plants
over time in three dimensional space.Representative nuclear
magnetic resonance (NMR) images ofroot systems at the end of the
experiment are shown in Fig. 3.We calculated the percentage of
roots that was located inthe inner half of the soil volume,
furthest away from wall andbottom and the percentage of roots
present in the outer 4mm ofthe pot. Only 2025% of the root biomass
was in the inner partof the pot (Table 1), whereas ~50% was found
in the outer 4mm(20% of the total volume). The proportional
distributionremained remarkably constant over time. Hence, if
theseobservations have wider validity, we conclude that a
relativelylarge fraction of roots is close to the edge of the pot,
whereunfavourable environmental conditions, for example,
largetemperature uctuations and impedance of the pot wall
maynegatively impact growth.
D Functional Plant Biology H. Poorter et al.
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4. When does pot size limitation starts?
In sections 1 and 2 we considered for each experiment theoverall
effect on plant growth, morphology and physiologywhen pot size was
doubled. However, it is to be expected thata plant of a given size
will be constrainedmore in a small than in alarge pot. That is,
young plants are initially not affected by potsize, but as plants
grow older, the pot size effect becomes morepronounced, even in
medium-sized pot volumes (Fig. 1a). When
experiments last for sufciently long time, even the largest
potsize might not be large enough for unrestricted growth, i.e.
thesaturating part of the curve extends beyond the largest pots
used.For the experimental data this implies that the
relationshipbecomes close to linear again. In fact, many of the
curves inFig. 1b show a linear relationship. For experiments where
onlytwo pot sizes were used, it is impossible to deduce whether
theresponse of the plants is indeed linear or not. But even in many
ofthe other experiments in Fig. 1b no clear saturation is
shown.What is the reason for that?
One objective way to relate plant and pot size acrossexperiments
is to calculate the plant biomass that is present ata given volume
of rooting space. This variable, for which we useBVR as an acronym
(total plant biomass : rooting volume ratio;g L1), has, to our
knowledge, been used only by Kerstiens andHawes (1994). BVR values
vary widely and ranged in ourdatabase from as low as 0.01 in work
by Climent et al. (2008)to over 300 g L1 in work reported by Biran
and Eliassaf (1980).The median value in the pot size experiments is
around 9.5 forexperiments both with herbaceous and woody species
(Fig. 4b).We tested whether the BVR could explain the form of
thedoseresponse curves in the 65 experiments shown in Fig. 1b,by
calculating for each point what the slope of the doseresponsecurves
was, as well as the BVR. In order to be consistent withFig. 2, we
derived the percentage increase in biomass with potsize doubling
for these data as well. We found that very fewexperiments
hadBVRvalues lower than 2 g L1 (Fig. 4b). Hence,for this part of
the analysis we included not only data of the lastharvest, but
alsodata of earlier harvestswhere available.Thismayimply that not
all data points in the analysis are formallyindependent, but it
increases the power of our analysis in thiscrucial range.We binned
all values in ve BVR ranges and showthe resulting distribution in
Fig. 5a. Estimation of slopes isalways more challenging than
determining the absolute valuesper point and this may be one of the
reasons that there isconsiderable variation within each category.
In the categorywith a BVR between 1 and 2 g L1 the effect of pot
size isclearly noticeable. Pot size effects are saturating when
BVRvalues exceed the 2 g L1 (P < 0.01).
Another way to obtain a greater insight into the
relationshipbetween plant biomass and pot size is to express the
biomass of
(a) (b)
Fig. 3. (a) NMR image of a Hordeum vulgare plant grown in a pot
with avolume of 1.3 L for 44 days. Roots in the inner 50% of the
soil volume(furthest away from wall and bottom) are colour-coded
yellow, roots in theouter 50% blue. The stem part that was masked
from the analysis is shown inred. (b) Idem for a Beta vulgaris
plant 48 days after sowing. The developingstorage root is
colour-coded red and was not included in this case.
Table 1. The proportion of the root mass that is present in the
inner 50% of the pot volume (more than 12.5mm fromthe wall or
bottom) and in the space less than 4mm from wall or bottom of the
pot, for Hordeum spontaneum and
Beta vulgaris plants growing in 1.3 L potsValues are based on
six plants followed over time. Time is days after sowing. Standard
error of the mean was on average 1.6
and 2.9 percentage points for the inner half and outer 4mm
respectively
Hordeum vulgare Beta vulgarisTime(days)
Root mass ininner half (%)
Root mass inouter 4mm (%)
Time(days)
Root mass ininner half (%)
Root mass inouter 4mm (%)
26 32.0 43.2 31 20.3 53.528 28.9 50.0 33 20.8 53.131 17.7 55.3
34 21.5 54.133 21.3 52.7 39 22.9 52.634 23.1 51.7 41 23.0 52.339
25.2 48.7 48 22.3 52.041 28.2 48.0 44 32.4 44.7
Pot size matters Functional Plant Biology E
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plants grown at various pot sizes relative to the biomass atthe
largest pot size and plot these values against the BVR(Fig. 5b).
Experiments where pot sizes are limiting growththroughout the full
range of pot sizes are characterised by linesthat decline linearly
with BVR. However, as long as biomass isnot affected, the linewill
remain around 1 and only drop at greaterBVR values when pot size
starts to reduce growth. For the fewexperiments where this was the
case, we could show that thisinection point occurred somewhere
between 0.2 and 2 gL1
(Fig. 5b). Thus, from both the full sets of experiments
compiled,as for the more detailed analyses over time, we derive
that potsize effects are particularly strong when BVR values are
greaterthan 2 g L1.
5. How do these BVR values relate to otherexperiments and the
eld?
As mentioned in section 1, the range of pot sizes used in
thiscompilation is large. The median value is around 0.9 L (Fig.
4a).How does that compare to common practice in
ecophysiologicalexperiments? This will partly depend on the species
studied.
Arabidopsis, for example, is generally grown in much smallerpots
(with an interquartile range of 0.080.21 L) than Zeamays L. (1.85.0
L). An overall impression of used pot sizescan be obtained from the
metaphenomics database describedby Poorter et al. (2010), where the
response of ~900 differentspecies to 12 different environmental
factors is compiled for atotal of ~800 experiments from the
literature. The median potsize used in that compilation of
experiments turns out to be~23 times larger than those in the pot
size studies (Fig. 4a),whereas themedianBVRvalue is ~4-fold lower
(Fig. 4b). Hence,we conclude that most studies on pot sizes have
focussed onrelatively small pots and have grown plants to larger
sizesthan is common in ecophysiological experiments. In
contrast,experiments in plant biology generally use relatively
larger potsand harvest plants at younger stages, when the BVR value
is stillbelow 8 gL1.
0.01
0.1
1
10
100(a)
(b)
9.3
0.5
1.6
2.5
3.0
2.0
10.5
Pot s
ize
(L)
1.2
0.01
0.1
1
10
100
Herb. Woody
WoodyWoodyMetaphenomics
database
BVR
(g L
1 )
Pot sizeexperiments
Herb. Herb.
Fig. 4. (a) Distribution of pot volumes as represented in the
current meta-analysis of pot size studies and in a compilation of
~800 studies on the effectof 12 environmental factors on growth and
related ecophysiological traits(meta-phenomics database, Poorter et
al. 2010). (b) Total plant biomass : potvolume ratios (BVR) in the
current meta-analysis and the meta-phenomicsdatabase. The
distribution is characterised by boxplots (see legend Fig. 2).Blue
boxes indicate the values for herbaceous species, red ones for
woodyspecies. Numbers above/below each box show the median
values.
0.1 1.0 10.00.0
0.2
0.4
0.6
0.8
1.0
1.2
Scal
ed to
tal d
ry m
ass
(g g
1 )
BVR (g L1)
20
0
20
40
60
80
100
120(a)
(b)
>2012 25 520
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Most experiments with pots are conducted to eventuallyunderstand
how (agro-)ecosystems function. It may thereforebe relevant to
consider what normal BVR values are in the eld.Maximum dry matter
yield of major crops varies between10003000 gm2 (Unkovich et al.
2010; assuming 20%biomass in roots). If we assume a rooting depth
of 1m, thiswould correspond toBVRvalues in the range of 13 gL1.
Giventhat at least half of the DM production takes place
duringgrain lling, we can expect that during the vegetative state,
theBVR value will not exceed 1.5 g L1. Similar calculations
fornatural ecosystems are more difcult as large variation existsin
root depth and the standing biomass. Given a rooting depthof 0.35m
(Nagel et al. 2012) and a density of 500 plants per m2,a eld of
Arabidopsis thaliana Heynh. plants of 0.1 g dry mass,would have a
BVR of 0.15 g L1. Although we realise that plantsin the eld
experience conditions that are very different fromthose where
plants are grown singly in pots in controlledconditions, we
conclude from these rough estimates that BVRvalues around 1 are of
the same order of magnitude as those ofvegetative plants in the
eld.
6. Does pot size affect experimental conclusions?
Up to now we have considered the effect of pot size per se.
Mostresearchers are also interested in whether the outcome of
theirexperiments is affected by the choice of the pot size. Arp
(1991)was one of the rst to draw attention to the fact that pot
sizemight restrict the response to elevated CO2. This would limit
thepossibilities to drawconclusions fromexperiments that have
beenconducted in this eld.
The analysis by Arp (1991) was a compilation of differentstudies
that worked with different pot sizes. Kerstiens and Hawes(1994),
however, published a meta-analysis of the results of arange of
studies with trees in which they show that the biomassresponses to
elevated CO2 did not correlate with pot size, or evendecreased with
a BVR over 18 g L1. This suggests that small
pot sizes do not reduce responses to elevated CO2
universally.However, in both meta-analyses the evidence could only
becircumstantial, as pot size was not an experimental factor inthe
compilations. Here we analysed experiments where pot sizewas
specically included in the experimental design, not onlyfor
interaction with CO2 but also for nutrients, water andirradiance
(Fig. 6). Given that nutrient and water availabilityalready
increase when pot size is increased, we would expectless additional
effect on plant growth if more nutrients or waterwere supplied.
However, we would expect increasingly strongergrowth responses with
larger pot size when light or CO2 wouldbe increased, simply because
of the higher demand for nutrientsand water in larger plants.
Although some of the experiments doindeed follow the expected
trend, results are not equivocal. Asresults depend on the
variability in at least four different harvests,the number of
experiments is likely too small to draw any strongconclusion.
Besides possible interactions with abiotic factors,
interactionswith biotic factors have been shown. For example,
vesiculararbuscular mycorhizae (VAM) infection rates, which
wouldnormally increase with reduced nutrient availability,
arereduced in small pots (Bth and Hayman 1984; Koide 1991).As a
consequence, VAM colonisation is less benecial fornutrient uptake
and results in smaller growth increases whensmall pots are used
(Kucey and Janzen 1987; Koide 1991).Similarly, Baldwin (1988)
showed that pot-bound Nicotianaplants do not respond to leaf
damage, whereas repotted plantsdo. Thus, although we cannot draw a
rm conclusion here wesuggest that the use of small pots with
crowded roots carries asubstantial risk of inuencing the
experimental results.
7. Other considerations
This analysis focussed on the effect of pot volume on
plantgrowth. However, choosing a pot for an experiment not
onlyincludes choosing the right volume, but also the right
shape.Although shape is less important than volume (McConnaughayet
al. 1993), shallow and deep-rooting species may responddifferently
to the actual diameter and height of the pots, atequal pot volume
(von Felten and Schmid 2008). Pot height isalso an important factor
in determining the free-draining watercontent of pots and thereby
the water potential as well as theoxygen availability in the pots
(Passioura 2006).
An alternative to standard plastic pots are containers that
haveribbed inner sides and small air holes. Such containers
promoteself-pruning of roots close to the holes, which avoids
rootspiralling and promotes development of lateral roots
(Rune2003). Not only shape, but also the material (Bunt andKulwiec
1970) and the colour of the pot (Markham et al.2011) may affect
plant growth, mainly through their effect onsoil and root
temperature. For a broader discussion on the use ofpots for growing
plants in the context of experimental setup seePoorter et al.
(2012a).
Conclusions
A meta-analysis of the effects of pot size on growth showsthat
on average a doubling of the pot size results in 43% morebiomass.
In most cases reduced growth in small pots will becaused by a
reduction in net photosynthesis. It is the plant
0.1 1 10 1000.7
1
2
3
4
5
Rat
io b
iom
ass
high
vs.
low
(g g
1 )
Pot size (L)
Irradiance CO2 Nutrients Water
Fig. 6. Interaction of pot size with various abiotic factors.
Results aregiven for total plant biomass and are expressed as the
ratio between thebiomass at a high level of that factor and the
lower level. Data are fromBilderback (1985), Thomas and Strain
(1991), McConnaughay et al. (1993),NeSmith (1993), Ismail et al.
(1994), Nobel et al. (1994), Barrett and Gifford(1995), Will and
Teskey (1997), Houle and Babeux (1998), Centritto (2000)and R.
Pierik (unpublished data).
Pot size matters Functional Plant Biology G
- mass per unit rooting volume that is relevant rather than pot
sizeper se. Large plant mass per pot volume not only reduces
growthof plants but also carries the risk of inuencing therelative
differences between treatments. We conclude that it isimportant for
researchers to minimise such effects by choosingpots that are large
enough for their plants, even at later stages ofgrowth. Our advice
is to avoid plant biomass to pot volumeratios larger than 2 g L1
and preferably work with plant and potsizes where this ratio
is
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Appendix 1. List of references used for the meta-analysis
A. Pot studies
Cris and Stout (1929) Agric. Exp. Stat. MSC, No 6.; Stevenson
(1967) Can. J. Soil Sci. 47, 163174; Endean and Carlson (1975)
Can.J. For. Res. 5, 5560; Hocking and Mitchell (1975) Can. J. For.
Res. 5, 440451; Carlson and Endean (1976) Can. J. For. Res.
6,221224; Herold and McNeil (1979) J. Exp. Bot. 30, 11871194; Biran
and Eliassaf (1980) Sci. Hortic. 12, 385394; Peterson et al.(1984)
Agron. J. 76, 861863; Bilderback (1985) J. Env. Hortic. 3, 132135;
Krizek et al. (1985) J. Exp. Bot. 36, 2538; Carmi(1986) Field Crops
Res. 13, 2532; Hanson et al. (1987) HortSci. 22, 12931295; Kucey
and Janzen (1987) Plant Soil 104, 7178;Ruff et al. (1987) J. Amer.
Soc. Hort. Sci. 112, 763769; Tilt et al. (1987) J. Amer. Soc. Hort.
Sci. 112, 981984; Bar-Yosef et al.(1988) Plant and Soil 107, 4956;
Robbins and Pharr (1988) Plant Physiol. 87, 409413; Bar-Tal et al.
(1990) Agron. J. 82, 989995;Gurevitch et al. (1990) J. Ecol. 78,
727744; Koide (1991) Oecologia 85, 389395; Latimer (1991)
HortScience 26, 124126;Martini et al. (1991) J. Am. Soc. Hort. Sci.
116, 439445; Simpson (1991) North. J. Appl. For. 8, 160165; Thomas
and Strain (1991)Plant Physiol. 96, 627634; Dubik et al. (1992) J.
Plant Nutr. 15, 469486; NeSmith et al. (1992) J. Plant Nutr. 15,
27632776;Samuelson and Seiler (1992) Env. Exp. Bot. 32, 351356;
Beeson (1993) J. Amer. Soc. Hort. Sci. 118, 752756; McConnaughayet
al. (1993) Oecologia 94, 550557; NeSmith (1993) J. Plant Nutr. 16,
765780; Ismail et al. (1994) Aust. J. Plant Physiol. 21,2335;
Menzel et al. (1994) J. Hortic. Sci. 69, 553564; Nobel et al.
(1994) Physiol. Plant. 90, 173180; Ran et al. (1994) Agron. J.86,
530534; Barrett and Gifford (1995) Aust. J. Plant Physiol. 22,
955963; Liu and Latimer (1995) HortSci. 30, 242246; Mandreet al.
(1995) J. Amer. Soc. Hort. Sci. 120, 228234; Agyeman et al. (1996)
Ghana J. For. 2, 1424; Hsu et al. (1996) HortSci. 31,11391142;
Huang et al. (1996) Plant and Soil 178, 205208; Ismail and Noor
(1996) Sci. Hortic. 66, 5158; McConnaughay et al.(1996) Ecol. Appl.
6, 619627; Giannina et al. (1997) Acta Hortic. 463, 135140; Van
Iersel (1997) HortSci. 32, 11861192; Willand Teskey (1997) Tree
Physiol. 17, 655661; Houle and Babeux (1998) Can. J. Bot. 76,
16871692; Nishizawa and Saito (1998)J. Amer. Soc. Hort. Sci. 123,
581585; Ray and Sinclair (1998) J. Exp. Bot. 49, 13811386; Boland
et al. (2000) J. Amer. Soc. Hort.Sci. 125, 135142; Centritto (2000)
Plant Biosystem. 134, 3137; Haver and Shuch (2001) Plant Growth
Reg. 35, 187196; Yeh andChiang (2001) Sci. Hortic. 91, 123132;
Aphalo and Rikala (2003) New Forests 25, 93108; Loh et al. (2003)
Urban For. UrbanGreen 2, 5362; Ronchi et al. (2006) Funct. Plant
Biol. 33, 10131023; Arizaleta and Pire (2008) Agrociencia 42, 4754;
Chirinoet al. (2008) For. Ecol. Man. 256, 779785; Climent et al.
(2008) Silvae genetica 57, 187193; Goreta et al. (2008)
HortTechnology18, 122129; Kurunc and Unlakara (2009) New Zeal. J.
Crop and Hortic. Sci. 37, 201210; Oztekin et al. (2009) J. Food
Agric. Env.7, 364368; Climent et al. (2011) Eur. J. Forest Res.
130, 841850; Nord et al. (2011) Funct. Plant Biol. 38, 941952;
Mommer andDe Kroon (pers. comm.); Pierik (pers. comm.).
B. Hydroponically grown plants
Richards and Rowe (1977) Ann. Bot. 41, 729740; Carmi et al.
(1983) Photosynthetica 17, 240245; Tschaplinski and Blake
(1985)Physiol. Plant. 64, 167176; Hameed et al. (1987) Ann. Bot.
59, 685692; Peterson et al. (1991) J. Exp. Bot. 42, 12331240;
Thomas(1993) Plant Growth Reg. 13, 95101; Ternesi et al. (1994)
Plant Soil 166, 3136; Bar-Tal et al. (1995) Sci. Hortic. 63,
195208; Barand Pressman (1996) J. Amer. Soc. Hort. Sci. 121,
649655; Kharkina et al. (1999) Physiol. Plant. 105, 434441; Xu et
al. (2001)J. Plant Nutr. 24, 479501.
J Functional Plant Biology H. Poorter et al.
-
Appendix 2
For this meta-analysis we screened the literature of the last
100 years. A total of 63 publications plus two additional
unpublishedexperiments dealt with plants grown in pots of various
sizes, 11 with plants grown in hydroponics with different levels of
rootconnement. The references are listed in Appendix 1. As we were
interested in not only the physical aspect of container volume,
butalso in the resources that come with it, we compared pot size
treatments based on size, including the possible additional benets
ofincreased nutrient and water availability. The experiments with
hydroponically-grown plants are not included in the main
analysis.Differences in the shape of the pots were not
independently analysed either. In several publications only pot
diameter was reported.For a sample of 30 different pots ranging in
diameter () from 7 to 40 cm we derived an estimate for pot volume
(V) based on theempirical equation V =p(/2)2 (0.46 + 0.8397
0.0023072). If additional data were missing, we assumed pots to be
lledwith substrate up to the rim.
For each species or genotype in a given experiment, we
determined the biomass at the last harvest and separated this
variable inbiomass of leaves, stems, roots and reproductive mass as
far as data were provided. To capture the importance of pot size
for growth wecalculated for each experiment the proportional
increase in total plant dry mass relative to the proportional
increase in pot size, bycalculating the slope of the lines that
were tted through the observed log-transformed plant masses and pot
volumes, separately foreach species in each experiment. Because
experiments differed in the range of pot size used, we scaled these
slopes in such a way thatthe number reects the percent change in
biomass (or another variable) given a doubling in pot size (see
Appendix 3 for more details).For a more detailed analysis,
log-transformed data from experiments which included three or more
pot sizes were also tted with asaturating equation, as described in
Appendix 3 and calculated with the nls procedure in R (R
Development Core Team 2011).
Appendix 3
Let V1 and V2 be pot volume 1 and 2 and B1 and B2 the total
plant dry biomass that is observed at the respective pot
volumes:because proportional differences are the focus of interest,
we calculate the slope of the line that connects these points
as:
S logB2 logB1logV 2 logV 1 ; 1
Suppose we want to know the fraction f by which plant mass
increases when pot size doubles. Then
B2 f B1; 2and
V 2 2 V 1: 3Assuming a common slope s over the whole trajectory
of pot masses considered, this results then in
S log f B1 logB1log2 V 1 logV 1
logf B1B1 log2V 1V 1
log f log2 ; 4
and, after rearrangement
f 10slog2 loglog2S 2S : 5The same procedure applies if slope s
is determined by linear regression through more than two points.To
relate the slope of the line to the observed values of total plant
mass per unit pot volume (BVR), we took the following approach:
In cases where an experiment consisted of two pot sizes, a
linear regression as above was calculated and the resulting slope
attributedto both points. In case an experiment consisted of more
than two pot sizes and a second order polynomial showed no
signicantsaturating trend, we tted a straight line through all
points. Otherwise, a saturating curve was tted through the points,
of the form
B a P bP c ; 6
with the constraint that a, b and c should be positive. The
slope of the line at each pot size is then given by the
derivative:
S ac bV c : 7
After the slope and f were calculated for each pot volume in
each experiment, data were binned in ve categories of BVR.
Pot size matters Functional Plant Biology K
-
Appendix 4.
We measured the root distribution of Hordeum vulgare (barley)
and Beta vulgaris (sugar beet), grown in a glasshouse in
cylindricalcontainers with a volume of 1.3 L (length 26 cm, inner
diameter 8 cm). Measurements were done by non-destructive imaging
of rootsusing nuclear magnetic resonance imaging (MRI) as described
extensively by Jahnke et al. (2009). This method is able to detect
rootswith a diameter down to 300mM, which implies that ne roots go
unnoticed. Image segmentation of the root system was done
bythresholding the image and removing the stem and the sugar beet,
with the RegionGrowingMacro module in MeVisLab (ver.
2.2.1;MeVisLab, Bremen, Germany). After segmentation, the pixel
values were integrated for the inner and outer regions of the soil
core anddivided by the integral over the whole pot.
L Functional Plant Biology H. Poorter et al.
www.publish.csiro.au/journals/fpb