Biochemical Society Transactions ( 1 996) 24
EFFECT OF TRAMPLING ON PHOTOSYNTHESISAND
RESPIRATION IN PLANTAGO LANCEOLATA
John D Mills, Joanne Stevenson, Jane Nicholls and Tony Polwart,
Plant Biochemistry and Microbiology Group, Keele University,
Staffs, STS SBG, UK
Pluntugo lunceolutu is a perennial rosette herb commonly found in
grazed or mown grassland where it is relatively resistant to trampling
[ 1][2]. The exact forces produced by trampling are complex, and
hence the damage inflicted on plant tissues is difficult to reproduce in
the laboratory. However, it has long been known that photosynthesis
is rapidly inhibited when plants are subjected to the mechanical stress
induced by a dropping tamp [3]. Inhibition occurs within minutes of
sustaining damage, and is therefore unlikely to involve changes in
gene expression. Possible causes include 1) increased resistance to
CO, diffusion due to stomatal closure or increased mesophyll
resistance, 2) feedback inhibition on the Calvin Cycle due to altered
source-sink relationships, 3) inhibition of the primary reactions of
photosynthesis resulting from damage to thylakoid membranes.
We have investigated the latter possibility by measuring the activity
and coupling state of thylakoids prepared from plants subjected to
trampling. Trampling was simulated in the laboratory by dropping a
mechanical tamp on whole plants or detached leaves. Each blow
delivered 5.5J. As shown in Figure 1, whole leaf photosynthesis as
measured by infra-red gas analysis is rapidly inhibited in trampled
leaves. Slight recovery is observed over a time scale of 24h. Similar
results can be obtained using detached leaves of market spinach.
Figure 1.
E5ect of trampling and time after trampling on photosynthesis.
CO, exchange was carried out on attached leaves at a PPFD of 6OOpM rn-' s-! On
each of the 6 replicate pots, half of the leaves were given 8 tamps, the untrampled
leaves on the same pot being used as controls. Measurements were made as soon
as possible after trampling (15 minutes) and at 24 hours. Cuvette temperature
was controlled at 22"C-23"C.
Standard errors are shown. Trampling induces a significant decrease in
photosynthesis (F = 18.60;p<O.OOl). The recovery after 24 horn is not
significant.
B B l m
Using detached leaves, it was found that respiration was hardly
affected under conditions where photosynthesis was inhibited by up to
75%. Active thylakoids were therefore prepared from Pluntugo
lunceolutu by standard methods [4]. Table 1 shows the rates of
399s
Table 1.
The activity of thylakoids isolated from control or trampled leaves of Planlagc
lanceolata.
15g leaves were subjected to 0 (control) or 8 tamps (trampled) and thylakoids
isolated by polytron homogenisation in a medium containing 30 mM HEPES,
2 mM EDTA, 1mM MgCI,, 1 mM MnCI,, pH7.6 as described [4]. Coupled
and uncoupled electron transport were measured by oxygen uptake in a
medium containing 20 pg chlorophyll/ml, 30 mM Tricine, 5 mM MgCI,, 0.1
mM methyl viologen with or without 5 mM NH,CI. For ApH, NH,CI was
omitted and the medium contained 4 $4 9-aminoacridine [4]. Standard
errors are given (4 replicates).
electron transport
(pmoles O4mg chl.h)
coupled
uncoupled
control
trampled
134* 9
105 f 10
226k 51
165 20
*
3.62
coupled and uncoupled electron transport measured at pH 8.0 using
methyl viologen as electron acceptor, and the transthylakoid pH
gradient (ApH) that could be generated. The recorded activities from
control leaves are typical for thylakoids from traditional sources
such as spinach. Approximately 30% inhibition of electron transport
can be detected in thylakoids from trampled leaves, but this effect is
not large enough to account for the observed inhibition of
photosynthesis in whole leaves (62% after 24h), and the high ApH in
these thylakoids indicates that electron transport is still well coupled
to ATP synthesis. Inhibition of the primary photosynthetic apparatus
appears to account for about 50% of the decrease in photosynthesis.
The mechanism by which this occurs is currently under investigation,
and includes an effect on photosystem 11. We conclude that inhibition
of photosynthesis in leaves due to trampling is complex, but may
contain a large contribution from increased stomatal or mesophyll
resistance. Other preliminary measurements (not shown) made in this
laboratory implicate either mesophyll resistance or source-sink
feedback.
1. Speight, M.C.D. (1973). Outdoor recreation and its ecological
effects. Discussion papers in conservation 4. University College,
London.
2. Grime, J.P. ef ul(1988). Comparative Plant Ecology. Unwin
Hyman.
3. Jones, V. (1985). Physiological response of turfgrasses to
trampling pressure. Ph.D. thesis. Keele University.
4. Mills, J.D. (1986) in Photosynthesis: energy transduction: a
practical approach, eds. M.F. Hipkins, N.R. Baker, pp. 143-187,IRL
Press, Oxford