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Plant Physiol. (1990) 94, 1301-1307 Received for publication April 18, 1990
0032-0889/90/94/1301 /07/$01 .00/0 Accepted July 17, 1990
High Light-induced Reduction and Low Light-Enhanced
Recovery of Photon Yield in Triazine-Resistant
Brassica napus L.
Jonathan J. Hart' and Alan Stemler*
Department of Botany, University of California, Davis, California 95616
ABSTRACT our laboratory, however, revealed that resistant plants grown
Triazine-resistant and -susceptible Brassica napus L. plants under low PFD do not exhibit the decreased photon yield
grown under low photon flux density (PFD) have previously been seen in plants grown under moderate to high PFD conditions
shown to exhibit a similar photon yield. In contrast, high PFD- (15). A smaller difference in growth is also seen in resistant
grown resistant plants have a lower photon yield than high PFD- and susceptible plants grown under low PFD. (JJ Hart, un-
grown susceptible plants (JJ Hart, A Stemler [1990] Plant Physiol published data; 1). These observations suggest that reduced
94: 1295-1300). In this work we tested the hypothesis that high photon yield and growth in triazine-resistant plants is a con-
PFD can induce a differential decrease in photon yield in low sequence of exposure to moderate to high PFD and not simply
PFD-grown plants. We measured photon yield, variable fluores- a result of slowed Qa to Qb electron transfer per se.
cence/maximum fluorescence, and 02 flash yield in low PFD- This work was initiated to test the hypothesis that high PFD
grown resistant and susceptible leaf discs before and after ex- light causes the differential reduction in photon yield observed
posure to high PFD exposure. The results demonstrated that high
PFD exposure results in a greater decrease in photosystem 11 in resistant plants. We exposed low PFD-grown B. napus leaf
(PSII) activity in resistant plants. Characteristics of recovery and discs to high PFD and measured photon yield and other
other evidence suggest that the differential decrease in PSII photosynthetic traits. We found that exposure to high PFD
efficiency in resistant leaf discs is caused by photoinhibitory induces a differential decrease in photon yield in resistant
damage. We propose that the differential reduction in photon plants. We also observed that the characteristics of recovery
yield and photosynthesis often observed in resistant plants is the from decreased photon yield are consistent with the hypoth-
result of increased sensitivity to photoinhibition. esis that resistant plants are more sensitive to photoinhibition
of PSII than susceptible plants.
MATERIALS AND METHODS
The development of triazine resistance in crop plants is an Plant Material
attractive goal because of the potential benefits it offers for
weed control. The loss of vigor that accompanies the resist- Plants used in these experiments were grown from seeds
ance trait, however, is a serious drawback. Studies with nearly produced by reciprocal crossing of individuals of the com-
isogenic resistant and susceptible lines of Brassica napus L. mercially available Brassica napus L. varieties 'Regent' and
revealed that resistant plants grew more slowly (JJ Hart, 'Triton' (15). All plants were grown in either a high or low
unpublished data; 5) and had a lower photon yield (15, 18) light intensity growth chamber as described earlier (15).
than susceptible plants. Development of productive resistant
crop varieties will depend on our understanding of the molec- High PFD Treatment
ular mechanism that brings about the loss of photosynthetic
performance and growth in plants with the resistance trait. A 10 cm2 disc was cut from a leaf grown in the low PFD
Jursinic and Pearcy (18) presented evidence that decreased growth chamber. The disc was floated upside down on 25°C
photosynthesis in resistant plants could be due to the slow water in a glass Petri dish. A floating foam pad with a hole in
Qa2 to Qb electron transfer that results from the resistance the center surrounded the disc and held it in position. The
alteration in the Dl protein of PSII. Recent experiments in edges of the foam in contact with the leaf disc were water-
saturated to allow a path for water flow from the bulk water
'Present address: Department of Soil, Crop and Atmospheric into the cut ends of xylem cells at the periphery of the disc.
Science, Cornell University, Ithaca, NY 14853. Leaf discs remained fully turgid even after the highest PFD
2 Abbreviations:Qa, primary quinone electron acceptor of photo-
treatments. Light was provided by a Leitz projector fitted with
system II; Qb, secondary quinone electron acceptor of photosystem
II; Fm, maximum Chl fluorescence induced by a saturating pulse of a 250W Osram HLX Xenophot lamp. Light was reflected
light following incubation in the dark; Fo, Chl fluorescence induced upward by a mirror onto the adaxial surface of the leaf disc
by a weak measuring light; F,, the difference between Fm and F.; in contact with the water. PFD at the surface of the leaf disc
LHCII, light harvesting complex of photosystem II; PFD, photon flux was adjusted to 2000 Mmol m-2 s-'. All subsequent measure-
density. ments of photon yield, 02 flash yield and Chl a fluorescence
1301
~T
1 302 HART AND STEMLER Plant Physiol. Vol. 94,1990
0.11 solution. To generate a signal, the measuring beam at 1.6 kHz
was applied to the leaf disc followed by an actinic light of
0.10
0
about 60 umol m-2 s-'. The signal was recorded on a Tektro-
0
s nix 2230 Digital Storage Oscilloscope and transferred to a
1 10.
-J 0.09 Zenith microcomputer for processing.
z
0 0.08 I °~-------------_ o Recovery Conditions
0
0- 0.07~
0-0 Susceptible, To monitor recovery of photon yield, leaf discs were placed
0---0 Resistant
in the oxygen electrode chamber immediately following high
PFD treatment. Recovery took place under the conditions of
0.06 photon yield measurements, i.e. 5% C02; alternating light/
0 200 400 600 800 1000 1200 1400 dark cycles with various low light intensities; 28°C; 100% RH.
GROWrH PFD (,Lrmol M-2 s-1) Examination of leaf discs after removal from the electrode
chamber revealed no wilting or other visible damage. To
Figure 1. Photon yield of triazine-resistant and triazine-susceptible measure Fv/Fm recovery after high PFD treatment, the Petri
B. napus leaves grown in growth chambers at four levels of PFD. dish containing the leaf disc was simply repositioned over the
Measurements were replicated four times for each leaf disc over the fluorometer fiber optic probe. Recovery occurred under the
course of 2 h. Each data points represents the mean of two leaf
discs. Error bars represent ± 1 SE. Some error bars do not extend conditions of Fv/Fm measurement, i.e. floating upside down
outside data points. on water at room temperature (25°C); air; periodic saturating
light pulses. Immediately following high PFD treatment, discs
were left in darkness for 10 min. Discs then either remained
were made on the same (adaxial) leaf surface that was exposed in darkness for the duration of the experiment or were exposed
to high PFD treatment. to low PFD (about 40 ,umol m-2 s-'). Discs treated with 5 and
45 min high PFD received 30 min of low light. Discs treated
Photon Yield Measurement with 90 min high PFD received continuous low light inter-
rupted by 10 min dark periods prior to saturating pulses
A leaf disc was placed in the chamber of a Hansatech LD2 applied to record Fm.
gas phase oxygen electrode. The protocol for photon yield
measurement was outlined previously (15). Each photon yield
determination required approximately 30 min. In the recov-
ery experiments, the procedure was performed four times over , 0.10o
the course of 2 h following high PFD treatment. Results are
plotted as the midpoint time of each determination. 0
0.09
-J " -~~~~~-
L>' 0.08
Oxygen Flash Yield Measurement z 0.07
O I-- i
After high PFD treatment, the leaf disc was transferred to 0 *0* Susceptible
the oxygen electrode. The procedure and equipment used in I 0.06
CL --- -o Resistant
measuring oxygen flash yield were as described previously 0.05
(15).
0.8
Fluorescence Measurements 0.7
These measurements were made with a pulse amplitude N
UE Q..
0.6
modulated fluorescence measuring system (H. Walz, Effel-
trich, FRG) (29). The protocol for determining Fv/Fm values 0.5
was described previously (15). For the DCMU-induced fluo- 0.4
0-* Susceptible
rescence induction experiment, two leaf discs were cut from 0. - - O Resistant
m It I I I I
a single leaf and floated upside down on 25°C water. One disc U.J3 I
0 10 20 30 40 50 60 70 80 90 100
was exposed to low PFD (100 ,umol m-2 s-') and the other to
high PFD (2000 ,umol m-2 s-') PFD for 3 h. A saturating HIGH PFD PRETREATMENT (min)
pulse of light was then applied to record Fm in each disc. Both
discs were then transferred to a solution of 4 x 10-4 M DCMU. Figure 2. Time course of change in photon yield and Fv/Fm of low
With the abaxial side of the leaf disk exposed to air and the PFD-grown triazine-resistant and -susceptible B. napus leaf discs
following exposure to a PFD of 2000 Amol m-2 s-1 at 25°C. Photon
foam pad providing a path for movement of solution, condi- yield measurements were replicated four times for each leaf disc over
tions were favorable for transpiration to assist in drawing the course of 2 h. For photon yield, each data point represents the
solution into the interior of the disk. Complete inhibition as mean of two determinations. For Fv/Fm each data point represents
indicated by rapid rise to constant maximal fluorescence was the mean of three determinations. Error bars represent ± 1 SE. Some
noted about 1 h after introduction of the leaf disc to DCMU error bars do not extend outside data points.
PHOTON YIELD LOSS AND RECOVERY IN TRIAZINE-RESISTANT BRASSICA 1 303
Table I. Percent Increase in Fo and Percent Decrease in Fm
following High Light Treatment
Leaf discs of triazine-resistant and -susceptible B. napus plants
grown in a low PFD growth chamber were exposed to high PFD Susceptible
(2000 Amol m-2 s-') for the indicated length of time. Values represent
means and standard deviations of at least three determinations.
Exposure F. (% increase) Fm (% decrease) C,)
Time Susceptible Resistant Susceptible Resistant
min
°I 11 Susceptible
5 4.7 ± 2.2 8.5 ± 2.0 20.9 ± 1.6 24.7 ± 1.7
10 3.7 ± 1.3 7.1 ± 2.5 30.9 ± 2.6 34.7 ± 3.2
20 5.6 ± 2.0 8.6 ± 3.0 34.6 ± 5.7 41.1 ± 2.2
45 9.4 ± 1.0 13.0 ± 2.2 44.2 ± 4.4 49.6 ± 3.0
90 10.8 ± 4.7 20.6 ± 3.4 52.9 ± 3.5 55.5 ± 1.4 tY
X < acL ont ~~~Resistant |
RESULTS Ipulm ont pc.o ac on
High PFD-Induced Decrease in Photosynthetic Efficiency
0 10 20 30 40 50
Resistant plants grown under low PFD (about 100 Amol
m-2 s-') had a photon yield nearly equivalent to susceptible Time (sec)
plants (Fig. 1). At higher growth PFD (450 ,umol m-2 s-' and
above) photon yield decreased in the resistant variety to a Figure 4. Fluorescence induction transients of DCMU-treated tria-
greater extent than in the susceptible variety (Fig. 1). Plants zine-resistant and -susceptible B. napus leaf discs exposed to 3 h of
low (A) and high (B) PFD. Discs were incubated on 4 x 10-4 M DCMU
grown under low PFD and then exposed to various durations
solution for 1 h. Fo was generated by tuming on the weak pulsed
of high PFD showed a decrease in photon yield (Fig. 2). measuring beam. Transients were produced by exposure of leaf discs
Photon yield was reduced to a greater extent in resistant plants to actinic light of about 60 limol m-2 s-'.
than in susceptible plants following exposure to high PFD.
F- increased and F decreased in both varieties as exnAosure
time t(o high PFD increased (Table I). The rise in F. in the varieties agrees well with the decrease in Fv/Fm (Fig. 2).
resistaint variety was nearly twice that of the susceptible variety Correlation between photon yield and Fv/Fm has been previ-
while the decrease in Fm was slightly greater in the resistant ously discussed (1 5). Because of the relative ease of measure-
variety{. High PFD also induced a decrease in Fv/Fm in plants ment, Fv/Fm was used as an indicator of photon yield in the
grown under low PFD (Fig. 2). The decrease was more pro- recovery experiments described below.
nounc ed in resistant plants than in susceptible plants. Oxygen flash yield was affected by high PFD exposure in a
The decrease in photon yield in resistant and susceptible manner similar to photon yield and Fv/Fm. Low PFD-grown
plants experienced a decrease in oxygen flash yield after
exposure to high PFD, with the resistant variety showing
greater sensitivity (Fig. 3).
100 Because the high PFD exposures described above resulted
2 in relatively small decreases in photon yield and Fv/Fm, we
C
0
" 1 E tested the response to prolonged high PFD exposure. Leaf
%
'' - 1 1 |Jdiscs were given 3 h of high PFD, then fluorescence transients
of DCMU-treated leaf discs were recorded. Induction tran-
-J
w
5::
sients of DCMU-treated material can be diagnostic for the
I 70 cause of photon yield reduction. DCMU-treated discs exhib-
-_
_ ___ T ited the induction transients seen in Figure 4. F. resulted from
N 60 ,
*- SUSCEPTIBLE turning on the low PFD pulsed measuring light and Fm was
0 0- --0 RESISTANT produced by the white actinic light. In the absence of high
50
PFD treatment, F. and Fm were higher in the resistant variety
0 20 40 60 80 100 (Fig. 4A). After 3 h of high PFD, leaf discs exhibited the
fluorescence traces seen in Figure 4B. Fm decreased in both
HIGH PFD PRETREATMENT (min) susceptible and resistant leaf discs. F. increased in both resist-
Figure:3. Time course of change in 02 flash yield of low PFD-grown ant and susceptible discs but only slightly in the resistant disc.
triazine--resistant and -suscentible RB 11C9tLJQ IqUC,I uI;O%; fnIInwinn e-
14A -11 g IL -4A
WL W II-,-I q-, ,KP4 ;w MA 7Mnanus leaf disr.s IVII%.VVIV IWyV;-
LJs
. Fv/Fm decreased to a greater extent in resistant discs.
posure to a PFD of 2000 ,gmol m-2 s-' at 250C. Values for untreated Recovery of Photon Yield following High PFD Treatment
control leaf discs are found in Hart and Stemler (1990). Each data
point represents the mean of three determinations. Error bars repre- Photon yield of susceptible plants decreased only slightly
sent ± 1 SD and do not extend outside all data points. after exposure to 20 min of high PFD and recovered fully
1304 HART AND STEMLER Plant Physiol. Vol. 94, 1990
-
0.10 -
0
0
20
0.82 Da*ark
Low PFD/dark 7
JIN ,
I''
0.09 0.78
E f~~~~
r .O 4-O------Q-~~~~~~~0---------
L-J 450
90 .F LL 0.74 S~~~~ ;M0*
0.08 20 °
z ,,-0 -
1
l.
0 0.70
0 45 °" ,v
I Susceptible
Resistant
n 0.07 0.66 -Dark
*-@ Susceptible ..............
30
go °' O----o Resistant
min low PFD
0.62
0.06 0 20 40 60 80 100
0 30 60 90 120
RECOVERY TIME (min)
RECOVERY TIME (min) Figure 6. Time course of recovery of Fv/Fm in triazine-resistant and
Figure 5. Time course of recovery of photon yield in triazine-resistant -susceptible B. napus leaf discs following 5 min of exposure to high
and -susceptible B. napus leaf disks following various exposure times PFD (2000 Amol m-2 s-1) at 280C. Following the 10 min dark period,
to high PFD (2000 umol m-2 s-1) at 280C. Numbers preceding each discs were either exposed to 30 min of low PFD (40 ,mol m-2 s-1) or
curve represent exposure time in min of each leaf disc prior to initiation remained in darkness.
of photon yield determination.
initial increase of Fv/Fm during the 10 min dark period
after 105 min (Fig. 5). Susceptible leaf discs exposed to 45 following high PFD treatment. A slow increase began several
and 90 min of high PFD showed a greater initial drop but min into the dark period (Fig. 8). The resistant leaf disc
recovered to nearly pretreatment level. Photon yield of resist- treated with low PFD recovered faster than one maintained
ant leaf discs decreased to a greater extent and showed less in darkness.
recovery than similarly treated susceptible leaf discs (Fig. 5).
Figure 6 illustrates recovery of Fv/Fm following 5 min of DISCUSSION
high PFD (2000 ,umol m-2 s-'). In both resistant and suscep- Figure 1 demonstrates that photon yield in triazine-resistant
tible varieties, there was a rapid recovery of Fv/Fm during the Brassica napus was dependent on the level of PFD during
first 10 min in darkness (Fig. 6). In leaf discs maintained in growth. Resistant plants grown under PFD as low as 400 ,mol
darkness, this rapid increase was followed by a constant level m-2 s-' had a significant reduction in photon yield. In pub-
of Fv/Fm. Leaf discs of both susceptible and resistant varieties lished reports of decreased photon yield in resistant plants,
exposed to 30 min of low PFD following the 10 min dark
period showed a further increase in Fv/Fm (Fig. 6). Fv/Fm
remained lower in resistant discs. However, the increase in
Fv/Fm in the resistant disc following the low PFD treatment -Dark Low PFD/dark Dr
Dark
0.80
was slightly higher than the increase in the susceptible disc.
Recovery of Fv/Fm following 45 min of high PFD showed 0.75
a slightly different pattern (Fig. 7). In both resistant and
susceptible leaf discs maintained in the dark, the rapid in- E
0.70 k
crease during the first 10 min was followed by a slow rise. 11
The amplitude of the initial fast rise was larger in susceptible 0.65 F
discs. In resistant discs, the low PFD-enhanced increase was
greater than in susceptible discs and originated during the low 0.60
PFD exposure. The final steady state level of Fv/Fm remained
lower in resistant as compared with susceptible leaf discs. 0.55
Following 90 min of high PFD, Fv/Fm of susceptible leaf
discs decreased about 40% (Fig. 8). They then showed the 0.50
same rapid initial increase in Fv/Fm as seen in discs exposed 0 20 40 60 80 100
to shorter durations of high PFD (cf. Figs. 6, 7, and 8). Fv/Fm
of susceptible discs maintained in darkness recovered at a RECOVERY TIME (min)
slower rate than discs exposed to low PFD during the recovery Figure 7. Time course of recovery of Fv/Fm in triazine-resistant and
period. -susceptible B. napus leaf discs following 45 min of exposure to high
Fv/Fm of resistant leaf discs also decreased about 40% after PFD (2000 Amol m-2 s-') at 280C. Following the 10 min dark period,
exposure to 90 min of high PFD (Fig. 8). In contrast to discs were either exposed to 30 min of low PFD (40 ,mol m-2 s-1) or
susceptible leaf discs, resistant discs did not exhibit the rapid remained in darkness.
PHOTON YIELD LOSS AND RECOVERY IN TRIAZINE-RESISTANT BRASSICA 1 305
0.85 ____
centers were transformed to fluorescence quenchers (4, 8, 22).
O Dark Low light / dark The greater reduction in variable fluorescence in resistant
,. 0 discs again suggests a greater loss of active PSII.
The collective responses of photon yield, Fv/Fm, 02 flash
0.75 yield and induction transients of DCMU-treated leaf discs
following high PFD treatment strongly suggest that PSII cen-
ters were rendered inactive. Several mechanisms can account
E~~~~ for loss of photochemical function in PSII. Transfer of ab-
0.65 L sorbed excitation energy away from PSII to PSI in a state I to
state II transition can cause a decrease in photon yield of 02
evolution (12). Nonradiative dissipation of excitation energy
~~ ~ ~ ~ ~ 0 Susceptible within the pigment bed can lower Fv/Fm and photon yield (9).
0.55 0.55 Photoinhibitory damage to PSII caused by excessive light can
a Resistant
L ¢ ~~~~~~Dark also bring about a decrease in photon yield (26) and Fv/Fm
(6, 14).
State I to state II transition is probably not the mechanism
0.45 responsible for reduced photon yield in the leaves measured
1 10 100 1000 in these experiments. The half-time of recovery for state
RECOVERY TIME (min) transitions is on the order of 5 min in barley leaves (27) and
in leaves of triazine-resistant and -susceptible lines of B. napus
Figure 8. Time course of recovery of Fv/Fm in triazine-resistant and similar to those used here (P Jursinic, personal communica-
-susceptible B. napus leaf discs following 90 min of exposure to high tion). The time needed for recovery to pre-high PFD treat-
PFD (2000 ,Amol m-2 s-1) at 280C. Following the 10 min dark period, ment levels of photon yield in our experiments was a mini-
discs were either exposed to low PFD (40 Mmol m-2 s-1) or remained mum of 100 min under the most favorable conditions (Fig.
in darkness. Leaf discs exposed to low PFD were allowed to incubate 5). Perhaps the most convincing evidence ruling out state
for 10 min in the dark prior to measurement. transitions is our 02 flash yield data. State transitions would
not cause a decrease in 02 flash yield. With saturating flashes,
every active PSI and PSII center will turn over regardless of
growth PFD (where specified) was higher than 400 umol m-2 the arrangement of antenna LHC. It is therefore unlikely that
s-' (17, 18, 25). The correlation between growth PFD and state transitions contributed much to the long-term decrease
photon yield in resistant plants suggests that light was involved in photon yield in our B. napus leaves.
in the photon yield depression. The results shown in Figure 2 Nonradiative dissipation can probably also be ruled out as
indicate that exposure of low PFD-grown resistant and sus- the cause of the long-term decrease in photon yield and F,/
ceptible plants to high PFD caused a decrease in photon yield. Fm observed in our leaf material. The recovery kinetics of
Clearly, resistant plants are more sensitive to light exposure photon yield (Fig. 5) and Fv/Fm (Figs. 6, 7, and 8) are
than susceptible plants. The results also indicate that the lower inconsistent with previous reports of recovery attributed to
photon yield reported many times in resistant plants grown radiationless dissipation. Recovery half-times of 30 min in
under moderate to high PFD conditions may well represent soybean (10) and 100 min in cotton (28) contrast with the
change caused by the light absorption itself. more than 20 h required for recovery of Fv/Fm in our high
The decrease in photon yield following exposure to high PFD-exposed leaf discs (Fig. 8).
PFD and the parallel response of Fv/Fm (Fig. 2) suggest a The increase in Fo following high PFD treatment observed
reduction in the number of active PSII centers. This is sup- in our leaf discs (Table I) is also inconsistent with radiationless
ported by the 02 flash yield response to high PFD exposure dissipation as the cause of decreased photon yield. The model
(Fig. 3). 02 flash yield has been used to measure the relative of Kitajima and Butler (20) predicts a decrease in F. as the
number of active PSII centers in algae ( 11) and more recently result of an increase in the rate constant of nonradiative
in leaf discs (7, 18). It should be pointed out that even before dissipation. Nonradiative dissipation following high PFD
high PFD exposure, resistant leaf discs had a lower 02 flash treatment has been experimentally correlated with a decrease
yield (15), probably due to incomplete recovery between in Fo (28).
flashes of some PSII centers (18). Because the ordinate of Finally, our 02 flash yield data argue against nonradiative
Figure 3 represents percentage of pre-high PFD treatment it dissipation as the cause of reduced photon yield. As pointed
still reveals a differential reduction in 02 flash yield in resistant out above, saturating flashes will turn over every functioning
leaf discs. The pattern of decrease of 02 flash yield in resistant reaction center regardless of energy diversion in the pigment
and susceptible leaf discs was similar to that of both photon system. The loss of 02 flash yield observed in our leaf material
yield and Fv/Fm, strongly suggesting a differential decrease in following high PFD treatment (Fig. 3) cannot be explained
active PSII complexes in resistant leaf discs. by an increase in nonradiative dissipation.
The change in fluorescence induction of leafdiscs infiltrated The relatively long recovery time following high PFD treat-
with DCMU following high PFD treatment (Fig. 4) is also ment (Fig. 8) is consistent with photoinhibitory damage as
consistent with loss of active PSII. Reduction in variable the cause of reduced photon yield. Recovery time in darkness
fluorescence in thylakoids similar to that exhibited by our leaf requiring hours has been reported in photoinhibited leaves (6,
discs in Figure 4 has been interpreted as evidence that PSII 13, 14, 27). Exposure of leaves to low PFD following photo-
1 306 HART AND STEMLER Plant Physiol. Vol. 94, 1990
inhibitory treatment has been shown to speed photon yield resistant discs (Figs. 6, 7, and 8) suggests that the mechanism
recovery (13, 14). Faster recovery of Fv/Fm was observed in that causes the relaxation was rendered inactive by high PFD
our low PFD-exposed discs (Figs. 6, 7, and 8). exposure. Again, clarification of the nature of this component
The increase in Fo seen here in B. napus leaf discs following will require additional experimentation.
high PFD-exposure (Table I) is another indication that pho- In this work, we have demonstrated that low PFD-grown
toinhibition was the primary factor in causing reduction of resistant B. napus plants experience a differential decrease in
photon yield. An increase in F0 following photoinhibitory efficiency of PSII following high PFD exposure. Based on a
treatment is predicted by the model of Kitajima and Butler number of criteria, we propose that the decrease is due to
(20) and has been demonstrated experimentally (14, 22). The greater sensitivity to photoinhibition in resistant plants. We
fluorescence induction traces of leaves infiltrated with DCMU suggest that the lower photon yield and diminished photosyn-
shown in Figure 4 are also consistent with photoinhibitory thetic capacity often observed in resistant plants are caused
damage. Similar changes in induction transients observed in by secondary effects of the slow Qa to Qb electron transfer
thylakoids following high PFD treatment were attributed to that results from the resistance mutation.
photoinhibition (4, 8, 22).
The reduction in 02 flash yield following high PFD expo- ACKNOWLEDGMENTS
sure in low light-grown B. napus (Fig. 3) is perhaps the most
direct evidence of photoinhibitory damage to PSII. The We thank Drs. Robert Pearcy and Steven Theg for valuable sug-
greater degree of apparent photoinhibitory damage seen in gestions and Dr. Rachel Scarth for providing B. napus seeds.
the resistant line is consistent with a recent report of increased
high light sensitivity in atrazine-resistant mutants of Synech- LITERATURE CITED
ocystis ( 1 9). 1. Ahrens WH, Stoller EW (1983) Competition, growth rate, and
The basis for increased sensitivity may lie in the alteration CO2 fixation in triazine-susceptible and -resistant smooth pig-
in the Dl protein that confers resistance. Electron transfer weed. (Amaranthus hybridus). Weed Science 31: 438-444
2. Arntz B, Trebst A (1986) On the role of the QB protein of PS II
from Qa to Qb has been shown to be slower in resistant B. in photoinhibition. FEBS Lett 194: 43-49
napus (15, 18). It is possible that slower electron transfer 3. Asada K, Takahashi M (1987) Production and scavenging of
results in a longer lifetime for Qa and Qb in the reduced active oxygen in photosynthesis. In DJ Kyle, CB Osmond, CJ
semiquinone state. Interaction of semiquinones with molec- Arntzen, eds, Photoinhibition, Topics in Photosynthesis, Vol
ular oxygen can lead to reactive species of 02 which can cause 9, Elsevier, Amsterdam, pp 227-287
4. Barenyi B, Krause GH (1985) Inhibition of photosynthetic re-
damage to membrane components (3). Kyle (23) argued that actions by light. Planta 163: 218-226
the basis for photoinhibition involves damage to the Dl 5. Beversdorf WD, Hume DJ, Donnelly-Vanderloo MJ (1988) Ag-
protein caused by oxygen radicals produced by interaction of ronomic performance of triazine-resistant and susceptible re-
Qa or Qb with 02- If the slow electron transfer in resistant ciprocal spring canola hybrids. Crop Sci 28: 932-934
6. Bjorkman 0, Demmig B (1987) Photon yield of 02 evolution
plants increases the lifetime of either semiquinone, there may and chlorophyll fluorescence characteristics at 77 K among
be a greater opportunity for production of reactive 02 species vascular plants of diverse origins. Planta 170: 489-504
and potential for photoinhibitory damage. Other workers 7. Chow WS, Hope AB, Anderson JM (1989) Oxygen per flash
argue that the primary site of photoinhibitory damage is not from leaf disks quantifies photosystem II. Biochim Biophys
the Dl protein but rather the reaction center itself (2, 4, 8). Acta 973: 105-108
8. Cleland RE, Melis A, Neale PJ (1986) Mechanism of photo-
Arntz and Trebst (2) concluded that both Qa and Qb can inhibition: photochemical reaction center inactivation in sys-
induce photoinhibition, although they did not offer a mech- tem II of chloroplasts. Photosynth Res 9: 79-88
anism for the role of these quinones in loss of PSII function. 9. Demmig B, Bjorkman 0 (1987) Comparison of the effect of
Van Mieghem et al. (30) recently proposed that the primary excessive light on chlorophyll fluorescence (77K) and photon
yield of 02 evolution in leaves of higher plants. Planta 171:
lesion of photoinhibition involves irreversible double reduc- 171-184
tion of Qa. Whether the primary damage occurs at the reaction 10. Demmig B, Cleland RE, Bjorkman 0 (1987) Photoinhibition,
center or the Dl protein, it appears that the slowed electron 77K chlorophyll fluorescence quenching and phosphorylation
flow from Qa to Qb in triazine-resistant plants contributes to of the light-harvesting chlorophyll-protein complex of photo-
photoinhibitory damage. system II in soybean leaves. Planta 172: 378-385
11. Emerson R, Arnold W (1932) A separation of the reactions in
Recovery from photoinhibition has been shown to involve photosynthesis by means of intermittent light. J Gen Physiol
removal and replacement of the Dl protein in thylakoid 15: 391-420
membranes (24). The slopes of plots of recovery of FV/Fm 12. Fork DC, Satoh K (1986) The control by state transitions of the
(Fig. 8) indicate a faster rate of recovery in resistant leaf discs. distribution of excitation energy in photosynthesis. Annu Rev
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