Plant Physiol. (1990) 94, 1295-13000032-0889/90/94/1 295/06/$01 .00/0

Received for publication April 18, 1990Accepted July 17, 1990

Similar Photosynthetic Performance in Low Light-GrownIsonuclear Triazine-Resistant and -Susceptible

Brassica napus L.

Jonathan J. Hart1 and Alan Stemler*Department of Botany, University of California, Davis, California 95616

ABSTRACT

Triazine-resistant plants grown under moderate to high photonflux density (PFD) conditions exhibit decreased photon yield,decreased light-saturated 02 evolution and slower growth thantriazine-susceptible plants. In this study we tested the hypothesisthat the comparable growth previously observed in resistant andsusceptible Brassica napus L. lines grown under low PFD wasaccompanied by comparable photon yield and light-saturated 02evolution. We measured photon yield, 02 flash yield, fluorescencedecay kinetics, fluorescence transient kinetics, and quenchingcomponents, Fv/Fm and light saturated 02 evolution in leaf disksof low PFD-grown triazine-resistant and susceptible B. napusisogenic lines. Results indicated that slow electron transfer fromthe primary to secondary quinone electron acceptors of photo-system 11 was still present in the resistant line but photon yieldand light-saturated 02 evolution were similar in the two B. napuslines. We conclude that the alteration in the DI protein thatconfers resistance does not necessarily cause decreased pho-tosynthefic performance. Decreased photon yield in resistantplants grown at high PFD is not a direct consequence of thealteration in Dl, but represents secondary damage.

Herbicide resistance has become an area of interest partlybecause ofthe potential for transferring the trait to crop plants.Among the likely advantages in weed control include in-creased crop-weed herbicide selectivity and elimination ofcarryover problems from slowly degrading herbicides in croprotations. Several crops including rapeseed (Brassica napusL.) (2), chinese cabbage (Brassica campestris L.) (25), andfoxtail millet (Setaria italica L.) (8) have already been devel-oped that are resistant to the photosynthesis-inhibiting tria-zines. Unfortunately, a significant reduction in yield accom-panies the resistance trait in most species studied. Field studiesof resistant rapeseed show decreased growth and crop yieldlosses (3, 10, 11).

Triazine resistance is brought about by a mutation in thechloroplast psbA gene which encodes the Dl protein of PSIIin thylakoid membranes (9). The Dl protein normally func-

Present address: Department of Soil, Crop and AtmosphericScience, Cornell University, Ithaca, NY 14853.

1295

tions to transfer electrons from the bound quinone Qa2 to anexchangeable plastoquinone, Qb (29). In susceptible plants,the Qb exchange site binds a triazine molecule which blockselectron transfer and ultimately causes plant death (26). Inresistant plants, the mutation causes substitution of a singleamino acid at or near the triazine binding site (13) whichgreatly reduces triazine binding (27). However, the bindingsite alteration also slows by 10-fold the normal electron trans-fer between Qa and Qb (5). Slow electron flow between Qa andQb has been suggested as the cause of the reduction in photonyield observed in resistant plants (14, 15, 19), and has beencited as the ultimate cause of decreased maximum photosyn-thesis in resistant plants ( 16).We have observed that under low PFD growth conditions,

triazine-resistant B. napus plants grow at a rate similar toplants of an isonuclear susceptible variety (our unpublisheddata), which suggests that photosynthesis in resistant plants isnot less efficient under low PFD growth conditions. This workwas undertaken to test the hypothesis that photosynthesis wasless suppressed in resistant B. napus when grown under lowPFD. Photon yield, oxygen flash yield, light-saturated 02evolution, fluorescence induction transient and Fv/Fm meas-urements were made on low PFD-grown plants and comparedwith an isonuclear susceptible variety. The results indicatethat Qa- to Qb electron transfer remains slower in low light-grown resistant plants, but that photon yield and light satu-rated 02 evolution are similar to those of susceptible plantsgrown in a similar environment. In the accompanying paper(12), we report on studies that investigated the effect of highPFD on PSII activity.

MATERIALS AND METHODS

Plant Material

Plants used in these experiments were grown from seedsproduced by a reciprocal cross of single individuals of the

2Abbreviations: Qa, primary quinone electron acceptor of photo-system II; ai, active ingredient; Fm, maximum Chl fluorescence in-duced by a saturating pulse of light following incubation in the dark;F., Chl fluorescence induced by a weak measuring light; F,, thedifference between Fm and F.; F,v, variable fluorescence induced byactinic light; F,(,), Chl fluorescence induced by a saturating pulse oflight during illumination; Qb, secondary quinone electron acceptor ofphotosystem II; qNp, quenching coefficient for nonphotochemicalfluorescence quenching; qp, quenching coefficient for photochemicalfluorescence quenching; PFD, photon flux density.

Plant Physiol. Vol. 94, 1990

triazine-resistant and -susceptible Brassica napus L. varieties'OAC Triton' and 'Regent' (2). Plants used in this study weregrown from seeds produced from the F2 generation of thereciprocal cross.

Seeds were planted in soil and germinated in a greenhouse.Three to 4 d old seedlings were moved to the appropriateenvironment within the greenhouse or transferred to a growthchamber. Greenhouse plants grown under full sunlight re-ceived a maximum incident PFD during midday of about2000 ,umol m-2 s-', as measured with a LICOR LI 185 photonflux meter. Shade-grown greenhouse plants were placed be-hind shade cloth where maximum PFD was about 150 ,molm-2 s-'. Greenhouse temperature was regulated by evapora-tive coolers. Temperatures varied from a maximum 30°Cduring the day to a nighttime minimum of about 14°C.For growth chamber grown plants, the photoperiod was 14

h light and 10 h dark and the temperature was 28°C duringthe light period and 16°C during the dark period. In the lowPFD growth chamber, light was furnished by a bank offluorescent lights supplemented with incandescent light. PFDat leaf height was 90 to 110Itmol m-2 s-'. Ambient relativehumidity in the growth chamber averaged 50%. Light in thehigh PFD growth chamber was provided by an array of metalhalide and mercury vapor lamps. Plants were arranged in thechamber so that PFD incident on the upper leaves was about1250 ,umol m-2 s-'. RH in the chamber was regulated at 75%.Greenhouse and growth chamber grown plants were watereddaily or as needed with half-strength Hoagland solution. Thefirst to fourth youngest fully expanded leaves were used in allexperiments.

Photon Yield

Photon yield measurements were made with a Hansatechmodel LD2 leaf disc oxygen electrode. Light was provided bya Leitz projector fitted with a 250W Osram HLX Xenophotlamp and attenuated with neutral density filters. PFD wasmeasured by placing a LICOR LI- 1 90SB quantum sensor atthe position of the leaf disk. The leaf disk chamber wasmaintained at 28°C by a circulating water bath.

Protocol for measurements was as follows. A 10 cm2 discwas cut from a leaf and placed in the electrode chamber. Thechamber was then flushed with 5% CO2. The electrode signalstabilized in about 12 min. During the stabilization periodthe disk was alternately left in darkness and exposed to lightof about 100 ,umol m-2 s-' PFD for periods of 90 s and 2min, respectively, to simulate light conditions present duringthe actual measurements.

After the signal had stabilized, the leaf disc was illuminatedfor 90 s, then allowed to remain in darkness for 60 s beforeagain being illuminated with a different PFD for 90 s. Toestablish the 02 evolution response, PFDs ranging from 10 to100 ,umol m-2 s- were applied. This procedure was performedfour times with randomly varied sequences of PFD levels.Leaf discs were never illuminated with more than 100 ,umol

-2 -Im s

After the final measurement was made, the center portionofthe leaf disc was cut out and placed in an integrating spherefor measurement of leaf absorptance. The same Leitz lightsource used for 02 evolution was used for absorptance meas-

urement. 02 rates were determined as the sum of evolutionin the light and uptake in the dark as described by Bjorkmanand Demmig (4).

Light-Saturated Photosynthetic 02 Evolution

A leaf disc was cut and placed in the same Hansatech LD2oxygen electrode that was used for photon yield measurement.The equipment and protocol were similar to those of photonyield determination with two exceptions. To prevent rapiddepletion ofCO2 in the chamber, a smaller (3.7 cm2) leaf discwas used. The Leitz light source was arranged to produce asaturating PFD of 1600 to 2000 umol m-2 s-'. Calculation of02 production rate was similar to that for photon yielddetermination.

Oxygen Flash Yield

Leaf discs were cut and placed in the electrode chamber asin the light saturated 02 evolution experiment. Saturatinglight flashes of 3 us duration were provided by an EG&Gmodel FX200 xenon strobe lamp operated at 1400 V with a5 ,uF capacitor. The flash lamp was fitted to a lens systemadjusted to focus the flashes at the plane of the leaf disc. Flashyield measurements were made at a flash frequency of 5 Hz.Flashing light was supplemented with far red light suppliedby a Sylvania 120W EKN Tungsten Halogen projector lampfitted with a Schott RG-9 filter and directed to the leaf diskwith a fiber optic light guide. Far red light was applied to keepPSI centers open to prevent their limiting noncyclic electronflow (7). Exposure of leaf discs to far red light alone producedno detectable 02 evolution. The protocol for measuring 02flash yield was similar to photon yield measurements. Flasheswere given for one min followed by 90 s of darkness. Theaverage for six flash/dark cycles was taken for each leaf disc.

Chi Determination

Chl concentration and Chl a/b in an 80% acetone extractwas measured by the method of Arnon (1).

Chi a Fluorescence Induction Transients

Chl a fluorescence measurements were made with a pulseamplitude modulated fluorescence measuring system (H.Walz, Effeltrich, FRG) (22). A low PFD (about 60 ,umol m-2s-') actinic light source produced by a Sylvania EKN projectorlamp and attenuated by passage through a fiber optic lightguide was connected to one of the four input fiber opticbundles. Another bundle carried a very high PFD (about 7000,umol m-2 s-') light produced by a similar lamp. A Corning4-96 filter was placed in the light path to remove red and farred light that interfered with the signal sensing system of thefluorometer.For fluorescence induction transients a disc was cut from a

leafand floated with its adaxial surface in contact with a smallvolume of room temperature (25°C) water in a glass Petridish. The end of the fiber optic bundle was positioned underthe Petri dish so that the distance from the end of the bundleto the adaxial surface was about 8 mm. The disc was main-tained in darkness for 15 min before any measurements were

1 296 HART AND STEMLER

PHOTOSYNTHESIS IN LOW PFD-GROWN TRIAZINE-RESISTANT BRASSICA

made to allow complete oxidation of Qa. To generate Fo, theweak modulated measuring signal was turned on at 1.6 kHz.The leaf disc was exposed to a one s pulse of the high PFDlight. The resulting fluorescence peak was designated Fm. Thedisc was then allowed to remain in darkness for 10 min. Theinduction transient was initiated by turning on the low PFDactinic light. Fluorescence induced during exposure to thislow PFD light was designated Fvar. Immediately following theinitial peak of fluorescence, a half-s high PFD pulse was givento obtain FV(S). Additional half-s pulses which generated F,(,)were given at varying intervals throughout the measurementperiod. Immediately before each high PFD pulse was giventhe measuring signal modulation was switched to 100 kHz toimprove signal to noise ratio and to improve time resolution.

Variable fluorescence (Fvar) was plotted as a function oftime after the start of actinic light. In addition, two fluores-cence quenching components designated qp and qNp werecalculated; qp represents photochemical quenching due to Qa-reoxidation and qNP is nonphotochemical quenching. Theseterms are equivalent to qQ and qE, respectively, describedby Schreiber et al. (23); 1 - qp therefore reflects therelative extent of Qa- reduction and is calculated as 1 - qp =Fvar/Fv(s) and 1 - qNp represents fluorescence not quenchedby nonphotochemical processes and is calculated as 1 -

qNP = Fv(s)/Fm.

Fluorescence Decay Measurements

Leafdiscs were prepared as described above for fluorescenceinduction transients. Following dark adaptation for 15 min,Fo was established by turning on the pulsed measuring beamat 100 kHz. The intensity ofthe measuring beam was adjustedlow enough to prevent an actinic effect from the measuringbeam itself. Fluorescence was excited by flashes generated bythe EG&G FX200 xenon flash lamp operated at 1300 V.Flashes were routed through one of the input fiber opticbundles of the PAM fluorometer after passing through a

Corning 4-96 filter. Flashes were given at a rate of 6 per min.The large fluorescence signal immediately following each flashoverloaded the measuring system and about 300 ,ts wererequired for the typical decay pattern to emerge. The fluores-cence signal was captured on a Tektronix 2230 Digital StorageOscilloscope. The digitized signal average from 10 decay traceswas transferred to a Zenith PC for processing.

Fv/Fm Measurement

Leaf discs were prepared and the fluorometer probe waspositioned as described above. After a 10 min dark adaptation,F0 was generated by turning on the pulsed measuring beamat 1.6 kHz. Fm was produced by exposing the disc to a 1 spulse of high PFD (7000 Mmol m-2 s-1) light that had passedthrough a Corning 4-96 filter. A separate experiment showedthat Fm produced by saturating light pulse was equal to thelevel of fluorescence produced by a disc treated with DCMU.For this measurement, Fv = Fm - F0.

RESULTS

Photon Yield

Susceptible Brassica napus plants grown under high PFDin the growth chamber had a higher photon yield (mol 02/

mol photon) than resistant plants grown under high PFD(Table I). Photon yield of resistant and susceptible varietiesgrown in the low PFD growth chamber was equivalent (TableI). Low PFD growth chamber-grown plants had a slightlyhigher photon yield than their shaded greenhouse-growncounterparts (Table I).

Photosynthetic Characteristics of Low PFD-Grown LeafDiscs

The rate of light-saturated 02 production, Chl concentra-tion, and leaf absorptance were the same in resistant andsusceptible leaf discs grown in low PFD light (Table II). Incontrast, several measurements did reveal differences betweenresistant and susceptible plants grown under low PFD light.Oxygen flash yield was 24% lower in resistant than susceptibleleaf discs (Table II). The Chl a/b ratio was lower in theresistant variety (Table II). Low PFD-grown resistant andsusceptible lines also exhibited differences in several fluores-cence characteristics. F0 and Fm were significantly (at 1%probability; Student's t test) higher in resistant leafdiscs (TableII). Conversely, Fv/Fm was about 2% lower (significant at0.1% probability; t test) in resistant leaf discs.

Figure 1 shows a typical fluorescence induction transientfor resistant and susceptible leaf discs taken from a low PFDgrowth chamber. Fvar and 1 - qp were consistently higher inresistant than susceptible leaf discs; 1 - qNp in the resistantleafdisc fluctuated relative to the susceptible disc, but becameequivalent to that of the susceptible disc after several minutes(Fig. 1). The relative level of 1 - qNp varied among samples,but at steady-state resistant and susceptible samples had equiv-alent values.Chl a fluorescence decay traces are shown in Figure 2. The

resistant variety had a slower decay, which is most obviousduring the first 8 ms after a flash and remained higher formore than 500 ms (data not shown).

DISCUSSION

The similar photon yield in low PFD-grown triazine-resist-ant and -susceptible Brassica napus observed here has notbeen previously reported. Other studies of photon yield intriazine-resistant plants were made with plants grown in rel-

Table I. Photon Yield Measured as Light-Dependent 02 Evolutionfrom B. napus Leaf Discs

Discs were taken from leaves of plants grown in the indicatedenvironment. Values represent means, standard deviations, and num-ber of replications.

Photon YieldGrowth Environment

Susceptible Resistant

mol 02/mol photonGrowth chamber

High PFD 0.095 ± 0.004 (4) 0.080 ± 0.002 (5)Low PFD 0.101 ± 0.001 (3) 0.099 ± 0.002 (3)

GreenhouseFull sun 0.094 ± 0.004 (2) 0.080 ± 0.001 (2)Shade 0.092 ± 0.002 (2) 0.092 ± 0.003 (6)

1 297

Plant Physiol. Vol. 94, 1990

Table II. Photosynthetic Characteristics of Leaf Discs Cut from Low PFD-grown B. napusFm was induced by irradiation with a one s high PFD pulse. Fv is Fm minus Fo. Light saturated 02 rate

is the rate of oxygen production from leaf discs with an incident PFD of at least 1600 Mmol m-2s-2 .

Values represent means and standard deviations with numbers of determinations in parentheses.

Measurement Susceptible Resistant

Light-saturated 02 rate (Amol 27.2 ± 6.5 (6) 24.7 ± 5.1 (8)02 m-2 S-1)

Chi (Mmol m-2) 426 ± 105 (38) 396 ± 81 (42)Leaf absorptance (%) 81.1 ± 1.2 (12) 79.9 ± 1.6 (14)02 flash yield (,mol 02 m-2 0.16 ± 0.02 (15) 0.12 ± 0.02 (16)

flash-')Chl a/b 2.89 ± 0.12 (38) 2.56 ± 0.07 (42)Fo 0.35 ± 0.03 (19) 0.45 ± 0.05 (20)Fm 2.10 ± 0.17 (19) 2.38 ± 0.28 (20)Fv/Fm 0.832 ± 0.006 (19) 0.812 ± 0.006 (20)

0.8

0.6

a

Ll0 0.4

0.2

0.00.8

a.CT

0.6

0.4

0.2

0.01.0

0.8zc 0.6 - ==

0.2 -

0.00 2 4 6 8 10

Time (min)Figure 1. Typical fluorescence transients of triazine-resistant and -

susceptible B. napus leaf discs taken from plants grown under lowPFD. Fvar represents the fluorescence trace induced by illuminationwith actinic light (PFD about 60 Amol m-2 s-'). 1 - qp representsfluorescence not quenched by photochemistry and 1 - qNp representsfluorescence not quenched by nonphotochemical processes.

atively high PFD (14-16, 19, 21). Resistant B. napus plantsgrown here in full sunlight in the greenhouse and in the highPFD growth chamber also exhibited a decrease in photonyield (Table I). We did not measure the PFD at whichreduction in photon yield begins, but we did observe a signif-icant (about 15%) lowering at a growth PFD as low as 400,umolm-2 S-'(12).The relative absence of photon yield reduction in low PFD-

grown resistant plants is supported by the similar Fv/Fm valuesseen in low PFD growth chamber-grown resistant and suscep-

1.0

0.8

0.6

0.40

0

0 2 4 6 8

0.81LUII

0 20 40 60 80

Time after flash (msec)

Figure 2. Fluorescence decay traces of triazine-resistant and -sus-ceptible B. napus leaf discs from plants grown under low PFD.Fluorescence was generated by exposing the leaf disc to a strongflash of white light. Decay traces were recorded on a digital storageoscilloscope and transferred to a PC for processing. Upper and lowerpanels are records of fluorescence decay traces of about 8 and 80ms duration, respectively.

1 298 HART AND STEMLER

PHOTOSYNTHESIS IN LOW PFD-GROWN TRIAZINE-RESISTANT BRASSICA

tible plants (TatButler (17) prediefficiency of PSIstrong correlatioat room temperphoton yield anmments. The correregression line e)of the good cornstatistically signiisuggests that a shave been preseras well.To explain thi

PFD-grown resisresistance is notconditions. Weand susceptible rha-' (field rate) (cation rates wereseen in any resi;expression of res

Analysis of theponents in Figutransfer occurs iIrelatively higherresistant leaf disquenching dueelectron transferical quenching, slevel of this fluoitible plants (Fig.The fluorescei

indicate slow QaSlower decay kinant leaf discs (1The kinetics of 1different from ti

0.12

0.101-8%s 0.08

bJ> 0.06z0

o 0.04

0.02

0.00 L

0.0

Figure 3. RelationFm of triazine-resistat room temperatLFv/Fm determined itreatment.

)le II). A model developed by Kitajima and similar B. napus isogenic lines. Decay in our resistant leaficts that Fv/Fm represents the photochemical discs is somewhat faster than theirs and the level of fluores-'I and Somersalo and Krause (24) reported a cence at 80 ms after flash is lower in both lines in ourn between photon yield and Fv/Fm measured experiments. The deviation in results is probably related to*ature. Figure 3 shows the relation between differences in measuring systems and protocols. Those authorsd room temperature Fv/Fm from our experi- recorded fluorescence decays of flashes given at two Hz. Theelation coefficient for the plot is high and the 10 s interval between flashes in our experiments probablyxtends close to the origin of the plot. Because allowed more complete reoxidation of the populations of Qa-elation between photon yield and Fv/Fm, the and plastoquinol, which may have in turn promoted fasterficant 2% lower Fv/Fm found in resistant lines fluorescence decay kinetics after a subsequent flash.,imilar small difference in photon yield may Further evidence that the resistance alteration causes slownt in low PFD growth chamber-grown plants Qa to Qb electron transfer in low PFD-grown plants comes

from the oxygen flash yield data. As seen in Table II, resistante absence of photon yield reduction in low leaf discs evolve 25% less 02 per flash than susceptible discs,tant plants, we first tested the possibility that indicating that at any flash there are fewer open PSII centers.expressed in plants grown under low PFD The lower rate in resistant discs is probably due to thesubjected several low PFD-grown resistant relatively incomplete reoxidation of Qa- that would occur atplants to a foliar application of up to 10 kg ai a flash rate of 5 Hz, as discussed by Jursinic and Pearcy ( 16).Df atrazine. All susceptible plants at all appli- We observed a lower Chl a/b in low PFD-grown resistant- killed within 10 d and no visible injury was plants (Table II). In studies using relatively high PFD-grownstant plant (data not shown), indicating full plants (6, 28), lower a/b was correlated with increased grana;istance at low PFD. stacking in resistant biotypes. We also observed a higher levelfluorescence transients and quenching com- of Fo and Fm in low PFD-grown resistant leaves with Chl

ire 1 suggests that slow Qa to Qb electron content similar to that in susceptible leaves (Table II). Robin-n resistant plants grown under low PFD. The son (20) also noted higher F. in resistant Chenopodium album.level of steady state fluorescence seen in Increased fluorescence levels in resistant leaves is consistent

scs can be accounted for by the decreased with a greater degree of grana stacking and larger amounts ofto Qa. reoxidation (Fig. 1). Slow Qa to Qb PSII light harvesting complex (LHC) (18). Lower Chl a/b andapparently has no effect on nonphotochem- higher F, and Fm levels suggest that increased grana stackingince there is no difference in the steady-state may be present in low PFD-grown resistant B. napus.rescence component in resistant and suscep- Decreased photochemical fluorescence quenching and1). slower fluorescence decay kinetics indicate that low PFDnce decay traces illustrated in Figure 2 also growth conditions do not alter the slow Qa to Qb electronto Qb electron transfer in the resistant line. transfer in resistant plants. However, there is a relative absenceietics have been reported previously in resist- of photon yield reduction in low PFD-grown resistant plants.6, 20) and in resistant chloroplasts (5, 20). This means that slow Qa- to Qb transfer is not directly linkedthe decays we recorded (Fig. 2) were slightly to photon yield reduction. To explain similar growth ratesiose reported by Jursinic and Pearcy (16) in and photon yield in the two varieties, the simplest hypothesis

is that Qa to Qb electron transfer is not the rate limiting stepin 02 evolution under low PFD conditions. This interpreta-tion requires that photons be absorbed and PSII reactioncenters turned over at a rate slower than the rate of transferfrom Q. to Qb. It would follow that at higher levels of absorbedPFD, slow Qa to Qb transfer should begin to limit 02 evolutionin the resistant variety. We do not, however, see a differencein light-saturated photosynthesis in resistant as compared withnormal leaf disks from plants grown under low PFD (Table

r= .938 II). This suggests that in low PFD-grown plants, factors other/O -.0004 .130 (Fv/Fm)

than the rate of Qa- to Qb electron transfer become limiting/ ¢=-.0004 + .130 (Fv/Fm) at high PFD. Limiting factors could include the size of theplastoquinone pool, number of Cyt b6/fcomplexes or capacityfor carbon fixation. This hypothesis, however, requires further

0.2 0.4 0.6 0.8 1.0 testing.0*2 0. .

The decreased photon yield seen in the higher PFD-grownFv/Fm resistant lines also requires explanation. Robinson (20) sug-

ship between photon yield of 02 evolution and F,/ gested that because of the change in the kinetics of electrontant and -susceptible B. napus leaf discs recorded transfer from Qa to Qb in resistant plants, there may be anire. Each data point represents photon yield or increased likelihood for damage caused by oxygen radicalsin different leaf discs following similar high PFD formed at the Qb binding site. If such damage occurs to the

Dl protein or associated reaction center, it could significantly

1 299

Plant Physiol. Vol. 94, 1990

decrease the efficiency ofcharge separation and result in lowerphoton yield. As we report in the accompanying paper (12),high PFD exposure does indeed cause a differential loweringof photon yield in resistant leaf discs. Our evidence suggeststhat the decrease in photon yield in resistant plants is due toincreased sensitivity of Dl to photoinhibitory damage (12).We conclude that the Dl protein alteration that brings

about triazine resistance does not necessarily cause a reduc-tion in photon yield or light saturated 02 evolution. The useof isogenic resistant and susceptible lines in these experimentshas allowed us to eliminate compensation by a nuclear-encoded trait as an explanation for the similarity in thesephotosynthetic measurements.

ACKNOWLEDGMENTS

We thank Drs. Paul Jursinic, Robert Pearcy, and Steven Theg forvaluable suggestions and Dr. Rachel Scarth for providing B. napus

seeds.

LITERATURE CITED

1. Arnon DI (1949) Copper enzymes in isolated chloroplasts. Poly-phenol oxidase in Beta vulgaris. Plant Physiol 24: 1-15

2. BeversdorfWD, Weiss-Lerman J, Erickson LR, Souza MachadoV (1980) Transfer of cytoplasmically-inherited triazine resist-ance from bird's rape to cultivated oilseed rape (Brassicacampestris and B. napus). Can J Genet Cytol 22: 167-172

3. Beversdorf WD, Hume DJ, Donnelly-Vanderloo MJ (1988) Ag-ronomic performance of triazine-resistant and susceptible re-ciprocal spring canola hybrids. Crop Sci 28: 932-934

4. Bjorkman 0, Demmig B (1987) Photon yield of 02 evolutionand chlorophyll fluorescence characteristics at 77 K amongvascular plants of diverse origins. Planta 170: 489-504

5. Bowes J, Crofts AR, Arntzen CJ (1980) Redox reactions on thereducing side of photosystem II in chloroplasts with alteredherbicide binding properties. Arch Biochem Biophys 200:303-308

6. Burke JJ, Wilson RF, Swafford JR (1982) Characterization ofchloroplasts isolated from triazine-susceptible and triazine-resistant biotypes of Brassica campestris L. Plant Physiol 70:24-29

7. Chow WS, Hope AB, Anderson JM (1989) Oxygen per flashfrom leaf disks quantifies photosystem II. Biochim BiophysActa 973: 105-108

8. Darmency H, Pernes J (1985) Use of wild Setaria viridis (L.)Beauv. to improve triazine resistance in cultivated S. italica(L.) by hybridization. Weed Res 25: 175-179

9. Erickson JM, Rahire M, Bennoun P, Delepelaire P, Diner B,Rochaix J-D (1984) Herbicide resistance in C. reinhardii re-sults from a mutation in the chloroplast gene for the 32kilodalton protein of photosystem II. Proc Natl Acad Sci USA81: 3617-3621

10. Grant I, Beversdorf WD (1985) Agronomic performance oftriazine-resistant single-cross hybrid oilseed rape (Brassica na-pus L.). Can J Plant Sci 65: 889-892

11. Gressel J, Ben-Sinai G (1985) Low intraspecific competitive

fitness in a triazine-resistant, nearly nuclear-isogenic line ofBrassica napus. Plant Sci 38: 29-32

12. Hart J, Stemler A (1990) High light-induced reduction and lowlight-enhanced recovery of photon yield in triazine-resistantBrassica napus L. Plant Physiol 94: 1301-1307

13. Hirschberg J, McIntosh L (1983) Molecular basis of herbicideresistance in Amaranthus hybridus. Science 222: 1346-1348

14. Holt JS, Stemler AJ, Radosevich SR (1981) Differential lightresponses of photosynthesis by triazine-resistant and triazine-susceptible Senecio vulgaris biotypes. Plant Physiol 67:744-748

15. Ireland CR, Telfer A, Covello PS, Baker NR, Barber J (1988)Studies on the limitations to photosynthesis in leaves of theatrazine-resistant mutant of Senecio vulgaris L. Planta 173:459-467

16. Jursinic PA, Pearcy RW (1988) Determination of the rate lim-iting step for photosynthesis in a nearly isonuclear rapeseed(Brassica napus L.) biotype resistant to atrazine. Plant Physiol88:1195-1200

17. Kitajima M, Butler WL (1975) Quenching of chlorophyll fluo-rescence and primary photochemistry in chloroplasts by di-bromothymoquinone. Biochim Biophys Acta 376: 105-115

18. Kyle DJ, Staehelin LA, Arntzen CJ (1983) Lateral mobility ofthe light-harvesting complex in chloroplast membranes con-trols excitation energy distribution in higher plants. ArchBiochem Biophys 222: 527-541

19. Ort DR, Ahrens WH, Martin B, Stoller EW (1983) Comparisonofphotosynthetic performance in triazine-resistant and suscep-tible biotypes of Amaranthus hybridus. Plant Physiol 72:925-930

20. Robinson H (1986) Non-invasive measurements of photosystemII reactions in the field using flash fluorescence. In WG Gen-sler, ed, Advanced Agriculture Instrumentation Design andUse. Martins Nijhoff Publishers, Dordrecht, pp 92-106

21. Schonfeld M, Yaacoby T, Michael 0, Rubin B (1987) Triazineresistance without reduced vigor in Phalaris paradoxa. PlantPhysiol 83: 329-333

22. Schreiber U (1986) Detection of rapid induction kinetics with anew type of high frequency modulated chlorophyll fluorome-ter. Photosynth Res 9: 261-272

23. Schreiber U, Schliwa U, Bilger W (1986) Continuous recordingofphotochemical and nonphotochemical fluorescence quench-ing with a new type of modulation fluorometer. PhotosynthRes 10: 51-62

24. Somersalo S, Krause GH (1989) Photoinhibition at chillingtemperature. Planta 177: 409-416

25. Souza Machado V, Ali A, Shupe J (1983) Breeding chloro-triazine herbicide resistance into rutabaga (Brassica napus L.)and Chinese cabbage (Brassica campestris L.) Eucarpia Cru-ciferae Newsletter 8: 21-22

26. Stanger CE, Appelby AP (1972) A proposed mechanism fordiuron-induced phytotoxicity. Weed Sci 20: 357-363

27. Steinback KE, McIntosh L, Arntzen CJ, Bogorad L (1981)Identification of the triazine receptor protein as a chloroplastgene product. Proc Natl Acad Sci USA 78: 7463-7467

28. Vaughn KC, Duke SO (1984) Ultrastructural alterations to chlo-roplasts in triazine-resistant weed biotypes. Physiol Plant 62:510-520

29. Velthuys, BR (1982) The function of plastoquinone in electrontransfer. In BL Trumpower, ed, Function of Quinones inEnergy Conserving Systems. Academic Press, New York, pp401-408

1 300 HART AND STEMLER