mutants of Oenothera, all having a defect in photosynthesis, were investigated to localize the ... plastome to specific biochemical events in photosynthesis. In 4 of ...
Plant Physiol. (1968) 43. 606-612
Studies
on
Electron-Transport Reactions of Photosynthesis in Plastome Mutants of Oenothera David C. Fork and Ulrich W. Heber'
l)epartment
of Plant Biology, Carnegie Institution of Washington, Stanford. California 94305 Received November 6, 1967.
A bstract. Fluoreseence characteristics and light-indueed absorbance changes of 5 plastome of Oenothera, all having a defect in photosynthesis, were investigated to localize the site of 'the block in their photosynthetic mechanism and 'to relate mutational changes in the plastome to specific biochemical events in photosynthesis. In 4 of the mutants examined photosystem 2 was largely, or completely, nonfunctional. Excitation of -system 2 did not cause reduction of oxidized cytoohrome f in these mutants. The system-2 dependent absorbance change at 518 mu seen in normal 'leaves was absent in the mutants. Moreover, the mutants had a high initial fluaorescence in the presence and in the ab'sence of 3-(3,4-dichlorophenyl)-1,1dimethylurea, which did not change during illumination, indicating that the reaction centers of system 2 were affected by the mutations. Photosystem 1 functioned normally. A fifth mutant had an impaired photosystem 1. Even high intensity far-red light did not lead to an aocumul-ation of oxidized cytochrome f as was seen in normal plants. Photosystem 2 was functioning, as evidenced by the fast reduction of the primary system-2 oxidant, and bN the characteristics of the 518-m., absorbance change. Because 1 of the 2 photosystems is functional in 'all mutants, and because they all have the enzymes of the photossynthetic carbon cycle, it appears that the effect of the mutation is specific. The results suggest that the plastome controls reactions within the electron-transport chain of photosynthesis. mutants
Genetic information has been shown to reside onlly in the nucletus but also in other parits of the cell. The genetic information contained in the chloroplasts is cailled the plastome (20). A number oif geneticallly weill-defined plastome mutants of Oenothera were 'recently investigated (15) in an attempt to determine the cause of photosyn'thetic (leficiencies observed in these planits. Alll the muitants appeared to have t-he enzymes needed for the reduction of carbon dioxide, and for the regeneration of the carbon dioxide acceptor, 'but in spite of this they photoreduced little o,r noGCO2. The results of the work of Halllier (14) suggested th,a,t electroni transport might be imipaired in some way. We have investigated electron-transport reactions in an attempt to ilocalize the site of deficiency. The results o'btained ifrom studies on time courses of fluorescence 'as wellll -as on 1'ight-induced absorbance changes indicated that 4 of the tmuttan,ts -have an impaired photosystem 2, antd another appears ito have a block near system 1. The resuilts demonstrate a relationship between the plastome anId electron transport of photosynthesis.
Materials and Methods
not
1 On leave from the Institute of Botany. Universitv of Dusseldorf, Germanv.
606
The photosyntheticallly-deficient plastome
mu-
tants used in this investigation were isolated from
Oenothera hookeri (plastome Ia, Ty, Th) and Oenoth era suaveolens (plastome IIa, Thy) by WAr. Stubbe. They are propagated by cross-pollinating flowers from normal and imutant tissue. As the plastid type iis transferre!d 'both by the pollen and by the embryo sac in Oenothera, the offspring will be heteroplastic. Somatic segregation of mutant and normal plastids then leads to variegation. Often variegated leaves have mutant tissue on 1 side of the midrib and normal on the other. The development of mutant tissue requires support from normal green parts. For the present work the different mutant plastid types were, combined with the genomes of Oenothera hookeri or of the hybrid albicans X hookeri and with the normal plastid types I and IV (fo,r differenit plastid types cf. 23, 24, 25). All of the mutants {are characterized by a reduced rate or complete lack of photosynthesis. Leaf sections containing only mutant tissue are pale green compared to normal parts. The mutants Iy and IIy have about 50 % less total chlorophyll than the normal, and mutants Iac, IIct, and I8, about
6)07
FORK AND HEBER-PHOTOSYNTHESIS IN PLASTOME MUTANTS OF OENOTHERA
25 % less. The chilorophyll a to b ratios are about the same for al!l the mutants and for normal tissuie (14). The multant pa,rts of the plants are photosensitive and do. no-t survive prolonged exposure to sunlight. Their biochemical and cytogenetic properties have been described (7, 14, 15). For the experiments ithe mutant leaf tissue was cluit away so as 'to avoid perielinal chimeras or normal tissue. Some experiments were performed comparing -the response of mutant tissue on 1 side of a leaf with that of noirmal tissue on the other side of the same lIeaf. However, since the experiments with different leaves, normall or mutant, generally agreed very wel,l we did not often have to use controils from other parts of the same leaf. or even from the same volant. For measurements of absorbance changes the sections of mutant or normal tissue were held on the surface of a lucite light pipe which was uised in tihe apparatus described previously (17). Monochromatic measuring light havinfg a half-4band wid'th of 1 m,u was obtain'ed from a Bausch and Lomb monooh'romator having a grating wit'h 1200 lines/ mm. The actinic light was filtereld throu,gh 27 mm o'f wq'ter andl combinaitions of colored glass and interference-type fi,lters. Red light was obtained by combining Cailflex C (Balzers, heat reflector) with RG 2 (Schott). Far-red light was obtained with Cailfilex C and RG 8, or Calf,lex C combined with a BalIzers 'interference filter having a peak trans-mission of 709 m,t, half-band wid,th 15 m,t. The intensities uised were measu'red with a sil;oon photocell calibrated with a thermopi'le. M\ easuirements of fluorescence were made wzith the same apparatuxs. slightly modified, so that light which excited fluorescence was incident onI the same side of the tleaf which faced the photomultiplier (EMI 9558B). A combination of interference aniid colored glass filters wuith a peak transmission of 684 nip, half-band width 15 mnu, was placed ifn front of the photomniultiplier to transmit flutorescent anld absoirb actinic light. Fluorescence was excitedl by a broad band of bluie 'light which was obtained by uising Calflex C in combination with Corning filters 4305 and, 5562. This band had a peak transmission nlear 440 mjL and a half band of 85 mu. Flulorescence was also excited by a bro'ad band of g,reen light havinlg a peak near 520 m,u, half-band wid,th 54 m,u. The latter was obtained using Callf'lex C in combination with Corning filters 9782, 4015 and Schott BG 18. When needed, 'leaves were treated with 3- (3,4dichlorophenyl)-1,1-di,methylurea (DCMU) by floating sections on a 10-4 M solution for 12 hours. All experiments were done at room temperature (220) with air as the gas phase.
Results Mutants Having Impaired Functioning of Photosystem 2. Reactions of the f-Type Cytochrome.
0 403
353
-20 420
-3
550
500
450
400
Wavelengfh,mp FIG. 1. Light-mintus-dark difference spectra for leaves of muitant 1T of Oenothera. TPhe spectrum with circles gives the response obtained after 3 seconds of illumination and the spectrum witih triangles after 0.1 second of illumination with light of 709 mju (14.2 neinstein cm-2 sec -1)
To test for system-2 activity in the mutants we examined tthe -reactions of ithe f-type cyt'ochrome since it has been well d-ocumented (2, 3, 9, 13, 19) that this compound fuinctions as a red-ox carrier in the electron-transport chain between the 2 photochemicail systems. It iis oxidized by system 1 and reduced by system 2. The light-minuis-dark difference spectrtum for mutant IT (fig 1) is characteristic of that produced ulfpon excitation of an f-type cytoc,hrome and had a large nega'tive Soret band slightly below 420 mt, 'a pos,iti-ve band near 403 mn Normal ~~~~a
s
a
Mutont I &
709 on
E 709on 0 4~~~~~ N Off
651 on Off
0
I
u
,&I/, = +5xl0-3
_
i
-a -0 c -0
a
d, 709 on
4
709on 651ion
t
;+f
-c
b
651 on _orm ; t $ 709off 651 off
651 off
a
* 709off
0
5
10
15
20
Off
0
5
10
e
15
20
Time,sec FIG. 2. A comparison of the kinetics of light-induced absorbance changes at 420 m,u caused by oxidation of the f-type cytochrome in leaves of normal Ocuiothcr-a and mutant Ij. The wavelength of the actinic light was 709 m,c (14.2 neinstein cMu2 sec-1 for traces a and b and 7.2 neinstein cm-2 sec-1 for traces c, d. and e). The intensity of the 6l- m,u light was 2.8 neinstein cm-2 sec1.
608
68PLANT
and ain
a band at 552
positive 518-m,u bands
mpl.
PhIYSIOLOGY
The negative 475 and
in the difference spectrutm for the initial deflection produced tupon illumination are discuissed in another section of the resull ts. It is clear that the bandl of cvtochronme f is suiperimposed on other large positive changes in thie green region which w1ill not be discussed here. The shoulder aroundi(l 435 m/ may be cauisedl ly oxidation of P700 (or of a b-type cytochrome). 'The alntagonistic effects of far-red and red light oni the oxidation state of cytochrome f (meastured at 420 m,u) in a normal leaf are shown in figuire 2. Far-red light (709 m/) cauise( rap.d loxidationi of the cytoc,hrome (seen in trace a downwardl (leflection). Dark reductioni of the cYtocbhrome after 709-m/ light was sloNv. Trace b of figuire sho.w,s the effect of tuirning off 709-mM lighit but at the same time turning Onl 651-mtu lighit. In this case rapid reduction of cytochrome was seen since 651-mM light excited systemii 2 effectively. The intensity of 651 m/t was low enoulgh so that this beam alone did not prodluce an appreciable deflectioni (cf. trace b la.bellled "651 off"). The accelerating effect of switching 651 for 709-mu light upoIn t'he reduiction of tihe cytocfhrome in normal leaves was abolishedl after inicuibatioIn in DCMU. AMoreover, after this treatment 651-mnu light alone produced slow oxidation of the cytochrome similar to that seen
a
a
as
No DCMU f +Of
+DCMU
Normal
NOff
E
* On
a1a
bb
* On
T
AI/, =10-2
Mufonf Hi'
,Off
-o
Off
0 0 -o
a
On
c
Off
I AI/x10-3
Mufant ll
*
'Xf"
Off
On 0
d
On
5
~~~~~~~4 On
4
e
10
0
f
5
10
Time,sec FIG. 3. A comparison of the kinetics of the lightilduce(d ahblsorhance changes at 548 mn, in leaves of niormcal Ocnothera anid mutanits ITy and hIa. Actinic illumination was provided 1w- broald band (620-80 mi) of red light of about 2.6 X 10' ergs cm l-2 eeC . a
prodluced by 709-mM light.
The accelerating effect of 651-mn light oni the reduction of cytochrome f coultld not be lenlonstrated for muitant To. Figure 2 (trace c) i'l;luistrates that in mutant 1I, as in ithe normal lIlant, illutminatioon with 709-mtt light l)roduced a fast oxidationi wvhich was followed by slow re(duiction inI the (lark. Illuimination of multant Th wvith weak 651-m,u light alonie p>rolhced a sloxv oxidlation (trace d) and wheni 651-im/.L switched fuor 709-m/, light (trace c) there was no reduiction an(l the cytochriome remained oxidized. System 2 thuis appears not to fulnictioni in this muttanit. The sanme general behcavior was foulind for multtanlts Ia, Iy, and Ily indicating that in these niiitanits photosystenm 2 is also essentially nonfunctional. Difference spectra oitainled by illuminiation of these muiitaints witlh re(l o,r far-red light were almost identical aId( exhibited the same chlaracterist.cs as those given in figulre 1 for multant lb. Miitant ly had, in addition, a smalil positive peak at 430 mM suggesting reducetion of a cytochrome of the b type. Absorbanec Chan'lcs (it 58,ip.. Absolrbance chaniges seen at 518 ma, in normiial Ocnothcera leaves fig 3) are complex butt typical of tho.se (trace produtce(l by other green planits containing chlorophyll b (11, 12, 27). Illtumination with a broa(l band of red lig,ht whic,h excited both system 1 aniid 2 produiced, initially, a fast rise which was followed by a Ilowver, and larger, inicrease to a steady state. Leaves poisoned witih DCAMU still ha(l the fast, but smaller, initial increase andl the steadv-state cihange Nvas ailniost completely inhibhited (trace b). \ctioiu a
was
a
spectra donie (11)
on
the
green
alga Ulva lobata
have shown tha,t the fast, iniitiall increase with or withou1t DCMU as well as the low steady-state change persisting in DCTMU are sensitized by system 1. The slow, larger absorhance inkcrease is sensitized by system 2. Mutant Ily had a 518-m,m change like that of
normal plants whose system-2 activity had been blocked by treatment wi,th DCMU. This plant had identical 518-m,u absorbance changes in the presence or absence o.f DCMTU (traces c and d) indicating that system 1 alone is functional in this plant. Very similar responses at 518 mp, were found for sonie other muititants (Iy, 1h and la). The relater spouses of mulftant Ila are discussedI in sectiOn1 The observations Onl the behavior of cytochrome f ini mutant (and also mutants y, ITa, and Hly) as welil as those oIn ithe 5l8-mp. change discousssed at,bove aill suggest that svstem 2 is almost completely nonfuinctional in these plants. Ftirther evidence tfhat system 2 was not functioning was ob)tained from their fluorescence behavior. Timte Courses of Fluorescence in7 DCMU-Treated Leaves. There is evidence (4, 5, 6, 8, 10) that the fluiorescence o,f chilorophyll in algae ancd higher plants originates mainly fro,m system 2 and th,at tlhe fluorescence increase which occurs around 685 mn. duiring illumination reflects the oxidation state of the primar) pho,tooxidant (Q) of system (10). When Q is in the oxidized form uit quenches chlolrophy ll flulo,rescence and whein in bhe reduced state a
.
609
FORK AND HEBER-PHOTOSYNTHESIS IN PLASTOME MUTANTS OF OENOTHERA
Mutant f l(No DCMU)
100
Mutant Ul.(No DCMU)
75Normal (DCMU) 0)
° 50
-
0)
25 -
0
5
0
10
IS
20
25
30
Dark fime,sec
FIG. 4. Fluorescence at 6&4 mg produced immnediately upon illumination of niormal leav-es of Qenothera and of mutanits HJy and 11a a-s a function of dark interval l)etween exposures to blue actinic light (267 ergs cm-2
s-ec-i).
(QH) it l)roduices increased fluorescence. In the albsence of DCMU, oxidation of QH may be bro-ught aibout by excitation of system 1. QH may ailso be oxidized by a dark reaction that proceeds in the presence or absence of DCMU. In the normal leaf treated with D-CMU the fluorescence *at 684 mJL produced initially was low. Duiring ililumination the fluorescence rose rapidly to a higher level as Q was reduced by system 2. Figure 4 showrs that in DCMU-treated normai leaves QH was half reoxidizeda in the dark in about 0.7 sec. A number of mutants ('Ia, Iy, Ib, and (Hy) were found to have a high initial fluorescence at 684 m/ comlpared r to normal plants whether poiso-ned with DCMU or not. Moreover, this highi fluorescence seen initially did not increase further during illumination and was not decreased even by a long dark interval (10 mm). Figure 4 shows this wacak of dark reoxidation of QH for mutant
II)Y.A
DCMU indicating that reduction of Q is at leaslt as effective in this mutant as in normall leaves. However, initiail f(luorescence after a 'long dark period was higher in tihe mutant than in the normal leaves and t'he mutant had somewhat less varia)ble ftluorescence. Therefore, the number of system-2 reaction centers may be somewhait reduced in mutant Iha when compared with the normal. AbsorUance Change at 5i8 m,u. Further evidence for the functioning of system 2 in mutant IIa wa,s seen in the behavior of the 5l18-mMu absorrbance change. Trace e of figure 3 shows that the change before poisoning with D'CMU consisted of a fast, initial rise fol,lowed by a second, slower and somewhat 'larger increase, characteri,stic of photosystem 2 acitivity. Thi,s second phase was smaller than in the normal plant. Trace f shows the effect of incubating the leaves in DCMU. A's in the normall plant the fast, initial transient was not inhibited, but even increased, and the second rise wa's abolished. Cytochromiie f. The 'beihavior of the f-type cytochrome suggested that mutant hIa had a block in electron transport near system 1. In thli,s mutant we were tunable to see accumulation of oxidized cytochrome f with 709-m,u actinic light as in the normal and mutant lb (cf. fig 2). In mutant HaI even ia broad band of 'high intensity far-red light gave no response until the leaves were incubated in DCMU. After this treatmenit accumulation of oxidized cytoch'rome f occurred both in high and low iintensity far-red ilight and the difference spectrum had negative bands at 553, 420 and a positive band at 405 mr,. Figure 5 compares the rate of cytocthrome f oxidaltion as a futnction of intensity
02
Muttant With Imipaired Functioning of Sys-
tem i. FlAuorescence. Mutant 1ha was found to
have a fluorescence behavigorintermediate between mautant ly or I>l and the normal plant. Figure 4 shows that for mutantIda QH was ha,lf reoxidized in the dark in a(bout 0.6 sec. The fluorescence behavior o-f this mutant was siimilar wi-th and without DCMU. Thus, unlike the other mutants described so far, mutant Ilae retains funrcFtional systaem-2 reaction centers because after Q is formed in a dark reaction it may be reduced again in the li.ght. To es-timate how welil photosystem 2 was functioning the half time of the increase oif variable filuorescence, whidh reflects the reduction of Q, was meassured at different light intensities. The rise, as judged from the half-time of QH formation, was found to be 30 to 50 % faster in DCMUAtreated mutant Tla than in normal -leaves treated with
C\
o O
_W
2
4
6
8
10K104
Intensity,ergs cm 2 sec' FIG. 5. The rate of decrease of absorbance at 420 for differences in the chlorophyll content of the leaves) caused by oxidation of the f-type cytochrome in normal leaves of Oeitothera and mutants IIhy and 11a as a function of the intenisity of a broad band of far-red light (675-800 m,u). The leaves were treated with DCMU as described in the text.
mA (unoorrected
610
PLANT
PHYSIOLOGY
of far-red light for normal Oeuothera, mutant Ily ancd Ila after treatment with DCMU. It is clear from this figure that the efficiency of cytochrome oxidation in mutant IIa is very low in comparison wvith normal OQnothera or muttant IIly. The differences apparent in figure 5 between mtttant hIa onl the 1 hand and normal Oenothera, or mutant Ihy, on the ot-her were even mulch more pronounced if electroIn flow from photosystem 2 was not blocked by DC'MU. Then excitation of system 2 by far-red light (5) was sufficient to couinterac.t oxidation of cytochrome f in multaint Tla buit not in other plants
tion was as efficient in the mutant as in normal leaves indicating that system 1 is functioning normally. Experiments with isolated chlloroplasts from the mutants also demionstrated that system 1 is functioning normally since high rates of ATP formation in a cyclic type of photophosphorylation were observed 'using PMS as a cofactor (14). Apart from a lower pginent content and increased fluorescence intensity, low-temperatture absorptionl spectra -and filuorescence-emission spectra of the mutants (not shown here) failed to revea.l abnormalities when com,pared with spectra of normal
having normal system-i
leavets.
activity.
The quanituim requ1irements for cytochrome oxi(lation were 10 for mutant ily, 16 for multant ly, 30 for normal Ocnot hera, and 170 for muitanit TIa. Stuch measurements in highly-scattering leaves are sullbject to relativelly large errors especially in the strongly-absorbing 420-mu region (1), but their relation shows again impaired functioning of photosy-stem in muitant ITa.
Discussion Mutants Hazing Impaired Photosysteml-2 Activity. The experiment's described in the first section of the Results demonstrate that mutants la, Ty, 1h and ITy have a 1block in photosystem 2 which is responsible for their inabiliity to perform photosy nthesis at a significant rate. The rate of l-ight(lependenit 14CO. fixation by these mlu'tants was less than 1 % of t'he rate of normail ;leaves (15) except for mutant Ia which, like multant Ila, couild almost compensate respiration (Hallier, personall commtnication).
The initiall flu1rescesnce of unt,reated
poisoned multants (as txypified by
mutant
or
DCMIU-
Ily, fig 4)
high and did not significantly increase d.uring illumination. Dark 'intervailis or illumination with far-red light did not lower this high initial fluorescence. This was even true for mutant Ia. As the ilight-induced rise in filuorescence reflects the trapping of energy by 'the reaction centers of system 2 (10), these ceniters welre all (multant ly) or nearly all (mutant 1a) nonfuinctional (reduiced and inecapable of reactiiig back in the (lark) or perhaps absent altogether. The inability of the photooxidant of system 2 to function as ain efficient trap for energy absorbed by the 'light-harvesting pigment molectules was also seen in the behav,ior of the 518-m,u absorbance chan'ge. That part of the change produced upon was
excitation
of
systemn
2
in
normal
leaves
(11)
was
absent in the 'mutants. In addition, cytochrome f oxi(lized bv far-red lighit was not, as in the normal plant, reduced UpoIn excitation of system 2. Thuis, in these respects, the uinlpoisoned mutants behaved just 1'ike normal plants treated with DCTMU. It appears that the effect of the mutation was specific for photosystem 2. Cytochrome f oxida-
The enzymes of the photosynthetic carbon cycle are present in the mutants and have abouit normal
aotivities. The very slow formation of radioactive sugtar phosphates from 14CO2 by the mutants (15) can be accounted for, at 'least in part, by photosystem-1 activity. The ATP content of mutant tissue increased tupon illumination (14), apparenitly due to svstem-i mediated photophosphorylation. Photooxidation of ascorbate by system 1, or perhaps transport of reducing equivalents from the cytoplasm, may lead to the slow formation of some NADPH. The reduiced ch'lorophyll. content of the leaves may be considered as a secondary consequence of the mutation. A block in the electron transport chain, which prevents 'light energy from being effectiveily channeled into photosynthesi!s, may be expected to cause photooxida'tive pigment destruction as observed in the mutants (14). Likewise, starvation phenomena stuch as increased .appearanice of free amino acids may also be considered as secondary events (15). A Mutant Hazving Imp/'aired Photosvstent-r Actizvitv. Mutant ITa wN-as different from all]l the other mutants investigated in that it had impaire(l functioning of system 1. In this multant even a high intensity far-recl light was insufficient to cause appreciahle oxidation of cyto'ch'rome f. A pronouinced oxidatilon coulld only be observed 'after addition of DCMU. Apparentliy there was sufficient excitation of photosystem 2 even in far-red light to couniteract the accuimuflation of cvtochrome f by system 1. In mu1tant Ha 'the rate of cvtochrome oxidati'on in far-red light after bilocking electron flow from sysitem 2 wvith DCM\U was about 12 times 'less onI a uinit chlorophyll basis than in the normal or in mutant IIy. The 518-mpt change of muitant Ila liad the fast, transient system-1 component as wel;l as a distinct second, slow phase, sensitive to DC\IIU, characteristic of tihat produced tupon excitation of sxstem 2. Photosystem 2, therefore, is operative in thfis miltant. However, it does not appear to be comipletely uinchanged in its activity. While the half times of the rise in fluorescence in DCMU-treaited leaves indicate even faster redtuction of QH in mutant IHa than in normall leaves, the nuimber of 'trappina centers of system 2 appears to be .somewhat redtuced,
FORK AND HEBER-PHOTOSYNTHESIS IN PLASTOMIE MUTANTS OF OENOTHERA
perhaps as a secondary consequence of the mutation. As discussed above, it is cjlear ithat photooxidative reactions induced by a block in system I should secondarily influence photosystem II. StilIl, the activity of photosystem 2 is much higher th-an that of photosystem 1 and the liatter is clearly limiting photosynthesis. Mutant IIa has an appreciable residual photosynthetic activity and can almost compensate respiration. As wiith the ot'her mutants, it conntains all the enzymes of the photosynithetic carbon cycle in about normal activities. Genetic-Physiologicail Aspects. There is an overwheilming body of evidence to ilink point changes of the genome caused by mutations to specific alterations of individual proteins. One of the aims of the present investigation was to determine whether it was possible to link, in a similar way, the changes caused by mutation of the plastome to specific events in the biochemical machinery of chloroplasts. Spontaneously-occurring plastome mutants are particularly well suited to prove tha-t the m-utated genetic material is associated closely with chloroplasts. The main criteria (cf. 18) are quantitative differences in the extent of variegation in hybrids from reciprocal crosses between normal and defective types. The nature of the plastome is still unknown. DNA Ihas 'been 'identified in recent years a constituent of chloroplasts and evidence for a relationship between chloroplast DNA and chloroplast development has been presented (21). Fturthermore, chloroplast DNA appears to be engaged in the formation of chloropl-ast ribosomes (22). However, in all mutants deficient in photosynthesis, listed in a recent survey by Kirk (16), that have been investigated biochemically and genetically, the muitations appear to be centered in the nucleus. The results of the present work indicate that the plastome exercises control over specific chloroplast reactions. Since none of the photosyntheticallydeficient mutanits investigated had a 1block in its carbon metabolism, and all were affected in their electron-transport chain, it appears that the plastome control steps of electron transport which occur in the lamellar struictture of chloroplasts. as
Acknowledgment It is a pleasure to thank Prof. Dr. XV. Stubbe who kiindly supplied the mutants used in this work and who provided valutable criticism of certain parts of this manuiscript.
Literature Cited 1.
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