PROMOTION OF LETTUCE SEED GERMINATION BY GIBBERELLIN 1"2
BOTANY DEPARTMENT, UNIV'ERSITY OF CALIFORNIA, Los ANGELES
Lettuce seeds do not germinate in darkness under
certain conditions, and their germination can then be
promoted by many factors.
presented preliminary reports on the promotion of
dark germination of Grand Rapids lettuce seeds by
Some details of their work and more
recent results of the study will be given here.
of the previously reported findings have been con-
firmed and amplified by researchers at The Hebrew
University of Jerusalem (7, 15).
has found that gibberellic acid promotes dark germi-
nation of a wild lettuce species, Lactuica scariola L.
Among the factors which may inhibit dark germi-
nation of lettuce seeds are: supra-optimal germination
(3, 5); pretreatment of imbibed seeds
with high temperature
(1, 2); and a process called
hibition is the reduction of germination in darkness
by the incorporation of appropriate amounts of os-
nmotically active solute in the germination solution.
The above inhibitory factors may all induce the same
ultimate block in the germination pathway.
lettuce see(ls whose germination is inhibited by one
of these factors will germinate if they are irradiated
briefly with red light.
Such seeds may be termed
When a red irradiation is
followed by a far red irradiation, the action of red
light is reversed (1, 2).
radiation, the promotion of germination can again be
potentiated by a red irradiation, and such a promotion
and reversal can be accomplished many times.
The effect of gibberellin upon germination was
determined on seeds that were made red-light respon-
sive by supra-optimal germination temperature, dark-
osmotic inhibition, pretreatment with high tempera-
ture, or irradiation with red light followed by irradia-
tion with far red light.
Gibberellin causes bolting and flowering of long
day plants in short days (12, 13).
action spectra for the control of flowering and seed
germination are similar (2), a part of this investiga-
tion was devoted to examining the relationship of gib-
berellin and light actions in seed germination.
Kahn et al (9, 10) have
Also, Lona (14)
Following the far red ir-
Because the light
'Received July 31,
2This work was supported in part by a grant made
hy the Lilly Research Laboratories to Professor Anton
Lang and in part by a research grant (RG-3939) to Pro-
fessors Lang and Sam G. Wildman from the National
Institutes of Health, U.S. Public Health Service.
MATERIALS AND METHODS
Three lots of Grand Rapids lettuce seed
(Lactuca sativa L.) were used in this study.
approximate germination range at 210 C in darkness
on water-saturated filter paper during this work and
their sources are given in table I.
same as that used previously to study dark-osmotic
GERMINATION CONDITIONS AND LIGHT SOURCES:
The standard experimental procedure has been de-
scribed in detail previously (8).
of this procedure will be noted.
general method employed.
ment consisted of 100 seeds sown in a Petri dish on
Whatman no. 1 filter paper saturated with an experi-
Unless a contrary statement is made,
from the time the seeds were sown the dishes were kept
21 + 10 C.
All operations requiring vision were per-
formed in dim green light.
from cool white fluorescent tubes and far red light
from reflector photoflood
Lot No. 1 is the
This, briefly, is the
Each replicate of a treat-
Red light was obtained
Two gibberellin preparations were used: A. A mix-
ture ([a]25 + 620) of gibberellin A1
A) and gibberellin A3 (gibberellic acid): and B. A
sample of gibberellin A3, 88.9 % pure.
term gibberellin will be used for both preparations;
in the description of experiments the second prepara-
tion will be distinguished from the first by calling the
second gibberellic acid.
DESCRIPTION OF GRAND RAPIDS LErrucE SEED LOTS USED
IN THIS STUDY
Ferry - Morse Seed
Mountain View, Cal., 1953
Dept. of Botany, Univer-
sity of Wisc., Madison;
now Dept. of Botany, Ind.
Ferry - Morse, Tip - Burn
Resistant strain, 1957
* On water-saturated filter paper at 210 C in darkness
PROMXIOTION OF GERAIINATION OF SEED Lor N
LIGHT AND BY GIBBERELLIN
o. 2 BY RED
RED LIGHT GIBBERELLIN
*Administered 2 hours 'after the beginning of imbibi-
RELEASE By GIBBERELLIN OF INHIBITION CAUSED
For different lots of Grand Rapids lettuce see(ls vari-
can be found
optimal for germination in darkness (5, unpublished
For lot No. 2, 210 C was too high to permit
more thani 20 to 35 % germination on water in dark-
ness, but a brief irradiation with red light strongly
promoted germination at that temperature.
motive effect of gibberellin and of red light on the
germination of this seed lot is shown in table II.
seed lot was peculiar in that maximal germination
was never obtained with a germination period of 48
Therefore, the saturation of the effects of light and
gibberellin that are evident in the table do not mean
that about 25 % of this seed lot was non-viable or in-
sensitive to light and gibberellin.
non-germinating seeds from experiments with this lot
germinated within several days when left in their
dislhes on a laboratory bench.
EFFECT OF GIBBERELLIN ON DARK-OSMOTIC IN-
The nature of dark-osmotic inhibition has
been analyzed previously
solution containing sufficient solute to prevent ger-
mination in darkness, germinate fully
briefly with red light.
When gibberellin (35 mg/i)
is included in a germination solution of 0.15 MI man-
nitol, the effectiveeness of dark-osmotic inhibition is
This response is slhown in table III.
red light protects the seeds against dark-osmotic in-
Almost all of the
Seedls sown on a
EFFECT OF GIBBERELLIN ON DARK-OSMOTIC INHIBITION*
*Seed lot No. 1
whether or not gibberellin pretreatment also can re-
Seeds may imbibe water for several hours, and their
transfer to an osmotic solution
seeds allowed to imbibe gibberellin (50 mg/l) or water
for 6 hours were washed thoroughly with water, and
transferred to 0.15 M mannitol.
promotive effect of gibberellin upon the germination
of seeds on 0.15 M mannitol when gibberellin is sup-
pliedl during a pretreatment.
gibberellin supply was withdrawn from the seeds after
6 hours of uptake, the pretreatment with gibberellin
Gibberellin did not necessarily act during the pre-
treatment, as gibberellin that probably remained with-
in the seeds, despite washing, could have acted later.
Table IV shows the
Although the external
EFFECT OF GIBBERELLIN- ON INHIBITION OF GER-
MINATION CAUSED BY HIGH TEMPERATURE PRETREAT-
Borthwick et al (1, 2) have shown that let-
tuce seeds of some varieties germinate well in dark-
ness at a particular temperature (20-25° C), but do
EFFECT OF GIBBERELLIN PRETREATMENT ON
*Seed lot No.
germinated on 0.15 M mannitol after
water or gibberellin
not germinate well at that temperature if transferred
to it after imbibing water and being maintained at
30 to 350 C for one to four days.
be called "heat sensitization", and seeds that are dark-
dormant at 210 C because they have had such a treat-
ment will be referred to as "heat sensitized".
germination of such seedls can be promoted by red
To study the effect of gibberellin on lheat sensitiza-
tion, seedls of lot No. 1 were allowe(d
kept at 36 +- 1° C for 117 hours.
less than 1 % (luring this perio(l.
Table V gives the results of this experiment.
This fin(ling can be explaine(d
two ways: A. seedls do not become heat sensitize(l in
the presence of
see(ls to circunmvent that block to germinationl
resuilts from lleat sensitization.
This treatment will
to imbibe eithler
After this timle all
to -ermlinate at 21° C for 49.5
is present (lurilng
hiigh temperature no effect of heat sensitization
B. gibberellin permits
KAHN-PROMOTION OF GERMINATION BY GIBBERELLIN
GERMINATION* AT 210 C FOLLOWING HIGH TEMPERATURE
PRETREATMENT** IN PRESENCE OR ABSENCE OF
12,26, 28, 16, 20
*Seed lot No. 1
** 36 ± 1c C for 117 hours
The following experiment was performed to de-
termine if gibberellin can promote the germination
of seeds that had become heat sensitized.
lot No. 1 were sown on water-saturated filter paper
and stored at 3641° C for 93.5 hours.
then dried in darkness at 360 C for 23.5 hours without
removal from the dishes.
and the filter paper substratum, sufficient amounts of
water or of gibberellin solution were added to each
dish to saturate the filter paper.
kept at 210 C for 49.5 hours before germination counts
Table VI shows that gibberellin treat-
ment following heat sensitization promotes germina-
Thus, gibberellin enables the seeds to overcome
or circumvent the germination block that results from
One cannot test to learn if gib-
prevent heat sensitization,
gibberellin that is supplied during the high tempera-
ture treatment cannot be removed from the seeds.
It may cause germination after the seeds are shifted
to a lower temperature.
After drying of the seeds
All seeds were then
PROMOTION OF GERMINATION OF SEEDS THAT HAD
BEEN IRRADIATED WITH RED LIGHT FOLLOWED BY FAR
Promotion of lettuce seed germination
is frequently potentiated by a brief red irradiation, and
this effect is reversed by a brief far red irradiation
to potentiate germination and soon thereafter suffi-
cient far red light to reverse the potential to germi-
nate will be called "far red inhibited", although far
red light may do no more than nullify the action of
red light and restore the seeds to their original condi-
Red light can repotentiate the germination of
far red inhibited seeds
experiments were performed to find out whether gib-
berellin also promotes the germination of far red in-
Two alternatives are given for the
length of high temperature pretreatment, tempera-
ture during drying of the seeds, gibberellin concen-
tration, and length of the germination period.
Seeds that have received sufficient red light
The following two
PROMOTION OF GERMINATION AT 210 C OF HEAT
SENSITIZED SEEDS* BY GIBBERELLIN
58, 47, 31
*Seeds of lot No. 1 were sown on water and kept at
36 ± 10C for 93.5 hours.
360 C aind allowed to reimbibe on experimental solutions.
81, 55, 68
They were then air-dried at
each case, the first of the alternatives refers to condi-
tions of experiment 7, and the second to conditions of
experiment 8 (table VII).
sown on water-saturated
334±2° C for 28 2/3 or 31 hours.
periods, some seeds received 30 minutes of red light;
some 30 minutes of red light followed by 1 minute
Seeds of lot No. 3 were
filter paper and kept at
At the end of these
PROMOTION OF GERMINATION OF FAR RED INHIBITED SEEDS*
BY GIBBERELLIC ACID
IRRADIATION DURING PRETREATMENT
LIGHT (1 MIN)
(1,000 or 1,500
(1,000 or 1,500
*Seeds of lot No. 3 were partially heat sensitized, irradiated as shown in the table, air dried, allowed to reimbibe
on the solutions shown in the table, and germinated at 210 C.
36, 43, 52, 56
89, 88, 85, 77
72, 70, 78, 81
of far red light; alndl sonme received no irradiation.
The seeds were then air (lrie(I in darkness at high
temperature or at 210 C.
wvater or gibberellic acid (1,000 or 1,500 mg/l) to
saturate the filter papers was added to each dish, and
the seeds were then allowed to germinate at 210 C for
74 or 51 hours before germination counts were made.
Table VTI shows that the far red irradliation fol-
lowing the red irra(liation re(luced germinability of
the see(ds on water from 76 to 13 %.
tion of seeds pretreated with red followed by far red
light was promoted to 78 CX
Therefore, gibberellic acid promotes
thle germination of far
gibberellic aci(d concentrations were used, because in
earlier experiments with the same seed lot. 400 mg/1
elicited only a small promotion of germination of
see(ds receiving similar, but longer high temperature
pretreatmiient and either no irradiation or red followed
by far red irradiations.
No seeds germinated dur-
by treatment with gib-
ARE MVIECHANIS_MS OF ACTION OF LIGHT AND GIB-
can substitute for re(d light in prionioting the germina-
tion of lettuce seedls that (1o not germinate in darkness
because of relatively lowv supra-optimal germination
tenmperature, dark-osmotic inhibition, heat sensitiza-
tion, or far red inhibition.
of two factors, however, can never be taken as evi-
dlence for identical meclhanisms of action.
whlether or not there is sonme kin(d of relationship be-
twreen the mechanisms of action of light and gibberellin
in lettuce seed germination.
Since the primary action of red light can be fully
reverse(d bv a subsequent, brief far red irradiance
(1, 2, 8). an attempt was ma(le to find out if promo-
tion of germination by gibberellin could be negated
by far re(d light.
Seeds of lot No. 2 were sown on
Water or gibberellin (10 ng/1).
The identical end effect
The promotion of
approximately the lowest that coukl be (letected \vitlh
Half the dishes of seeds on each sub-
stratum were irradiate(I for- 2 iminutes with far red
light on three occasions: 4. 19'2, 2and 2534 hours after
the beginning of imbibitioln.
two minutes of far red light were always sufficient
to reverse fully the actioni of a satuirating (lose of red
light given immediately before the far redl irra(liationl.
The total germination pelrio(dwas 49.5 hours.
first group of results in table VIII shows that the
promlotionof gel-rmiinationi by gibberellin was nlot r-e-
duce(d by three brief irradiations with far red light.
Two similar experinments done \vith seed lot No. 1
are suimmarize(d bv the second group of data in table
without gibberellin (100 mg 1l) incorporated in the
after they were sown.
wvere 50 or 53.5 hours.
light were ample to reverse fully the effect of a prior
light of seeds suppliedl with gibberellin was somiiewhlat
greater than tllat of seedls lacking gibberellin.
far re(l light may have reduced
slightly in this case, but as the dlifferences are slmlall,
this conclusion rests on an infirm basis and is incon-
sistent with the first group of (lata in the table.
Though in these experiments brief irradiances of
far red light did not negate promotion of gernmination
by gibberellin. it is still conceivable (especially Stip-
posing that gibberellin
One might assumile that anly effect pro(luced by
gibberellin prior to a brief far red irra(liationi is in
fact reversed by the irraldiation, hut that residual -ib-
berellin in the see(d imme(liately acts again to promlote
in wlhichi far re(l light wvas given continuously.
of lot No.
27-+±10 C) that w-ere suipra-optimal for (lark germina-
At the intensity use(l,
Seeds w\ere so\\n on 0.22 'M nmannitol witlh or-
Seeds were irra(liated three or four hours
The total gernminatioln periods
Twenty secon(ls of far red
Reduction of gernmination by far redl
(loes reverse gibberellin
that far red light
Therefore. experimiielntswere performlled
3 were usedland temperatures
STABILITY OF GIBBERELLIN EFFECT TO BRIEF IRRADIANCES OF FAR RED
18, 21,18, 20, 27
29, 22, 23, 27, 21
41, 35, 38, 39,25
33, 43, 46, 29. 30
*Seed lot No. 2
**Seed lot No.
3 X 2tmin
3 X 2 min
28, 21, 23
14, 15, 14
49, 48, 40
16, 32, 26
97, 93, 92
20, 17, 9
KAHN-PRO'MOTION OF GERMINATION BY GIBBERELLIN3
INHIBITION OF GERMINATION* BY CONTINUOUS FAR RED LIGHT
0, 0, 0, 0
(400 or 200 mg/1)
(400 or 200 mg/i)
Thiourea (0.5 or 0.2 %)
Thiourea (0.5 or 0.2%)
*Seed lot No. 3 at 26 or 27+1° C with 72 or 70 hours allowed for germination
** Pretreated at 4+10 C in darkness for 8 days
tion of this lot were employed.
mote dark germination
optinmal temperature were compared with gibberellic
They were low-temperature pretreatment (3)
anid thiourea (16).
The results of two such experi-
ments are sunmmarized in table IX.
a cold pretreatment wvere kept imbibed in darkness at
4±1° C for 8 days.
Thiourea was used as a 0.5 or
0.2 % solution, gibberellin was used at a concentration
of 400 or 200 mg/l, and germination counts were made
after 72 or 70 hours.
It is evident that the low tem-
perature pretreatment. gibberellic acid, and thiourea
all promoted germination in darkness but not in con-
tinuous far red light.
In experiment 19 (table IX), seeds which after
72 hours on 400 mg/l gibberellic acid in far red light
had germinated 4 %. were tested further.
ing and transfer to darkness were done by the light of
the far red source.
Two of the four replicates were
left in conltinluous far red light, and two replicates
were shifted to dlarkness, where they remained for an
additional 48 hours.
At the end of this period, seeds
remaining in far red light had germinated 68 % and
those shifted to dlarkness had germinated 58 %.
10 % difference is not consideredl significant.
far red light under conditions of these experiments
does not prevent gibberellin-promoted germination.
but it does delay it markedly.
action of far red light on gibberellin-promotion of
germination may not be specific, since continuous
far red light also prevents or delays the promotion
of germination by other factors that enhance lettuce
seedl germination in darkness.
Another approach to the question of whether a
relationship exists between the mechanisms of action
of red light and gibberellin was a study of the com-
bined effects of these two treatments.
the effects of two different factors upon a given re-
sponse are more than additive, a synergistic interaction
of the factors
Two factors that pro-
lettuce seeds at
However, the delaying
As a rule. if
Per cent germination
being an expression of ultimate all-or-none responses
by individuals comprising a sample of a population,
however, attempts at deciding whether the actionis of
gibberellin and red light are additive or synergistic.
based on the percentage responses of the seeds, must
be considered tentative.
The combined effects of red light and gibberellin
upon germination have been studied in many experi-
Whenever feasible, data from such experi-
ments have been grouped for presentation in table X.
Experiments that were united always yielded the same
senting 6 or more replicates (each of 100 seeds) per
treatment are based upon the averaged results of two
or three separate
were not always repeated in precisely the same man-
ner, in some cases alternative mannitol or gibberellin
concentrations or irradiances of red light are given in
the grouped data.
The table shows that the com-
bined effects of gibberellin and red light are additive
or somewhat less than additive, if the irradiation is
given four or fewer hours after the seeds are sown.
When the irradiation is given 20 or more hours after
the seeds are planted, however, the combined effects
of red light and gibberellin are somewhat greater than
This apparent synergistic effect is relatively small
but quite consistent.
It is most evident in the last
experiment in table X.
Here a 5 second red irradi-
ance given 28 hours after the beginning of imbibition
did not produce a measurable effect by itself, but, when
combined with 50 or 100 mg/l of gibberellic acid, the
red irradiance increased germination from 16 to 35
and from 36 to 58 %, respectively.
presence of gibberellic acid seems to have resulted in
an increased responsiveness of seeds to red light.
All groups of data repre-
In other words,
A close parallel has been established between the
effects of gibberellin and red liglht on lettuce seed ger-
which would not germinate because of relatively low
supra-optimal germination temperature, heat sensiti-
zation, dark-osmotic inhibition, or far red inhibition.
Brief irradiation of seeds with far red light does
not negate gibberellin action (see table VIII).
haps promotion of germination by gibberellin is re-
duced slightly by a brief far red irradiance, but such
an effect is not shown consistently.
experienced the same results (see their tables IV and
Because of this lack of consistency, and because
of the difficulty
in interpreting ineffectiveness
brief far red irradiations as noted previously, the com-
bined(ldata at hand do not permit us to decide whether
or not brief far red irradiances can reverse any part
of the promotion caused by gibberellin.
these (lata do not allow a conclusion about the relation
of the effects of light an(d gibberellin in lettuce seed
The effect of prolongedl irra(liations with far red
light also permit no conclusion about the relation of
light and gibberellini inlettuce
Clearlyv continuous far red light of rather high in-
Both promote dark germination of seeds
Evenari et al (7)
tensity delays the germination of seeds promoted by
This need not indicate specific reversi-
bility of gibberellin action by far red light comparable
with the reversal of the action of red light by sub-
sequent brief far red radiation, since the germination
of seeds promoted by thiourea or a low temperature
pretreatment is also delayed or prevented by continu-
ous far red light.
Experiments in which gibberellin an(d red light
were given in combination showed that their effects
are simply additive when seeds are irradiated early
in the germination period, but are nmore than additive
when a red irradiance is given 20 or 28 hours after
Mayber et al (15; see their table IV) also show more
than additive effects when lettuce see(ds are irra(liated
with red light 48 hours after the beginninig of imbibi-
tion on gibberellin solution.
lin not only promotes lettuce seed germination in com-
plete (larkness, but also, after a sufficiently long perio(d
of action, it seems to increase the responsiveness of
lettuce seeds to red light.
siveness of imbibing lettuce seeds to re(d light under-
It is known that the respon-
PROMOTION OF GERMINATION BY COMBINATIONS OF GIBBERELLIN AND RED LIGHT TREATMENTS
20 or 25
20 or 25
0.10 or 0.15
6 or 8
4 or 10
4 or 10
10 or 6
10 or 6
3 or 4
*Hours from beginning of imbibition to irradiation
**Mixture of gibberellin A and gibberellic acid
f Increase over controls expected for simple addition of effects of red light anid gibberellin alone
KAHN-PROMOTION OF GERMINATION BY GIBBERELLIN
goes a rapid initial rise, plateaus, and then declines
rapidly after 12 to 20 hours of water uptake by the
Since synergism between gibberellin
and red light action, as measured by per cent germina-
tion, becomes substantial about 20 hours after sowing
of the seeds, it seems possible that a function of gib-
berellin is to maintain the responsiveness of imbibed
seeds to red light at the maximum level for a longer
An alternative notion is that gibberellin must
be transformed or affect a preparative process before
it influences responsiveness of seeds to red light, and
about 20 hours are required for the gibberellin action
to become effective.
These results lead to the suggestion that gibberel-
lin may affect seed germination along two different
pathways: a. Along a red light dependent pathway,
and b. Along an alternate pathway that functions in
darkness and is not necessarily related to the former.
Possibly these two pathways are developed in varying
degrees in diverse species of seeds.
In lettuce seeds and in seeds of Arabidopsis thali-
ana (L.) Heynh. (11), where gibberellin also is able
to replace red light in germination,
germination in total darkness, the major part of gib-
berellin action seems to be along the pathway which
is not dependent on light.
of Kalantchoe blossfeldiana v. Poellnitz, where gib-
berellin greatly reduces the daily light requirement
for germination but at a concentration of 100 mg/l
fails to bring about any dark germination (4), the
major or entire gibberellin action may be to alter the
i.e., to induce
On the other hand, in seeds
Gibberellin causes dark germination of lettuce
seeds that would not germinate in the absence of a
promoter because of a relatively low supra-optimal
germination temperature; dark-osmotic inhibition; in-
hibition induced by a high temperature pretreatment,
and inhibition produced by far red light.
Thus, gibberellin is capable of replacing re-I
light in every situation examined in which light pro-
motes germination of lettuce seeds.
Brief irradiances of far red light, which re-
verse fully the potential promotion of germination by
red light, do not negate gibberellin-mediated promo-
tion of germination.
Continuous far red light delays gibberellin-
mediated promotion of germination. but under the con-
ditions of these experiments, dloes not prevent germi-
The promotive effects of gibberellin and red
light are additive or less than additive when red light
is administered 4 hours after seeds begin to imbibe
on a gibberellin solution.
istered 20 hours or longer after seeds are sown on
gibberellin, the combined promotion of germination is
greater than can be predicted by simple addition of
the two single effects.
These results suggest that a
part of gibberellin action in light-sensitive seeds in-
creases the responsiveness of the seeds to red light
When red light is admin-
or maintains red light responsiveness at its maximum
In lettuce seeds, this may be only a small part
of the gibberellin effect, but in other seeds it may be
the major or sole gibberellin action.
I thank Professor Anton Lang for his suggestions,
advice, and criticism throughout this study.
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