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
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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Action of light on lettuce-
I. Light, temperature, and coumarin as ger-
III. The effect of light on