BRIEF PAPERS
supplied with fixed nitrogen. Plant Physiol. 27:
223-230. 1952.
7. MULDER, E. G. Importance of molybdenum in the
nitrogen metabolism of microorganisms and higher
plants. Plant and Soil 1: 94-119. 1948.
8. NASON, A. and EVANS, H. J. Triphosphopyridine
nucleotide-nitrate reductase in Neurospora. Jour.
Biol. Chem. 202: 655-673. 1953.
301
9. NICHOLAS, D. J. D., NASON, A., and McELROY, W. D.
Effect of molybdenum deficiency on nitrate reductase in cell-free extracts of Neurospora and
Aspergillus. Nature 172: 34. 1953.
10. VIRTANEN, A. I. and RAUTANEN, N. Nitrogen assimilation. In: The Enzymes, Vol. II. Part II.
J. B. Sumner and K. Myrbiick, Editors. Academic Press, Inc., New York. 1952.
POSSIBLE INTERACTION BETWEEN LIGHT-DARK CYCLES AND
ENDOGENOUS DAILY RHYTHMS ON THE GROWTH
OF TOMATO PLANTS 1
HARRY R. HIGHKIN AND JOHN B. HANSON 2
EARHART LABORATORY, DEPARTMENT OF BIoLoGY,
CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA
In the course of an experiment in which plants
were grown under various alternations of light and
darkness, it was found that some plants grown under
conditions which deviated too greatly from a 24 hour
cycle of 12 hours light and 12 hours darkness were
inhibited in their growth.
Sunflowers, peas, and tomatoes were grown from
seed under cycles of 6 hours of light-6 hours of darkness, 12 hours of light-12 hours of darkness, and 24
hours of light-24 hours of darkness. The plants were
grown in vermiculite at 23'C and about 1000 fc of
artificial light from warm white fluorescent lamps
supplemented with light from incandescent bulbs. In
a 48 hour period all plants received the same quality
and quantity of light.
The most striking effects were those on the growth
of the tomato plants (fig 1). The peas showed some
inhibition of growth in the 6-6 and the 24-24 hour
regimes as compared to the 12-12 hour treatment, but
the sunflowers were not significantly affected. Garner
and Allard (3) have shown that short alternations of
light and dark inhibit the growth of many plants.
Arthur, Guthrie, and Newell (1) found that tomato
plants were extremely sensitive to long day length and
will not survive under continuous illumination. The
leaf injury symptoms reported by these authors are
similar to those observed here-small, stiff, yellow
leaves with dark necrotic spots. In addition it was
noted that the number of nodes per plant was not
significantly reduced by the abnormal light-dark treatment. Arthur et al found that the injury to the
tomato plants was not due to an excessive accumulation of carbohydrates, and considered it to be due to
a breakdown of the photosynthetic process. However,
the results reported here were obtained with plants
receiving equal quantities of light in every 48 hour
period so that if photosynthetic mechanisms are involved, they must be sensitive to some superimposed
periodic phenomenon.
BTinning (2) has formulated an hypothesis that
Received August 24, 1953.
Present address: Dept. of Agronomy, University of
Illinois, Urbana, Illinois.
1
2
FIG. 1. The effect on the growth of tomato plants of
12, 24, and 48 hour cycles of alternating light and dark
periods.
the endogenous daily rhythm of plants determines
such photoperiodic responses as flowering. He has
proposed that during one phase (the photophile) of
the endogenous daily rhythm, light promotes the assimilatory activities of the plant, such as those that
affect flowering. At the other extreme of the endogenous daily rhythm, there exists a phase (the scotophile) in which light has no promoting effect or is
even inhibitory.
The effect which light-dark periodicity has on the
growth of tomato plants might well be explained on
the basis of Bunning's hypothesis. Thus plants grown
in the 6-6 and the 24-24 hour regimes would receive
light during all or some portion of the scotophile
phase, if the tomato has an endogenous daily rhythm
of 24 hours (fig 2). That the tomato does have such
a rhythm is shown by such a phenomenon as diurnal
rhythms in bleeding (4). Continuous light should
have the same effect as the alternation of light and
dark periods in the 6-6 and the 24-24 hour regimes,
since any light period in excess of about 16 to 18 hours
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302
PLANT PHYSIOLOGY
would expose the plants to light during advanced
stag,es of the scotophile phase.
If Buinning's hypothesis is applicable to the growth
of some plants as well as to such a phenomenon as
flowering, a light interruption during the normal dark
period (the scotophile phase) should be inhibitory.
An experiment was designed to examine this point.
Tomato plants were grown at a constant temperature
of 23°C, at high intensity greenhouse light for 8 hours,
from 8 A.M. to 4 P.M., followed by supplementary,
artificial light of 1000 fc from 4:00 P.M. to midnight.
After initial growth rates were determined, half
of the plants were given their supplementary light
from 4 P.M. to 10 P.M. and from 2 A.M. to 4 A.M.,
thus interrupting the dark period with 2 hours of
light. Both the control and the treated plants continued to receive a total of 16 hours of light of the
same quality and intensity, with the night interruption being the only variable.
The results of this experiment are illustrated in
figure 3. Dry and fresh weights fully confirm the
inhibition shown by height measurements. Although
highly significant statistically, the inhibition is not as
great as expected. This confirms the observations of
Arthur et al that plants grown in the greenhouse
during the day are more resistant to the inhibitory
effect of artificial light given during the night than
are plants grown entirely in artificial light. A repetition of the dark interruption experiment with the
plants growing entirely in artificial light of about
1400 fc and a constant temperature of 23°C shows a
far greater inhibition of growth.
Biinning's hypothesis, although it gives us no
mechanism to test, might be applicable to growth of
plants such as tomatoes, which are photoperiodically
LEGEND
1
24 hrs
ScotophUie
rN\,
(rN\
A
4
\l-/
m
=3m
E
20-
10-
DAY S
4
FIG. 3. Growth curve showing the effect of 2-hour
light interruption of the normal dark period. The control plants received continuous light for 16 hours, and
8 hours of darkness. The treated plants received 14
hours of continuous light with a 2-hour light interruption of the dark period.
indeterminate with respect to flowering, as well as to
flowering of photoperiodically sensitive plants.
LITERATURE CITED
liqht
_ dark
photophile
50-
I
FIG. 2. Photophile phase: Phase of endogenous daily
rhythm in which light promotes assimilatory activities
of the plant. Scotophile phase: Phase of endogenous
daily rhythm in which light is inhibitory or has no promoting effect.
1. ARTHUR, J. W., GUTHRIE, J. D., and NEWELL, J. M.
Some effects of artificial climates on the growth
and chemical composition of plants. Amer. Jour.
Bot. 17: 416-482. 1930.
2. BUiNNING, G. Uber die Photophile und Scotophile
Phase der endogenen Tagesrhythmik. Planta 38:
521-540. 1950.
3. GARNER, W. W. and ALLARD, H. A. Effect of abnormally long and short alterations of light and darkness on growth and development of plants. Jour.
Agr. Res. 42: 629-651. 1931.
4. WENT, F. W. Plant growth under controlled condi-
tions. III. Correlation between various physiological processes and growth in the tomato plant.
Amer. Jour. Bot. 31: 597-618. 1944.
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