Plant Physiol. 29(3): 301-2 (1954)


1 Received August 24, 1953.
2 Present address: Dept. of Agronomy, University of Illinois, Urbana, Illinois.

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.

Bünning (2) has formulated an hypothesis that 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 Bünning'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 would expose the plants to light during advanced stages of the scotophile phase.

FIG. 1. The effect on the growth of tomato plants of 12, 24, and 48 hour cycles of alternating light and dark periods.

If Bünning'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 S hours, from S 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.

Bünning's hypothesis, although it gives us no mechanism to test, might be applicable to growth of plants such as tomatoes, which are photoperiodically indeterminate with respect to flowering, as well as to flowering of photoperiodically sensitive plants.

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.   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.


  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. BÜNNING, 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 conditions. III. Correlation between various physiological processes and growth in the tomato plant. Amer. Jour. Bot. 31: 597-618. 1944.