Originalarbeiten . Original Papers

Department of General Biology, Vanderbilt University, Nashville, Tennessee, U.S.A.

Effects of Light Quality upon the Growth of Avena Coleoptiles

FREDERICK T. WOLF

Received May 18, 1978 . Accepted August 23,1978

Summary'

Growth of decapitated Avena sativa coleoptile segments, of coleoptile segments which included the tip, and of coleoptiles of intact seedlings was compared in darkness and in light of different wavelengths. In all instances, both in light and in darkness, coleoptile growth in intact seedlings exceeded that of decapitated coleoptile segments, while the growth of decapitated segments was greater than that of segments which included the tip. The expansion of decapitated coleoptile segments is insensitive to light, the resulting growth being comparable to that in darkness. The growth of apical coleoptile segments is inhibited, in comparison with control segments incubated in darkness, at 510-565 nm, but stimulated at 605-700 nm. Growth of the coleoptiles of intact Avena seedlings is inhibited at 455-565 nm and at 660-700 nm, though not at intermediate wavelengths.

Key words: growth, coleoptile segments, light quality, Avena sativa.

Introduction

The widely used Avena coleoptile segment growth test of BONNER (1933) is customarily performed in darkness. With respect to the effects of light upon the growth of coleoptile sections, discordant results are mentioned in the literature. LIVERMAN and BONNER (1953) established that, in the presence of IAA, growth of coleoptile segments is enhanced by red light and reversed by far red light. AGHION et al. (1962) reported that extension in blue light equalled that in darkness, while growth in red light was 1.3-1.4 times as great, and in far red light was 1.5-1.7 times as great as that of dark controls. GrNTILE and KLEIN (1964) however, found no statistically significant effect of continuous blue, green, red, or far red light upon the growth of Avena coleoptile segments, either in the presence or absence of IAA. In experiments involving lOoo-fold differences in light intensity, growth was comparable to that obtained in darkness in all instances. HOPKINS and HILLMAN (1965), working with excised Avena coleoptile sections which included the tip of the coleoptile, reached conclusions regarding the response to red and far red light similar

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2 FREDERICK T. WOLF

to those of LIVERMAN and BONNER (1953) with coleoptiles lacking the tip. In the case of Zea coleoptile segments, EVANS et al. (1977) stated «The growth performance of isolated apical sections with the tip intact is puzzling. Even though these sections have intact tip cells as a source of auxin they grow very slowly in comparison to segments without tips».

It is well known that plants grown in the absence of light are usually taller than light-grown plants under otherwise comparable conditions. Stated in another way, light results in inhibition of growth. In the case of intact Avena seedlings, it has been known since the work of VOGT (1915) and Du Buy and NUERNBERGK (1929) that coleoptiles of plants which had been allowed to develop in light are shorter than dark-grown coleoptiles, by a degree which is proportional to the quantity of light to

which they had been exposed. This conclusion was supported by AVERY et al. (1937) who compared the coleoptile growth of 20 varieties of oats in darkness, during the period until the first leaf protruded, with that of plants grown for 10 hours in light followed by darkness. Coleoptile length was found to decrease with increased light intensities. Analyses of the growth rate of coleoptiles of Victory oats over an extended time period by THOMSON (1950) revealed an increased rate of elongation during the period of most rapid growth in light, as compared with dark controls, but a shortened duration of that period, resulting in a reduction in the final length of the coleoptiles. Continuous irradiation of germinating oat seedlings with very low intensities of light has been reported by JOHNSTON (1937) to increase growth of coleoptiles in comparison with dark controls at wavelengths of 550-720 nm.

Materials and Methods

The oats, var. Carolee, were purchased from Carolina Biological Supply Company, Burlington, North Carolina. Seeds were germinated, and seedlings were grown in plastic trays containing paper towelling and cheesecloth, in darkness, in a cabinet maintained at 25°C.

The experiments were of three different kinds, involving excised coleoptile segments without the tip, coleoptile segments which included the tip, and intact seedlings. In the first instance, segments 10 mm in length were cut from the apical portion approximately 3 mm back from the tip of 4-day-old dark-grown coleoptiles using an instrument consisting of two razor blades mounted in parallel. Groups of 10 coleoptile segments each were placed in beakers containing 5 ppm of indoleacetic acid (IAA) and 2 Ufo sucrose. The light source for this series of experiments consisted of a bank of four Sylvania Gro-Lux fluorescent bulbs. The interference filters used in all experiments were Type Filtraflex-K, nos. 1-7, obtained from Balzers Aktiengesellschaft, Balzers, Liechtenstein. The median wavelengths of the filters were 410, 455, 510, 565, 605, 660, and 700 nm; the half band width in each case was ca. 50 mm. Water to a depth of 8 cm was placed between the light source and the filters to minimize heating. Light intensities were measured using a Yellow Springs Instrument Company Radiometer, Model 65. The light intensity in this series of experiments was 2.0 X 103 ergs!cm2 /sec. After growth for 24 hrs., the lengths of the coleoptile segments were measured to the closest 0.5 mm. The data presented are based upon eight replications of the experiment. Two variations from this experimental design will be mentioned in connection with the results.

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Effects of light quality 3

In a second series of experiments, segments 10 mm in length including the tip were cut from four-day-old dark-grown coleoptiles. Groups of 10 coleoptile segments each were placed in beakers containing 2 % sucrose. The light source was a 750 watt General Electric light bulb. The light intensity at the level of the coleoptile segments was 5.2 X 105

ergs/ cm2 / sec. Coleoptile lengths were measured after exposure for 24 hrs. The data presented are averages of five replications of the experiment.

In the third series of experiments, intact Avena seedlings were used. Because in some instances the first leaf protruded from the tip of the coleoptile after four days of growth, the experiments were performed using three-day-old dark-grown seedlings. Each experiment involved groups of 10 seedlings whose coleoptiles initially measured 10 mm in length, which were placed in beakers containing distilled water. The light source was the same as that used with coleoptile segments which included the tips. Coleoptile lengths were measured after 24 hrs. The data presented are averages of five replications of the experiment.

Results

Growth of the excised coleoptile segments without tips, in the presence of both IAA and sucrose, in the light of varying qualities, was in all instances the same as that in darkness (Table 1). This results is confirmatory of the findings of GENTILE and KLEIN (1964). With reference to the results of these workers, HOPKINS and HILLMAN (1965) suggested that the low pH and high IAA concentration employed in their experiments, which gave rise to a large growth increment, may have obscured any possible light effects. Accordingly, in other experiments which will not be reported in detail, the IAA concentration was reduced from 5 ppm, and 0.05 M phosphate buffer, pH 7.0, was added to the incubation medium. In no instance was any significant difference noted between growth in darkness and that under the various wavelengths of light. Whatever the explanation of insensitivity to light may be, it is not attributable to either of these factors.

Table 1: Final lengths of four-day-old dark-grown Avena coleoptile segments following growth in light of different qualities for 24 hours. Initial length was 10 mm.

Wavelength Coleoptile length Per cent change nm mm

410 20.11 + 1.4% 455 19.81 -0.2 % 510 20.21 + 1.9 % 565 20.50 + 3.3 % 605 20.19 + 1.8 Ofo 660 20.23 + 2.0 % 700 20.04 + 1.0 % Dark control 19.84

Significant differences from the dark controls were found in certain instances in the case of coleoptile segments which included the tip, incubated in 2 Ofo sucrose

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4 FREDERICK T. WOLF

(Table 2). Wavelengths in the region 510-565 nm were inhibitory, and those ranging from 605 to 700 nm were stimulatory to coleoptile expansion.

Quite different results were obtained in the case of the height growth of coleoptiles of intact seedlings upon exposure to different wavelengths of light (Table 3). Marked inhibition of growth, as compared with that of dark controls, was found in the region

Table 2: Final lengths of four-day-old dark-grown Avena coleoptile segments (including tip) following growth in light of different qualities for 24 hours. Initial length was 10 mm.

Wavelength Coleoptile length Per cent change nm mm

410 14.55 + 1.4 Ofo 455 14.30 - 0.4 % 510 13.78 - 4.0 Ofo 565 13.71 - 4.5 Ofo 605 15.73 + 9.6 Ofo 660 15.62 + 8.9 % 700 15.25 + 6.3 % Dark control 14.35

Table 3: Final lengths of coleoptiles of intact Avena seedlings, following growth in darkness fo 3 days and in light of different qualities for 24 hours. Length at 3 days was 10 mm.

Wavelength Coleoptile length Per cent change nm mm

410 25.16 - 1.33 0/0

455 22.67 -11.10% 510 21.93 -14.0% 565 23.28 - 8.81 % 605 25.28 - 0.87 Ofo 660 23.05 - 9.61 % 700 22.65 -11.18 Ofo Dark control 25.50

of 455-565 nm, and also at 660-700 nm, with an intermediate neutral region near 605 nm in which neither inhibition nor stimulation occurred. The plot of degree of inhibition as a function of wavelength thus resembles a bimodal curve.

Discussion

In considering the present data, it should be borne in mind that the decapitated coleoptile segments were provided with both exogenous IAA and sucrose, while the apical segments were furnished only sucrose, and the intact seedlings received no

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Effects of light quality 5

supplement whatever, thus depending upon endogenous sources for both IAA and a metabolizable energy source.

While there was considerable quantitative variation within each type of experiment from one replication to another under presumably indentical conditions, nevertheless the direction of the change (inhibition vs. stimulation) with respect to the dark controls was remarkably consistent in almost all instances.

The lack of differential response of the decapitated coleoptile segments to light is confirmatory of the findings of GENTILE and KLEIN (1964). Since this insensitivity is apparently not attributable either to IAA concentration or to pH, it is tempting to speculate that it might be due to the absence of a special light perception property of the coleoptile tips, such as that which characterizes their phototropic response (WENT, 1926). This is difficult to reconcile however, with the results of experiments with coleoptile segments which include the tip, in which slight inhibition of expansion in comparison with dark controls was observed in the range of 510-565 nm. Definite stimulation of growth was found in the red region of the spectrum (605-700 nm), possibly representing a phytochrome-mediated response.

Upon comparison of Tables 1 and 2, it will be noted that the expansion of segments which include the coleoptile tip, both in the dark controls and in all of the light treatments, is consistently much less than that of decapitated coleoptiles. This finding is in agreement with that of EVANS et al. (1977) with Zea coleoptile segments, which led these workers to postulate the production of a growth inhibitor by the coleoptile tip. Since the growth of coleoptile segments including the tip is also much reduced in comparison with that of coleoptiles of intact plants, it seems probable that the difference in response in the two cases is due to growth promoting compounds contributed to the coleoptile from other parts of the seedling.

In the case of coleoptiles of intact Avena seedlings, all of the light treatments resulted in reduced growth with respect to dark-grown controls (Table 3). The small amount of inhibition found at 410 and at 605 nm is probably insignificant. Reduced expansion of the coleoptiles was very pronounced both at 455-565 nm and at 660-700 nm. It is noteworthy that the response at 605 nm in this case is quite different from that obtained with apical coleoptile segments (Table 2). The writer has been unable to find earlier references to quantitative data in experiments of this kind, and any explanation of these complex interactions of IAA and light with coleoptile growth must await further study.

References

AGHION, J., c. JOUGLARD, and A. LOUSTIoux: Effets d'eclairement colon~s de faible energies sur la croissance de coleoptiles d'avoines in vitro. Physiol. Plantarum 15, 452-456 (1962).

AVERY, G. S., Jr., P. R. BURKHOLDER, and H. B. CREIGHTON: Polarized growth and cell studies in the first internode and coleoptile of Avena in relation to light and darkness. Bot. Gaz. 99,125-143 (1937).

z. P/lanzenphysiol. Bd. 91. s. 1-6. 1979.

6 FREDERICK T. WOLF

BONNER, J.: The action of the plant growth hormone. Jour. Genl. Physiol. 17, 63-76 (1933). Du Buy, H. G. and E. NUERNBERGK: Weitere Untersuchungen iiber den EinfluB des Lichtes

auf das Wachstum von Koleoptile und Mesocotyl bei Avena sativa. Proc. K. Akad. Wetenschappen, Amsterdam 32, 808-817 (1929).

EVANS, M. L., M. SINON, and M. J. VESPER: Further characterization of the spontaneous growth response in Zea coleoptile segments. Plant and Cell Physiol. 18,441-452 (1977).

GENTILE, A. C. and R. M. KLEIN: Absence of effect of visible radiation on elongation of decapitated Avena coleoptile sections. Physiol. Plantarum 17, 299-300 (1964).

HOPKINS, W. G. and W. S. HILLMAN: Response of excised Avena coleoptile segments to red and far-red light. Planta 65,157-166 (1965).

JOHNSTON, E. 5.: Growth of Avena coleoptile and first internode in different wave-length bands of the visible spectrum. Smithsonian Misc. ColI. 96, (6) 1-19 (1937).

LIVERMAN, J. L. and J. BONNER: The interaction of auxin and light in the growth responses of plants. Proc. Nat!. Acad. Sci. (U.S.) 39,905-916 (1953).

THOMSON, B. F.: The effect of light on the rate of development of Avena seedlings. Amer. Jour. Bot. 37, 284-291 (1950).

VOGT, E.: Dber den EinfluB des Lichts auf das Wachs tum der Koleoptile von Avena sativa. Zeitschr. f. Bot. 7, 193-270 (1915).

WENT, F. W.: Concerning the difference in sensibility of the tip and base of Avena to

light. Proc. K. Akad. Wetenschappen, Amsterdam 29, 185-191 (1926).

FREDERICK T. WOLF, Department of General Biology, Vanderbilt University, Nashville, Tennessee, U.S.A.

Z. PJlanzenphysiol. Bd. 91. S. 1-6. 1979.