GERMINATION OF THE LIGHT-SENSITIVE SEEDS OF OCIMUM AMERICANUM LINN.

Transcription

1 New Phytol. (1968) 67, GERMINATION OF THE LIGHT-SENSITIVE SEEDS OF OCIMUM AMERICANUM LINN. BY C. K. VARSHNEY Department of Botany, University of Delhi {Received 30 June 1967) SUMIVT.'\RY A brief light exposure is essential for the germination of dark imbibed seeds of Ocimum americanum Linn. Seeds with testa broken at the radicular end were independent of light requirement for germination. Red and far-red light were found to affect germination in a fashion typical of other photomorphogenic reactions mediated through a phytochrome system. The stimulatory effect of light was not constant if the imbibed seeds were maintained in darkness for long periods. Imbibed seeds entered a secondary dormancy (skotodormancy) under prolonged darkness. This was relieved by cold treatment. The ecological implications of skotodormancy are discussed. INTRODUCTION Light triggers seed germination in a number of plants. Detailed studies on the role of light in the germination of light-sensitive seeds were made by Flint and McAlister (1935, 1937). Later, Borthwick et al. (1952) demonstrated the presence of a reversible red and far-red sensitive pigment system called 'Phytochrome' in Grand Rapids variety of Lactuca sativa seeds. Since then it has been considered that such red and far-red sensitive reactions in seed germination must be mediated through the phytochrome system. Although a large number of light-sensitive seeds have been discovered, only a few have been studied in detail. The present report deals with the germination of seeds of Ocimum americanum. Experiments were done to study the effects of white (W), red (R) and farred (FR) radiations and dark incubation on germination. Attempts were also made to evaluate the role of seed coats in the photocontrol of germination. MATERIALS AND METHODS Seeds for study were collected in February 1965, from plants growing on the Old Delhi Ridge, near the University of Delhi campus. The seeds were cleaned and stored for 8 months at room temperature in glass-stoppered bottles. They were germinated in petri dishes containing Whatman filter paper (No. i) soaked in glass-distilled water. Experiments with W light were done under a light bank consisting of four daylight fluorescent tubes of 40 watts each. R light was obtained by filtering the W light of the fluorescent tubes through a red cellophane paper. Light from three incandescent bulbs of 200 watts each was filtered through blue and red perspex filters and served as a source of FR. Dark treatment was given by covering the petri dishes with aluminium foil and storing in a cardboard box inside a cupboard. Seeds were exposed to light for 5 minutes, except otherwise mentioned, after 10 hours of dark imbibition. Each treatment (with fifty seeds in each replicate) was replicated thrice. Observations were made after 3 days 125

2 126 C. K. VARSHNEY of treatment. Any necessary manipulations were done under a dim green safe light. All experiments were carried out at 25 + i C. The seeds of Ocimum americamim are hard and polished and secrete a large amount of mucilage when wetted. This makes the decoating of seeds difficult. Thus seeds with broken testa at the radicular end only were used for analysing the role of testa in the light requirement of seeds. RESULTS Experiments 7vit/i ivhite light In preliminary trials it was found that a brief exposure to light was necessary for germination. Experiments were therefore designed to determine the minimum light requirement of the seeds to complete germination. The percentage of germination increased with an increase in the length of light treatment up to three minutes (Fig. i). 80r Light Irealmeni ( minutes) Eig. I, The effect of dur.ition of light treatment on the germination of Ocimum americamim seeds. Further increase in exposure time had no stimulatory effect. In all subsequent experiments seeds were exposed to light for 5 minutes to ensure light saturation. The promoting effect of W light on germination was, howe\ er, decreased with a corresponding increase in the period of dark incubation (Fig. 2) before the light treatment. Light treatment induced germination in 60" of the seeds up to 5 days of dark incubation, but decreased rapidly when the period of dark incubation was prolonged further (Fig. 2). After 10 days of dark imbibition only 18 of the seeds germinated in response to the W light treatment. In order to restore the light-sensitivity of the dark imbibed seeds they were subjected to high (60 C), low (15" C) and room (28-32 C) temperatures for 24 hours (Table i) before light treatment. Cold treatment was found to restore the light-sensitivity of the seeds imbibed in the dark for a long period. Experiments with red and far-red light Dark imbibed seeds when exposed to R light for short periods were found to germinate readily. This stimulatory effect of R light on germination indicates the participation of a phytochrome system in controlling the germination of Ocimum seeds.

3 Seed germination in Ocimum americanum o- o 50 40! No of days (dark imbibition) Fig. 2. The effect of dark imbibition on the germination of Ocimum americanum seeds. Table i. Effect of temperature on restoration of Ught-sensitivitv of Ocimum americanum seeds {period of dark imbibition =15 days) Temperature treatment before exposing to white light 60 C for 24 hours 28-32^ C for 24 hours 15' C for 24 hours 25' C throughout the experiment 0/ /o germination Dark period between Rand FR treatments (hours) fig. 3. Effect of dark period, intercalated between red and far-red, on germination of Ocimum seeds. I N.P.

4 128 C. K. VARSHNEY To test this hypotlicsis, dark imbibed seeds were exposed for 5 minutes each to R and FR radiation in a sequence given in Table 2. Red light was found to promote germination. Seeds failed to germinate when irradiated with FR alone or after exposing them first to R radiation. In a series of R and P^R light treatments (Table 2) germination was found to be controlled by the last treatment. In another experiment seeds treated with R light were given different periods of darkness before exposure to the FR treatment (Fig. 3), The inhibitory influence of FR decreased with the corresponding increase in the length of the dark period following the R light treatment. Seeds failed to respond to the inhibitory FR radiation when given after hours of R light treatment. Table 2, Germination of Ocimum americanum seeds after exposure to red and far-red in sequence [each exposure for 5 minutes) Radiation Red Rc'd-FR Red FR-Rcd Red-FR-Red-FR Red-FR-Rcd-FR-Red Red-FR-Red-FR-Red-FR Rcd-FR-Red-FR-Red-FR-Red Dark Mean germination (%) 62, ,0 4,0 61,4 4,0 6s,o 6.0 Germination of seeds with broken testa To evaluate the role of the testa in the photoreaction, seeds with testa broken at the radicular end were used (Table 3). Seeds kept in continuous darkness showed 35/0 germination as against 6% among those in which testa was intact. Seeds with broken testa with W light treatment gave 38/0 germination which is comparable to the response in continuous darkness. In one experiment, seeds were exposed to FR after 10 hours of dark imbibition. The FR treated seeds also germinated (Table 3), It may, however, be noted that the percentage germination of seeds with broken testa was less than those of the intact seeds exposed to W light (i,e,, cf, data in Fig. i). Further, the germination percentage of the seeds with broken testa was not constant in successive trials. The low germination of such seeds in dark or light and in successive trials may result from injury to embryos during decoating operations. Table 3. Germination of seeds zcith damaged testa Treatment / germination Control* 6 Continuous darkness 35 Continuous light 38 Fxposure to FR for 5 minutes after lo hours dark imbibition 29 * Intact seeds maintained in continuous darkness. DISCUSSION Seeds of Ocinmm americanum need a brief light exposure to complete germination. Since the classic paper of Borthwick et al. (1952) it is believed that the effect of ligh' upon germination is based on a reversible photoreaction controlled by R and FR

5 Seed germination in Ocimum americanum 129 radiation. The reversibility of any photomorphogenic process by R and FR radiation is a reliable evidence for the participation of a phytochrome system. Results obtained from R and FR treatments of Ocimum seeds show that the light reactions of the seeds are mediated through a phytochrome pigment system. The reversing influence of I'R was, however, found to be dependent on the period of dark incubation between the R and FR treatments. The inhibitory influence of F'R was lost after hours of R light treatment. This appears to be a common feature among all such seeds (Evenari, 19^6). It has been suggested that if the biochemical reactions initiated by R light have proceeded to a certain point they cannot be reversed by FR treatment (Evenari, 1965). The amount of energy required for promoting or inhibiting the germination by R and FR radiation was not estimated. However, the preliminary findings suggest low energy requirement as was shown for lettuce seeds of Grand Rapids variety (Borthwick et al., 19^2) and Anagalis (Grant, Lipp and Ballard, 1963). It is interesting to note that the W light lost its promoting effect when the seeds were imbibed m dark for long periods. Prolonged darkness induced secondary dormancy known as skotodormancy (Evenari, 1956). After 3 days of dark incubation the seeds rapidly lost their light-sensitivity and ability to germinate. The skotodormancy could be released by subjecting the seeds to low temperature treatment for 24 hours as was shown by Evenari (1956). The interaction of light, temperature and darkness in seed germination appears to be of considerable ecological value. What proportion of the seed crop is maintained in the skotodormant condition in nature is a matter of speculation. But the decided advantage of the skotodormant condition to a species in competition and colonization cannot be underestimated. In addition, allseeds in any particular replicate do not respond to a particular treatment uniformly; rather, they represent various degrees of physiological maturity. The ecological importance of such variability in a population of seeds has been emphasized by Salisbury (1942). The role of testa in the light-sensitive seeds poses a serious problem, and no satisfactory solution is yet available. Ocimum seeds with damaged testa were found to be independent of the light requirement for germination. The way in which damage to the testa replaces the light requirement is not known. Further work is being carried out on this problem. ACKNOWLEDGMENTS I am grateful to Professor B. M. Johri for interest and encouragement. I am also thankful to Dr H. Y. Mohan Ram for critically reading the manuscript and to Professor S. C. Maheshwari for valuable counsel. REEERENCES BORTHWICK, H. A., HENDRICKS, S. B., PARKER, M. W., TOOLE, E. H. & TOOLE, V. K. (1952)..A reversible photoreaction controlling seed germination. Proc. natn. Acad. Sci. U.S.A., 38, 662. EVENARI, M. (1956). Seed germination. Radiation Biology (Ed. by A. Hollender), Vol. 3, pp New York. EVENARI, M. (1965). Photoenvironment: Introductory Lecture. In: Recent Progress in Photobiology (Ed. by E. J. Bowc'n). Oxford. FLINT, L. H. & MCALISTER, E. D. (1935). \Va\-elength of radiation in tbe visible spectrum inbibiting tbe germination of light-sensiti\'e lettuce seed. Smithson. misc. Colhis, 94, i. FLINT, L. H. & MC.'\LISTER, E. D. (1937). Wavelengths of radiation in tbe visible spectrum promoting the germination of light-sensitive lettuce seed. Smithson. misc. Colhis, 96, i. CHANT LIPP, A. E. & BALLARD, L. A. T. (1963). Germination patterns shown by the light-?ensiti\e seed of Anagalis ttrvensis. Aust.J. biol. Sci., 16, 572. ',-, E. J. (1942). The Reproductive Capticity of Plants: Quantitative Studies in Biology. London

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