Environmenl~landExperiment~lBotany, Vol. 22, No. 3, pp. 285 to 291, 1982 0098-8472[82/030285-07 $03.00/0 Printed in Great Britain ~ 1982. Pergamon Press Ltd. EFFECT OF LOW G A M M A R A D I A T I O N E X P O S U R E S ON RICE SEEDLING D E V E L O P M E N T * A. O. ABIFARIlNt n d J . l ~ . n . R I Z I ~ F , R International Institute of Tropical Agriculture, PMB 5320, Ibadan, Nigeria and U.S. Department of Agriculture, Agronomy Department, University of California, Davis, CA 95616, U.S.A. ( Received 20 February 1979; in revised form 6 ffuly 1981; infinalform 11 November 1981 ) ABIFARIN A. O. and RUTOERJ. N. Effect of low gamma radiation exposures on rice seedling development. ENVIRONMENTAL AND EXPERIMENTAL BOTANY 22, 285--291, 1982.--In two of four experiments, low dosage gamma radiation significantly increased height of rice seedlings. In the two experiments in which roots were measured, the number of secondary roots per seedling was increased by an exposure of 0.5 kR applied after the rice seeds had been pre-soaked for 24 hr. However, the stimulatory effects of low exposures of gamma rays on rice seedlings were generally small and do not appear useful in stimulating seedling development. INTRODUCTION RADIATION-INDUCED stimulation of plant growth has been observed by many investigators, but there is considerable disagreement on the magni- tude and reproducibility of stimulatory ef- fects, tl'a) For example, BURTON et al. ~a'4) did not observe any significant stimulatory effects of low doses of gamma rays on forage and silage yields of pearl millet, sorghum or corn. Similarly, FOWLER and MAC(~UEEN t5) did not observe any practical change in yield or other agronomic traits of spring wheat, although a significant yield response occurred in 1 of 3 yr. Fowler and MacQ.ueen hypothesized that most of the reported stimulatory effects of low dosage radiation are due to increased seedling vigor, early modifications in auxiliary bud develop- ment, and changes in initial rate of floral dif- ferentiation. SIDRAK and SUESS (9) obtained yield increases from low doses of radiation on tomatoes. BHATTACHARYA and J o s m (2~ observed that low doses of radiation increased seedling height of rice, with the greatest stimulation occurring at 8% seed moisture content. KAWA! and SATO ¢7) also observed stimulation of rice seedling height, when seeds with a 12% moisture content were irradiated. Stimulation of rice seedling height by any means would be useful in rice culture in California, where rice seeds are sown by airplane directly into 15 cm of cool (15-18°C) water. Under these conditions seedlings with vigorous growth and fast root development during the first 2 weeks after sowing would be highly desirable. Even with present tall rice cultivars, seedling vigor as determined by rapid increase of seedling height and root development is often less than desired; the shift to short stature cultivars, which often have less seedling vigor, may create a more serious problem. Also in African countries where rainfall is irregular during the early growth stage of upland rice, and in some deep water rice, there is a need for early growth vigor to obtain good * Research carried out at University of California Agronomy Department during the Senior author's sabbatical leave there. f Present address: West Africa Rice Development Association, P.O. Box 1019, Monrovia, Liberia. 285 286 A .O . ABIFARIN and J. N. RUTGER stand establishment. Therefore the present study was initiated to determine if low gamma radiation exposure of seeds would result in more vigorous seedlings alad better root establishment. MATERIALS AND METHODS Four experiments were conducted with rice seeds which had been subjected to low exposures of 6°Co gamma rays. In Experiment 1, seeds of the cultivars CS-M3 and M7 were irradiated at a 9% moisture content. Radiation exposures of 0.5, 0.75, 1, 2, 4, 6, 8 and 16 kR were given at the Crocker Nuclear Laboratory, University of California, Davis. Immediately after irradiation, seeds were soaked in water for 24 hr at 25°C. This seed soaking period approximates grower prac- tice in California, in which seeds are soaked for 21 18 hr so that they will be heavy enough to sink when sown by airplane directly int• water. The slant board laboratory technique, which shows good correlation with field measurements, t6~ was used to measure seedling vigor. Twenty seeds from each treatment and a control were placed on slant boards. The slant board was held in position in racks placed in Pyrex® trays containing water. The trays were kept in a dark grown chamber for a week at 25°C. This temperature was first used to obtain information on whether growth would be stimulated under opt imum temperature con- ditions. In other experiments 18°C was used to approximate California growing conditions. Height of each etiolated seedling, an indication of seedling vigor, t6~ was recorded and averaged for the seedlings in each replication. The experi- mental design was a randomized block with four replications. In Experiment 2, six cultivars, CS-M3, M7, M9, TOs 2300, Ai-Nan-Tsao-1, and CI 11032, were irradiated at 9 and 14% moisture content. Those with 9% moisture content were taken directly from seed storage bins. The 14% moisture content was achieved by daily additions of 1 ml of water to 100 g of the 9% seed kept at 3-5°C. Radiation exposures were the same as in Experiment 1. Immediately after irradiation the seeds were soaked for 24 hr and placed on slant boards as in Experiment 1. The trays were placed in a dark 18°C growth chamber for 17 days. There were l0 seeds per sample, with four replications in a randomized complete block design. In Experiment 3 seeds of cultivar M7 were soaked for 0 (same as in Experiments 1 and 2 at 9% seed moisture content), 12, 24, 48 and 72 hr before exposure to 0.5, 1, 2, 4, 8, and 16 kR gamma rays. Seeds of the treatments and their controls were placed on slant boards as in Experiment 1 and 2 and kept at 17-18°C in a dark growth chamber. One set of seeds was placed at the top of the slant boards to allow measurement of the roots; another set was placed near the bottom for shoot measurement. Each set consisted of ten seeds. The experimental design was a randomized complete block with four repli- cations. After two weeks of growth, length of primary roots and those secondary roots of 10 mm or greater were measured. The number of 10 mm or greater secondary roots per seedling was also recorded. Seedling height was determined after 18 days in the growth chamber. In Experiment 4 seeds of cultivar M7 were soaked for 0 and 24 hr before exposure to 0, 0.5, 1, 2, 4, 8, 16, 24, and 32 kR. Other aspects of this experiment were as in Experiment 3. Analyses of variance were determined for the data and Duncan's multiple range tests were calculated to determine mean differences. RESULTS In Experiment 1, highly significant differences from the F-test determinations for exposures, cultivars, and their interaction were observed (Table 1). Averaged over both cultivars, the principal effect of the gamma radiation was to depress seedling vigor at the higher exposures (Table 2). The seedling vigor of the short stature cultivar M7 was considerably less than the vigor of the tall cultivar CS-M3, their respective average seedling heights being 83 and 101 ram. Examination of the exposure by cultivar interac- tion revealed that the 0.5 kR treatment signifi- cantly increased the seedling vigor of CS-M3, but did not increase the vigor of M7 (Table 2). In Experiment 2, highly significant differences for exposures, cultivars and the exposure by moisture interaction were observed (Table 1, Expt. 2). On average all the cultivars produced taller EFFECT OF RADIATION ON RICE SEEDLINGS 287 p. c~4 %.. o ~ o9 N Z N o9 o9 o9 u'3 ~D f, : : I ~ t ~ ~ q A v ,+.- 0 .,J ° ~ ~ 3 e~ o9 X o9 ~9 ° ~ 5. o9 = ~a ~3 u o9 , g o9 "~ o9 ~ ~+-- Table 2. Effect of gamma radiation on seedling height of two rice cultivars in Experiment 1" Gamma ray exposure (kg) Seedling height (ram) Cultivars CS-M3 M7 Mean 0 (control 108 bc 90 ab 99 a 0.5 116 a 88 abc 102 a 0.75 114 ab 91 a 103 a 1 109 abc 90 ab 99 a 2 114 ab 91 a 103 a 4 109 abc 88 abc 99 a 6 103 cd 83 bc 93 b 8 100 d 82 c 91 b 16 36 e 42 d 39 c S.E. = 2.36 * Values having same letter within a column are not significantly different at the 5% level according to Duncan's multiple range test. seedlings at the 0.5 kR treatment although the difference between the height of 0.5 kR and that of the control was not significant at 5% level (Table 3). At 14% seed moisture content for the 6 kR treatment seedling height was significantly dif- ferent from the control (Table 3). Relative to the control, the 0.5 kR treatment significantly increased seedling vigor as deter- mined by seedling height, when averaged over both seed moisture contents (Table 3). Mean heights without exposure to gamma rays of the six cultivars, CS-M3, M7, M9, TOs 2300, Ai-Nan- Tsao-1, and CI 11032, were 97, 77, 98, 47, 86 and 96 mm, respectively. The exposure by moisture interaction appeared to be principally due to a greater reduction of seedling vigor by the 16 kR treatment on the 9% moisture seeds than in the 14°/o moisture seeds (Table 3). In Experiment 3 all sources of variation were highly significant (Table 1, Expt. 3). For seedling height, primary root length, and secondary root length, no significant stimulatory effects of gamma radiation were observed; for the low exposures compared to the control at the higher exposures depressive effects occurred (Table 4). A significant stimulatory effect was observed for the 0.5 and 1 kR treatments on the number of 288 A. O. ABIFARIN and J. N. RUTGER Table 3. Effect of gamma radiation on mean seedling height of six rice cultivars at two moisture contents in Experiment 2* Average over both 9% moisture 14% Moisture moisture control Seedling Seedling Seedling Exposure height Exposure height Exposure height (kR) (mm) (kR) (mm) (kR) (mm) 0.5 88 a 6 86 a 0.5 86 a 1 8 6 a 1 85 ab 1 86 ab 2 86 ab 2 85 ab 2 85 ab 6 85 ab 4 84 ab 6 85 ab 4 84 ab 0.75 84 ab 4 84 ab 0 (control) 84 ab 0.5 84 ab 0.75 84 ab 0.75 83 b 8 83 ab 8 83 ab 8 83 b 0 (control) 81 be 0 (control) 82 b 16 71 c 16 79 c 16 75 c S.E. = 1.43 * Values having same letter within a column are not significantly different at the 5% level according to Duncan's multiple range test. secondary roots/seedling, as well as depressive effects at the two highest radiation levels. Examination of the exposure by hours of soak interactions revealed that stimulatory interaction effects occurred only for the number of secondary roots per seedling. Significant stimulatory effects occurred in the seed treatments soaked for 0 and 24 hr before irradiation. All exposures within the 0 hr of soaking treatment significantly increased the number of secondary roots/seedling (Table 5). In the 24 hr soaking treatment, the number of secondary roots per seedling was significantly increased by the 0.5, 1, and 2 kR exposures (Table 5). In Experiment 4, highly significant effects on all four characters were noted for exposures and hours of soak, but significant exposure by hours of soak interaction was observed only for the number of secondary roots per seedling (Table 1, Expt. 4). The significant exposure effect was generally due to a depressive effect of the higher dosages (Table 6). The dosage by hours of soak interaction for secondary root number yielded a significant stimulatory effect at the 0.5 kR ex- posure of the 24 hr soaking treatment (Table 7). Expression of number of secondary roots/seedling (and also of the other three characters; data not shown) was greater at 24 than 0 hr of soaking (Table 7). D I S C U S S I O N These four experiments demonstrate that the stimulatory effects of low gamma radiation ex- posure on etiolated seedling height of rice are small. In Experiment 1 an exposure of 0.5 kR stimulated the seedling height of the tall cultivar CS-M3 by about 7%, but did not significantly stimulate seedling height of the short stature cultivar M7 (Table 2). In Experiment 2, and 0.5 kR exposure on average over both moisture contents stimulated seedling height of all six cultivars, including CS-M3 and M7, about 5% (Table 3). Since the need for increased seedling vigor is more pressing in the short stature cultivar M7 than in the tall cultivar CS-M3, only M7 was included in the subsequent Experiments 3 and 4. However, in Experiments 3 and 4, low level exposure to gamma rays failed to stimulate the seedling height of M7 at all pre-soaking treat- ments including the 0 hr which is the same for experiments 1 and 2 at 9% seed moisture contents. During the first two experiments it was gener- EFFECT OF RADIATION ON RICE SEEDLINGS 289 i p,.. ~S ~ o laO eq eq eq eq ,-.; ,--; ¢:5 ~5 S ~t0 O t t~ a ,fi ~ 0 ,..I ¢...) ~a0 ;> . +.a O o a.a O t~ t t ~ 8 t t3 o ,.. 8 ii tt~ v ~ r . / 2 i t, ~o 6 Z , ,q 6 ¢,q 6 Z 6 tt~ t t3 t ~ c , 4 ~ c5 8 £ ~ao c~ tt.3 . ,a S _= 8 r¢ "3 290 A. O. ABIFARIN and J. N. RUTGER Table 6. Effect of gamma radiation on shoot and root characters of the rice cultivar M7 seeds, averaged over two periods of pre-treatment soaking in Experiment 4" Mean Seedling Primary root secondary root Number of Exposure height length length secondary (kR) (ram) (mm) (mm) roots/seedling 0 (Control) 78 ab 104 ab 29 a 2.2 ab 0.5 77 ab 104 ab 29 a 2.4 a 1 80 a 102 ab 30 a 2.3 ab 2 76 ab 106 a 29 a 2.4 a 4 75 ab 98 b 28 a 2.1 ab 8 74 b 97 b 27 a 2.2 ab 16 67 c 76 c 23 b 2.0 b 24 56 d 45 d 18 c 1.0 c 32 35 e 25 e 11 d 0.6 d S.E. = 1.65 2.28 0.97 0.10 * Values having same letter within a column are not significantly different at the 5% level according to Duncan's multiple range test. ally observed that radia t ion was s t imulat ing root development. Root studies in the last two experi- ments revealed that the n u m b e r of secondary roots per seedling was sometimes increased by low radiat ion exposures. A consistent increase in these two experiments was observed only for the 24 hr pre-soaking 0.5 kR t rea tment (Tables 5 and 7); in both experiments Table 7. Effect of gamma radiation on number of secondary roots~seedling of rice cultivar M7 seeds given two different periods of pre-irradiation seed soaking in Experiment 4* 0 hr 24 hr Exposure Exposure (kR) Root No. (kR) Root No. 0 (Control 2.1 a 0.5 2.8 a 1 2.1 a 2 2.7 ab 2 2.1 a 8 2.6 ab 0.5 2.0 a 1 2.5 ab 4 1.9 ab 16 2.4 ab 8 1.8 ab 4 2.3 ab 16 1.6 b 0 (control) 2.2 b 24 0.5 c 24 1.6 c 32 0.1 d 32 1.1 d S.E. = 2.33 * Values having same letter within a column are not significantly different at the 5% level according to Duncan's multiple range test. this specific t rea tment increased the n u m b e r of secondary roots per seedling by about 25% over the non- i r radia ted control. In general it appears doubtful that radiat ion- induced st imulat ion of rice seedling development is large enough to be commercial ly useful. Much larger s t imulatory effects on rice seedling develop- ment have been produced by gibberellins, tl 0) and appear more promising than radia t ion- induced stimulation. REFERENCES 1. ANONYMOUS (1966) Effectsoflowdosesofradiation on crop plants. Foreword. Inter. Atomic Energy Agency Tech. Rept. Ser. 64. Vienna, Austria. 2. BHAT'rACHARYA S. andJosnI R. K. (1977) Factors modifying radiation induced stimulation in plants: Pre-irradiation seed moisture content. Radiat. Environ. Biophys. 14, 47-51. 3. BURTON G. W., POWELLJ. B. and CONSTANTIN M. J. ( 1971 ) Forage production of pearl millet hybrids grown from seed exposed to low doses of gamma rays. Radiat. Bot. 11, 447451. 4. BURTON G. W., MONSON W. G., HANNA W. W. and CONSTANTtN M. J. (1975) Silage production and quality of pearl millet, sorghum, and corn hybrids grown from seed exposed to low doses of gamma rays. Radiat. Bot. 15, 33-38. 5. FOWLER D. B. and MACQUEEN F. K. (1972) Effect of low doses of gamma radiation on yield and other EFFECT OF RADIATION ON RICE SEEDLINGS 291 agronomic characters of spring wheat (Triticum aestivum). Radiat. Bot. 12, 349-353. 6. JoNEs D. B. and PZTERSON M. L. (1976) Rice seedling vigor at suboptimal temperatures. Crop Sci. 16~ 102-105. 7. KAw~ T. and SATO H. (1966) Some factors modifying the effects of radiation in seed treatment in flee. Pages 151-171 in Mutations in Plant Breeding, IAEA, Vienna. 8. SAx K. (1963) The stimulation of plant growth by ionizing radiation. Radiat. Bot. 3~ 179-186. 9. Smv.AK G. H. and SuEss A. (1973) Effects of low doses of gamma radiation on the growth and yield of two varieties of tomato. Radiat. Bot. 1~ 309-314. 10. Wu Y. L. and PETEnSON M. L. (1979) Effects of growth substances on cool-temperature seedling vigor in rice. Bot. Bull. Acad. Sinica 20, 27-38.