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Effects of radiation on the photoperiodism of Portulaca smallii
Radiation Botany, 1963, Vol. 3, pp. 265 to 269. Pergamon Press Ltd. Printed in Great Britain.
EFFECTS OF RADIATION ON THE P H O T O P E R I O D I S M O F PORTULACA S M A L L H DAVID J. COTTF~ Department of Biology, Alabama College, Montevallo, Alabama (Received 4 March 1963) A b s t r a c t - - F l a t s of Portulaca smallii were exposed in the Emory University Cobalt field to six radiation treatments: 1000 r, 5000 r, 15,000 r at three dose rates, and 30,000 r. These plants and controls were exposed to two photoperiods: a natural photoperlod of 14 hr which favored vegetative growth or a 10 hr photoperiod which approximates maximum floral induction. Increased radiation was found to suppress both vegetative and reproductive potentials, and that reproduction was more sensitive to radiation than growth. Differences in photoperiod responses remained evident but were less marked as dosage increased. 30,000 r was found to exceed the LDs0 dose. R~smm~---A l'Universitd d'Emory, on a exposd des groupes de Portulaca smaUii en plein champ aux rayons du cobalt. Six traitements ont dtd rdallsds: 1000 r, 5000 r, 15000 r & trois ddbits de dose et 30.000 r. Ces plantes et les tdmoins ont dtd exposds & deux photopdroldes: une photopdriode naturelle de 14 heures qui favorise la croissance vdgdtative ou une photopdriode de 10 heures qui se rapproche du maximum d'induction florale. On a trouvd que des doses accrues supprimalent ~ la lois les potentiels vdg6tatif et reproducteur et que la reproduction dtalt plus sensible aux rayons que la croissance. Des diffdrences de rdponses & la photopdriode sont dvidentes mais moins marqudes lorsque la dose s'accroit. On a trouvd que la dose de 30.000 r ddpasse la dose DLs0. Z u s A m m e z t f a s s , m g - - K / i s t e n mit Portulaca smallii wurden im Cobalt-Feld der Emory Universit~it 6 Strahlenbehandlungen ausgesetzt. Letztere waren eine Dosis yon 1 000 r, eine yon 5 000 r, drei yon 15 000 r (bei Anwendung dreier verschiedener Dosisraten) und eine yon 30 000 r. Die bestrahlten sowie Kontrollpflanzen wurden 2 verschiedenen Photoperioden ausgesetzt: Einer natfirlichen Photoperiode yon 14 Std., die das vegetative Wachstum begfinstigte, bzw. elner 10 stfindigen Photoperiode, die ungeFaahr maximaler Blfihinduktion entspricht. Steigende Strahlendosen hemmten sowohl die F/ihigkeit zu vegetativem wit zu reproduktivem Wachstum. Das reproduktive Wachstum war strahlenempfindlicher als das vegetative. Mit steigender Strahlendosis wurden die Untersehiede in der photoperiodischen Reaktion geringer. Die Dosis yon 30 000 r lag fiber der LDso. INTRODUCTION IN A previous d e t a i l e d s t u d y COTTER a n d PLATTO) h a v e shown the i m p o r t a n c e o f light, a n d especially the significance o f p h o t o p e r i o d ism, in the ecological life history o f Portulaca smallii. T h e p r e s e n t s t u d y was u n d e r t a k e n to a s c e r t a i n the effect o f r a d i a t i o n on its p h o t o p e r i o d i c responses. HILLMAN(2) has p r e s e n t e d a g e n e r a l review on p h o t o p e r i o d i s m . R a d i a t i o n effects on p h o t o p e r i o d i s m , p a r -
t i c u l a r l y changes in p h e n o l o g y a n d seed p r o ductivity, h a v e obvious a n d i m p o r t a n t effects on p l a n t p o p u l a t i o n s , y e t v e r y little w o r k on the specific effect o f r a d i a t i o n on p h o t o p e r i o d i s m has been a t t e m p t e d . M o s t studies m e r e l y state t h a t t h e r e is a d e l a y or a n a d v a n c e in t h e t i m i n g o f flowering responses, following e x p o s u r e to r a d i a t i o n , w i t h o u t a n y specific a t t e m p t to a c c o u n t for the effects on the p l a n t s e x p o s e d o r to s u b s e q u e n t generations. VLITOS et al.(e)
265
266
EFFECTS OF RADIATION ON PORTULACA SMALLII
reported a change in the M a r y l a n d M a m m o t h tobacco from a short day plant to one which flowered under long day conditions after exposure to chronic g a m m a radiation of 50 or more r/day for two months. SPARROW et el.(4) described irregular and suppressed flowering as a consequence of reduced branching in tobacco plants exposed to chronic radiation for eleven weeks at dose rates from 50 to 210 r/day. SPeNCeR(5) irradiated sixteen varieties of corms and bulbs with 5200 r and observed a significantly earlier flowering in seven varieties the first year, but did not find significantly earlier flowering times the second year. MCCORMICK and PLATW(3) observed both stimulation and inhibition of flowering in several species of granite outcrop plants following exposure to chronic g a m m a radiation over a wide range of doses. MATERIALS A N D M E T H O D S
Plants were collected shortly after germination from their natural habitat on the margin of M o u n t Arabia, a granite outcrop in D e K a l b County, Georgia, and transplanted into flats for exposure to radiation at the Cobalt-60 radiation field at Emory University, Atlanta, Georgia. These plants were divided into seven groups, one serving as a control which received no radiation, and the other six groups respectively exposed to 1000 r, 5000 r, 15,000 r at a low dose rate (six days), 15,000 r at the normal rate (2 days) and 15,000 r at a high rate (6 hr), and 30,000 r. After irradiation the plants were transported to the campus of Alabama College, Montevallo, Alabama, and 15 plants of a uniform four leaf size were transplanted into 10 in. clay flats. From each of the seven groups two sets of flats were prepared. One of the sets was exposed to the natural photoperiod of approximately 14 hr duration during the experiment while the other was exposed to a rigid 10 hr exposure to light, which approached the m a x i m u m induction cycle.O) To control the light exposure of the plants in the latter group a light-proof chamber was constructed. The chamber was opened and closed daily by an apparatus consisting of two electric timers, two mercury switches, an
electric motor with a reduction gear, and a metal a r m to raise and lower the chamber. Observations began on July 25, 1961, and continued on a twice-a-week pattern for five weeks when the experiment was terminated. RESULTS AND DISCUSSION
Results of the differences in radiation exposure were immediate and obvious. Plants which received more than 1000 r were adversely affected, illustrating the typical retardation of vegetative and reproductive growth which follows exposure of plants to high doses of radiation. Several apparent growth abnormalities were noted in the irradiated plants, and although no specific data were collected on these structural anomalies they appeared to increase with dosage. The 30,000 r group demonstrated a rather dramatic response to the irradiation with almost complete cessation of growth. This group alone manifested the typical responses that these plants undergo following prolonged exposure to drought, i.e., pronounced stunting, reddening of leaves and appression of leaves against the stem.O~ They also failed to demonstrate any response to the different photoperiod regimes and all died from one to four weeks after irradiation, with only two of the approximately sixty plants maintained in the greenhouse showing any growth or reproduction. Apparently this dose level exceeded the LDs0 for P. smallii and exposure to this level of radiation induces an apparent physiological drought, perhaps due to a cessation of root development. A biweekly examination of the other plants ensued to determine any temporal responses to the treatments. Fig. 1 shows the results of three radiation treatments at the conclusion of the differential photoperiod treatment. Plants exposed to the shorter photoperiod consistently bloomed earlier than those in the longer light exposure. Radiation apparently delayed the flowering response only slightly, perhaps to a m a x i m u m of three days. Typically the plants in the optimum, shorter photoperiod had twice as m a n y reproductive units as the 14-hr plants, while the 14-hr plants had three times as great a vegetative growth (Table 1). This photoperiod
FIo. 1. 10-in. flats of P. smallii showing effects of b o t h photoperiod a n d radiation treatments. F r o m left to right flats have been exposed to 1000 r, 5000 r a n d 15,000 r high rate. Those at the top of the picture have keen exposed to 14 h r of light for five weeks a n d those at the bottom, 10 hr.
DAVID J. C O T T E R
267
Table 1. A summary of the effects of radiation and photoperiod exposure leaves/plant
reproductive units/plant
seeds/capsule*
reproductive potential
seeds/leaf
Natural 10 hr
180 55
17.4 31.7
23.70 20.80
412 659
2"29 11.98
1000 r Natural 10 hr
162 55
14.0 31.8
28.49 16.92
399 538
2.46 9"78
5000 r Natural 10 hr
102 34
9.8 18.3
13.70 12.37
134 226
1.31 6.64
15,000 r High rate Natural 10 hr
119 24
11-7 11.2
12.69 8.85
148 99
1.24 4.13
56 19
9.3 8.8
--t 9.26
-82
-4"32
101 16
8"7 10.4
11.01 5.31
96 53
0.95 3.31
Treatment Control
15,000 r Normal rate Natural 10 hr 15,000 r Low rate Natural 10 hr 30,000 r Natural 10 hr
*All figures for seeds/capsule are significant at the 1 per cent level when compared to any other figure, with the exception of those at the 15,000 r level at different rates, tData for seed productivity invalidated by damage by an insect larva before seed harvest.
response was similar t h r o u g h o u t the different radiation treatments, but b e c a m e less m a r k e d at the higher doses. W h e n the average n u m b e r of seeds per capsule is considered, rather t h a n the m o r e obvious a n d less tedious n u m b e r of reproductive units, there is no set pattern illustrated as before. It m a y be noted from T a b l e 1 that in the natural p h o t o p e r i o d a dose o f 1000 r increased the productivity per capsule, perhaps a slight stimulatory effect, but this response is not duplicated in a n y of the other treatments, all of
w h i c h showed a decreased productivity with increased exposure to radiation. I f considered in terms of reproductive potential calculated by multiplying the n u m b e r of seeds by the n u m b e r o f reproductive units, the a p p a r e n t increase o f reproduction by the shorter induction cycle is not as m a r k e d as before a n d this differential decreases "with increased exposure to radiation. These data also indicate t h a t it w o u l d be erroneous to assume that the n u m b e r o f gross reproductive units is truly indicative of the reproductive
EFFECTS OF RADIATION ON POR"TULACA SMALLH
268 12.
h
,o.:
8
w t~
4
,
o
;
';
'
,'5
3o
DOSAGE IN kr GEOMETRIC SCALE
Fro. 2. Effect of radiation on seed/leaf ratio. potential of a plant when exposed to different photoperiods or radiation exposures. Fig. 2 shows that the effects of radiation on reproduction are greater than its effects on growth. The number of seeds per leaf would remain the same if the effects of radiation on reproduction and growth were equal; increase if the effects on growth were greater; and decrease if the effects on reproduction are greater. It is probable that seed production is dependent to some degree on vegetative growth, but the extent of this relationship could not be determined. It is evident that different radiation or photoperiod treatments will change this relationship (Fig. 2). Since the slopes of the two different photoperiods are not completely linear more data on the effects of intermediate doses would be necessary to firmly establish their slopes. The data do not warrant an attempt to compare the effects of radiation on the seed-leaf ratio of the different photoperiods. Differences in the rate of exposure to radiation were not of sufficient magnitude to allow for comparison of the three different dose rates at 15,000 r with one another, but serve to enunciate the trend of reducing the response of the plants, both vegetatively and reproductively, regardless
of the photoperiod to which they were exposed. Increased radiation was found to decrease the total size, number of leaves, branches and reproductive units, seeds, reproductive potential, and the seed-leaf ratio. In addition to the decrease in the vegetative and reproductive expression with radiation, the differences due to photoperiodic treatment were also decreased by radiation. The vegetative potential was emphasized in the 14-hr photoperiod, while the reproductive potential was accentuated by the short 10-hr photoperiod. Increases in radiation exposures decreased the expression of both potentials, but have a greater effect on the reproductive state. The definite differences in the seed bearing capacity of the plants due to exposure to different photoperiods became less marked with increased radiation; nevertheless, the photoperiod remained the dominant controlling factor in the plants' growth except at the highest exposure level. An attempt to determine the ultimate effects of the treatments on the next generation by carrying through the life cycle to a study of germination rates failed. Only about 20 per cent germinated and they were equally divided between the photoperiod treatments and showed no relationship to radiation dose. Normally germination rates of 70 per cent can be obtained with P. smallii.
Acknowledgements The author would like to express his appreciation to Dr. F~Nx~ McCo~mK, Vanderbilt University for his assistance in the collection and radiation of the plants, Mr. M. T. WPaGHT,Alabama College student for his work in the photoperiod exposure mechanism and for making some of the observations of the plants, and for the financial support of a grant from the Alabama College Research Funds. REFERENCES
I. Cow'r~.gD.J. and PLATTR. B. (1959) Studies on the ecological life history of Portulaca smallii. Ecology 40, 651-668. 2. HmLMANW. S. (1962) The physiology offlowering. Holt, Rhinehart and Winston, Inc., New York, pp. 164. 3. McCoRMmKJ. F. and PLATr R. B. (1962) Effects of ionizing radiation on a natural plant community. Radiation Botany 2, 161-188.
DAVID J. C O T T E R 4. SPARROWA. H., GUNCKELJ. E., SCX-~aRERL. A. and HAOEN G. L. (1956) Tumor formation and other morphogenetic responses in an amphidiploid tobacco hybrid exposed to chronic gamma irradiation. Am. 07. Botany 43, 377-388. 5. SPENCERJ. L. (1955) The effect of X radiation
269
on the flowering of certain cultivated bulbs and corms. Am..7. Botany 42, 917-920. 6. VLrros A. J., SHAPIRO S. and MEUDT W. (1955) Responses of photoperiodically sensitive plants to continuous (chronic) gamma radiation. Plant Physiol. 30 (Suppl.): XX. (Abstr.).
EFFECTS OF RADIATION ON THE P H O T O P E R I O D I S M O F PORTULACA S M A L L H DAVID J. COTTF~ Department of Biology, Alabama College, Montevallo, Alabama (Received 4 March 1963) A b s t r a c t - - F l a t s of Portulaca smallii were exposed in the Emory University Cobalt field to six radiation treatments: 1000 r, 5000 r, 15,000 r at three dose rates, and 30,000 r. These plants and controls were exposed to two photoperiods: a natural photoperlod of 14 hr which favored vegetative growth or a 10 hr photoperiod which approximates maximum floral induction. Increased radiation was found to suppress both vegetative and reproductive potentials, and that reproduction was more sensitive to radiation than growth. Differences in photoperiod responses remained evident but were less marked as dosage increased. 30,000 r was found to exceed the LDs0 dose. R~smm~---A l'Universitd d'Emory, on a exposd des groupes de Portulaca smaUii en plein champ aux rayons du cobalt. Six traitements ont dtd rdallsds: 1000 r, 5000 r, 15000 r & trois ddbits de dose et 30.000 r. Ces plantes et les tdmoins ont dtd exposds & deux photopdroldes: une photopdriode naturelle de 14 heures qui favorise la croissance vdgdtative ou une photopdriode de 10 heures qui se rapproche du maximum d'induction florale. On a trouvd que des doses accrues supprimalent ~ la lois les potentiels vdg6tatif et reproducteur et que la reproduction dtalt plus sensible aux rayons que la croissance. Des diffdrences de rdponses & la photopdriode sont dvidentes mais moins marqudes lorsque la dose s'accroit. On a trouvd que la dose de 30.000 r ddpasse la dose DLs0. Z u s A m m e z t f a s s , m g - - K / i s t e n mit Portulaca smallii wurden im Cobalt-Feld der Emory Universit~it 6 Strahlenbehandlungen ausgesetzt. Letztere waren eine Dosis yon 1 000 r, eine yon 5 000 r, drei yon 15 000 r (bei Anwendung dreier verschiedener Dosisraten) und eine yon 30 000 r. Die bestrahlten sowie Kontrollpflanzen wurden 2 verschiedenen Photoperioden ausgesetzt: Einer natfirlichen Photoperiode yon 14 Std., die das vegetative Wachstum begfinstigte, bzw. elner 10 stfindigen Photoperiode, die ungeFaahr maximaler Blfihinduktion entspricht. Steigende Strahlendosen hemmten sowohl die F/ihigkeit zu vegetativem wit zu reproduktivem Wachstum. Das reproduktive Wachstum war strahlenempfindlicher als das vegetative. Mit steigender Strahlendosis wurden die Untersehiede in der photoperiodischen Reaktion geringer. Die Dosis yon 30 000 r lag fiber der LDso. INTRODUCTION IN A previous d e t a i l e d s t u d y COTTER a n d PLATTO) h a v e shown the i m p o r t a n c e o f light, a n d especially the significance o f p h o t o p e r i o d ism, in the ecological life history o f Portulaca smallii. T h e p r e s e n t s t u d y was u n d e r t a k e n to a s c e r t a i n the effect o f r a d i a t i o n on its p h o t o p e r i o d i c responses. HILLMAN(2) has p r e s e n t e d a g e n e r a l review on p h o t o p e r i o d i s m . R a d i a t i o n effects on p h o t o p e r i o d i s m , p a r -
t i c u l a r l y changes in p h e n o l o g y a n d seed p r o ductivity, h a v e obvious a n d i m p o r t a n t effects on p l a n t p o p u l a t i o n s , y e t v e r y little w o r k on the specific effect o f r a d i a t i o n on p h o t o p e r i o d i s m has been a t t e m p t e d . M o s t studies m e r e l y state t h a t t h e r e is a d e l a y or a n a d v a n c e in t h e t i m i n g o f flowering responses, following e x p o s u r e to r a d i a t i o n , w i t h o u t a n y specific a t t e m p t to a c c o u n t for the effects on the p l a n t s e x p o s e d o r to s u b s e q u e n t generations. VLITOS et al.(e)
265
266
EFFECTS OF RADIATION ON PORTULACA SMALLII
reported a change in the M a r y l a n d M a m m o t h tobacco from a short day plant to one which flowered under long day conditions after exposure to chronic g a m m a radiation of 50 or more r/day for two months. SPARROW et el.(4) described irregular and suppressed flowering as a consequence of reduced branching in tobacco plants exposed to chronic radiation for eleven weeks at dose rates from 50 to 210 r/day. SPeNCeR(5) irradiated sixteen varieties of corms and bulbs with 5200 r and observed a significantly earlier flowering in seven varieties the first year, but did not find significantly earlier flowering times the second year. MCCORMICK and PLATW(3) observed both stimulation and inhibition of flowering in several species of granite outcrop plants following exposure to chronic g a m m a radiation over a wide range of doses. MATERIALS A N D M E T H O D S
Plants were collected shortly after germination from their natural habitat on the margin of M o u n t Arabia, a granite outcrop in D e K a l b County, Georgia, and transplanted into flats for exposure to radiation at the Cobalt-60 radiation field at Emory University, Atlanta, Georgia. These plants were divided into seven groups, one serving as a control which received no radiation, and the other six groups respectively exposed to 1000 r, 5000 r, 15,000 r at a low dose rate (six days), 15,000 r at the normal rate (2 days) and 15,000 r at a high rate (6 hr), and 30,000 r. After irradiation the plants were transported to the campus of Alabama College, Montevallo, Alabama, and 15 plants of a uniform four leaf size were transplanted into 10 in. clay flats. From each of the seven groups two sets of flats were prepared. One of the sets was exposed to the natural photoperiod of approximately 14 hr duration during the experiment while the other was exposed to a rigid 10 hr exposure to light, which approached the m a x i m u m induction cycle.O) To control the light exposure of the plants in the latter group a light-proof chamber was constructed. The chamber was opened and closed daily by an apparatus consisting of two electric timers, two mercury switches, an
electric motor with a reduction gear, and a metal a r m to raise and lower the chamber. Observations began on July 25, 1961, and continued on a twice-a-week pattern for five weeks when the experiment was terminated. RESULTS AND DISCUSSION
Results of the differences in radiation exposure were immediate and obvious. Plants which received more than 1000 r were adversely affected, illustrating the typical retardation of vegetative and reproductive growth which follows exposure of plants to high doses of radiation. Several apparent growth abnormalities were noted in the irradiated plants, and although no specific data were collected on these structural anomalies they appeared to increase with dosage. The 30,000 r group demonstrated a rather dramatic response to the irradiation with almost complete cessation of growth. This group alone manifested the typical responses that these plants undergo following prolonged exposure to drought, i.e., pronounced stunting, reddening of leaves and appression of leaves against the stem.O~ They also failed to demonstrate any response to the different photoperiod regimes and all died from one to four weeks after irradiation, with only two of the approximately sixty plants maintained in the greenhouse showing any growth or reproduction. Apparently this dose level exceeded the LDs0 for P. smallii and exposure to this level of radiation induces an apparent physiological drought, perhaps due to a cessation of root development. A biweekly examination of the other plants ensued to determine any temporal responses to the treatments. Fig. 1 shows the results of three radiation treatments at the conclusion of the differential photoperiod treatment. Plants exposed to the shorter photoperiod consistently bloomed earlier than those in the longer light exposure. Radiation apparently delayed the flowering response only slightly, perhaps to a m a x i m u m of three days. Typically the plants in the optimum, shorter photoperiod had twice as m a n y reproductive units as the 14-hr plants, while the 14-hr plants had three times as great a vegetative growth (Table 1). This photoperiod
FIo. 1. 10-in. flats of P. smallii showing effects of b o t h photoperiod a n d radiation treatments. F r o m left to right flats have been exposed to 1000 r, 5000 r a n d 15,000 r high rate. Those at the top of the picture have keen exposed to 14 h r of light for five weeks a n d those at the bottom, 10 hr.
DAVID J. C O T T E R
267
Table 1. A summary of the effects of radiation and photoperiod exposure leaves/plant
reproductive units/plant
seeds/capsule*
reproductive potential
seeds/leaf
Natural 10 hr
180 55
17.4 31.7
23.70 20.80
412 659
2"29 11.98
1000 r Natural 10 hr
162 55
14.0 31.8
28.49 16.92
399 538
2.46 9"78
5000 r Natural 10 hr
102 34
9.8 18.3
13.70 12.37
134 226
1.31 6.64
15,000 r High rate Natural 10 hr
119 24
11-7 11.2
12.69 8.85
148 99
1.24 4.13
56 19
9.3 8.8
--t 9.26
-82
-4"32
101 16
8"7 10.4
11.01 5.31
96 53
0.95 3.31
Treatment Control
15,000 r Normal rate Natural 10 hr 15,000 r Low rate Natural 10 hr 30,000 r Natural 10 hr
*All figures for seeds/capsule are significant at the 1 per cent level when compared to any other figure, with the exception of those at the 15,000 r level at different rates, tData for seed productivity invalidated by damage by an insect larva before seed harvest.
response was similar t h r o u g h o u t the different radiation treatments, but b e c a m e less m a r k e d at the higher doses. W h e n the average n u m b e r of seeds per capsule is considered, rather t h a n the m o r e obvious a n d less tedious n u m b e r of reproductive units, there is no set pattern illustrated as before. It m a y be noted from T a b l e 1 that in the natural p h o t o p e r i o d a dose o f 1000 r increased the productivity per capsule, perhaps a slight stimulatory effect, but this response is not duplicated in a n y of the other treatments, all of
w h i c h showed a decreased productivity with increased exposure to radiation. I f considered in terms of reproductive potential calculated by multiplying the n u m b e r of seeds by the n u m b e r o f reproductive units, the a p p a r e n t increase o f reproduction by the shorter induction cycle is not as m a r k e d as before a n d this differential decreases "with increased exposure to radiation. These data also indicate t h a t it w o u l d be erroneous to assume that the n u m b e r o f gross reproductive units is truly indicative of the reproductive
EFFECTS OF RADIATION ON POR"TULACA SMALLH
268 12.
h
,o.:
8
w t~
4
,
o
;
';
'
,'5
3o
DOSAGE IN kr GEOMETRIC SCALE
Fro. 2. Effect of radiation on seed/leaf ratio. potential of a plant when exposed to different photoperiods or radiation exposures. Fig. 2 shows that the effects of radiation on reproduction are greater than its effects on growth. The number of seeds per leaf would remain the same if the effects of radiation on reproduction and growth were equal; increase if the effects on growth were greater; and decrease if the effects on reproduction are greater. It is probable that seed production is dependent to some degree on vegetative growth, but the extent of this relationship could not be determined. It is evident that different radiation or photoperiod treatments will change this relationship (Fig. 2). Since the slopes of the two different photoperiods are not completely linear more data on the effects of intermediate doses would be necessary to firmly establish their slopes. The data do not warrant an attempt to compare the effects of radiation on the seed-leaf ratio of the different photoperiods. Differences in the rate of exposure to radiation were not of sufficient magnitude to allow for comparison of the three different dose rates at 15,000 r with one another, but serve to enunciate the trend of reducing the response of the plants, both vegetatively and reproductively, regardless
of the photoperiod to which they were exposed. Increased radiation was found to decrease the total size, number of leaves, branches and reproductive units, seeds, reproductive potential, and the seed-leaf ratio. In addition to the decrease in the vegetative and reproductive expression with radiation, the differences due to photoperiodic treatment were also decreased by radiation. The vegetative potential was emphasized in the 14-hr photoperiod, while the reproductive potential was accentuated by the short 10-hr photoperiod. Increases in radiation exposures decreased the expression of both potentials, but have a greater effect on the reproductive state. The definite differences in the seed bearing capacity of the plants due to exposure to different photoperiods became less marked with increased radiation; nevertheless, the photoperiod remained the dominant controlling factor in the plants' growth except at the highest exposure level. An attempt to determine the ultimate effects of the treatments on the next generation by carrying through the life cycle to a study of germination rates failed. Only about 20 per cent germinated and they were equally divided between the photoperiod treatments and showed no relationship to radiation dose. Normally germination rates of 70 per cent can be obtained with P. smallii.
Acknowledgements The author would like to express his appreciation to Dr. F~Nx~ McCo~mK, Vanderbilt University for his assistance in the collection and radiation of the plants, Mr. M. T. WPaGHT,Alabama College student for his work in the photoperiod exposure mechanism and for making some of the observations of the plants, and for the financial support of a grant from the Alabama College Research Funds. REFERENCES
I. Cow'r~.gD.J. and PLATTR. B. (1959) Studies on the ecological life history of Portulaca smallii. Ecology 40, 651-668. 2. HmLMANW. S. (1962) The physiology offlowering. Holt, Rhinehart and Winston, Inc., New York, pp. 164. 3. McCoRMmKJ. F. and PLATr R. B. (1962) Effects of ionizing radiation on a natural plant community. Radiation Botany 2, 161-188.
DAVID J. C O T T E R 4. SPARROWA. H., GUNCKELJ. E., SCX-~aRERL. A. and HAOEN G. L. (1956) Tumor formation and other morphogenetic responses in an amphidiploid tobacco hybrid exposed to chronic gamma irradiation. Am. 07. Botany 43, 377-388. 5. SPENCERJ. L. (1955) The effect of X radiation
269
on the flowering of certain cultivated bulbs and corms. Am..7. Botany 42, 917-920. 6. VLrros A. J., SHAPIRO S. and MEUDT W. (1955) Responses of photoperiodically sensitive plants to continuous (chronic) gamma radiation. Plant Physiol. 30 (Suppl.): XX. (Abstr.).