Radiation-induced changes in rates of photosynthetic CO2 uptake in soybean plants

Environmental and Experimental Botany, 1977, Vol. 17, pp. 27 to 34. Pergamon Press. Printed in Great Britain.

R A D I A T I O N - I N D U C E D CHANGES IN RATES OF P H O T O S Y N T H E T I C C O 2 U P T A K E IN SOYBEAN PLANTS D. J. URSINO, H. SCHEFSKI and J. McCABE Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada

(Received 22 March 1976) UnslNo D. J., ScHEvSKIH. and McCABEJ. Radiation-induced changes in rates of photosynthetic CO 2 uptake in sq),beanplants. ENVIRONMENTALANDEXPERIMENTALBOTANY17, 27--34, 1977.--Soybean plants 18-21 days post germination received acute doses of 3.75 or 11.25 krads of gamma radiation from a 6°Co source. Using infrared gas analysis, rates of net photosynthetic CO 2 uptake were measured at 21 ~'o and 1% oxygen prior to and 5 min to 4 hr following irradiation. At both radiation doses reductions occurred in the rates of net photosynthetic CO2 uptake without significant changes occurring in the rates of CO/evolution in the dark. Furthermore, the extent of the reductions were similar at 21 ~o and 1'~,ooxygen and no differences in stomatal aperture or leaf diffusion resistance were observed between the irradiated and unirradiated plants. These observations suggest that the reduced rates of CO a uptake are the result of a radiation effect on the photosynthetic apparatus and not due to increased stomatal resistance or to accelerated CO 2 evolution from light or dark respiration. In addition, shoot growth, leaf expansion and new leaf emergence were monitored during a 10day period following exposure of the soybean plants to radiation. Reduction in the rate of shoot growth and leaf expansion were evident even at a dose of 750 rads.

INTRODUCTION

contribute towards a better understanding of the relationship between leaf photosynthesis, photoassimilate export and translocation patterns in plants. For such investigations, however, pine is considerably less suited particularly if biochemically-oriented studies are likely to be made. W e have therefore chosen soybean as research material to continue our investigations. Soybean is frequently used in f u n d a m e n t a l physiological research studies and has been shown to be m o d e r a t e l y radiosensitive. (e2) In this report of our initial studies with soybean we are p r i m a r i l y showing the effects of an acute dose of 3-75 krads on rates of photosynthetic C O a uptake measured at the n o r m a l oxygen concentration of 21°/jo. Rates of C O 2 uptake at 1°~ oxygen as well as rates of C O 2 evolution in the d a r k were also measured to d e t e r m i n e if the reduced rates of C O 2 uptake were the result of increases in either light or d a r k respiration. At 1 ~,, 0 2 both the oxygenase activitity of R u D P

T w o PREVIOUS publications resulting from work in our l a b o r a t o r y have focused on the effects of ionizing radiation on various physiological processes in young white pines, t24,z6) O n e of these reported the effects of beta radiation from internal 14C initially photoassimilated as 14CO2 on the translocation p a t t e r n of photosynthetic products. T h e other reported the effects of external g a m m a radiation on rates of a p p a r e n t photosynthesis and dark respiration measured 1-21 days following acute doses ranging from 230 rads to 7.5 krads. For these studies white pine served as suitable research material p a r t i c u l a r l y since this species has been shown to be highly radiosensitive and therefore responsive to low levels of ionizing radia t i o n / 1, ~5.20, 21,28~ T h e studies with white pine demonstrated r a d i a t i o n - i n d u c e d changes in the processes of photosynthesis and translocation and suggested that further studies using ionizing radiation might D

27

28

D. J. URSINO, H. SCHEFSKI and J. McCABE

carboxylase and rates of light respiration are minimal. ~3) A second report (2v) describes the effects of gamma radiation on the translocation of photo-assimilates and the modulation of the responses by post-irradiation application of the phytohormone, IAA. MATERIALS AND M E T H O D S

Soybean seeds (Glycine max. cultivar Harosoy '63) were germinated in vermiculite and when the seedlings were 3 - 5 c m in height, they were transferred from the greenhouse to a controlled environment chamber where they were maintained on a 16-hr light period (27°C) and 8-hr dark period (22°C). Lighting of 700 ft-c intensity measured at the level of the first mature trifoliate leaf was provided by a combination of incandescent and florescent lamps. The pots were watered daily and regularly fertilized with a mineral nutrient solution (Plant Products 20-2020). Plants were selected for use when the length of the first trifo!iate leaf was approximately twice the length of the second trifoliate leaf. Normally the first leaf was 8.9-t-0.6 cm long, the second leaf4.5 -t-0.4 cm and the plants 18-21 days old. Soybean plants while still potted were irradiated singly in the dark radiation chamber of a Gammacell 220 6°Co unit manufactured by the Atomic Energy of Canada Ltd. The average dose rate during the period of this study was 750 rads/ min. Control plants were placed in the dark for a period equivalent to that of the experimental ones in the Gammacell. Rates of CO 2 uptake were obtained by enclosing leaf tissue in a Plexiglas chamber attached to a closed gas-flow system containing a Beckman 215A infrared gas analyzer, a pump and flow meter. Either normal laboratory air (21 °/o 0 2) or specially purchased air (1.0~?/o 02, 0.035'I/o COz, balance N2) was circulated in the system. The volume of the closed system was either 0.32 1. or 0.511. depending on the leaf chamber used. The flow rate was maintained at 4.01 l/rain, the temperature at 25°C. The light intensity was provided by an illumination chamber containing both fluorescent and photoflood incandescent bulbs. The light was filtered through a flowing water shield 4.5 cm in depth. Rates of respiration were obtained using the same system by monitoring rates of CO 2 evolution in the dark.

Measurements of stomatal aperture and leaf resistance to water diffusion were determined on darkened leaves and on illuminated leaves taken from non-irradiated and irradiated plants. During the post-irradiation period which lasted either 15 rain or two hours, the plants were illuminated at either 1600ft-c or 2800ft-c and maintained at approximately 23°C. Stomatal apertures were measured directly from microscope observation of transparent films obtained following addition of acetone: commercial colourless nail polish (1 : 1 ) to leaf impressions made in silicone rubber. ~ls) Impressions made from the lower epidermis of leaves were obtained from at least six plants per treatment. Leaf resistance to water diffusion was measured with a diffusion resistance meter and sensor (Model Li-20s) manufactured by the Lambda Instrument Company. The Mann-Whitney U test was employed for the statistical analysis of all data. The test was performed using the ratio of the post-radiation or final rate to the pre-radiation or initial rate for each time and for each treatment (non-irradiated and irradiated). Differences referred to as significant in this paper are significant at the 5°,i~ level. In all tables, values in parentheses indicate standard deviation. These values were not utilized in determining significant differences. RESULTS

In the first series of experiments, rates of CO2 uptake of intact mature trifoliate leaves were obtained immediately prior to exposure to gamma radiation and at specific times afterwards, ranging from five minutes to four hours. Prior to the initial determination, the plant was retained in the illumination chamber for 30min at a light intensity of 2800 ft-c measured at the level of the first trifoliate leaf. Following the five-minute irradiation period (3.75krads) the plant was returned to the illumination chamber and retained under 2800 ft-c of illumination until the rates of CO2 uptake were again measured, first at 21 ° o O 2 then at 1°/o 0 2. The volume of the closed system was 0:321. and the rates, determined at the two oxygen concentrations, were calculated for an average CO2 concentration of 300#1/1 (ppm). Control plants (non-irradiated) were treated similarly to the experimental ones except that instead of receiving the five-minute irradiation,

PHOTOSYNTHETIC CO 2 UPTAKE IN SOYBEAN the plants were placed in the d a r k for a five-minute period. T h e results of this e x p e r i m e n t are presented in T a b l e 1. Rates of photosynthetic CO2 uptake, whether d e t e r m i n e d at 2 1 % 0 2 or 1% 0 2 (Table 1 ), were always significantly lower following irradiation, irrespective of the time elapsed following irradiation. T h e extent of the reduction was a b o u t 10% . T h e observation that rates of C O 2 uptake at 1% O 2 are higher than at 21% 0 2 was not unexpected since at the low oxygen concentration there is considerably less competition between oxygen and carbon dioxide for the ribulose 1,5-diphosphate carboxylase active site and considerably less metabolism of glycollate with the concomitant release of carbon dioxide in the light. °) Control plants, not exposed to ionizing radiation

29

had rates of C O 2 uptake which did not change significantly over the period from 15 min to four hours (Table 1 ). W h e n the rates were d e t e r m i n e d only five minutes after the d a r k treatment, however, they were found to be 7% less than the initial rate. Nevertheless, in the i r r a d i a t e d p o p u l a t i o n the decrease was 23%, a value significantly higher than that obtained from the controls. In the second study, rates of C O 2 uptake and d a r k respiration were d e t e r m i n e d i m m e d i a t e l y prior to irradiation and two hours following doses of 3"75 or 11-25krads. T h e volume of the closed system was 0-511., the oxygen concentration 21 ~!,, and the rates were calculated for an average C O 2 concentration of 310 # 1 / l . For one hour before the initial rates were d e t e r m i n e d a n d for the two-hour period following irradiation, the plants were

Table 1. Rates of net photosynthetic CO 2 uptake at 21% and 1% oxygenfor leavesof non-irradiated soybean plants and plants which receivedan acute dose of 3.75 krads (mgCO2 hr- l g.fr.wt- 1) 21~'o 0 2 Time following irradiation (rain)

1% 0 2

I ni tial or pre-radiation rate

Final or post-radiation rate

12-2 (2.0)

11-3 (2.3)

12"9 (1.6)

12'9 (1-6)

0

14-5 (0.9)

11.1 (0.9)

-23

15

14.2 (1.I)

12.5 (1.2)

- 12

19.3 (1.1)

17.0 (1.5)

-- 12

30

12.5 (1.1)

11.9 (1.2)

--5

17.4 (1.1)

16.2 (1.5)

- 7

60

14.3 (l.0)

13.3 (1-4)

- 7

19.2 (1.7)

17.2 (1.9)

- 10

240

13-1 (1-9)

11-6 (1.9)

-- 11

17.6 (1.5)

15.2 (2.7)

- 14

Non-irradiated plants 5

15-240 Irradiated plants 5

l,ight intensity = 2800 ti-c, average [C02] = 300/A/1. Values in parentheses show standard deviations.

Per cent change

Initial or pre-radiation rate

Final or post-radiation rate

17.0 (1.1)

17.0 (1.2)

Per cent change

-7

.

.

.

0

.

30

D. J. URSINO, H. SCHEFSKI and J. McCABE

retained under 1600 ft-c illumination. A new seed source was employed for this study and the experiments were conducted during the summer period rather than in the winter as in the first study (Table 1 ). These differences likely contribute to the observation that the rates of C O z uptake at 1600ft-c (Table 2) were higher than those at 2800 ft-c (Table 1 ). Nevertheless, in this study as in the first one, significant reductions in the rates of C O 2 uptake were observed two hours following irradiation (Table 2). I n fact, the extent of reduction was essentially the same despite the fact

the greenhouse where shoot and leaf lengths were measured for a 10-day period following irradiation. During this period control plants exhibited a continuous increase in shoot length leading to an increase of 3.9 cm (Table 3). The two trifoliate leaves continued to expand reaching a final length of approximately 11 cm. From the data in Table 3 it is evident that those plants exposed to g a m m a radiation showed a reduction in shoot elongation and leaf expansion. Furthermore, only in the control plants and plants receiving a dose of 750 rads did a third leaf emerge and expand, to a

Table 2. Rates of net photosynthetic CO2 uptake and dark respiration for leaves of soybean plants prior to and 2 hr following exposure on a single occasion to gamma radiation (mgCO 2 hr- 1g.fr.wt. - a )

Net photosynthetic CO 2 uptake

Dark respiration

Dose (krads)

Pre-radiation rate

Post-radiation rate

Per cent change

Pre-radiation rate

Post-radiation rate

Per cent change

3.75

15-6 (I.2)

13-2 (1.2)

- 15

1-5 (0.2)

1.6 (0.5)

N.S.

11.25

17.7 (1.1)

15-6 (1-2)

- 12

1.5

1.8

N.S.

(0.3)

(0.2)

(O2) = 21%, av (CO 2) = 310 #1/1, light intensity = 1600 ft-c.

that one population received a radiation dose considerably higher than the other. In contrast, no significant changes were observed in the rates of C O z evolution in the dark at either of the radiation doses. Analysis of the epidermal tissue from control plants and plants receiving doses of either 3.75 or 11.25 krads showed no observable differences with regard to the per cent of stomata open or in the average width of the open stomata. For the plants illuminated at 1600 ft-c following treatment approximately 72~!/o of the stomata were open, the average width being 2"3 #m ( + 0" 3). For the plants illuminated at 2800f t-c, the average width was also 2"3 pm although the per cent of stomata open had increased to 760/0. With regard to leaf resistance to water diffusion no differences were observed between the leaves from irradiated plants and the illuminated controls. In the final study, plants 18-21 days old were irradiated and immediately afterwards returned to

length of 5-8 cm and 5.1 cm, respectively. In all these responses, the extent of the response was related to the magnitude of radiation dose. DISCUSSION

The dose of g a m m a radiation primarily used in this study was 3.75krads received on a single occasion during a five-minute period. Quite clearly, this dose was sufficient to reduce the rates of photosynthetic CO2 uptake and to retard normal growth and development monitored as shoot and leaf elongation and leaf emergence. The effects of g a m m a radiation on soybean growth observed in this study are not inconsistent with results reported by others despite the large influence which dose rate, species diversity, stage of growth, season, environmental conditions and time post-irradiation for scoring can have on the magnitude of the response. ~4' s, x0,11) For example, KILLION and CONSTANTIN(10) reported significant reductions in soybean (Glycine max L. Merril var.

PHOTOSYNTHETIC CO 2 UPTAKE IN SOYBEAN

31

Table 3. Changes in lengths of shoot and trifoliate leavesof non-irradiated and irradiated soybeanplants 10 daysfollowing exposure to gamma radiation

Shoot length

1st trifoliate

°/o of nonirradiated controls

Increase (era)

2nd trifoliate

°Jo of nonirradiated controls

Increase (cm)

c~, of nonirradiated controls

Exposure (krads)

Increase (cm)

0

3-9 (O.8)

100

1.5 (0.4)

100

5.7 (0.8)

100

0-75

2.4 (1.0)

62

1.2 (0.5)

80

4.9 (2.4)

86

3.75

1.4 (0.5)

36

0.8 (0.3)

53

3.7 (1.6)

65

11.25

0.3 (0.1)

8

0.4 (0.2)

27

2"2 (0.9)

39

Values in parentheses show standard deviation. Hill) shoot growth, vegetative yield and seed yield following acute doses to the soybean plants at the unifoliate stage ranging from 1"2 to 4.8 krads. A dose of 2.8 krads delivered at 17-8 rads/min was sufficient to reduce the height to 50% of the controls when scored approximately 2 months following irradiation. Using a different variety (var. Kent) of soybean, KXLL1OYet al. ~11~ reported that stem elongation was unaffected at doses of 1.0 and 1"5 krads, reduced by 50% at 2"8 krads and essentially arrested at doses of 3.0 krads and above when irradiated 18 days after emergence and scored at maturity, 55 days post-irradiation. Furthermore, they emphasized the influence that time of scoring (post-irradiation) will have on the extent of the radiation-induced damage. With regard to stem growth inhibition, they identified the most radiosensitive period as the one between 11-25 days after emergence. In o u r study 18-2t day old soybean plants receiving a dose of 3"75krads showed a 64% reduction in stem elongation when scored 10 days post-irradiation (750 rads gave a reduction of 38%). In addition to the retardation of shoot elongation, soybean plants receiving a dose of 3-75krads also showed significant reductions in leaf expansion and inhibition in the emergence on new leaves during the 10-day post-irradiation

period. These significantly large responses can partially be attributable to the high dose rate of 750 rads/min. (4~ WITHERSPOON(29) also using a high dose rate (461 rads/min), reported complete growth inhibition within 1 week when soybean seedlings in the unifoliate stage received 6-0 krads. Despite these apparently large growth responses by soybean to ionizing radiation, its radiosensitivity (with regard to 50% yield reduction) in comparison to 88 other genera of economic plants, places it only in the moderately-sensitive range.~22) Broad bean and pea were considerably more radiosensitive; okra and winter rape the most radioresistant. The reduction in shoot growth reported in this study was evident 1 day after the soybean plants received a dose of 3.75 krads, ~19) for at the end of the initial 24-hr post-irradiation period the irradiated plants showed a stem length increase of only 0.2cm (__+0.04) in comparison to the nonirradiated plants which increased 0.5 cm (__+0.1 ). Although the rate of stem elongation following irradiation was not analyzed earlier than the initial 24-hr period, rates of net CO 2 uptake (apparent photosynthesis) were as reported; the response of this process to ionizing radiation was both rapid and significant. Within five minutes after receiving a dose of 3.75 krads, the rate of CO z

32

D. J. URSINO, H. SCHEFSKI and J. McCABE

uptake showed a reduction of 23%, a value which was significantly higher than the 7'~o reduction observed for the non-irradiated controls. At all other time intervals, the rates of apparent photosynthesis for the control plants did not show any significant change whereas the rates in all irradiated plants were reduced between 5 and 15°i, (Tables 1 and 2). In another study ~19) following a dose of 22.5 krads, the reduction after three hours was 36O'o. Although the rates are expressed in fresh weight units, the same responses were observed when the rates were calculated on a leaf dry weight or leaf area basisJ ~2) Reports of changes in rates of photosynthesis following exposure of plants to ionizing radiation are not numerous although reductions have been reported for several genera of pine, c/' 6, 7, 24, 26) as well as for Viciafaba, ~lv) wheat, (3°) barley, (z3) and algae.(9,14, 30~ In contrast to the published data in which reduced rates of net photosynthesis have normally been observed several hours or days tbllowing irradiation, the data in this paper show a very rapid response--within five m i n u t e s ~ f this process to ionizing radiation. Indeed, the technique used in this study to measure the rate of CO 2 uptake does not permit the monitoring of this response any faster than the minimum five-minute period used. Although the primary site of radiation damage responsible for the reduction in photosynthesis is not identifiable from our data, some possibilities have been eliminated. The reduced rates do not appear to be the result of increased stomatal resistance to CO 2 diffusion since no change in either the per cent of stomata open or in stomatal diameter was observed. Not only is this observation true for soybean but also for white pine (unpublished data) in which needle impressions were obtained using Letraset Protective Coating (Matte). Analysis of the impressions showed over 80% of the stomata of the irradiated pines were fully open one day following a dose of 7.0 krads of gamma radiation. This value, as well as the diameter of the aperture, were equivalent to the non-irradiated illuminated controls. With time, however, particularly after the seve~)th day, the per cent of open stomata in the irradiated pines did decrease, reaching only 14% by the fourteenth day. Reduction in stomatal aperture several days following irradiation has also been reported for broad bean. ~16'17) The 2-3 pm

average width of the stomata observed in our study is considerably less than the 10 pm value which has been obtained for stomata on the lower epidermis of soybean. ~s~ Such a large average width was observed, however, for plants subjected to very high temperatures and light intensities (in excess of 30'~C and 9000ft-c, respectively) and for plants grown under different environmental conditions than ours. The observation that ionizing radiation reduces the rates of apparent photosynthesis at low oxygen concentration (Table 1) by an equivalent per cent as at high oxygen concentration is consistent with our previous data from white pine ~26) and suggests that reduction in the rate of CO 2 uptake following irradiation is not likely the result of an accelerated rate of CO 2 evolution in the light through photorespiration or damage to a site specifically sensitive to 0 2. The suggestion ~2m that a particularly radiosensitive site may be associated with the 'photo' component of photosynthesis has been supported by recent studies ~13'25~ idfintifying the process of photophosphorylation in vivo as being particularly radiosensitive. Unlike one other study, ~3°~complete repair of the radiation damage to the process of photosynthesis was not observed. In contrast to the response which photosynthesis showed, doses of 3.75 or l l.25krads of gamma radiation did not result in any significant change to the rate of dark respiration (Table 2). At a dose of 22-5 krads to the soybean, however, a 31°i~ increase in dark respiration was observed three hours following irradiation. ~xg)These results show a differential radiosensitivity of processes associated with different cellular organelles in higher plants and exclude the possibility of respiratory enhancement contributing to the observed reduction in net photosynthesis even though the extent of mitochondrial respiration in the light is uncertainJ 3~In Chlorella, the differential sensitivity of processes associated with the chloroplast and mitochondrion observed for the soybean and for white pine was not evident. ¢14) Although reduction in the rate of COz uptake in the light has been reported for several genera of plants, differences in experimental procedures make it difficult to compare the radiosensitivity of the photosynthetic process in the various species. Certainly, irradiation of pines ~7'24) and broad bean ~16) with radiation doses considerably less

PHOTOSYNTHETIC CO 2 UPI"AKE IN SOYBEAN than the ones used in the present study with soybean gave larger reductions in the rate of net photosynthesis. However, in those studies, rates were not d e t e r m i n e d as r a p i d l y following irradiation. I n comparison to vascular plants, it would a p p e a r that the photosynthetic mechanism in algae is considerably more radioresistant. ~13, 28) I t would also a p p e a r that C O 2 uptake as an index of the photosynthetic capacity of the algae is more responsive to radiation than the process of oxygen evolution.t13) As stated earlier, o u r selection of soybean as experimental material was to facilitate the comp a r a t i v e e x a m i n a t i o n of various physiological processes to ionizing radiation. T h o u g h app a r e n t l y not as sensitive to radiation as pines, nevertheless the soybean did show a n u m b e r of similar responses, in particular, a r a p i d response of the photosynthetic apparatus, monitored as net C O 2 uptake in air having either n o r m a l or low oxygen concentrations. T h e photosynthetic response was not the result of increased stomatal resistance or accelerated C O 2 e v o l u t i o n - - e i t h e r from light o1" dark r e s p i r a t i o n - - a n d at low doses the photosynthetic response was not a c c o m p a n i e d by any detectable change in m i t o c h o n d r i a l respiration. T h e second p a p e r in this series (27) utilizing similar experimental material and radiation doses describes some r a d i a t i o n - i n d u c e d changes in the export and distribution of photoassimilated carbon. Acknowledgements We thank Mr. JIM DOWL[NGfor his excellent technical assistance in preparing, photographing and examining the epidermal tissue of the leaves. This study was supported by a research grant from the National Research Council of Canada to D.J. Ursino. REFERENCES

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33

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D. J. URSINO, H. SCHEFSKI and J. McCABE

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