IBMP experiment “Seeds”: Biological effects in Arabidopsis embryos

Nucl. Tracks R&at. Mm., Vol. 20, No. I, pp. 217-225, 1992 ht. J. Radial. Appl. In&am., Port D F’rintcd in Great Britain

0735-245X/92 SS.00 + .OO Pcrgamon F’rcss pk

THE ESA/IBMP EXPERIMENT “SEEDS”: BIOLOGICAL EFFECTS IN ARABZDOPSIS EMBRYOS (PI); K. GARTENBACH;M. ZIMMERMANN; E. &XoPPE.a,t B. Bm2AN.t TH. WENDNAGEL,f J. U. SCHOTT$and C. HEKMANN§ *Botanical Institute and TInstitute of Nuclear Physics, J. W. Goethe-University, P.O.B. 111 932, D-6000 Frankfurt am Main 11, F.R.G.; SInstitute of Aerospace Medicine, Biophysics Division, DLR-German Aerospace Research, P.O.B. 90 60 58, D-5000 KBln 90, F.R.G. and #Centre de Recherche NuclCaire, rue du Loess 62, F-67084 Strasbourg, France

A. R.

b.hNZ

(Received 13 May 1991; injina/Jorm

11 June 1991)

Al&ret-The aim of the experiment “Seeds” flown on the Soviet satellite Biokosmos 9 was a more detailed investigation of biological effects caused by cosmic radiation (CR) at special geometric loci of dry seed embryos of ArubidopsisthalianuHeynh. (L.) representing radiation-sensitive targets. Combined with statistical evaluation, the damage endpoints are assigned to individual interpenetrating, ionizing cosmic particles and their physical parameters, following the Biostack principle. Two types of ESA-ESTEC exposure units were flown: a low-shielding unit, Type I, mounted at the surface of the satellite (1.4 g cm-* shielding) and, for comparison, an identical item inside (16 g cmez shielding) using nuclear emulsion as track detectors. A Type II unit, flown inside (18 g cm-’ shielding) was supplied with AgCl-detectors; the layout is briefly described. A first set of dosimetric and biophysical data obtained from AgCl-detectors of the inside unit II and subdivided into charge- and LET-groups hints at a considerable contribution of the intermediate LET-group (350-1000 MeV cm-‘) due to medium heavy (2 = 610) particles, Exemplified biophysical data from the exposure of non-metabolic plant seeds of Arubidopsisare compared. The exposed seed embryos representing different biological targets, the stem cell initials (di/tetraploid cell), meristemic tisues (shoot/root), organs (cotyledon, hypocotyl, radicle) and organisms (seed embryo) proved to be successful model systems for a biotest of the physiological stress (germination delay), somatic defects (tumorization) and of the lethal damage induced by the cosmic radiation. The exposure inside and outside the spacecraft, shows evident shielding effects.

1. INTRODUCTION FREE-FLYERbiostacks have been part of a series of biophysical experiments in space during the last two decades. Using the original Bucker-biostack sandwich container (Kranz, 1990) in which layers of biological subjects such as bacterial and fungus spore, plant seeds, or invertebrate eggs alternate between sheets of nuclear track detectors, comparison of damage (lethality) in the test organisms has been made corresponding to the impact parameter of heavy ions (HZE-particles) determined for single hit and non-hit events. Recent experience (Kranz et al., 1990), however, suggests a need to pay attention to the accompanying background of the other components of the CR, too. Mutagenic effects have been observed in Arabidopsis seeds which had not been hit by an HZE-particle. The experiment “Seeds” is aimed at differentiating the effects of CR for various components and their quality factors as well as to distinguish the biological damage for the smaller target of diploid and the larger one of tetraploid meristem cells for the root and shoot of the seed embryo. Therefore, the conditions of the Biokosmos 9 (Cosmos-2044) experiment differ in many respects from those of the Biokosmos 8 flight (Zimmermann et nf., 1988; Kranx

et al., lWO), i.e. as to the detector hardware as well as to the genotype of the biological subject. 1.1. Physical and biological objectives The spectral composition of CR in the near-polar orbit of the Biokosmos 9 mission (e = 82.7”) is different from that of the Biokosmos 8 (@ = 63’), the SL 1 (8 = 57”), and from the planned ERA-mission (@ = 27”), as a consequence of the geomagnetic cut-off. In near-polar orbits, particles of lower rigidity are admitted, i.e. particles of lower energy and of higher charge and LET (Heinrich et al., 1989). Table 1 presents an estimate of expected fluxes of various components of the CR and the energy transfer onto the Arubidopsis seeds during the 352 h mission of Biokosmos 9. The Table shows a weak background-burden of the single seeds by 2 x 10’ fast protons (line 1); they produce nuclear disintegrations (stars) and their fragments in the surrounding matter (line 2). Enders of protons (and of other light particles, see line 4) may contribute to the biological effects with respect to the increasing Quality Factor towards the end of their range (Et&e, 1971; Folkhard, 1989). For the same reason we have listed in line 3 the flux of albedo-neutrons of < 10 MeV energy @chopper et al., 1967; Dudkin et al., 1990) 217

A. R. KRANZ

218

et al.

Table I. Expected fluxes and energy-transfer onto Arabidopsb during the Biokosmos 9 Mission E MeV/nucl.

LET I Mev/cm&O n/cm* x d

m=r.F “-dY%tti hlto NW

t&m’

flgun oi ovuli

z/e/c

fsst p

stars p, 0 from

12

n (slbedo)

IO

p, enders

2eEc2.4

40

%

1.5Xld

2x10’

5

1 x10’

1.4 x I@

3 (p)

400

3

40

5 x lo-a

ZlOOO

-20

280

0.7

\\ \\ \\ \\

-19

/I

lo

-1.2

30 pm HZE

@srs

N-4

3.5 x lo? N’d”

4.9x lo) 0”

1.1 x lo-’

40

28

-5

0.5

F=2.5x10’cmZ

and their equivalent of secondary protons from (n,p) reactions in the tissue. As enders, we are considering only protons entering the seed with energies between 2 and 2.4 MeV, and stopping within a layer at 70 + 15 pm depth, the expected locus of the seed meristem. Accelerator experiments complementing our argument are now under way. In order to cover an adequate spectrum of these various groups, we decided on detectors with a broad range and low threshold of sensitivity (LET ,. 10MeV cm-’ H20): AgCl-detectors and K2nuclear emulsion on glass-support, recording protons up to -40 MeV (Baican et al., 1990a). They also allow us-at least in some characteristic cases-to localize vertices of stars originating inside the seed, from their outgoing prongs. The biological contribution of these stars should not be neglected (line 6).

2. DESCRIPTION

(4

Reference cross Projection

Bio-Objects I

Gloss- Frames

I

,I

IL

Glass Support 170pm 130pm /Iir Gap

,

Errhsion

Scaling

(i’lass Plate FIG.

l(a).

Cross-section

of

a compartment

200pm lOOO*m

of Type 1 unit.

OF THE EXPERIMENT

2.1. Hardware construction 2.1 .l. Biorack Type I (ESA ). A low-shielding unit was mounted at the outside of the satellite (OFI) protected during the flight by 1.4 g cm-’ stainless steel; for comparison an identical unit was flown inside the satellite (IFI) behind about 16 g crnS2 Al. Figure 1 shows its layout: the detectors, consisting of 200~pm thick K2-emulsion on glass plates (Ilford Ltd), are fixed on glass frames (A) and kept at a small, well-defined distance by the glass frame (B) from the biological layer; the latter consisted of seeds fixed with Luviskol upon a 170~pm thick glass plate which was mounted at frame (C). This structure allowed the emulsion to be kept in a closed small gas volume at the 60% constant humidity (and thickness) needed, separated by the thin glass plate from the

FIG. l(b). Mounting of the compartments. Compartments of the Type I units: (A) support frame of the nuclear emulsion (0.75 mm); (B) distance frame (1.50 mm); (C) support frame of the 0.17 mm glass plates bearing the seeds (0.75).

ESA/IBMP

EXPERIMENT

“SEEDS”

219

seeds which were kept in a closed volume of dry argon gas. The frames ABC, glued to each other during the Sight as a compartment, could be separated after the flight. Reference marks of about 10pm width, evaporated onto the thin glass plate support of the bio-objects, and projected by light onto the emulsion surface, made the following possible:

contact. The detectors were mounted on glass plates which were kept apart by glass frames, thus forming gas-tight compartments tilled with dry argon.

(1) to localize and to assign biological objects and particle tracks coordinated by the reference marks on the glass and the emulsion; (2) to handle and to store the bio-objects and the detectors in separate procedures; and finally (3) to evaluate them with our electronic imageanalyzing and computer-controlled microscope systems separately (Baumgardt et al., 1986a, b; Baican et al., 1986).

(1) l/l container Biorack Type I for outside exposition (OFI, OBI); (2) l/l container Biorack Type I for inside exposition (IFI, IBI); (3) l/l container Biorack Type II for inside exposition (IFII, IBII).

The nuclear emulsions were developed in Strasbourg (CRN) after a test development at the IBMP, Moscow. Two out of the five emulsion sheets of the units OF1 and three of IF1 were evaluated by IBMP. 2.1.2. Bioruck Type II-unit (ESA). This unit of higher shielding (_ 18 g cm-* Al) was flown only inside the satellite and supplied with AgCl-detectors (Fig. 2). A matrix of LEDs, powered by four Li batteries (selected for constant high power by the manufacturer SONNENSCHEIN/LITHIUM, Biidingen, F.R.G.) provided the yellow light necessary for the stabilization of the tracks in the AgCl-detectors during the tlight (Baican et al., 1986, 1990a). The monocrystalline, 150~pm thick layers of AgCl (Cd) on glass supports sealed with a 2-pm thick coating (Epoxite), carried the Arubidopsis seeds in direct

Glass AgCl +Bio

LED

4 Li -Batt

FIG. 2. Cross-section of Type II unit with AgCl detectors.

2.2. Experimental conditions Flight- and backuphardware respectively, sisted of the following ESA/ESTEC-units:

con-

Biological objects were as follows: Dry seeds of Arubidopsis thulium of the following ecotypes (1) En-2 2n (AIScharge 2590, seed size by sieving > 250 pm); (2) En-2 4n (AIScharge 999, seed size by sieving >250 pm). Detectors used were: AgCl-mono crystalline layers on glass support; nuclear track emulsions K2 (Ilford) fixed on l-mm thick glass support. Fixing of biological objects: On the detector surface with small droplets of Luviskol VA64 (10 g(lO0 ml)-’ w/v ethanol 50%). The dry seeds, containing embryos of the crucifer plant Arubidopsis thuliunu (L.) Heynh. were flown for 352 h (for further flight data see elsewhere in this issue). The seeds were fixed on glass supports, in the Biorack-units type I as described above. One unit (OFI) was exposed outside; the others were installed inside the satellite. The temperature profiles of the flown seeds inside and outside the satellite were simulated on the ground with an identical backup control sample (IBI, OBI). Additional controls were: lab control CI and SC, using the original seed sample, the latter not fixed on detector sheets. One point deserving attention is the influence of the gas atmosphere and outgassing of structure material onto the seeds and the detectors during the pre-flight, flight and post-flight phase of storage. Earlier compatibility-tests extended over several months with various structure materials (plastic, glass, quartz glass, glueing material) and various gases (oxygen, argon, humidity) have partly shown strong influence on certain fungus spores in particular after long exposure, but non-significant effects on Arubidopsis seeds. To be on the safe side, we have therefore chosen inert glass for the structure material and dry argon gas for the seeds (Baican ef al., 1990b).

A. R. KRANZ

220

&,

Y,2,

AgCl

FIG. 3. Assignment particle/seed (schematic). Scheme of a cross-section showing the geometry of the assignment of tracks to a seed locus (hatched).

2.3. The particle -track assignment

The track in the AgCl- and the nuclear emulsiondetectors of the Frankfurt group have been evaluated at two image-analyzing and computer-controlled microscopes installed at the Institute of Nuclear Physics of the University of Frankfurt (Baumgardt et al., 1986a, b), concerning: (1) number and geometric data (coordinates of the tracks); (2) densitometric data (LET-values and charge) of the particles. Figure 3 shows schematically the geometrical conditions of the assignment of particle tracks to special loci of the seed, here for the AgCl-detectors in unit II. The description of the geometry in unit I, using nuclear emulsion, will be published separately, together with biological results from unit I. We determined the coordinates (x,, . yo. q,) of all tracks (penetration point of the detector-surface) and the regression equation (i.e. cos cp, 9) of every particle

et al.

above a given LET-threshold, from five points of its track. A measure of the precision of these data may be the following: the coordinates x, y of the intersection point of the extrapolated track with a plane at z = 150 pm above the detector surface have a mean error of k 5 pm, sufficiently small for the assignment to a spherical meristem of 50 pm diameter inside the seed embryo (Fig. 4, Table 4).

2.4. The seed-track assignment Data relevant for the assignment of track to seed by the biologists are stored off-line for transfer to a compatible analyzing system, installed at the Botanical Institute of Frankfurt University. This PC-assisted (Tandon Pat 386sx, Leitz Orthoplan + video camera) system allows the three-dimensional measurement of the seed and its embryo enclosed inside with its shoot and root meristem cells (Fig. 4) (Bork and Kranz, 1983). Finally, all data of the track distance to the target of the seed embryo-the impact parameter-and its correlation to biological damage endpoints are listed by a specific software program. With AgCl-detectors the seeds remain fixed by Luviskol adhesive to the detector and stored in the dry atmosphere of an Optivac-container, until the end of the seed embryo measurement and the final biotest. For this last step the seeds are removed from the detector for an individual biological investigation under aseptic in t&o culture of a growth chamber (Bork et al., 1986). Finally, the detectors are transferred back to the physicists for the evaluation of those tracks which have not been detected. being obscured below the non-transparent seeds above.

FIG. 4. SEM-photos of the seed embryo (left) of the diploid Arabidopsis the dimensions of the shoot (s) and root (r) meristem

lhaliana (WT line 151-2) showing (left and right).

ESA/IBMP

EXPERIMENT

“SEEDS” IO-2

This method of evaluation-the seeds staying in their original position above the detector also during the development of the latter-while offering the advantage of high geometric precision of the assignment of track to seed, has the disadvantage that those seeds provided for precise assignment are not available for biological investigation until the end of the phase of all geometric track and seed measurements. For a future designed experiment of this kind (BION 10) we are proposing a layout with AgCl-detectors similar to that applied in unit I with nuclear emulsion (Fig. 1) which allows separate but simultaneous treatment of detectors, tracks and seeds.

3. PRESENT

221

r

-6-b

CC

RESULTS

3.1. Physical data

IO.71 IO’

IO’

IO’

Data from the physical evaluation of a part of the detectors are presented in Table 2 and Fig. 5. Table 2 contains global data from both units, showing a decrease of the number of particles with LET >20 MeV cm-’ by a factor of about 4.5 behind the shielding (16 g cm-*) of the inside unit, whereas the number of stars is changing much less. Figure 5 presents the integral LET-spectrum measured at the inside unit II with AgCl-detectors. Fading effects of the latent tracks in both types of detectors, caused by the time-delay between flight and development as well as by increased temperatures during the landing phase (> 60°C at the outside units) are corrected by inflight calibration using the prongs of nuclear stars. In Table 3 the LET-distribution is subdivided into charge- and LET-groups; besides the absolute numbers of particles it shows the percentual ratio within the various charges and groups (a, b, c, in Fig. 5). The

104

LET McV/cm

Water

FIG. 5. Integral LET-spectrum, Biokosmos 9, Unit II inside K2 emulsion (shielding 16 g cmm2);AgOdetectors (shielding 18 g cm-*).

argument for the choice of these groups is the following: corresponding to present knowledge and to data of the literature (Letaw er al., 1987) we are regarding them as typical regimes of LET, to which we tentatively attribute the mean values of relative efficiency crrl of 1 or 8 or 20, respectively. As the product n x t,, we find numbers representing the relative contribution of the various particle groups, for a global evaluation of the radiation field. They hint at a considerable contribution of the intermediate group b, belonging to light and intermediate nuclei (to be compared with our experimental findings in Table 5).

Table 2. Dosimetric data from Biokosmos 9, seeds experiment

Unit I outside II inside

Shielding (g cm-?)

Dose rad in LiF

Detector

0.005* 1.4 16.0 18.0

130 1.2 0.2 -

K2 K2 AgCl

LET MeVcm-’ water >20 >20 >50

Particles N(>LET) cm2 s-i

Stars g-’ d-’

Prongs

1.7 x 1o-2 3.8 x lo-’ 2.7 x lo-’

386 f 56 301 k 69 395 f 18

7.4 f 0.6 9.0*0.1 6.1 kO.2

*By Kapton foil. Table 3. LET spectrum on Biokosmos 9 behind 18 g cm-’ Al. LET-groups attributed to charge groups %

N (LET) % (MeVcm-’ 150-350 350-1000

Z

Ncn-l-rXS

l-5 p, a enders 6-10

6.47 x IO-’ 2.50 x lo-’ 7.15 x IO-4

44.5 (2.0) 49.5

34.5 15.0

10.0 (2.0) 31.4

226 11-25 z &I N x c,~

3.40 xx 1O-J 10-J 5.20 1.52 x IO-’

2.3 3.6

-49.5

- 1.2 42.6

I -50

water) >I000

3.1 ::r: 7.8 20 -155

222

A. R. KRANZ

et al.

Treotmrnt 0 SC 2n + CI 52n l

x

OBIl2n OF1 12n

A SC4n

4

2

0

6

8

12

IO

Days

FIG. 6. Effect of Right conditions on seed germination. (SC) fresh control; (CI 5) control Type I container (No. 5); (OBI 1) outside backup Type I container (No. 1); (OF1 1) outside flight Type I container (No. 1).

3.2. Biological data 3.2.1. General vitality test. In a first step, quick

information about the general condition of the flown and backup (control) material of seeds was obtained by testing the rate and dynamic of germination. Lethality of seedlings obtained at the end of the germination process shows clearly the higher sensitivity of the seeds exposed in space (OFI) in comparison to the control samples SC CI, OBI (cf. Fig. 6). An additional result obtained is shown by the different growth dynamic observed for the seed germination and for the flowering of seedlings. Both processes are delayed depending also on the shielding position inside vs outside. The larger target size of the tetraploid cell nuclei results in maximum delay mainly of flowering (Fig. 7).

The physical background of such growth delay induced by cosmic ionizing radiation is an interesting aspect for future experiments. We assume that hormonal control by auxin and/or gibberellic acid (GA) is concerned. Earlier findings by Reinholz (1967) have already shown that GA (0.002 to 0.2%) reduced the inhibiting effect of X-irradiation on seed germination in the range of 100400 kR. The beneficial effect of GA (0.1%) and cysteine (0.1%) on y-irradiation seeds (36-78 kR) had been oberved additionally by Kucera (1966). We will make use of this earlier result in the proposed Biokosmos 10 experiment with Arabidopsis, prepared for the Kosmos-flight mission in 1991. Investigations of this type may contribute to the general problem of radioprotective substances applied to biological subjects exposed

loo-

80

.7 .

.

*

x

-

0s

/

v)

20 “0

.

Go-

Trcotmrnt

z

sczn

+

CI 5 2n

+

t ,o LL

0

40

-

-, .

.-

20

I

/

. .

”

20

21

22

23

24

2,

26

27

20

29

20

31

.

081 l2n

x

OFI 1213

.

SC 4n

.

CL 54n

A

081 14n

0

OF1 1411

32

Days

FIG.

7. Effect of flight conditions on the temporal development of first open flower.

ESA/IBMP

EXPERIMENT

“SEEDS’

223

Table 4. Biological damage of ArobidopsiP seedlings individually correlated to impact parameters (< or > 50 rm) between cosmic nuclear tracks (AgCl monocrystal detector) and the embryonic .mehstem(s). The ts-test of A% relative to BC (Sokal and Rohlf, 1987) Diploid line En-2 Arabidoosis _ thaliana Distance of impact Sample size N Survival Germination delay (I >120h) Somatic defects Normal plants

Flight unit (IF II) detector AgCl 84119 Hit (root or shoot) meristems d <50pm 25 16 (64%y 9 (56%). 6 (38%)* 1 (6%).

Non-hit meristems d >SOgm 224 137 (61%p 53 (39%).

Backup

(BCI , _ Stacked

ground

control loo 69 (69%)

40 (29%)* 61 (45%).

1 (1%)

0 (0%) 68 (99%)

Seed control (SC) , ,

Non-stacked seed sample 100 13 (73%) 0 (0%) 0 (0%) 73 (100%)

lP < 0.001 (significance for a at p).

to the space environment and, thus, the basic question of stress and risk of space mission (Kranz, 1990). 3.2.2. Specific nuclear track correlated biological endpoints. The usual situation in radiation biophysics is to transfer to the target volume a homogeneous dose. The cells are considered to respond to radiation by lethality or survival as independent entities and the cell nucleus must be hit with its DNA-molecules as the final target. This model fits to unicellular conditions (e.g. spores of bacteria and fungi) rather than to multicellular tissues of individuals such as embryos, seeds, etc. Their larger target volumes have to be considered in toto. The nuclei of meristem cells in Arabidopsis have a volume of 20 pm3; the embryonic meristems of a seed cover an estimated volume of -. 50 pm in diameter, and the whole seed embryo resembles an elipsoid of 350 x 500 pm diameter (Figs 3 and 4). Furthermore, in the case of CR in space these targets are confronted with a mixture of particles of various mass, charge and energy. It is thus reasonable to consider the influence of their specific type of energy deposition in targets of different size. A special analysis of this kind has not been attempted or published elsewhere. Table 4 gives an example of the result of an analysis of particle track-seed assignment with the nuclear AgCl track detector (sheet No. 84/19 of experiment Biokosmos “Seeds”) and the diploid Arabidopsis (WT line En-2). Response to radiation was investigated by the endpoints: survival, seed germination delay, somatic defects (tumorization, abnormal growth), and the number of normal or abnormal plants. Root or shoot meristems hit by an HZEparticle (impact parameter d < 50 pm) are considered. The differences in survival between flight unit (IFII) and controls (BC, SC) are non-significant; germination delay, however, increases significantly as well as the number of seedlings showing somatic defects; in total the frequency of normal plants decreases drastically with particle hits inside the root or shoot meristem.

With larger objects like our seeds an additional aspect should be considered: most of the seed embryos so far analyzed were hit, in fact, by several cosmic particles. Really non-hit seeds are rare; with a mean number m = 4.5 hits seed-’ by particles above N 350 MeV cm-’ LET-threshold, the probability for 1 hit seed-’ is Pi (m) = 3 x 10m2.The sample size of the observed 25 meristem-hit seeds from a total of 250 seeds fits well with the flux of particles of a LET threshold of -350 MeV cm-’ (Fig. 5) hitting a spherical target of 50pm diameter. Table 5 contains the result of a preliminary attempt to correlate the distribution of the biological endpoint in meristem-hit seeds to the three groups of LET in Table 3 of interacting particles. With respect to the small number of seeds falling into the group of LET > 1000 MeV cm-’ with a total sample of 250 seeds, we have combined the groups of LET 350-1000 MeV cm-’ and > 1000 MeV cm-‘. Although the sample sizes of present available data are too small for statistical tests of significance, the tendency to different responses is evident. It Seems that with increasing LET, while survival does not decrease, more somatic defects at the cost of normal plants are obtained, the contribution of the group “a” is not negligible. An example of a somatic defect occurring in a meristem-non-hit embryo is given in Fig. 8; it shows the photograph of a tumor in seed 130, hit by three particles.

Table 5. Distribution of biological endpoints in “meristemhit” seeds (impact distance to calculated center of meristem < 50 pm) diploid Arabidopsisthaliana,line En -2, depending on the LET classification of interacting HZE-particles Class of LET Sample size (N) Survival Germination delay Somatic defects Normal plants

<350 MeV cm-’

> 350 MeV cm-’

:06(63%)

9 5 (63%) 5 (50%)

3 (60%) 4 (40%) 2 (20%)

2 (40%) 0 (0%)

224

A. R. KRANZ

-

et al.

-2

track track

LET

number

MeV/cm

(pm)

from

Z

germina-

root

shoot

seed

meristem

meristem

centre

811

280

8

258

1272

160

4 8

4200

distance

1

137

116

69

426

218

108

477

286

tion

time (h)

384

suble

-

thal plant

+

FIG. 8. Tumor production below the shoot apex in a sublethal seedling grown from seed No. 130 of diploid fhaliuna (WT line En-2) hit by three cosmic particles are listed. The data obtained for the tracks below the figure.

Arubidopsis

4. CONCLUDING

REMARKS

We should keep in mind, again, that the exposure of the seeds in our experiment is characterized by: (1) predominantly multiple hits by particles from various LET-regimes; (2) the admixture of a permanent, spatially moreor-less homogeneous low-LET component to the microscopic hits by nuclear particles of higher LET. Both factors may lead to an enhancement of the biological effect of the nuclear particles. Although peculiar damage endpoints of the Biokosmos 9 experiment “Seeds” may be caused by single track events and hits inside the embryo targets mentioned above, multiple hits of cosmic particles with low, intermediate and high LET occur obviously. Detailed estimations in this respect are possible using the improved technique and methods of this experiment. In this way new and original results are expected to be obtained in the near future showing

peculiar effects of the complex spectrum of cosmic ionizing radiation. Furthermore, dose fractionation studies (Miller et al., 1990) with neutrons and charged particles of intermediate LET have shown enhancement effects on oncogenic transformation depending on the time interval of radiation exposures. Additional effects of this kind may be relevant in space. too, and

should be studied in future experiments with new and improved nuclear track detectors which offer a timely record of nuclear events in space (Benemann er al.. 1990). Detectors of this type will be indispensible for growing or moving subjects like seedlings or insects in the non-resting stage Acknowledgemenrs-We wish to thank the scientific and technical management of ESA/ESTEC, Paris/Noordwijk (Drs H. Oser, D. Mesland and W. Jansen) as well as the colleagues of IBMP, Moscow (Drs Kovalev, Vikrov, Dudkin and Potapov) for their competent contribution in making the experiment successful. and Dr Seltz, CRN Strasbourg, for valuable cooperation. The experiment was supported by BMFT. Bonn (Grant No. 01 QV 85650).

ESA/IBMP

EXPERIMENT

REFERENCES Baican B., Schopper E. and Schott J. U. (1986) AgCldetectors in space biophysics. Nucl. Tracks Radiat. Meas. 12, 519-522. Baican B., Schopper E., Wendnagel Th. and Schott J. U. (1990a) Biokosmos I-experiment, dosimetric measurements with AgCI-detectors. Nucl Tracks Radiat. Meas. 17, 173-177. Baican B., Schopper E., Wendnagel Th., Schott J. U. and Heilmann C. (1990b) Seeds experiment on Biokosmos 9: dosimetric part. Proc. 28th COSPAR Mtg Den Haag 1990 (in press). Baumgardt H. G., Amend W., Baican B., Staudte R. and Schopper E. (1986a) A computerized video-electronic device for particle tracks. Nucl. Tracks Radiat. Meas. 12, 265-268.

Baumgardt H. G.. Baican B. and Schopper E. (1986b) Microscopic track structure of heavy ions, video-electronically measured. Ado. Space Res. 6, 83-86. Benemann A., Bodena A., Brlunig D., Brumbi D., Klein J. W., Schott J. U., Seifert C.-Ch., Spillekothen H.-G. and Wulff F. (1990) The effects of radiation on electronic devices and circuits. Atomenergie Kerntechnik. Indep. J. Energ. Syst. Radiat. 55, 2-8.

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“SEEDS”

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