Some effects of acute gamma radiation in giant Sequoia seedlings

Rarliafiort Bolurg, 1968, Vol. 8, pp. 67 to 70. Pcrgamon

SOME EFFECTS OF ACUTE SEQUOIA

Press. Printed in Great Britain.

GAMMA RADIATION SEEDLINGS*

IN GIANT

F. G. TAYLOR Jr. Radiation Ecology Section, Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A. (Received

11 November

1966; Revised 25 October

1967)

Abstract-Actively growing Giant Sequoia seedlings [Sequoia gigrrnlea (Lindl.) Decne.] rcceived acute y-radiation from a @OCosource at various dose rates ranging from 42 to 1119 rad/24 hr. While maximum radiation levels (697 and 1119 rad) proved to be sublethal, growth of primary stems was inhibited 28 percent by 134 days postirradiation. Microscopic examination of foliar tissues receiving from 346 to 1119 rad revealed abnormal development of the vascular strand as the result of a reduction of elements per xylem row, extensive development of transfusion tissues, and increased cell wall thickness of some tracheids. Double 01 twin resin ducts also were observed in leaves receiving 697 rad. R&mm&Des plantules de Stquoia g&ant [Sequoia giganlea (Lindl.) Decne.] en croissance active ont reFu une dose aigue de rayons gamma provenant d’une source deooCo z+des dtbits de dose variCs allant de 42 B 1119 rad par 24 h. Bien que les doses maximales de radiation (697 et 1119 rad) s’averent sub&ales, ii y a une inhibition de croissance des tiges primaires de 28 pour cent le 134tme jour aprb irradiation. L’examen microscopique des tissus foliaires qui ont regu des doses allant de 346 & 1119 rad a rtvtlC un developpement anormal des tissus vasculaires rbultant d’une rkduction des tlkments de chaque rang&e de cellules de xyleme, un dkeloppement intense des t&us conducteurs et un epaississement de la paroi cellulaire de certaines trachkides. Dans les feuilles irradiCes par 697 rad, on a aussi observk des canaux rCsinif&s doubles ou jumdts. Zusammenfassung-Wachsende Keimlinge von Mammutbaumen (Sequoia giganlea Lindl. Decne) wurden akuter Gamma-Strahlung aus einer E°Co-Strahlenquelle ausgesetzt. Es wurden Dosen zwischen 42 und 1119 rad pro 24 Stunden gewLhlt. Wghrend maximale Strahlendosen (697 und 1119 rad) sich als subletal erwiesen, war das Wachstum des Primfrstammes 134 Tage nach Bestrahlung urn 28% verringert. Mikroskopische Untersuchungen von Blattgewebe, das zwischen 346 und 1119 rad erhalten hatte, zeigten eine abnorme Entwicklung der Geftisstringe: Red&ion von Elementen pro Xylemstrang, starke Entwicklung des Transfusionsgewebes und Zunahme der Wandstirke einiger Tracheidcn. Doppelte oder Zwillings-Harzgsnge konnten ebenfalls beobachtet werden in Bllttern, die 697 rad erhalten hatten. INTRODUCTION known that the nuclei shoot apical meristems are sensitive

IT

IS WELL

in cells of to ionizing

radiation,(“) and that the resultant morphological or physiological changes are parameters often used to assess radiation damage. (sp@) More

*Research sponsored by the U.S. Atomic Energy Commission poration. 67

under contract

with the Union Carbide

Cor-

68

&‘. G. TAYLORjr.

specifically, a criterion characterize

stem or shoot elongation has been used by many investigators to radiation effects in conifers.(10*13~15) BRANDENBURG(~) reported growth inhibition in pinyon pine after exposure to acute y-irradiation, and anomalous development of vascular tissue in the leaves. In this study giant Sequoia seedlings, Sequoia gigantea (Lindl.) Decne., were irradiated to determine whether this gymnosperm conformed to the same degree of radiosensitivity as their more abundant coniferous relatives.

MATERIALS AND METHODS Three-month-old giant Sequoia seedlingscultured in planters containing top soil (Emory silt loam)-received acute y-radiation from an indoor source consisting of 30 Ci of OOCo. Exposures were chosen, based on interphase chromosome volumes (211 = 22) during active growth, to produce a spectrum of effects from mortality to minor growth ranging anomalies.(ll) Dose rates ranged from 1.75 to 46.62 rad/hr. Total radiation doses received over 24 hr were 42 f 7 S.E., 346 f 3697 f 9, and 1119 f 23 rad. Each treatment contained six plants. Plants were maintained following irradiation in a plant growth chamber (86°F day, 72” night, 2000 ft-c., 16-hr day length) for one growing season, and scored for growth rate and morphological anomalies,

FIG.

1.

At 44 days post irradiation leaves were collected, killed in a Nawaschin type fixative, dehydrated-infiltrated in a standard tertiarybutyl alcohol series, and embedded in Paraplast. Cross section were cut 10-p thick, stained with safranin and fast green, and mounted into permanent slides.

RESULTS AND CONCLUSIONS Exposures necessary to produce slight growth inhibition among six genera of gymnosperms have been reported to vary from 150 to 700 R, while exposures necessary to produce severe growth inhibition ranged from 200 to 1250 R.tll) Based on that study and the chromosome volumes of the Sequoia seedlings, 24~s f 0.9 S.E. (n = 40), one would predict that approximately 1000 R would be lethal to this species, while 600 R would inhibit gro&h severely. At 134 days postirradiation growth of primary stems (first order lateral stem from main axis) was inhibited 28 percent. Differences in average lengths between control and irradiated treatments were compared using the multiple range and multiple f-tests.cO) Plants receiving 697 and 1119 rad (2 length of primaries = 12.1 mm f O-8 SE. and 12.3 f 0.8) differed significantly at the 5 percent level from those receiving 346, 42 rad, and controls (15.9 mm -& 1-O S.E., 17.6 f l-2 and 16.8 f 0.9 respectively). Plants at radiation doses of 346 and

Cross sections of vascular tissues in leaves of Sequoiu seedlings days postirradiation (xylem is adaxial).

at 44

(a) Control; note the oval configuration of the vascular strand, and the orderly development of xylem elements into radial rows (x360).

(b) 346 rad; increased volume of transfusion tissue to the left of the vascular strand ( x 360). (c) 697 rad; reduction in number of elements per xylary row, and extensive development of transfusion tissues, note bordered pits of transfusion tracheids ( x 260). (d) 1119 rad; note the absence of xylem parenchyma cells between the xylem rows, and the extreme thickness of individual tracheids ( x 360).

R.B. f.p. 68

SOME

EFFECTS

OF ACUTE

42 rad were not significantly different between treatments or from controls Secondary stems had not developed on any seedlings by the time of irradiation. At 50 days postirradiation plants which had received 697 and 1119 rad developed secondary stems, whereas secondary stems did not develop on controls or other irradiated plants until 104 days postirradiation. Tertiary stems appeared on plants which received 346, 697 and 1119 rad by 104 days postirradiation, but did not appear on plants receiving 42 rad or on controls throughout the growing season. After I-yr postirradiation growth it was evident that no seedling received sufficient radiation to produce death. A modification of the normal leaf fusion process, and a reduction of vascular tissues in pinyon pine needles has been described following y-radiation.c4) In the present study effects of radiation on foliar tissues were also o!)served. Needles from primary stems of plants receiving 697 and 1119 rad were typically gnarled and swollen. The normal, needle-like leaf [Fig. l(a) and Fig. 2(a)], characteristic of first season Sequoia seedlings, contains a single vascular strand, located centrally between the adaxial and abaxial surfaces. The vascular strand is compact, oval to circular with the xylem oriented on the adaxial and the phloem on the abaxial side. The elements of the xylem are arranged in tabular rows, with xylem parenchyma interspaced between the rows. Transfusion tissue occurs to the left and right of the vascular strand. Vascular strands of irradiated plants showed anomalous development of xylem and transfusion tissue, while the phloem appeared un-

FIG. 2.

Cross sections through (resin duct is abaxial).

Sequoia

GAMMA

RADIATION

affected. A reduction in xylem in plants receiving 346 rad was observed with an increase in volume of transfusion tissue [Fig. 1 (b)]. The vascular strand in Fig. 1 (c) is typical of 50 percent of needles examined from seedlings receiving 697 rad. Transfusion tissues and transfusion tracheids were extensively developed extending nearly to the epidermis, while the number of elements in each xylem row appeared to be reduced. The same aberrant vascular tissues were evident in 58 percent of examined needles receiving 1119 rad [Fig. l(d)]. In addition the xylem exhibited a profound deviation from the radial seriation (tabular rows), characteristic in normal leaves. This anomalous character is apparently due to the inhibited differentiation of xylem parenchyma cells, and pressures exerted by the mesophyll. Number of phloem cells per row indicates normal mitotic activity at each radiation treatment, while the number of xylem elements Ctracheids) per row suggests inhibited cell divisions. This presents an interesting and yet unexplained problem, since both xylem and phoem are derived from the same precursor tissue, the procambium. At radiation doses above 346 rad mesophyll cells were generally enlarged and necrotic with ruptured cell walls. Chloroplasts seem unaffected as indicated by differential staining. Cell divisions in the peripheral zone of the apical meristem, which initiate leaf primordia, are correlated with the divisions pertaining to the initiation of the procambium. It is thought that the procambium in the region of the leaftrace differentiates both acropetally to the site of primordial initiation and basipetally, before the leaf buttress is visible.t7) Thus the anomalous development of the vascular strand in the

leaves illustrating

Portion of control leaf depicting the normal vascular strand (I’S) and resin duct (Rd), ( (b) Leaf containing twin resin ducts (697 rad), (c) 2 resin ducts, separate and near the abaxial ( X 200). (a)

69

twin resin ducts

development of the x 260). ( x 350). epidermis (697 rad),

70

F. G. TAYLOR

Sequoia leaves was probably preceded by radiation damage to precursor cells (xylem initials) either in the peripheral tissue zone or procambium. Anatomical damage of this nature could inhibit mineral uptake(@ and translocation between the vascular strand and the mesophyll, thus affecting the whole plant at later periods when demands for nutrients or water might be greater. Of leaves examined microscopically from plants receiving 697 rad, two had twin resin ducts. In cross section a normal leaf (control) usually possesses a single resin duct, located between a central vascular strand and the abaxial surface [Fig. 2(a)]. The anomalous resin ducts observed were located above the normal duct [Fig. 2(b)], or separate from the duct, and near the abaxial surface [Fig. 2(c)]. Evidence in the literature concerning the ontogeny of some resin canals in secondary xylem of conifers indicates that the fusiform initials are sensitive to mechanical injury, and wounding often results in the development of resin ducts. (1--9p12) It therefore seems plausible that the same mechanism could be triggered by cell damage induced by ionizing radiation. More specifically, one or more meristematic precursors or parenchyma cells in the leaf primordia could have been damaged or killed, thus forming resin ducts in a schizogenous manner as described by EsAu.(‘) These results suggest that actively growing Sequoia seedlings might not be as radiosensitive as predicted from nuclear characteristics.(“) This apparent discrepancy is believed to be due to minimal environmental stresses during the post irradiation growth period in environment chambers, and not a conflict with the nuclear or chromosome volume concept. Other studies with forest tree speciest 14) have shown that postirradiation environmental factors greatly influence radiosensitivity.

REFERENCES 1. BAILEY I. W. and FAULL A. F. (1934) The cambium and its derivative tissues, IX. Struc-

Jr.

2. 3. 4.

5.

tural variability in the redwood, Sequoia semfiervirens, and its significance in the identification of the fossil woods. Arnold Arboretum J. 15, 233-254. BANNAN M. W. (1936) Vertical resin ducts in the secondary wood of the Abietinea. Nero Phytologist 35, 1 l-46. BLOCH R. (1941) Wound healing in higher plants. Botan. Rev. 7, 110-146. BRANDENBURG M. K., MILLS H. L., RICKARD W. D., and SHIELDS L. M. (1962) Effects of acute gamma radiation on growth and morphology in Pinus monophylla Torr. and Frem. (Pinyon Pine). Radiation Botany 2, 251-263. BROWN G. N. and TAYLOR F. G. (1966) Zinc-65

distribution and radiosensitivity of growth and @Zn uptake in red oak seedlings. Radiation Botany 6, 519-525. 6. DUNCAN D. B. (1955) Multiple range and multiple f tests. Biometrics 11, l-42. 7. ESAU KATHERINE (1953) Plant Anatomy. J. Wiley, New York, 719 pp. a. GUNCKEL J. E. (1965) Modificaions of plant growth and development induced by ionizing radiations. Encyc. Plant Physiol. XV(2), 365-387. 9. GUNCKEL J. E. and SPARROW A. H. (1961) Ionizing radiations: Biochemical, physiological, and morphological aspects of their effects on plants. Encycl. Plant Physiol. XVI, 555-611. IO. MERCEN F. and THIELGES B. A. (1966) Effects of chronic exposures to Co” radiation on Pimrs rigida seedlings. Radiation Botany 6, 203-210. II. SPARROW RHODA C. and SPARROW A. H. (1965) Relative radiosensitivities of woody and herbaceous spermatophytes. Science 147, 1449-1451. 12. THOMSON R. G. and SIPTON H. B. (1925) Resin canals in the Canadian spruce [Picea canadensis (Mill.) B. S. P.], an anatomical study, especially in relation to traumatic effects and their bearing on phylogeny. Roy. Sot. London Phil. Trans. Ser. B,

214,63-l 11, 13. WITHERSPOON J. P. (1965) Radiation damage to forest surrounding an unshielded fast reactor. Health Phys. 11, 1637-1642. 14. WITHERSPOON J. P. and TAYLOR F. G. (1965) Health Physics Division Annual Oak Ridge National Laboratory,

45-50. 15. WOODWELL Predicted radiation

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G. M. and SPARROW A. H. (1963) and observed effects of chronic gamma on a near-climax forrst ecosystem.

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