Translocation of strontium from leaves of bean and corn plants

Radiation Botany, 1964, Vol. 4, pp. 259 to 265. Pergamon Press Ltd. Printed in Great Britain.

TRANSLOCATION OF STRONTIUM FROM LEAVES OF BEAN AND CORN PLANTS* J O H N E, A l V m I ~ R t U.S. Softs Laboratory, Soil and Water Conservation Research Division, Agricultural Research Service, USDA Beltsville, Maryland (Received 4 December 1963) A b s t r a c t - - T r a n s l o c a t i o n of Sr 85 from leaves of bean and corn plants to plant parts was measured at successive stages of growth. Translocation was greater from leaf applications that were periodically rewetted than from applications allowed to air dry and stay dry. The maximum amount of Sr 85 translocated from rewetted bean and corn leaves was about 0.6 and 0.5 per cent, respectively, of that applied. In bean plants the greatest translocation occurred from the youngest leaf (fourth trifoliate), treated 20 days after the straightening of the hypocotyl. In corn this occurred from the youngest leaf (leaf nearest the tassel) treated 10 days before pollination. Translocatlon of Sr s5 into roots and fruit parts, from rewetted applications, was less than 0.006 per cent of that applied. From most foliar treatments, aUowed to air dry and stay dry, translocation was less than 0.01 per cent of that applied. R6suna6---Chez la f~ve et le ma/s, on a mesur6/t diff~rents stades successffs de la croissance, la transloeation d'Sr 85 allant des feuftles/L diff~rentes parties de la plante. La translocation ~tait plus rapide A partir de feuilles r6humidifi~es p&,-iodiquement que pour des feuilles qui 6taient s6ch~es t~ Fair et rest6es s&ches. La quantit~ maximale de Sr 86 transloqu¢~ A partir de feuilles r6humidifi*es de f~ve et de mais 6tait d'environ 0,6 et 0,5 % respectivement de la substance appliqu6e sur la feuftle. Chez la f~ve, les translocations maximales surviennent pour la feuille la plus jeune (la 4~me feuiUe trifoli6e) trait6c 20 jours a p r ~ le redressement de l'hypocotyle. Chez le reals, ce maximum se produisait pour la fenille la plus jeune (la plus proche de l'inflorescence) trait6e 10 jours avant la pollinisation. La translocation du S r " dans les racines et dans les fruits apr~s r,humidification comportait moins de 0,006 pour cent de la substance appliqu6e. Pour la majorit~ des traitements des feullles s6ch6es ¢t l'air et rest6es s6ches, la translocation 6tait de moins de 0,01 pour cent de la substance appliqu~e. Z u s a m m e n f a s s u n g - - A n Bohnen und Maispflanzen wurde in aufeinanderfolgenden Wachstumsstadien die Translokation yon Sr 8s aus B1attern in verschie-dene Pflanzenteile gemessen. Die Translokation war gr6sser, wenn die behandelten Bl~itter periodisch wieder befeuchtet wurden, sie war kleiner, wenn die behandelten Bl~itter in der Luft troeknen konnten und so belassen wurden. Der H6chstbetrag yon Sr 85, der yon wiederbefeuchteten Bohnen und Maisbl/ittern abgeleitet wurde, lag bei 0,6 bzw. 0,5 Prozent der applizierten Menge. Bei Bohnenpflanzen ergaben diejiingsten B1/itter (4.dreibl~ittrige s), die 20 Tage nach der Streckung des Hypocotyls behandelt worden waren, die grOsste Translokation. Beim Mais zeigte sich die gr6sste Translokation beim jiingsten Blatt (der Rispe am ngchsten), das 10 Tage vor der Best/iubung behandelt worden war. Die Translokation von Sr s~ in Wurzeln und Fruehttefte war nach Behandlung mit Wiederbefeuchtung gerin er als 0,006 Prozent der applizierten Menge. Wenn die behandelten Bl~itter in der Luft trocknen konnten und trocken belassen wurden, war die Translokation geringe r als 0,01 Prozent der applizierten Menge. *This study was supported in part by the U.S. Atomic Energy Commission. fPlant Physiologist. 259

260

TRANSLOCATION OF Sr FROM LEAVES

INTRODUCTION WHEN exposed parts of plants are contaminated with fallout radiostrontium, nonexposed parts may become contaminated by translocation of the strontium. In the present work the amounts of foliar-applied strontium translocated to other plant parts from certain leaves, at certain growth stages, and under wet and dry conditions were studied. Translocation from a foliar application of radiostrontium consists of absorption by the leaf and subsequent movement to other parts. Therefore, factors that affect absorption may affect the overall process. Age of leaf, and humidity, have been reported to be such factors. Young leaves were found to often exceed older leaves i n the absorption of foliar-applied materials. (6'9,z°,lz) MmDLETON (5) found that wheat plants sprayed at ear emergence and exposed to rainfall contained about 50 per cent more Sr s 9 in the grain than did protected plants. PALLAS(V)showed that the absorption of certain foliar-applied materials increased with humidity. Also, PALL/kSand WILLIAMS(8) reported that increased soil-moisture tension decreased translocation of certain foliar-applied materials. Studies of the translocation of foliar-applied radiostrontium have been conducted by BLrKOVAC and W~r,W'ER (I) and ITO et al. (4) In the former, Sr s9 was applied to one of the primary leaf blades of bean plants. Although about 30 per cent of the Sr s9 application was absorbed by the leaf in four days, it was not translocated from the leaf to other plant parts and was considered immobile. In the latter study, Sr 9° was applied to leaves of sugar beet plants growing in nutrient solutions with a constant Ca plus Sr molarity, but with variable Ca:Sr ratios. Translocation of Sr 90 occurred in these plants whenever Ca was completely replaced with Sr. In these cases, the Sr 90 was translocated to other leaves and to the roots, and was excreted gradually into the nutrient solution on the eighth day of the experiment.

solution containing Sr a5 was applied to a single leaf of each plant. The foliar-applied solution was allowed to air dry. Thereafter, some of the treated leaves remained dry while others were periodically rewetted. The plants were harvested at maturity and the distribution of radlostrontium in various plant parts determined. Bean plants (Phaseolus vulgaris L. vat. Stringless Green Pod) and corn plants (yea rnays L. vat. Golden Midget) were grown in sand or solution cultures during the summer months. The initial nutrient and nutrient renewal solutions had the following composition (in g/l) : Ca(NOa)~.4H,O, 0"24; KH~PO4, 0-10; and MgSO,.TH,O, 0.10. T h e concentration of minor elements (in ppm) was: Fe, 3; B and Mn, 0.5; Zn, 0"05; Cu, 0.02; and Mo, 0.001. The initial p H of the nutrient solution was 6.5. The nutrients in the sand or solution cultures were renewed at 10-day intervals by leaching or by complete solution replacement, respectively. Each of the treatments described was replicated on three plants grown in separate pots. A TREATED AREA A. TRIFOLIATE LEAVES

%

FLORAL'APEX 4TH TRIFOLLATE

B. PRIMARY LEAVES 3RO TRIFOLIATE

2ND TRIFOLIATE

+

IST TRIFOLIATE

PRIMARY LEAVES

MATERIALS AND METItODS

General The experiments reported herein were carried out with bean and corn plants grown in a greenhouse. At various stages of growth, a

Fxo. 1. Schematic diagram of a bean plant illustrating how leaves were numbered and where treatments were applied.

JOHN E. AMBLER TREATED AREA

A. MID LEAF

261

first rewetting each day was made at 9 a.m. In the other, the unwetted group, the air-dried treatments remained dry until plant harvest.

BAGGED TASSEL

IST LEAF

B. ONE INCH FROM STEM

2ND LEAF 5RD LEAF 4TH LEAF EAR 4TH LEAF

5TH LEAF 6TH LEAF 7TH LEAF

Fro. 2. Schematic diagram of a corn plant illustrating how leaves were numbered and where treatments were applied.

solution of carrier-free Sr s5 in 0.1 N KC1 was applied to certain areas of the upper surfaces of selected bean and corn leaves during the vegetative or early fruit stages of growth (Figs. I, 9). The p H of the Sr 85 solution was adjusted with K O H to correspond to that of the expressed plant sap. T h e solution contained 4 &c of Sr s~ per ml at the start of the experiment. It was applied by means of a micropipette so that 1 ml was distributed uniformly over the treated area in approximately 125 droplets. Foliar treatments then were allowed to air dry. T h e effect of periodically rewetting some of the treated leaf areas upon the translocation of Sr 85 was also investigated. The foliar-treated plants were divided into two groups of equal numbers of plants. In one, the rewetted group, the air-dried treatments were periodically rewetted until plant harvest. The treated leaf areas were rewetted four times daily at 2-hr intervals with 0.6 ml of deionized water. The

Bean The rewetted group of bean plants was grown individually in sand cultures to facilitate the handling necessary for rewetting the air-dried foliar treatments. The unwetted group of plants was grown in solution cultures. Twenty days after the straightening of the hypocotyl the bean plant had formed the plant parts shown in Fig. 1. Fruit emergence occurred at this time from the floral-apex structure. Foliar treatments (solution p H 6.3) were begun after the respective leaves unfolded (Table 1). The primary leaves were treated 4, 8, 16, 20 or 28 days after the straightening of the hypocotyl. The first trffoliate leaves were treated 8, 12, 16, 20 or 28 days after the straightening of the hypocotyl. The second, third, and fourth trifoliate leaves were treated in a similar manner. The bean plants were harvested 60 days after the straightening of the hypocotyl. Leaves were detached from the plant at the petioles, and the stems were sectioned at the basal part of each node. Thus, the dismembered parts consisted of roots, stem sections, individual leaves and fruits (primary, first trifoliate, etc.), and axillary leaves of treated leaves. ~0Fn Corn plants were grown individually in sand cultures. Tassels appeared about 40 days after seedling emergence and were enclosed in polyethylene bags. One ear of each plant was handpollinated 50 days after seedling emergence with pollen obtained from plants not in the vicinity of the experimental plants. Foliar treatments (solution p H 5.3) were made 10 days prior to pollination, at pollination, or 10 days after pollination. The Sr s5 solution was applied 1 in. from the stem to the leaf at, just above, or just below the first emerging ear. In other treatments, the Sr 8s was applied, midway between the leaf tip and stem, to the six youngest leaves. The surface area of the two sites of treatment was approximately the same, 4 in.2

262

TRANSLOCATION OF Sr FROM LEAVES

Table 1. Translo¢ation of Sr e6from individual bean leaves treated and then re-wettedfour times daily until harvest 60 days after straightening of hypocotyl (values in parentheses transformed by.formula 2 + log x). Percentage of applied Sr a5 translocated from treated leaf* Treated leaf 4

8

Day of treatmentt 12 16

Fourth trifollate Third trifoliate Second trifoliate First trifollate Primary leaves

0-14 (1.15)

0.45 (1.65) 0.12 (1.10)

0.38 (1.58) 0.32 (1.50) ..

0.15 (1.19) 0-20 (1.32) 0.06 (0.81) 0-08 (0.93)

20

28

0.59 (1-77) 0.24 (1.38) 0.17 (1.24) 0.16 (l.21) 0.10 (1.03)

0.18 (1.27) 0"05 (.077) 0"07 (0.89) 0.06 (0.82) 0.01 (0.20)

*Standard deviation, applicable to transformed values, is O"13. tDays elapsed after straightening of hypocotyl.

T h e corn plants were harvested 20 days after pollination. Stems were dismembered at the basal part of each node and numbered as stem section 1, etc., starting from the tassel. Leaves were also numbered from the ta, sel. T h e leaf and ear arising from the same node were indicated by the same number, fourth leaf, fourth leaf ear, etc. T h e dismembered parts consisted of roots, stem sectiom, individual leaves, tassel less pollen, pollen, and individual fruit parts (grain, cob, cob stem, silk, husk and undeveloped ears).

Analysis The dismembered fresh plant parts, including the treated leaf, of the bean and corn plants were placed in celluloid tubes and analyzed for Sr B5 with a g a m m a ray spectrometer. Similar plant parts from untreated plants were analyzed in a similar manner to determine background levels. One ml of the original Sr s5 solution was used as a standard, and its count rate determined each day that plant-parts analyses were made. I n all cases, more t h a n 99 per cent of the applied Sr 85 was recovered in the treated leaf. T h e percentages of the applied Sr s5 transslocated into other plant parts were calculated.

Because of the extreme variation in Sr as transslocated to various plant parts, a logarithmic transformation was applied to the data before the standard deviations were calculated. RESULTS

General Translocation of Sr 85 from leaves of bean and corn plants was greater in rewetted treatments than in treatments allowed to stay dry. I n the latter the Sr 85 translocated to other plant parts was essentially zero, i.e. less than twice the applicable standard deviation. In rewetted treatments translocation of Sr 8s was also essentially zero to fruits and roots; however, translocation to other plant parts was substantial. Bean and corn plants behaved similarly in their translocation of Srss in some of the rewetted foliar treatments. For example, the fourth trifoliate bean leaf (Table 1), treated 20 days after the straightening of the hypocotyl, translocated about 0-6 per cent of the applied Srss. T h e youngest corn leaf (Table 3), treated 10 days before pollination, translocated about 0.5 per cent of the applied Sr ss. I n general, young bean and corn leaves translocated more Sr s5 than older leaves treated

JOHN E. AMBLER at the same time. There also appeared to be more translocation by leaves of bean and corn plants when foliar treatments were made during early fruit development.

263

site of treatment. Approximately 40 per cent of the Sr ss translocated was to the primary leaves. Practically no Sr ss was translocated to fruits, to stem tissue below the primary feat~ node, or to roots.

The distribution of Sr s5 in plant parts varied in other rewetted treatments. The distribution of Sr s5 in leaves was related to position of the treated leaf. From the primary leaves more Sr s5 was translocated to the first trifoliate leaf than to other leaves. From the first, second, and third trifoliate leaves more Sr s5 was translocated to the primary, first and second trifoliate leaves, respectively, than to other leaves. From the fourth trifoliate leaf more Sr s5 was translocated to the second trifoliate leaf. In general, more Sr s5 was translocated to stem sections when a particular leaf was treated at a younger age. Among the applications of Sr s5 to bean leaves allowed to stay dry until harvest, there was only one in which a significant percentage was translocated (greater than twice the standard deviation). This occurred when the fourth Table 2. Translocatlon of Sr~ from first trifoliate bean leaf trifoliate leaf was treated 20 days after the treated eight days after straightening of hypoco~yl and then straightening of the hypocotyl. The Sr ss transre-wettedfour times daily until harvest (values in parentheses located was 0.027+0.009% of the applied treatment. transformed byformula 4+log x).

Bean

Measurable amounts of Sr s5 were translocated in all rewetted treatments (Table 1). When treated prior to fruit emergence, leaves translocated less Sr ss as they became older. However, in treatments at fruit emergence, 20 days after the straightening of the hypocotyl, translocation of Sr ss was generally greater than in the treatment four days earlier. I n treatments eight days after fruit emergence, translocation of Sr s5 decreased to less than half of the values obtained at fruit emergence. The complete analysis for Sr s5 in plant parts from one rewetted treatment (the first trifoliate leaf treated eight days after the straightening of the hypocotyl) is shown in Table 2. Translocation was acropetal and basipetal from the

Percentage of applied Sr~ translocated into separate parts* Plant section Separate parts of plant section

Fourth trifoliate Third trlfoliate Second trifoliate First trifoliatet Primary

Leaves

Stems

0.035 (2.534) 0.085 (2.927) 0-080 (2.901) 0-033 (2.521) 0.151 (3.179)

0.009 (1.944) 0.007 (1.863) 0.005 (1.704) 0.003 (1.467) 0-002 (1.230)

*Standard deviation, applicable to transformed values, is 0.245. Translocations into stem tissue below primary node and into roots were 0.001 and into fruits, 0.002. tAxillary growth from axil of treated first trifoliate leaf.

C0tT/ Translocation of Sr s5 from rewetted treatments was influenced by site of application on individual leaves, position of treated leaf on the stem, and maturity of the plant (Table 3). Translocation of Sr s5 was greater from leaves treated midway between leaf tip and stem than from leaves treated 1 in. from the stem. More Sr sa was translocated from the first leaf (nearest tassel) when treated 10 days before pollination than from any other leaf treatment. Translocation of Sr ss from treated corn leaves was mainly to other leaves, with movement being acropetal and basipetal. In the foliar treatments I in. from the stem, Sr ss was translocated a distance of one to two leaves from the treated leaf. I n the foliar treatments midway between leaf tip and stem, Sr s5 was translocated distance of two to five leaves from the treated leaf. I n general, Sr ss was found in greater amounts in the first or second leaf from the treated leaf than in any other plant part.

264

TRANSLOCATION OF Sr FROM LEAVES

Table 3. Translocation of SrSa from individual corn leaves treated and then re-wettedfour times daily until harvest 20 days after polliuation (values in parentheses transformed by Jbrmula 2 + log x). Percentage of applied Sr 85 translocated from treated leaf* Treated leaf

Day of treatment 10 days before pollination At pollination One inch from stem

Nearest tassel

..

Second from tassel

..

Third from tassel Fourth from tassel Fifth from tassel Sixth from tassel

Region of leaf treated Midway between One inch leaf tip and stem from stem ..

Midway between leaf tip and stem

0.05

0.58 (1"77) 0.44 (1.65) 0.38

0.02

0.28 (1.45) 0.17 (1.25) 0"21

(0.70)

(1.58)

(0.4O)

(1.33)

0.03 (0.47) [0-02 (0.30) ..

0.30 (1.48) 0.15 (1.19) 0.08 (0.93)

0.02 (0-30) 0.02 (0.33) 0.01 (0.13)

0"I0 (1.03) 0.02 (0.39) 0.01 (0.21)

..

*Standard deviation, applicable to transformed values, is 0.13 DISCUSSION The periodic wetting of foliar treatments enhanced the translocation of Sr ss from bean and corn leaves, except for treatments to corn leaves 10 days after pollination. The latter may be accounted for by the senescence of the corn plants at this time. A plausible explanation, for the enhanced translocation of Sr sa, is reverse translocation via the xylem to transpiring leaves. I n those treatments in which rewetting enhanced the translocation of Sr sS, the distribution of Sr s5 varied in individual leaves of a treatment. I n general, more Sr sa was translocated to expanding leaves which were larger, at the time of treatment, than other expanding non-treated leaves. The rewetting of treated leaves, thus, allowed the Sr sa, in solution, to be absorbed by the xylem for movement to all transpiring leaves. There may be a relationship between transpiration rate and amount of Sr s5 translocated to individual leaves. The Sr ss content of fruits and roots was essentially zero, therefore Sr ss trans-

location via the phloem was considered negligible in these experiments. CLOg et al.(2,3) investigated infiltration of cotton plant tissue by soluble organic compounds. They reported that high humidity increased absorption and produced a reverse movement in the xylem. Moisture, condensed on the foliar application, they explained, could maintain a supply of tracer, in solution, to be absorbed by the xylem for movement to all transpiring leaves. The effect of moisture condensation and rewetting upon foliar applications, as related to reverse movement in the xylem, appear to be similar. In some cases, differences in translocation may be related to differences in absorption. Cuticle development and proportion of lea f area occupied by stomates change with age of leaf. The expected changes are consistent with reduced translocation from older leaves and from sites of application nearer to the corn stem.

J O H N E. AMBLER T h e results of these experiments indicate that radiostrontium fallout stays primarily at the site of deposition. Certain edible plant parts m i g h t be c o n t a m i n a t e d directly by exposure to fallout, but there would be relatively little contamination with radiostrontium translocated from above-ground plant parts. REFERENCES 1. BmcovAc M. J. and Wrrrwxg S. H. (1957) Absorption and mobility of foliar applied nutrients. Plant Physiol. 32, 428-435. 2. CLOR M. A., CRAFTS A. S. and YAMAOUCm S. (1962) Effects of high humidity on translocation of foliar-applied labeled compounds in plants I. Plant Physiol. 37, 609-617. 3. CLOg M. A., CaAFTS A. S. and YAMAOUCHIS. (1963) Effects of high humidity on translocation of foliar-applied labeled compounds in plants. II. Translocation from starved leaves. Plant Physiol. 38, 501-507. 4. ITO S., TAKENAGAH., MumA T. and MOROOKA N. (1961) On the foliar application and the distribution of Sr 9e in sugar beet plants. Pro¢. Crop Sd. Soc. 07apan 29, 376-378.

265

5. MIDDLETONL. J. (1958) Absorption and translocation of strontium and caesium by plants from foliar sprays. Nature 181, 1300-1303. 6. Ot.~'+D K. and OPh.~D T. B. (1956) Uptake of magnesium by apple leaves. Physiol. Plantarum 9, 401-411. 7. PALLAS,JR., J. E. (1960) Effects of temperature and humidity on foliar absorption and tramlocation of 2,4-dichlorophenoxyacetic acid and benzoic acid. Plant Physiol. 35, 575-580. 8. PALLAS,JR., J. E. and WILLIAMSG. G. (1962) Foliar absorption and translocation of pas and 2,4-dichlorophenoxyacetic acid as affected by soil-moisture stress. Botan. Gaz. EL~, 175-189. 9. T~oP.J~ G. N. (1958) Factors affecting uptake of radioactive phosphorus by leaves and its translocation to other parts of the plant. Ann. Botany 22, 381-398. 10. Tummy H. B., WXT'rW~RS. H. and BuKovAc M. J. (1961) Absorption of radionuclides by aboveground plant parts and movement within the plant. 07. Agr. Food Chem. 9, 106-113. 11. WALLIH~a~E. F. and I'~YMnNN-I-I~gsCHBEROL.. (1956) Some factors affecting absorption and translocation of zinc in citrus plants. Plant Physiol. 31, 294-299.