The development of the leaf abscission zone in Ginkgo biloba trees and seedlings was studied. Leaves from Ginkgo seedlings were debladed to enhance anatomical changes in the abscission region. The development of an abscission layer was observed six days after deblading. The formation of the zone was not complete, but an abaxial to adaxial pattern of development was apparent. No evidence of separation was noted. Debladed Ginkgo petioles remained green and did not abscise during the six weeks period of study. The development of an abscission layer was observed in leaves from a Ginkgo tree one week prior to separation. The zone occupied an area approximately eleven cells wide at the junction of the petiole and stem. Separation of the petiole occurred through an apparent dissolution of the middle lamella. The separation was observed in the distal region of the zone between the ninth and tenth cell layers. The pattern of leaf abscission zone development is similar in Ginkgo trees and seedlings

Mosher, G. Wayne

Smith, Daryl D.

Name not available

A Study of Abcission Zone Development In Leaves of Ginkgo Biloba L.

University of Northern Iowa

Proceedings of the Iowa Academy of Science Volume 77 Annual Issue Article 7 1970 A Study of Abcission Zone Development In Leaves of Ginkgo Biloba L. G. Wayne Mosher Daryl D. Smith University of Northern Iowa Let us know how access to this document benefits you Copyright ©1970 Iowa Academy of Science, Inc. Follow this and additional works at: https://scholarworks.uni.edu/pias Recommended Citation Mosher, G. Wayne and Smith, Daryl D. (1970) "A Study of Abcission Zone Development In Leaves of Ginkgo Biloba L.," Proceedings of the Iowa Academy of Science, 77(1), 23-31. Available at: https://scholarworks.uni.edu/pias/vol77/iss1/7 This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu. A Study of Abcission Zone Development In Leaves of Ginkgo Biloba L. G. WAYNE MOSHER[ & DARYL D. SMITH 2 Abstract. The development of the leaf abscission zone in Ginkgo biloba trees and seedlings was studied. Leaves from Ginkgo seedlings were debladed to enhance anatomical changes in the abscission region. The development of an abscission layer was observed six days after deblading. The formation of the zone was not complete, but an abaxial to adaxial pattern of develop-ment was apparent. No evidence of separation was noted. Debladed Ginkgo petioles remained green and did not abscise during the six weeks period of study. The development of an abscission layer was observed in leaves from a (;inkgo tree one week prior to separation. The zone occupied an area ap-proximately eleven cells wide at the junction of the petiole and stem. Sep-aration of the petiole occurred through an apparent dissolution of the mid-dle lamella. The separation was observed in the distal region of the zone between the ninth and tenth cell layers. The pattern of leaf abscission zone development is similar in Ginkgo trees and seedlings. Abscission refers to the natural separation of any part of a plant. It is most noticeable as the means by which deciduous plants shed their leaves prior to dormancy. Other examples are the sep-aration of flowers, fruit and bark. The process could also be con-sidered a self-pruning device utilized for the removal of injured or senescent organs ( 1). The abscission zone is a general term for the region at the base of the abscising organ through which separation eventually occurs. Separation denotes the physical separation of accessory organs from the main axis of the plant. Those cells involved in separation are referred to as the separation layer. Separation is usually within or distal to the tiers of cells formed during abscission. The primary studies on abscission were anatomical. Von Mohl ( 10) reported an abscission layer where separation occurred. Fur-ther, he recognized two independent phases of the process: the separation of the organ and development of protective tissue. This early work has led to investigations of the abscission zone and changes taking place in this region prior to separation. Plants such as Coleus, maple and oak have a structurally distinguishable zone where abscission is favored. Rubenstein and Leopold ( 12) re-ported Gymnocladus to have ninety zones in one compound leaf. Yager ( 19) reported that the abscission zone consisted of twelve tiers of cells in tobacco flowers. He distinguished the cells of the separation layer as being smaller than surrounding cells. Yager 1 10908 St. Boniface Lane, St. Ann, Missouri 63074 'Department of Biology, University of N. Iowa, Cedar Falls, Iowa 50613 23 1Mosher and Smith: A Study of Abcission Zone Development In Leaves of Ginkgo BilobaPublished by UNI ScholarWorks, 197024 IOWA ACADEMY OF SCIENCE [Vol. 77 inferred that separation cells do not develop in the same proportion as surrounding cells, thereby explaining their small size. Addicott ( 1) agreed, referring to the abscission layer as a zone of arrested development. Facey ( 4) demonstrated that the leaf abscission zone of Frax-inus was twelve to sixteen cells thick and differentiated from cells on either side. She indicated that separation is preceded by cell division beginning in the proximal cells, proceeding distally through the entire zone. Further, lignification of abscission zone cells, with the exception of the narrow separation region, was observed several weeks before leaf-fall. Rubenstein and Leopold ( 12) proposed that cell division is not essential to abscission, but it does have an active role in the production of the protective scar. The experiments of Scott et al. ( 15) suggested that the abscis-sion zone might be a product of localized cell senescence brought about, in part, by water stresses due to tylose formation in the vas-cular elements. This evidence suggests a role for the specialized abscission layer in the process of organ separation. However, Ga wadi and Avery ( 5) showed that cotton, poinsettia and pepper leaves fall without the development of an abscission zone. Based on this evidence they suggested that the term "abscission zone" be discontinued. Separation has been reported to occur in one of three manners. Lee (8) described the dissolution of the middle lamella between primary walls as a factor leading directly to abscission. More re-cently Yager ( 19) has observed a similar situation in the abscission zone of tobacco flowers. Tison ( 17) observed a dissolution of the middle lamella and primary wall leaving a thin cellulose wall over the protoplast. Lloyd ( 9) also found this to be the case in M irabilis. Hannig ( 6) reported a third type of separation resulting from a dissolution of entire layers of cells in two species. However, Lloyd (9) questioned the validity and occurrence of Hannig's observa-tion. Observing one of the species Hannig used, Lloyd noted only a dissolution of the middle lamella and primary wall. More recent-ly Webster ( 18) reported a similar type of separation in Phaseolus. Rubenstein and Leopold ( 12) are of the opinion that anatom-ical changes during abscission vary somewhat from plant to plant. This was also noted by Pfeiffer ( 11) in 1928. It was the purpose of this investigation to determine the gen-eral characteristics of abscission zone development in leaves of Ginkgo biloba L. MATERIALS AND METHODS Ginkgo seeds were germinated in shallow trays lined with Sphagnum. Germinated seeds were potted and grown in the Uni-versity of Northern Iowa greenhouse for one month prior to use. 2Proceedings of the Iowa Academy of Science, Vol. 77 [1970], No. 1, Art. 7https://scholarworks.uni.edu/pias/vol77/iss1/71970] ABCISSION ZONE DEVELOPMENT 25 The seedlings were maintained in the greenhouse during the e:xper-imental period. The latter part of this investigation involved Ginkgo trees adjacent to the Parkway Central Senior High School in St. Louis, :Missouri. Leaves of three two-month old seedlings in the greenhouse were debladed to enhance possible anatomical changes in the abscission region. Non-debladed seedlings were maintained under the same greenhouse conditions as controls. The seedlings were exposed to 15-20% relative humidity, 22° C and approximately 150 foot-candles of light intensity. The basal millimeter of the petiole prox-imal to the stem was considered to be the abscission zone. The abscission zone as well as some adjacent stem and petiole tissue was excised from different seedlings each day for six days. Excised sec-tions were aspirated in 5 milliliters of Craf III fixative and stored for two weeks. The infiltration embedding procedures sugges~ed by Sass ( 14) were followed. Ten micron longitudinal sections through the abscission zone and accompanying tissue were prepared on a rotary microtome and stained with safranin and fast green. Other abscission regions were taken from Ginkgo trees grown under park conditions. The sections were hand-cut, prepared ac-cording to wet mount procedures and observed immediately. RESULTS AND D1scuss10N Debladed petioles from Ginkgo biloba seedlings showed no macroscopic evidence of abscission at the end of three weeks. This is in contrast to debladed petioles of Coleus which abscise in four to five days ( 13). Debladed Ginkgo petioles remained green, up-right and resisted pressure applied to the adaxial surface. Microscopic analysis indicated that an abscission zone was lack-ing in non-debladed Ginkgo petioles (Fig. 1). Figure 2 shows the preliminary development of the Coleus leaf abscission zone twenty-four hours after deblading. The zone is noticeable as a transversf' layer of smaller cells. These immature cells ( 1) were not char-acteristic of the Ginkgo leaf abscission zone. Although zones were taken from Ginkgo seedlings over a six-day period, the most significant anatomical change appeared on the sixth day after deblading (Fig. 3). On the sixth day an abscis-sion zone was visible on the abaxial side of the petiole. This is in agreement with the pattern of abscission zone development in de-bladed petioles of Coleus. The cells on the proximal side of the zone lacked nuclei. These cells also had greater accumulations of safranin in their cell walls as compared to surrounding cells. This indicates 3Mosher and Smith: A Study of Abcission Zone Development In Leaves of Ginkgo BilobaPublished by UNI ScholarWorks, 197026 IOWA ACAEMY OF SCIENCE [Vol. 77 Figure 1. Abscission region m a non-debladed Ginkgo petiole. I. petiole Figure 2. Coleus leaf abscission zone twenty-four hours after deblading. 1. petiole, 2. abscission layer the possibility of lignin as a component of these cell walls. Lignified cell walls were observed in Fraxinus abscission zones several weeks before leaf-fall ( 4). The formation of an abscission zone in Ginkgo seedlings is incomplete six days after deblading. 4Proceedings of the Iowa Academy of Science, Vol. 77 [1970], No. 1, Art. 7https://scholarworks.uni.edu/pias/vol77/iss1/71970] ABCISSION ZONE DEVELOPMENT 27 Figure 3. Gin kgo leaf abscission zone six days after deblading. 1. pe~iole , 2. abscission layer Yager ( 19) reported the abscission zone in tobacco flowers to be twelve tiers of cells in width. Initially, the abscission zone in debladed petioles of Ginkgo seedlings was three to four cells wide (on the abaxial side (Fig. 3) . The width of the zone could be greater, however, since the formation of the abscission . layer was probably incomplete. A clos'er examination of the abscission region shows a difference in the cells on either side of the zone. The distal cells appeared to be vacuolated and lacking nuclei (Fig. 3) . Proximal cells were nucleated and had a large number of inclusions or granules which might have been carbohydrates. This could have resulted from .a movement of materials from distal to proximal cells as was ob-served by Scott and Leopold ( 16) in bean explants. They believed that increased metabolism resulting from cell division could serve as the driving force for the mobilization. Facey ( 4) noted that cell division accompanied the development of the abscission zone in Fraxinus. She reported that the division began in proximal cells and spread distally through the zone. The irregular size and shape of proximal cells and the prominence of chromatin within their nuclei suggested the possibility of cell divi-sion in Ginkgo. Some of these cells appeared to be in telophase; however, no clearly defined mitotic figures were observed in the abscission zones during the six-day period. It is also possible that those cells· thought to be in telophase could, conceivably, have been 5Mosher and Smith: A Study of Abcission Zone Development In Leaves of Ginkgo BilobaPublished by UNI ScholarWorks, 197028 IOWA ACADEMY OF SCIENCE [Vol. 77 binucleate cells. This was observed by Webster ( 18) in Phaseolus after treatment with ethylene. She explained the bi-nucleate condi-tion as a result of mitotic divisions which were not followed by cytokinesis; or as a result of the dissolution of existing middle lamel-lae and walls between adjacent cells during ethylene treatment. The development of the abscission zone in Fraxinus petioles was noted several weeks before leaf-fall ( 4). This also seems to be true. of Ginkgo seedlings since the abscission zone was obvious by the sixth day after deblading, but separation requires longer than three weeks. Webster ( 18) and Scott et al. ( 15) have suggested that lack of water due to tylose plugs in vascular elements contributed to stress on distal cells. This could hasten senescence and influence abscission. Tylose was not evident in the tracheids of Ginkgo seed-lings. The fact that the petioles were green and not abscising is strong evidence that water transport was not interrupted. Hormones have been shown to be a factor in retention of de-bladed petioles. Kuster ( 7) noted quite early that different amounts of leaf blade would serve to prevent leaf abscission in Coleus. The Ginkgo petioles were green, suggesting heal thy tissue capable of production of hormones which could help retain the organ. No zone of separation was noted. Apparently, physiological changes involving the middle lamella and primary cell wall that lead to separation are initiated later. These changes are probably correlated with induced senescence of the debladed petioles. Petioles from a Ginkgo tree were examined for abscission zone development one-week prior to separation. All the petioles exam-ined were found to have abscission zones at various stages of devel-opment. Figure 4 is a representation of the Ginkgo abscission zone which begins to fonn on the abaxial side of the petiole at the junc-tion of the petiole and stem. This is in agreement with the initial developmrnt of the abscission layer in Ginkgo seedlings (Fig. 3). The completed abscission zone was recognized as a layer approx-imately eleven cells wide. This was similar to the size of the abscis-sion layer observed in tobacco flowers ( 19). Facey ( 4) reported a similar situation in Fraxinus leaves. The proximal part of the Ginkgo abscission zone was composed of five layers of non-nucleated cells. The cell walls in this layer were brown and easily differen-tated from cells on either side. Since a stain was not applied to these sections it was impossible to determine the components of the brown cell walls. It is possible that these cells were lignified. This possibility was also observed in Ginkgo seedlings (Fig. 3). The cells on the stem side of the abscission layer were green, nucleated and seemed to have more inclusions than their counter-parts in the distal petiole. This w::is also observed in petioles taken 6Proceedings of the Iowa Academy of Science, Vol. 77 [1970], No. 1, Art. 7https://scholarworks.uni.edu/pias/vol77/iss1/71970] ABCISSIO N ZONE DEVELOPMENT 29 Figure 4. Ginkgo leaf abscission zone one week prior to separation. 1. vascu lar tissue, 2. petiole, 3. distal section of developing abscis-sion zone, 4. proximal section of developing abscission zone Figure 5 . Completed Ginkgo lea f abscission layer. 1. petiole, 2. vascular tissue, 3. abscission layer 7Mosher and Smith: A Study of Abcission Zone Development In Leaves of Ginkgo BilobaPublished by UNI ScholarWorks, 197030 IOWA ACADEMY OF SCIENCE [Vol. 77 from Ginkgo seedlings. Many of the cells on the petiole side of the zone were green and contained nuclei. These characteristics stood in direct contrast to the cells of the abscission layer. Most of the leaves abscised on November 4, 1969. All the petioles examined on that day showed a complete abscission layer (Fig. 5). Separation was observed to occur through the apparently suberized section of the abscission layer between the ninth and tenth Figure 6. Completed Ginkgo leaf abscission layer showing separation lay-er. 1. petiole, 2. distal part of abscission zone, 3. proxmal part of abscission zone, 4. vascular tissue, 5. separation layer cell layers (Fig. 6 ) . The separation region is in the distal part of the abscission zone. Distal areas of separation were reported m Fraxinus (4), Nicotiana (19) and Gossypium (2). The mode of separation followed an abaxial to adaxia l pattern. This pattern could have been influenced by the method of sectioning. The cells of the separation layer were not disrupted indicating that the act of separation results from a dissolution of middle lamellae between separation cells. This is in agreement with patterns of separation in Brassica (3) andMirabilis (9). The pattern of abscission zone development is similar in Ginkgo biloba trees ,and seedlings. The evidence suggests that the abscission zone has a direct relationship to the separation of Ginkgo leaves. -8Proceedings of the Iowa Academy of Science, Vol. 77 [1970], No. 1, Art. 7https://scholarworks.uni.edu/pias/vol77/iss1/7• ·~ 1970] ABCISSION ZONE DEVELOPMENT 31 ACKNOWLEDGEMENTS Thanks to Dave Mixon for his assistance in the preparation of photographs used in Figures 4 through 6 . 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. Bibliography ADDICOTT, F. T. 1965. The physiology of abscission. Handbuch der Pflanzenphysiologie XV (2): 1094-1126. BoRNMAN, C. H. 1967. South Afr. four. Agr. Sci. 10: 143-154. CORMACK, R. 1955. Science 122: 1019-1022. FACEY, V. 1950. New Phytologist 49: 103-116. G"-WADI, N. G. & G. S. AVERY. 1950. Amer.]. Bot. 37: 172-180. HANNIG, E 1913. Zeitschr. Bot. 5:416-193. KusTER, E. 1916. Ber. Deut. Botan. Ges. 34: 184-193. LEE, E. 1911. Ann. Bot. (London) 25:51-107. LLOYD, F. E. 1916. Botan. Gazette 61: 213-230. MoHL, H. VON. 1860. Pflanzenorgane. Bot. Zeitung 18: 273-274. PFEIFFER, H. 1928. Die pflanzlichen Trennungagewebe. In Linsbauer, Handbuch Pflanzenanatomie, Abt. 1, Tei! 2, Bd. 5, Lief 22. p. 336. RUBENSTEIN, B. & A. C. LEOPOLD. 1964. Quart. Rev. Biol. 39:356-372. SALISBURY, F. B. & R. V. PARKE. 1965. Vascular plants: form and function. Belmont, California: Wadsworth Publishing Company. SAss, J. E. 1958. Botanical microtechnique. V. Ames, Iowa: The Iowa State College Press. SCOTT, P. C., B. D. WEBSTER & A. C. LEOPOLD. 1964. Plant Phys. 39: (suppl.) abstr. xiv. ScoTT, P. C. & A. C. LEOPOLD. 1966. Plant Phys. 41: 826-830. TsION, A. 1900. Mem Soc. Linn. Normandie 20: 121-327. WEBSTER, B. D. 1968. Plant Phys. 43: 1512-1543. YAGER, R. E. 1959. Proc. Iowa A cad. Sci. 66: 86-90. 9Mosher and Smith: A Study of Abcission Zone Development In Leaves of Ginkgo BilobaPublished by UNI ScholarWorks, 1970

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A Study of Abcission Zone Development In Leaves of Ginkgo Biloba L.

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