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1 Blossom-Bud Differentiation in Citrus Trees Author(s): Chas. E. Abbott Reviewed work(s): Source: American Journal of Botany, Vol. 22, No. 4 (Apr., 1935), pp Published by: Botanical Society of America Stable URL: Accessed: 23/12/ :13 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Botanical Society of America is collaborating with JSTOR to digitize, preserve and extend access to American Journal of Botany.

2 BLOSSOM-BUD DIFFERENTIATION IN CITRUS TREES CHAS. E. ABBOTT- (Received for publication September ii, I933) The time of blossom-bud differentiation for nearly all of the cultivated deciduous fruits has been determined, but very little work has been done toward determining the time of blossom-bud formation of evergreen fruit trees. This perhaps is due in part to the fact that evergreen fruit trees are not so widely cultivated in the United States as are the deciduous fruits. Evergreen fruit trees, especially those belonging to the genus Citrus, present an entirely different problem from that of most deciduous trees, in that it is impossible to distinguish with any degree of accuracy between a fruit bud and a vegetative bud, either by the position of the bud on the tree or by the size of the bud prior to the beginning of a growth flush. The literature on the subject of fruit-bu differentiation and development in horticultural plants in the United States is very extensive. However, no attempt is made in this paper to give a resume of such literature, inasmuch as it does not bear directly on the question as related to citrus trees. Though there is a recent Japanese publication (Hiroto, I931) dealing with blossombud differentiation of several species of Citrus-namely, C. grandis Osbeck; C. sinensis Osbeck (the Washington Navel orange); C. tacnkan Hayata; C. poon.ensis Hort. ex Tanaka; and C. msicrocarpa Bunge-growing in northern Formosa, apparently no data are available on the time of blossom-bud differentiation in grapefruit, orange, satsuma, or the kumquat in the United States. MATERIALS AND METHODS The materials used in this study were collected from three species of Citrus: grapefruit (C. paradisi Macf.), sweet orange (C. sinensis Osbeck), and satsuma (C. nobilis Lour. var. Unshiu, Swingle). A study also was made of kumquat (Fortunella inargareta Swingle). The varieties studied were the Duncan grapefruit, Pineapple orange, Owari satsuma, and Nagami kumquat. The collection of materials was started in November, I927, and since that time collections have been made fromn orange and grapefruitrees at weekly intervals during the greater part of each year until March i6, I932, with two exceptions. During late December, I927, low temperatures destroyed the buds and in many cases killed the branches of the grapefruit and orange trees back several feet, thus destroying all bearing wood. Then again in the spring of I930 it was thought unnecessary to collect materials due to the weakened 476

3 April, I935] ABBOTT - CITRUS 477 condition of the trees following the maturinlg of a very heavy crop of fruit during the previous seasonl. The satsuma trees were not injured as badly by the low temperature as were the grapefruit and orange trees, and therefore it was possible to make a complete series of collections duringv I928, as well as in I929. The kumquat buds were gathered throughouthe season of I930. The buds were collected at Gainesville, Florida, from grapefruit, orange, and satsuma trees growing on the grounds of the College of Agriculture, and' the kumquat buds were taken from trees growing on the grounds of the Experiment Station. The trees vary in age from 14 to 24 years. The soil is a Norfolk sandy loam and slopes gently to the south. All the trees with the exception of the kumquat have received practically the same fertilizer and cultural treatment.- Prior to I926 the trees received clean cultivation with cover crop, but for the past five years the trees have received no cultivation and at present are growing in a sod of Bermuda grass with a portion of the block covered with other grasses and weeds of native species. Each collection consisted of twenty twigs taken at random from each group of trees, from which four to six buds nearest the terminal portion were taken as a sample. Collections were made from the spring, summer, and fall flushes of growth on grapefruit, orange, and satsuma trees. Standard histological methods were used in the preparation- of the buds for sectioning and photographing. ' Alcohol was employed in killing and dehydrating, xylol in clearing, paraffin for embedding, and Delafield's haematoxylin in staining. A rotary microtome was used in sectioning, and sections were cut at I2 microns. PRESENTATION OF DATA In this investigation blossom-bud differentiation in grapefruit, orange, satsuma, and kumquat was considered to have begun when longitudinal sections of the bud showed a broadening of the growing point with concurrently developing' lobes (fig. IC, IF, 2B, and 2G, respectively). Grapefruit (C. paivadisi Macf.). A vegetative bud from a Dun1can grapefruit tree of bearing age is shown in figure IA. The first evidence of inflorescence differentiation in I929 appeared in materials collected January 26. In 193I and I932 the first differentiation appeared in materials collected January I8. Figure IA represents a bud collected January I8, I93I. The period of blossom-bud differentiation in I929 and I93I was found to occur over a period of about two weeks, apparently varying with the uniformity and rapidity with which the tree forced its buds into growth. During the spring of I932 blossom-bud differentiation was found in materials collected between January I8 and March I6, inclusive. The broadening or flattening of the growing point which precedes differentiation is shown in figure ib. Figure IC shows the first differentiation of individual flower parts, the sepals, in the form of a slight swelling on either side of the central axis, and figure ID illustrates a bud collected January 26, and shows later development of these

4 478 AMERICAN JOURNAL OF BOTANY [VoI. 22, _ v.:.f''s s - l ~~~1,061 _l I - :.:... 3,;... I. _ 1 1, F... u.... Il _ q D*ell_- 1FIG. 1 I I _

5 April, I935] ABBOTT - CITRUS 479 parts. Figure ie, also collected January 26, shows an early stage in the development of the petals, and a very large flattened central portion, indicating that the stamens and pistil are not differentiated. Axillary buds on growth of the current season, which are to become blossom buds, develop and differentiate into blossom buds after the development and differentiation of the terminal blossom bud of the flowering shoot. Progress of development of axillary buds, from a very early vegetative stage through the period of differentiation, is shown in figure id and E. Figure id shows a broad and rounded axillary bud indicating a stage of development shortly before differentiation. Figure ie shows the axillary bud after differentiation of the first individual flower parts formed. Ora,nge (C. sinensis Osbeck). Early development in the vegetative bud from Pineapple orange trees is shown in figure if. The first evidence of inflorescence differentiation in 1929 appeared in materials collected January 29. In 193I the first differentiation appeared in materials collected January i8, and in 1932 differentiation appeared in materials collected January 12. The trees from which materials were collected in I932 had not been allowed to mature fruit during the previous season; therefore they started growth earlier than did the trees from which the fruit had not been harvested. These trees also started growth earlier than did the grapefruitrees which matured no fruit. Figure if represents a bud collected January I8, 193I. Figure ig and H represent two buds collected January i8 and show progressive stages in the development of the individual flower parts noted in figure if. Figure II represents a bud collected February i, and shows well-developed sepals and petals, together with the early stage of stamen formation. Satsu'ma (C. nobilis Lour.). Figure 2A shows an early vegetative stage of a bud from an Owari satsuma tree collected July 2. The first evidence of inflorescence differentiation in 1928 appeared in materials collected February I3. In I929 the first inflorescence differentiation appeared in materials Fig. i. A to E, inclusive, are grapefruit buds. A, very early vegetative stage of a bud from a tree of bearing age, July 27. X 86. B, bud from a tree showing a broad, flat growing point at what is considered to be the pre-differentiation stage. This bud also shows a very early stage in the development of the axillary bud. January i8. X IOO. C, a very early stage of blossom-bud differentiation evidenced by the lobes, representing individual flower parts, developing concurrently on the crown. This bud shows an increase in development of the axillary bud. January 26. X 84. D, progress of the development of the sepals. The petals, stamens, and pistil are not differentiated. The axillary bud is broad and rounded, indicating a stage shortly before differentiation. January 26. E, advancement in the development of the sepals, together with an early stage of petal formation. The axillary bud shows early development of individual flower parts. January 26. X 86. F to I, inclusive, are orange buds. F, a very early stage of blossom-bud differentiation, evidenced by lobes advancing concurrently on the crown. January i8.. X 98. G and H, progress of development of buds like those shown in fig. if, also collected January i8. X IOO. I, an early stage of the development of the complete flower; the sepals, petals, stamens, and pistil primordium are now evident, February i.

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7 April, I9351 ABBOTT - CITRUS 48I collected January 26. Figure 2B represents a bud collected on that date, which shows the broad, flattened growing point that is characteristic of the "pre-diff erentiation " stage. Figures 2C and D representwo buds also collected January 26, and show progressive stages in the development of the individual flower parts already laid down. Figure 2D shows a very large, flattened portion of the growing point, indicating that the petals, stamens, and pistil are not differentiated. Figure 2E represents another bud collected January 26, I929, and, shows an advanced stage in the development of the sepals, well-formed petals, and stamens and pistil not yet differentiated. Kumquat (Fortunella: margarita Swingle). A vegetative bud from Nagami kumquat tree is shown in figure 2F. The first evidence of inflorescence dlifferentiation in I930 appeared in materials collected May 20. Figures 2G and H representwo stages of buds collected on that date; G shows an early stage in the differentiation of the bud, while H shows the sepals and petals already formed. Figure 2I represents a bud collected May 26, and shows advancement in the development of the flower parts previously noted, with an additional formation representing the stamens. Effect of ringing. To determine the influence of ringing on the time of blossom-bud differentiation, six branches, ranging from one to one and a half inches in diameter on each grapefruit, orange, and satsuma tree, were ringed March 25, I930, by removing one-fourth inch section of bark completely surrounding the branch. Each wound was carefully wrapped with budding tape to prevent drying. Upon examining the wounds May I, it was found that every ringed branch on grapefruit, orange, and satsuma trees had healed perfectly, and the foliage, which had become slightly yellow about two weeks after ringing, had returned to a healthy dark green color. Materials collected at weekly intervals from these branches showed no increase in the number of blossom buds differentiated due to ringing as compared with unringed portions of the various trees. On May 3 other branches were ringed on grapefruit, orange, and satsuma trees, the same as above, but precautions were taken to prevent the rapid filling-in and healing of the wounds. Buds were collected from these branches at weekly intervals throughouthe season. The leaves soon became pale yellow in color, and all visible vegetative growth ceased. The leaves Fig. 2. A to E, inclusive, are satsuma buds. A, early vegetative stage of a bud. July 2. X 96. B, a broad, flat growing point, at what is considered to be the pre-differenltiation stage. January 26. X 98. C and D, progress of development of the sepals, the first flower parts formed. January 26. X 98. E, advancement in the development of the sepals, together with an early stage of petal formation. January 26. X 98.- F to I, inclusive, are kumquat buds. F, very early vegetative stage of a bud from tree of bearing age. May 5. X 96. G, a very early stage of blossom-bud differentiation. May 20. H, bud as it appeared a few days after differentiation, showing the sepals well advanced, together with petal formation. The stamens and pistil are not differentiated. May 20. X 98. I, an early stage in the development of the complete flower; the sepals, petals, stamen-s, and pistil primordia are evident. May 26. X 87.

8 482 AMERICAN JOURNAL OF BOTANY [Vol. 22, remained yellow throughouthe growing season and many dropped during late summer. The ringed branches did not make a " June flush" of growth as did the unringed portions of the various trees. Materials collected from the branches September 6, at the time when the buds were just swelling and starting growth, showed the first evidence of blossom-bud differentiation, and on September 27 the, ringed branches on all trees were in full bloom. The influence of prolonged drought on blossom-bu differentiation. During I93I it became necessary to irrigate the grapefruit and orange trees following the prolonged drought of the summer and fall. The trees were badly wilted when water was applied October 25 and 26. To each of two Pineapple orange trees five pounds of nitrate of soda were applied October 27 to accelerate growth further. Buds were collected at the time of applying the nitrate and at five-day intervals thereafter for several weeks. The bucds were prepared for microscopic examination the same as buds used in other studies. Nine days after the application of water and nitrate the trees showed visible signs of bud swelling and growth. Materials collected October 27, at the time of applying the nitrate, showed no evidence of blossom-bud differentiation when examined under the microscope. Materials collected November 6, at the time when the buds were swelling and pushing into growth, showed an abundance of blossom-bud differentiation. Differeit flushes of growuth. Examination of buds collected from the spring, summer, and fall flushes of growth on grapefruit, orange, and satsuma trees failed to show any difference in time of blossom-bud differentiation during the subsequent spring period of inflorescence formation. The " June bloom " or blossom buds which usually occur during the summer following a moderately dry period are d ifferentiated the initiation of the June or summer flush of growth on trees or parts of trees that set few or no fruit during the spring. Buds collected in I930 from the spring flush of growth on grapefrui trees from which the fruit set during the spring had been removed showed blossom-bud differentiation May 3I. In I93I June blossom buds were not differentiated until about the middle of July. This evidently was due to the limited rainfall during the months of April, May, and June, associated with a light set of fruit during the spring. DISCUSSION Differentiation of blossom-buds in the species of Citrus studied does not take place until the beginning of growth in the spring or upon the resumption of growth at any other season of the year following a period of environmental conditions favorable and of sufficient duration for the accumulation of a reserve food supply. Therefore the time of differentiation will vary slightly from year to year with climatic and seasonal variations. This is evidenced by the occasional blossoming of citrus trees during the summer or early fall when forced into growth following a prolonged dry period, or after branches have been ringed for a sufficient time and later forced into growth. The pro-

9 April, I935] ABBOTT - CITRUS 483 longed check in growth during the winter months in the absence of limiting factors seems to be especially favorable for bringing about the proper conditions for abundant blossom-bud differentiation during the spring, especially in the grapefruit, orange, and satsuma trees. This is somewhat in line with work reported on other fruits. Arndt (I928), working with coffee, found at the Haitian Coffee Experiment Station that the floral buds mature during the dry season and the flowers open almost exactly eight days after the first rain subsequent to the winter dry season. Lees (I926), working with apples, pointed out that in England the dry seasons are more conducive to flower-bud formation than are the wetter ones. The growth and fruiting habits of the Nagami kumquat differ somewhat from those of the Citrus species studied, in that the majority of the fruit buds formed for the crop of the current season are differentiated during late May and early June on wood that was formed during the spring of the current season. However, a few flower buds were observed to come out on older wood which was produced during the previous season, or perhaps earlier. The failure of the Nagami kumquat to differentiate and push out blossoms in great numbers at the first flush of growth in the spring of the year following the prolonged winter check in growth is evidently due to specific and generic characteristics. In the grapefruit and orange it was found that most blossom buds are formed toward the outer extremity of the last flush of growth on the branch, regardless of whether the flush of growth was made during the spring, summer, or fall. However, blossom buds have been observed to occur farther back on the branches, where the wood is much larger and older. This is especially true on trees that blossom very heavily following prolonged moderately dry seasons, which apparently are most favorable for stimulating blossom-bud formation. The unequal distribution of irrigation water during the fall of I93I is probably responsible for the orange trees starting growth and differentiating blossom buds earlier during the spring of I932 than did the grapefruitrees. The continued drought until late in the spring, associated with an insufficient and variable water supply, perhaps is responsible also for the irregularitv with which the various parts of the trees were forced into growth, thus prolonging the period over which blossom-bud differentiation occurred. The ringing of* branches on the grapefruit, orange, and satsuma trees March 25, when the trees were in active growth, and protecting the wounds so that complete healing occurred in from three to four weeks, did not increase blossom-bud differentiation. Apparently the duration of time over which the influence of ringing was effective in this case was not sufficiently long for the accumulation of a reserve food supply or the bringing about of other necessary conditions favorable for blossom-bud differentiation. On the other hand, branches ringed May 3, and treated in such a way as to prevent rapid healing, had a stimulating effect on the number of blossom buds formed. In

10 484 AMERICAN JOURNAL OF BOTANY [Vol. 22, this case the blossom buds were differentiated during the first week in September, at the first flush of growth made by the girdled branches subsequent to the girdling. This indicates that ringing during the spring of the year will have little effect on the formation of blossom buds during the following spring unless the ringing be severe enough to make its influence effective over the greater part of the growing season. Perhaps ringing in late summer or early fall would have more influence on increasing the number of blossom buds formed during the spring period of inflorescence differentiation. SUM MARY Blossom-bud differentiation in the species of Citrus studied takes place at the initiation of growth in the spring or upon the resumption of growth at any other season of the year subsequent to a period of environmental conditions favorable to and of sufficient duration for the accumulation of a reserve food supply. Blossom-bud differentiation in the Duncan grapefruit in I929 appeared in materials collected January 26. In I93I and in I932 differentiation appeared in materials collected January I8. Blossom-bud differentiation in the Pineapple orange in I929 appeared in materials collected January 29. In I93I the first differentiation appeared in materials collected January i8, and in I932 differentiation appeared in materials collected January I2. Blossom-bud differentiation appeared in the Owari satsuma in I928 ill materials collected February I3, and in I929 differentiation appeared in materials collected January 26. Blossom-bud differentiation in the Nagami kumquat in I930 appeared in materials collected May 20.. Lateral blossom-bud differentiation growth of the current season, in the citrus trees studied, does not occur until after the terminal blossom bud of the shoot has differentiated. The ringing of grapefruit, orange, and satsuma branches on March 25, followed by rapid healing of the wounds, did not stimulate blossom-bud f ormation. Branches ringed on May 3, and treated in such a way as to prevent rapid healing of the wounds, increased the number of blossom buds differentiated at the beginning of the September flush of growth. The September flush was the first growth made by the branches subsequent to ringing. " June bloom " is differentiated during the initiation of the summer flush of growth, subsequent to a prolonged dry period, and on trees or parts of trees that set few or no fruit during the spring. Prolonged moderately dry periods which cause an extended check in growth favor blossom-bud formation at the subsequent resumption of growth.

11 April, I935] ABBOTT CITRUS 485 The writer wishes to express his thanks and appreciation to Dr. L. H. MacDaniels, of Cornell University, for suggestions in planning the work and for examining the microscopic materials; to Major W. L. Floyd, Professor H. H. Hume, and Professor J. Francis Cooper, of the University of Florida; and to Professor V. R. Gardner, of the MVfichigan State College, for criticism and suggestions in the preparation of the manuscript. UNIVERSITY OF FLORIDA, GAINESVILLE, FLORIDA LITERATURE CITED ARNDT, C. H. I928. Configuration and some effects of light and growth on coffee arabica. Amer. Jour. Bot. i6: I73. HIROTo, N. ig3i. The flowering habit and the fruit bud formation in Citrus. Reprinted from Studia Citrologia, Tanaka Citrus Experiment Station, volume 5, no. I. LEES, A. H. I926. The influence of a summer rainfall and previous crop on fruiting of apples. Jour. Pomology and Hort. Sci. 5: I78-I94.