Pollination and Floral Studies of the Minneola Tangelo

MUSTARD, ET AL: MINNEOLA TANGELO 277

POLLINATION AND FLORAL STUDIES OF THE MINNEOLA TANGELO

Margaret J. Mustard, S. John Lynch stigmas and styles were made with a pair of metric vernier calipers. and Roy O. Nelson The viability of the pollen was determined by the hanging drop technique (5) using 20% Division of Research and Industry sucrose media. After 24 or 48 hours storage at University of Miami room temperature, percentage germination and average pollen tube length was determined Coral Gables for representative fields of each hanging drop The Minneola tangelo, a hybrid of the slide. Bowen grapefruit pollinated with the Dancy In thespring of 1955, approximately 200 tangerine (7), is considered an excellent fruit Minneola flowers were hand-pollinated with because of its attractive color, fine flavor, few Seminole andLake tangelo pollen. The pollen seeds, and good shipping quality. The com was obtained by collecting unopened flower mercial value of this fruit has been limited due buds of the two varieties and storing them iii to its failure to set adequate crops when plant a warm room for a couple of days until the ed in solid grove plantings. anthers had dehisced. Minneola flower buds which were about to open but which had not A previous report (3) showed that the yet begun to shed pollen were emasculated by failure of the Minneola tangelo to set adequate running a scalpel around the base of the bud fruit could not be directly attributed to a de removing the petals and stamens. The pollen ficiency of nitrogen, phosphorus, or boron al was applied by dusting it from the dehisced though an increase in nitrogen level did re anthers of the other varieties onto the recep sult in a slight increase in yield. Likewise, tive sticky stigmas of the emasculated flowers. Butcher (1) has shown that honey bees as In the case of the self-pollinated Minneolas, pollinating insects have a beneficial effect on flower buds which were about to open were Minneola fruit set but of insufficient magni bagged without emasculation. AH excess flow tude to provide an adequate explanation to the ers and unopened buds adjacent to the pol overall problem. linated flowers were removed prior to bag The purpose of the present paper is to pre ging. The same basic procedure was followed sent data dealing with the effect of pollen in 1956 except that Valencia and Pineapple source on the yield of Minneola tangelos and orange and Seminole and Minneola tangelo to present a few observations concerning the pollen was used; the Minneola flower buds, to floral anatomy of Minneola and other varieties be self-pollinated, were emasculated and hand- of tangelos. These data cover research con pollinated with Minneola pollen; and alumi ducted during the past two years. Although a num foil caps were substituted for craft paper brief summary of part of the first year's work bags for covering the hand-pollinated flowers. has been presented (6), it has not yet been The fallowing adaptation of the Oppenheimer published and is again summarized here as aluminum foil technique suggested by Men the second year's work is a continuation and del (4) proved much faster than the tradi confirmation of the observations of the previ tional bagging technique. Small aluminum foil ous year. caps were formed by pressing a piece of Materials and Methods aluminum foil, measuring approximately 2 by 2 inches, over the eraser end of a lead pencil. The Minneola trees were four years old at After hand-pollinating the flower, one of the the initiation of these studies and are on rough aluminum caps was placed over the emascu lemon rootstock growing on oolitic limestone lated flower and pressed^ in around its base in South Dade County. (Fig. 1). The craft bags an<#*aluminum jFoil Measurements of the length of the indivi caps were removed after approximately: three dual pistils and the combined length of the weeks and in those instances where fruit had 278 FLORIDA STATE HORTICULTURAL SOCIETY, 1956 set were replaced with cheesecloth sacks. Both the sperm and egg cell of the Min- These sacks were made large enough to allow eola are functional as evidenced by the abili for the future development of the fruit. ty of Minneola pollen to develop pollen tubes in artificial media and by the ovules to de Results and Discussion velop into normal seeds following cross-pollin The hypothesis that self-unfruitfulness ation. might be a contributing factor to low crop In checking the tangelo flowers, it ap yields in solid plants of Minneolas seemed peared that Minneola flowers had longer pis feasible because of the similarity of the prob tils than either Lake or Seminole flowers. lem to that sometimes encountered in solid Measurement of one hundred pistils of each plantings of other fruits and the fact that some variety confirmed this observation as can be tangelos are known to be self-sterile (7). seen from the data in Table 1. These data, Observations of the gross anatomy of tange coupled with the fact that Minneola pollen lo flowers: It has been observed in other va produced shorter pollen tubes when cultured rieties of citrus (7) that self-pollination may on artificial media than did the other tangelo be inadequate, in the absence of insects, due pollens used in cross-pollination (Table 2), to the maturation of the anthers before the suggests the possibility that self-unfruitfulness stigmas become receptive. The stigmas and in the Minneola might be associated with the anthers of the Minneola tangelo were found failure of Minneola pollen tubes to penetrate to mature at the same time. The anthers start the full length of the stigma and style to to dehisce just prior to or at the time of flow fertilize the ovule. Crane (2) has referred to er opening when the stigma has secreted stig- similar cases of self-sterility in other plants matic fluid and is receptive to pollen. The due to the failure of the plant's own pollen to anthers are so located with reference to the travel the full length of the style. To date, no stigma that there is ample opportunity for procedure has been found by which the de seU-polHn a tion. velopment of these pollen tubes can be satis-

Fig. I. Hand-pollination of tangelo flower. Left: Unopened flower just prior to emasculation. Center: Emasculated flower ready for pollination. Right: Hand-pollinated flower covered by aluminum cap. MUSTARD, ET AL: MINNEOLA TANGELO 279

TABLE 1. MEASUKENIENTS OF TANGELO PISTIIS

No. of flowers Average Length (micra)* Variety checked Pistil Stigma & Style Ovary

Minneola 100 13,500 9,800 3,700

Lake 100 12,100 8,900 3,200

Seminole 100 12,200 8,900 3,300

* These figures have been converted from centimeters to micra to facilitate comparison with the data in Table 2. factorily followed through the stylar canals ceived comparable cultural treatments. A of the tangelo although a number of stains check on the average number of seeds in have been tried both on crushed and sectioned twenty mature fruit resulting from open pol tissues. lination showed that the fruit in the Minneola Effect of proximity to other citrus varieties block, three rows from a row of Seminole on fruit and seed production of Minneola tan- trees contained an average of 3.6 seeds per gelos: It had been observed during past sea fruit; whereas, twenty fruit from the Minneo sons that a single Minneola tree located in a la tree located in the mixed citrus planting mixed citrus planting had consistently born a contained an average of 16.1 seeds per fruit. heavier crop of fruit than had the Minneola Yield records were taken on a block of trees in the solid Minneola planting although Minneolas consisting of six rows of thirteen the trees were of the same age and had re trees each, having a single row of Seminoles

TABLE 2. VIA3ILITY OF POLLEN CULTURED ON ARTIFICIAL 1/EDIA

No. of Germination Percent Average Length Year Pollen Cultures Period Germination of Pollen Tubes

!.:icra

1955* Lake tangelo 8 24 hrs. 59 202.9

Seminole tangelo 8 24. hrs. 65 218.3

I&nneola tangelo 8 24. hrs. 44 174.5

1956 I&nneola tangelo 8 48 hrs. 13.6 23.4

Seroinole tangelo 8 48 hrs. 56.4 177.4

Pineapple orange 8 48 hrs. 8.2 27.7

Valencia orange 8 48 hrs. 9.9 43-3

* 1955 pollen cultures made from, freshly collected pollen, whereas the 1956 cultures ;vere made using pollen stored at room terperature for five days. FLORIDA STATE HORTICULTURAL SOCIETY, 1956 280 on one side of the block and several rows of The viability of each type of citrus pollen Lake tangelos on the other. It is evident from used in the hand-pollinations was determined Fig. 2 that there was a marked increase in and these data summarized in Table 2. The yield with decrease in distance from either difference in the viability and pollen tube de velopment noted on the two consecutive years of the pollen sources. for pollen from the same variety of citrus may be partially attributable to the fact that the 1955 cultures were made from freshly col lected pollen; whereas, the 1956 cultures were made from pollen following storage for five

12 days at room temperature. It will be noted from the 1955 data in Table 3 that no Minneola flowers set fruit o |O when self-pollinated; whereas, 18.9% ancj 9.3% set fruit when the flowers were cross-pollin ated with Seminole and Lake tangelo pollen, respectively. Subsequent shedding of fruit was greater in the case of the Minneola which had been pollinated with Lake pollen than in those pollinated with Seminole pollen. No seeds were found in the fruits pollinated with Lake _L 12 3 4 pollen, while those pollinated with Seminole

Number rows from other tangelo varieties pollen averaged 39.8 seeds per fruit. Based on these data, the Lake tangelo was not consid Fig. 2. Effect of Proximity of Lake and Seminole Tangelos on Yield of Minneola Tangelos. ered a desirable pollen source for the Minneola tangelo. Effect of hand-pollination on Minneola In the spring of 1956, additional hand- fruit set: In view of the above observations, it pollinations were made using Valencia and was deemed advisable to determine the effect Pineapple orange and Seminole and Minneola of cross-pollination on Minneola fruit set. tangelo pollen. The pollen of the two varieties

TABIE 3. EFFECT CF POLLEN SOURCE ON MINNEOIA FKUIT

% Set After % Reaching No. Seeds / Fruit Year Pollen Source No. Pollinated Ifcturity Average Range

28 dare

9.3 2*8 0 1955 Lake tangelo 107 16.0 39.8 4- (4o2)* 53*31 "Seminole tangelo 106 18.9

0 —•" —*■ Minneola tangelo 91 0

17 days 47 days

41.0 (6.3)* 52-32 1956 Valencia orange 86 29.1 25.6 35.4 (15.6)* 45-24 Pineapple orange 84 58.3 50.0 36.9 37.5 (9.7)* 50-29 Seminole tangelo 82 31.7 27.5 20.7

0 Minneola tangelo 81 8.6 1.2

ii ■

* Number of small aborted seeds. MUSTARD, ET AL: MINNEOLA TANGELO 281 of orange were used to determine if pollen ness of Pineapple pollen in the open pollina from some other citrus having greater com tion of Minneola tangelos and to learn some mercial value than the Seminole tangelo could thing concerning the number and distribution be found which would prove equally effective of trees which should be topworked to a as a pollinizer for topworking trees in solid pollinizing variety to provide for cross-pollina Minneola plantings. It can be seen from Table tion in solid Minneola plantings. 3 that Pineapple and Valencia orange pollens were about equal or superior to Seminole pol Summary len in increasing the set of hand-pollinated The commercial value of the Minneola tan Minneolas. It is not known whether the gelo has been limited due to its failure to set marked increase in fruit set following hand- adequate crops in solid grove plantings. A two- pollination with Seminole pollen in 1956 as year experiment dealing with the effect of compared to 1955 could be attributed to the pollen source on the yield of Minneola tange use of the aluminum caps instead of the craft los is presented together with some observa paper bags for covering the hand-pollinated tions concerning the gross anatomy of tangelo flowers. The use of aluminum caps eliminated flowers. It was found that a marked increase the wilting of leaves adjacent to the hand- in fruit set resulted from the hand-polJination pollinated flowers such as occurred following of Minneola flowers with the pollen of either their enclosure in the craft paper bags. The Seminole tangelo, Lake tangelo, Valencia slightly chlorotic appearance of the set fruit orange, or Pineapple orange as compared to enclosed in the aluminum caps soon disap self-pollination. Based on these observations, peared following their exposure to sunlight. a five acre block of Minneola trees has been The use of pineapple orange pollen resulted partially topworked to Pineapple orange to de in a higher yield of mature fruit than did the termine its effect through open pollination on other pollens used in the 1956 hand-pollina the yield of the Minneola planting. tions. Topworking solid Minneola blocks to pro Acknowledgement vide for cross-pollination: Based on the above The workers wish to acknowledge their in observations, a five acre block of four year old debtedness to Mr. F. H. MacDonald of Coral Minneolas. has been partially topworked to Gables for his financial assistance and personal Pineapple orange to determine the effective interest in this project. ness of this pollen source in the open pollina tion of Minneolas. The five acre block was REFERENCES divided into two plots consisting of two and 1. Butcher, F. Gray. Honey bees as pollinators of Minneola tangelos. Proc. Fla. State Hort. Soc, 68:313. one half acres each. In one plot, containing 1955. thirteen rows of thirteen trees each, the third, 2. Crane, M. B. Report on tests of self-sterility and cross-incompatibility in plums, cherries, and seventh, and eleventh rows have the third, apples at the John Innes Horticultural Institution. II. Jour, of Pomol. and Hort. Sci. Ill: 1-18, 1923. seventh, and eleventh trees topworked to 3. Lynch, S. John and Margaret J. Mustard. Pineapple orange giving a total of nine top- Studies on the unfruitfulness of Minneola tangelos in Dade County. Proc. Fla. State Hort. Soc. 68:299- worked trees as pollen sources. In the other 301. 1955. 4. Mendel, K. Unpublished information of Dr. C. plot, containing the same number of rows and Oppenheimer. Personal correspondence. 1956. trees per row, the third, fifth, ninth and elev 5. Mustard, Margaret J., Su-Ying Liu, and Roy O. Nelson. Observations of floral biology and fruit- enth rows have the third, sixth, ninth and setting in lychee varieties. Proc. Fla. State Hort. Soc, 66: 212-220. 1953. twelfth trees topworked to Pineapple orange 6 Progress report on the un giving a total of sixteen topworked trees as fruitfulness of the Minneola tangelo. Presented April 7, 1956, to Caribbean Section of the Am. Soc. for pollen sources. By taking yield records from Hort. Sci. 7. Webber. H. J. and L. D. Batchelor. The citrus the individual trees in these two plots, it industry. Vol. I. History, botany and breeding. 1st should be possible to determine the effective ed. 952 p. Berkeley and Los Angeles: University of California Press. 1943.