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 and Roy O. Nelson Division of Research and Industry University of Miami Coral Gables The, a hybrid of the Bowen grapefruit pollinated with the Dancy tangerine (7), is considered an excellent fruit because of its attractive color, fine flavor, few seeds, and good shipping quality. The com mercial value of this fruit has been limited due to its failure to set adequate crops when plant ed in solid grove plantings. A previous report (3) showed that the failure of the to set adequate fruit could not be directly attributed to a de ficiency of nitrogen, phosphorus, or boron al though an increase in nitrogen level did re sult in a slight increase in yield. Likewise, Butcher (1) has shown that honey bees as pollinating insects have a beneficial effect on fruit set but of insufficient magni tude to provide an adequate explanation to the overall problem. The purpose of the present paper is to pre sent data dealing with the effect of pollen source on the yield of s and to present a few observations concerning the floral anatomy of and other varieties of s. These data cover research con ducted during the past two years. Although a brief summary of part of the first year's work has been presented (6), it has not yet been published and is again summarized here as the second year's work is a continuation and confirmation of the observations of the previ ous year. Materials and Methods The trees were four years old at the initiation of these studies and are on rough lemon rootstock growing on oolitic limestone in South Dade County. Measurements of the length of the indivi dual pistils and the combined length of the stigmas and styles were made with a pair of metric vernier calipers. The viability of the pollen was determined by the hanging drop technique (5) using 2% sucrose media. After 24 or 4 hours storage at room temperature, percentage germination and average pollen tube length was determined for representative fields of each hanging drop slide. In the spring of 1955, approximately 2 flowers were hand-pollinated with and Lake pollen. The pollen was obtained by collecting unopened flower buds of the two varieties and storing them iii a warm room for a couple of days until the anthers had dehisced. flower buds which were about to open but which had not yet begun to shed pollen were emasculated by running a scalpel around the base of the bud removing the petals and stamens. The pollen was applied by dusting it from the dehisced anthers of the other varieties onto the recep tive sticky stigmas of the emasculated flowers. In the case of the self-pollinated s, flower buds which were about to open were bagged without emasculation. AH excess flow ers and unopened buds adjacent to the pol linated flowers were removed prior to bag ging. The same basic procedure was followed in 1956 except that Valencia and Pineapple and and pollen was used; the flower buds, to be self-pollinated, were emasculated and handpollinated with pollen; and alumi num foil caps were substituted for craft paper bags for covering the hand-pollinated flowers. The fallowing adaptation of the Oppenheimer aluminum foil technique suggested by Men del (4) proved much faster than the tradi tional bagging technique. Small aluminum foil caps were formed by pressing a piece of aluminum foil, measuring approximately 2 by 2 inches, over the eraser end of a lead pencil. After hand-pollinating the flower, one of the aluminum caps was placed over the emascu lated flower and pressed^ in around its base (Fig. 1). The craft bags an<#*aluminum jfoil caps were removed after approximately: three weeks and in those instances where fruit had

27 FLORIDA STATE HORTICULTURAL SOCIETY, 1956 set were replaced with cheesecloth sacks. These sacks were made large enough to allow for the future development of the fruit. Results and Discussion The hypothesis that self-unfruitfulness might be a contributing factor to low crop yields in solid plants of s seemed feasible because of the similarity of the prob lem to that sometimes encountered in solid plantings of other fruits and the fact that some s are known to be self-sterile (7). Observations of the gross anatomy of tange lo flowers: It has been observed in other va rieties of citrus (7) that self-pollination may be inadequate, in the absence of insects, due to the maturation of the anthers before the stigmas become receptive. The stigmas and anthers of the were found to mature at the same time. The anthers start to dehisce just prior to or at the time of flow er opening when the stigma has secreted stigmatic fluid and is receptive to pollen. The anthers are so located with reference to the stigma that there is ample opportunity for seu-polhn a tion. Both the sperm and egg cell of the Mineola are functional as evidenced by the abili ty of pollen to develop pollen tubes in artificial media and by the ovules to de velop into normal seeds following cross-pollin ation. In checking the flowers, it ap peared that flowers had longer pis tils than either Lake or flowers. Measurement of one hundred pistils of each variety confirmed this observation as can be seen from the data in Table 1. These data, coupled with the fact that pollen produced shorter pollen tubes when cultured on artificial media than did the other pollens used in cross-pollination (Table 2), suggests the possibility that self-unfruitfulness in the might be associated with the failure of pollen tubes to penetrate the full length of the stigma and style to fertilize the ovule. Crane (2) has referred to similar cases of self-sterility in other plants due to the failure of the plant's own pollen to travel the full length of the style. To date, no procedure has been found by which the de velopment of these pollen tubes can be satis- Fig. I. Hand-pollination of 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 Variety No. of flowers checked Pistil Average Stigma Length (micra)* & Style Ovary 1 13,5 9, 3,7 Lake 1 12,1,9 3,2 1 12,2,9 3,3 * These figures have been converted from centimeters to micra to facilitate comparison with the data in Table 2. factorily followed through the stylar canals of the although a number of stains have been tried both on crushed and sectioned tissues. Effect of proximity to other citrus varieties on fruit and seed production of s: It had been observed during past sea sons that a single tree located in a mixed citrus planting had consistently born a heavier crop of fruit than had the trees in the solid planting although the trees were of the same age and had re ceived comparable cultural treatments. A check on the average number of seeds in twenty mature fruit resulting from open pol lination showed that the fruit in the block, three rows from a row of trees contained an average of 3.6 seeds per fruit; whereas, twenty fruit from the Minneo la tree located in the mixed citrus planting contained an average of 16.1 seeds per fruit. Yield records were taken on a block of s consisting of six rows of thirteen trees each, having a single row of s TABLE 2. VIA3ILITY OF POLLEN CULTURED ON ARTIFICIAL 1/EDIA Year Pollen No. of Cultures Germination Period Percent Germination Average Length of Pollen Tubes!.:icra 1955* Lake 24 hrs. 59 22.9 24. hrs. 65 21.3 I&nneola 24. hrs. 44 174.5 1956 I&nneola 4 hrs. 13.6 23.4 Seroinole 4 hrs. 56.4 177.4 Pineapple 4 hrs..2 27.7 Valencia 4 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.

2 FLORIDA STATE HORTICULTURAL SOCIETY, 1956 on one side of the block and several rows of Lake s on the other. It is evident from Fig. 2 that there was a marked increase in yield with decrease in distance from either of the pollen sources. 12 o O _L 12 3 4 Number rows from other varieties Fig. 2. Effect of Proximity of Lake and Tangelos on Yield of Tangelos. Effect of hand-pollination on fruit set: In view of the above observations, it was deemed advisable to determine the effect of cross-pollination on fruit set. The viability of each type of citrus pollen used in the hand-pollinations was determined and these data summarized in Table 2. The difference in the viability and pollen tube de velopment noted on the two consecutive years 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 days at room temperature. It will be noted from the 1955 data in Table 3 that no flowers set fruit when self-pollinated; whereas, 1.9% ancj 9.3% set fruit when the flowers were cross-pollin ated with and Lake pollen, respectively. Subsequent shedding of fruit was greater in the case of the which had been pollinated with Lake pollen than in those pollinated with pollen. No seeds were found in the fruits pollinated with Lake pollen, while those pollinated with pollen averaged 39. seeds per fruit. Based on these data, the Lake was not consid ered a desirable pollen source for the. In the spring of 1956, additional handpollinations were made using Valencia and Pineapple and and pollen. The pollen of the two varieties TABIE 3. EFFECT CF POLLEN SOURCE ON MINNEOIA FKUIT Year Pollen Source No. Pollinated % Set After % Reaching Ifcturity No. Seeds Average / Fruit Range 2 dare 1955 Lake " 17 16 9.3 1.9 2* 16. 39. 4- (4o2)* 53*31 91 " * 17 days 47 days 1956 Valencia 6 29.1 25.6 41. (6.3)* 52-32 Pineapple 4 5.3 5. 36.9 35.4 (15.6)* 45-24 2 31.7 27.5 2.7 37.5 (9.7)* 5-29 1.6 1.2 ii * Number of small aborted seeds.

MUSTARD, ET AL: MINNEOLA TANGELO 21 of were used to determine if pollen from some other citrus having greater com mercial value than the could be found which would prove equally effective as a pollinizer for topworking trees in solid plantings. It can be seen from Table 3 that Pineapple and Valencia pollens were about equal or superior to pol len in increasing the set of hand-pollinated s. It is not known whether the marked increase in fruit set following handpollination with pollen in 1956 as compared to 1955 could be attributed to the use of the aluminum caps instead of the craft paper bags for covering the hand-pollinated flowers. The use of aluminum caps eliminated the wilting of leaves adjacent to the handpollinated flowers such as occurred following their enclosure in the craft paper bags. The slightly chlorotic appearance of the set fruit enclosed in the aluminum caps soon disap peared following their exposure to sunlight. The use of pineapple pollen resulted in a higher yield of mature fruit than did the other pollens used in the 1956 hand-pollina tions. Topworking solid blocks to pro vide for cross-pollination: Based on the above observations, a five acre block of four year old s. has been partially topworked to Pineapple to determine the effective ness of this pollen source in the open pollina tion of s. The five acre block was divided into two plots consisting of two and one half acres each. In one plot, containing thirteen rows of thirteen trees each, the third, seventh, and eleventh rows have the third, seventh, and eleventh trees topworked to Pineapple giving a total of nine topworked trees as pollen sources. In the other plot, containing the same number of rows and trees per row, the third, fifth, ninth and elev enth rows have the third, sixth, ninth and twelfth trees topworked to Pineapple giving a total of sixteen topworked trees as pollen sources. By taking yield records from the individual trees in these two plots, it should be possible to determine the effective ness of Pineapple pollen in the open pollina tion of s and to learn some thing concerning the number and distribution of trees which should be topworked to a pollinizing variety to provide for cross-pollina tion in solid plantings. Summary The commercial value of the tan gelo has been limited due to its failure to set adequate crops in solid grove plantings. A twoyear experiment dealing with the effect of pollen source on the yield of tange los is presented together with some observa tions concerning the gross anatomy of flowers. It was found that a marked increase in fruit set resulted from the hand-poljination of flowers with the pollen of either, Lake, Valencia, or Pineapple as compared to self-pollination. Based on these observations, a five acre block of trees has been partially topworked to Pineapple to de termine its effect through open pollination on the yield of the planting. Acknowledgement The workers wish to acknowledge their in debtedness to Mr. F. H. MacDonald of Coral Gables for his financial assistance and personal interest in this project. REFERENCES 1. Butcher, F. Gray. Honey bees as pollinators of s. Proc. Fla. State Hort. Soc, 6:313. 1955. 2. Crane, M. B. Report on tests of self-sterility and cross-incompatibility in plums, cherries, and apples at the John Innes Horticultural Institution. II. Jour, of Pomol. and Hort. Sci. Ill: 1-1, 1923. 3. Lynch, S. John and Margaret J. Mustard. Studies on the unfruitfulness of s in Dade County. Proc. Fla. State Hort. Soc. 6:299-31. 1955. 4. Mendel, K. Unpublished information of Dr. C. Oppenheimer. Personal correspondence. 1956. 5. Mustard, Margaret J., Su-Ying Liu, and Roy O. Nelson. Observations of floral biology and fruitsetting in lychee varieties. Proc. Fla. State Hort. Soc, 66: 212-22. 1953. 6 Progress report on the un fruitfulness of the. Presented April 7, 1956, to Caribbean Section of the Am. Soc. for Hort. Sci. 7. Webber. H. J. and L. D. Batchelor. The citrus industry. Vol. I. History, botany and breeding. 1st ed. 952 p. Berkeley and Los Angeles: University of California Press. 1943.