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Seed Development in Citrus

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Seed Development in Citrus SEED DEVELOPMENT IN CITRUS L. K. JACKSON; Revisedby F. G. GMITTER, JR. ProfessorEmeritus and AssociateProfessor, Department of Horticultural Sciences,University of Florida, Citrus Researchand EducationCenter, Lake Alfred, FL 33850 Becauseseed development is necessarilya part of the developmentof fruit. it is important to considerthe processesresponsible for seeddevelopment and someof the reasonsfor the lack of seedsin somecases. We will then turn attentionto the actual growth and developmentof the seed within the fruit. Lastly, we needto considerthe horticultural significanceof seedproduction and especiallysome of the uniquefeatures of citrus seedswhich makethem both an assetand a liability for growersand researchscientists. Citrus Flower and Fruit MoQ1holo~. There are 2 main types of bloom in citrus. Leafy bloom occurswhen flowers form on and with new vegetativegrowth in the spring. The vegetative growth of the elongating shoot is transformedinto a tenninal inflorescenceof 1 or more flowers about the time lateral shootsemerge from bud scales.Other flowers are produceddirectly in leaf axils of previous growth flushesand are termedbouquet bloom. As a vegetative growth point is transfonned into a flower bud, it becomes broadened and flattened, forming a floral apical meristem. Each flower part is fonned from the outside in (acropetally). The sepal (button) primordia are fonned first, then those of the petals, the anthers and finally the carpels which will form the segments. The female flower parts (pistil) are composed by fusion of a whorl of approximately 10 carpels. The cavities fonned by fusion of the carpels are called locules. Each locule contains 2 vertical rows of ovules (Fig. 1), that can develop into seeds. Fig. 1. Diagrammaticportrayal of opencitrus flower. 133 The ovules are arrangedin rows near the central axis (axile placentation). The inner integumentis differentiatedfirst. The outer integumentis developedlater. Both integumentstake part in the fonnation of the seedcoat andthe micropylewhich providesthe openingthrough which the pollen tube entersthe ovule. The First Stel2sto Seed Formation: Me~asl2oro~enesisand Embrxo Sac Develol2ment. Within the ovule, the archesporia!cell forms nearthe apexof the nucellusbefore the integuments arecompletely developed. The cell is distinguishedfrom thosesurrounding it by its larger size and larger nucleus.The archesporia!cell divides to form a tapetalcell and an embryo sacmother cell (megasporocyte).The tapetal cell divides rapidly and the megasporocyteis soon surroundedby tissuenear the centerof the nucellus.The megasporocyteundergoes reduction division, producing 4 haploidcells with only one setof chromosomesper cell. Thesecells, the megaspores,are arranged in a longitudinal row in the nucellus. Three of the 4 megasporesdegenerate and the remaining megasporedevelops into the megasporemother cell and later, the embryo sac.(Fig. 2). MJcellus inner in~ment outer integument outer ntegument inner integument archesporial cell nucellus ~megasporocyte ~II -:; 'lis.2a. Developing ovule Fig.2b. Developing ovule with archesporial with megasporocyte. cell. ooter in~ment integument in~ment inner integUment tetrad 3 degenerated m~ores -7 functional megaspore '1a.2c.. Developing ovule vi th -a.spore Fig.2d. Developing ovule tetrad. vith'.eg&spore lIother cell. Megasporogenesis. 34 micropyle outer integument inner integument developing embryo sac I / First stage of embryo sac developaent in Two-celled stage of Pig.3b. ovule - aegaspore .other cell. embryo sac developD8Dt in ovule. Four-celled stage of Fig. 3~. Eight-celled stage eabryo sac developaent of e8bryo sac in ovule. developaent in ovule. Fig. 3. Embryo sacdevelopment. 35 The functionalmegaspore divides to fonD 2 daughternuclei which migrateto oppositeends of the sacand remain there until the next nucleardivision. Meanwhile,the cytoplasmis reducedto a thin layer in the periphery of the embryo sac,except around the nuclei. The 2 daughternuclei divide again,producing 4 nuclei which fonD 2 pairs at oppositeends of the sac.The nuclei divide againto form the final 8 nuclei. The 4 nuclei in the micropylar end of the sacdevelop into the egg, 2 synergids,and upper polar nucleus. The 4 nuclei in the chalaza!end fonD the 3 antipodalsand lower polar nucleusto completeembryo sacdevelopment (Fig. 3). The time requiredfor embryosac development varies with the cultivar. The embryo sacsare reported to be in the 2- or 4-nucleatestage at anthesisin most ovules of 'Foster'grapefruit, while 'Pineapple'sweet orange embryo sacsare mostly in the 4- or 8-nucleatestage at anthesis. Maturation of the embryo sac reportedly occurs several days prior to petal opening (anthesis) in 'Foster' grapefruit, at anthesis in 'Pineapple' sweet orange and several days after anthesis in 'Washington' navel sweet orange and 'Satsuma' mandarin. The process of embryo sac formation varies from ovule to ovule within the same fruit. Formation of the mature embryo sac with respect to floral development is apparently cultivar-dependent and may also be influenced by other external and internal factors. Megaspore or embryo sac development at anthesis ranges from the megaspore tetrad stage through the fully developed embryo sac. 'Pineapple' sweet orange embryo sacs apparently undergo a rapid development. Thirty-four percent of the ovules of this cultivar contain mature embryo sacsat anthesis,28% were in various stagesof development, and the remaining 38% aborted at some stage. Embryo sac formation is reportedly complete in some ovules several days prior to anthesis in 'Foster' grapefruit, although many ovules contain embryo sacs in the 1-, 2- or 4-nucleate stages at anthesis. Pollen Tube Growth. Fertilization.and Endos~rm Develol2ment.Pollen grains develop in the anthers, following reduction division of microsporocytesto produce microsporeswith the haploid chromosomenumber. Insectsvector the pollen to the stigma, where they germinateto producea pollen tube that growsthrough the style to the ovary. This tube penetratesthe micropyle of the developingovules, and the 2 spermnuclei aredischarged into the embryosac. One fuseswith the egg nucleusand the secondnucleus fuses with the 2 polar nuclei and forms a triploid endosperm nucleus.This doublefertilization apparentlyoccurs simultaneously. The zygote goesinto a resting stageafter fertilization for severalweeks before dividing (Fig. 4). The endospermnucleus divides successivelymany times, without cell wall development,to produce a multinucleate endosperm. Free nuclear endospermcan be seenup to 67 days after pollination. This endospermmaterial becomes cellular, with 3 layersof cells on the micropylar end and a single layer of cells further down the chalazalend. The endospermprovides a sourceof nutrition that is absorbedas the embryo develops;it is consumedby 100 days after pollination. Sterilitx. Gameticsterility is the inability to producefunctional pollen or embryosacs. Male sterility may resultfrom imperfectdevelopment of stamensbut usually is causedby defectivepollen development.Pollen sterility is found in varying degreesin many cultivarsof the genusC.i.tms. The 'Washington'sweet orange is essentiallyvoid of pollen. Satsumamandarin pollen also degenerates 36 degenerated synergids multinucleate endospenn de~netated antipodals Fig. 4. Fertilization and endospermdevelopment within the embryo sac. during formation,leaving only a few nonnal grainsat anthesis.Cross-pollination slightly increases seedset of commerciallyseedless citrus cultivars;their relative seedlessnessis associatedwith low pollen viability and meiotic anomaliesresulting from chromosomalstructural changes. Female sterility may result from aborted female flower parts or defective embryo sac development. Studies of'Satsuma' mandarin and 'Washington' sweet orange provide classic examples of female sterility. The megaspore mother cell of 'Washington' often aborts before division. The cell becomes long and narrow and is compressedupward by the surrounding nucellar tissue. Generally, the megaspore mother cells pass through nomIal divisions, producing the linear tetrad of megaspores.Soon after the nomIal degenerationof the 3 megasporesnearest the micropyle, the functional megaspore shrivels and aborts. Embryo sacs often abort in other stages of development. Many of the embryo sacsexhibit delayed development which indicates the egg is not mature and viable for fertilization at the proper time. Embryo abortion is another form of sterility. Functional gametes(male and female parts) may be produced and fertilization occur, yet the embryo may fail to develop. The occurrence of empty seed coats in fruit suggests embryo abortion. Citrus nonnally has 18 chromosomesbut various other multiples of chromosome levels. The Tahiti, or Persian, lime for example is a triploid citrus cultivar (i.e. 2n = 3x = 27). When triploid citrus attempt to undergo meiosis, gametes(eggs and pollen) are producted with unbalanced numbers of chromosomes because, quite simply, 27 is not evenly divisible by 2. Citrus breeders are attempting to exploit this fonn of sterility to develop seedlesscultivars. Unlike other cultivars such as 'Clementine' which are seedlessby virtue of self-incompatibility, but which will bear seededfruit when cross-pollinated by other citrus, triploids should be seedlessregardless of pollination by other cultivars. One method to develop triploids is to induce embryogenesisin endospenn cultural in vitro; 31; although it has been accomplished, the limited number of successesmakes this method
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