Cameron, James W. : Emeritus

Soost, Robert K. : Department of Botany and Plant Sciences

are largely physiological , but can be long - lasting . Breeding and development Yields of older , established nucellar budlines have often been higher than those of their parent lines . James W . Cameron , Professor of Horticulture But most importantly , many viruses and viruslike and Geneticist , Emeritus , and agents are eliminated at embryo formation . This Robert K . Soost , Professorof Genetics , has led to widespread propagation of nucellar lines Department of Botany and Plant Sciences and has contributed to the improvement of citrus plantings in many countries during the last two decades . hen Howard B . Frost began citrus breeding at the University of CaliforniaCitrus Ex - W Inheritance of nucellar embryony periment Station in 1914 , knowledge of One problem associated with citrus breeding - crossing relationships in the genus was limited to as with many tree fruits - is the scarcity of char - earlier studies in Florida by the U.S . Department acters controlled by one or a few genes . Most char - of Agriculture . Frost began a wide series of crosses acters are inherited quantitatively , involving many among edible types , but the numbers of hybrids genes . In addition , genes are apparently seldom first obtained were limited by nucellar embryony . homozygous ( that is , existing in two identical This is the phenomenon by which somatic cells of copies , or alleles ) within a clone . Thus , fruits the nucellus ( tissue in the ovule but outside the showing a wide range of size , shape , color , and embryo sac ) develop into embryos . Since these flavor may occur in a single progeny from two nucellar embryos develop asexually , with no male parents . cells contributing to their formation , they are usu - Because of the importance of nucellor embry - ally genetically identical with the seed parent . of ony , U.C . researchers began to study its pattern Nucellar embryony has been a major obstacle to inheritance . By 1959 , data indicated that its inher - F1 hybrids and self - the systematic production of itance was rather simple within common Citrus pollinated progenies . Many cultivars produce al - species . Crosses between strictly sexual parents most entirely nucellar embryos , and there may be regularly yielded hybrids that were themselves several in a single seed . Others form both nucellar sexually reproducing ; crosses between sexual and and zygotic ( sexual ) embryos . Studies have identi - nucellar parents usually produced hybrids of both fied additional cultivars that produce only sexual types . Two parents that could reproduce by nucel - progeny , and these have been largely used in more lar embryony sometimes produced offspring that recent crosses . were completely sexual . Tetraploid hybrids ( hav - The occurrence of nucellar embryony enabled ing four sets of chromosomes ) were also examined , Dr . Frost to compare carefully the tree and fruit of segregationpat - and they showed the same kind characters of recent nucellar seedling budlines terns . Nucellar embryony thus appears to be dom - with those of their older parental budlines . The inant over sexual embryony , and very few genes nucellar budlines were usually more vigorous , may be involved . faster growing , more thorny , and slower to fruit Citrus and its near relative , In crosses between than parental ones . Thesejuvenile characteristics Poncimcs , the inheritance is somewhat more com - plex . Many F , hybrids were sexually reproducing , even when both parents could reproduce by nucel - lar embryony . Yet backcrosses of these sexual hy - Citrus in experiment brids to sexually reproducing station tests still resulted in progeny that were essentially all sexual , adding to the evidence that strictly sexual plants may be homozygousfor this character . Self - incompatibilityin citrus Hybridizationin citrus can also be complicated by self - and cross - incompatibility , which occurs in a number of cultivars . Self - incompatibleforms set little or no seed , and often very little fruit , after Triploid orange ( with 3 sets of chromosomes ) self - pollination . Climatic factors , however , some - with fewseeds ( center ) times favor fruit set even in the absence of seeds ; is from cross of is called parthenocarpy . this condition Ruby orange tetraploid Self - incompatible cultivars may produce many by diploid King ( left ) fruitsand seeds after cross - pollination by particu - tangor ( right ) . lar pollen parents , indicating cross - compatibility . Reactions of this sort occur in a number of plant Right : CES - developed â?¬ amiliesand are considered to be caused by one or pum - seedless Chandler more series of self - incompatibilityalleles . Studies melo was grown in isola - tion ; seedy fruit at left reported by Dr . Soost in 1965 indicated that such received other citrus are present in several citrus taxa , including alleles pollen . 5ome pummelos and their hybrids , certain grape - Fruit hybrids such as Minneola and Orlando , and CR - 4 the Clementine mandarin . In such cases , cultivars from two or three basic histogenic layers , but a that can serve as pollinators are often needed to nucellar embryo stems originally from a single seed produce good crops of fruit . parent cell . The evidence indicates that the pink grapefruits carry genetic factors for color in only The use of polyploidy one or two of their histogenic layers . Thus nucellar 11 ) can either embryos ( believed to arise from layer Most common citrus is diploid , with two sets of lack a color factor or , conversely , can carry it in all chromosomes , but tetraploids often occur spon - their tissues . taneously among seedling populations . Dr . Frost was one of the first to identify and collect these Chemical factors as markers forms , among a series of citrus cultivars . Although Citrus taxonomy has long been controversial , not themselves commercially useful , tetraploids and the origin and relationships of many forms are have been important in breeding few - seeded tri - sets of chromosomes ) . Fruits uncertain . Intensive studies of a range of chemical ploids ( with three Oroblanco triploid grape with few seeds are desirable for eating out of and physical characters have been directed toward hand , but most diploid hybrids are seedy . Triploids solving these problems . Under the leadership of fruit hybrid , developed at can be produced by crossing diploids with tetra - R.W . Scora , U.C . scientists have investigated ter - U.C . in 1980 , is nearly penes of leaf and rind oils , leaf hydrocarbons , pro - seedless and earlier 1938 Frost had obtained limited ploids , and by tein and isozyme patterns , oxidase browning , virus ripening than the Marsh numbers of such hybrids . Recently , large popula - resistance , and momhologicalcharacters . The grapefruit . tions have been produced and are under study . When a partly sexual tetraploid is used as seed overall results suggest that there are four primary Below : Immature fruit of as pollen parent , triploids are parent and a diploid species , namely , the subtropical Citrusreticulata variegated orange . usually obtained , but the reciprocal cross results in and C . medica , the tropical C . grandis and the yellow rind sectors have recently discovered c.halimii . The latter is some - normal growth ; green few triploids and numerous hybrid tetraploids . U.C . , Riverside , studies showed that the what intermediate between the genera citrus and sectors are & Dressed . chromosomesoccasionally doubled in the diploid Fortunella . Other forms that have often been clas - as species can , in fact , be identified as mem - seed parent , followed by selective survival of the sified bers of complexes involving asexual reproduction . resulting tetraploid embryos . An unfavorable In studies of closely related biotypes , relationships balance between chromosomenumbers in the young embryo and endosperm ( the nutritive tissue ) were found between diploid and tetraploid forms , is responsible for this unusual behavior . However , among identical twin individuals , and among F , hybrids of Citrus and Pcm.cirus . such new tetraploid plants , some of which are U.C . researchers , in cooperation with the Uni - strictly sexual , have already been used as addi - versity of Kansas , have recently identified enzyme tional seed parents to produce more triploids . characters that are simply inherited and useful for The triploid embryos that arise from pollination also have a of tetraploid seed parents by diploids both breeding and taxonomy . The browning reac - tion in young shoot tissue results from oxidation of certain developmental disadvantage , since they sometimes tend to mature earlier and produce phenolic substrates . Presence of the substrate is smaller seeds . They can usually be germinated sat - under single - gene control , which often permits the isfactorily by use of in vitro techniques . separation of nucellar from zygotic seedling plants . Another reaction causes coagulation of shoot Mutations and chimeras tissue in some forms . Coagulationis recessive to so that , again , separation of dif - non - coagulation Somatic mutations occur frequently in citrus ; ferent genetic types is often possible . Analyses of they are often unfavorable , but occasionally highly several other leaf enzymes , also inherited in valuable . In Japan , mutations in the satsuma man - single - genefashion , have been especially reward - darin have resulted in closely related clones differ - The presence or absence of as many as eight ing . as color and ripening ing in characteristics such of these markers , taken together , can permit early At Riverside , Dr . Frost found that similar time . and rapid separation of nucellar from zygotic mutations had produced satsuma seedling lines plants in a great many crosses , and can assist in that varied in ripening time , productivity , and determining parentage of some long - established fruit shape . One of these nucellar lines has been cultivars . as an improved cultivar . used commercially arise in single somatic cells are Mutations that New citrus cultivars sometimes perpetuated in only a part of the tissues Efforts to produce early - maturing citrus fruits of of a plant or animal , resulting in a chimera . In moderate acidity have been aided by the use of a 1942 , chimeral citrus trees were identified at grandis ) that has essen - clone of pummelo ( Citrms Riverside in which certain histogenic , or tissue - tially no titratable acid . When U.C . plant breeders producing , cell layers were diploid , whereas others crossed this pummelo with cultivars of medium had become tetraploid . This caused differences in acidity , the resulting hybrids had acid levels aver - the height , shape , and leaf texture of these plants . at early mid - We have studied an interesting case of chimer - aging well within the edible range season . Compared hybrids with a medium - or high - ism among pigmented grapefruits , in cooperation ofAgriculture . It is as a parent were usually too sour . acid pummelo with the U.S . Department known that the Thompson and Foster pink grape - One of the acidlesspummelo hybrids , when selfed , fruits arose as limb sports on white grapefruit produced acidless individuals in proportions that trees early in this century . When nucellar seedling suggest single - gene action . These individuals may trees of these cultivars were grown in about 1960 , as new acidless parents . serve those of Thompson had no pink in their fruits , whereas those of Foster had deeper color than their parent . Various tissues in many plants arise CR - 5 two citremons showed certain good characteristics Two U.C . - developed hybrids in - with lemons , but overall they have not been suffi - volving the acidless pummelo ciently promising . The citrumelo behaved well in have been introduced : the some trials with orange , but showed severe prob - Chandler pummelo ( 1961 ) and the Oroblanco grapefruit hybrid lems in a Eureka lemon trial . ( 1980 ) . Both are of good flavor Identifying satisfactory rootstocks for lemons has been especiallydifficult . Sweet orange is fav - and are usually edible by De - orable but should not be used where gummosis is a cember in southern California plantings . The Oroblanco has problem . Eureka lemons are incompatible with very few seeds , because it is a as are some semidense Lisbons . Cleopatra Troyer , mandarin has been perhaps the most useful of the triploid . Studies of seed number in the Chandler indicate that it mandarin stocks . Several recent trials with Citrus is self - incompatible and , when macrophylla have shown that it gives high yields planted in isolation , can pro - be a with small tree size , but phloem necrosis can duce good yields of seedless limitingfactor . Pmirus and Citrus to fruit . As with other such New crosses between cultivars , this ~ h ~ racteristic obtain rootstocks have been made at Riverside . Two germinating seeds on right show polyembryony varies With climatic conditions Successfulrootstock cultivars must be highly of more than one embryo ) . Others are ( production monoembryonic . nucellar in their reproduction , and hybrids with and by years . Fruits with few sweet orange have appeared promising in this seeds can sometimesbe smaller in size . Phy - respect . Some of them show resistance to The first new citrus cultivars introduced at Riv - tophthwu , tristeza , and nematodes , but their ef - fect on yield and fruit quality is uncertain . One of 1935 ) were those produced by Dr . Frost . erside ( in They were the Kara , Kinnow , and Wilking man - the better ones , unfortunately , has a low yield of darins and the Trovita orange . The Kinnow has at - fruit and seeds . tained considerable importance in Pakistan and in Trifoliolateleaf of Poncirus parts of North Africa ; it continues to be grown on a small scale in California.The Wilking , usually Leaves of Poncirus are typically composed of too small for general use , has been a parent ofng three leaflets , in contrast to the simple ( monofoli - several mandarin hybrids with excellent eati olate ) leaves of common citrus . It has been of in - quality . In Japan the Trovita , which is an early , this trifoliolate terest to examine the inheritance of few - seededjuice orange , has been used in crosses leaf character . Nearly all of the older , long - with satsuma to produce a new tangor , the Citrus with Pmirus were established hybrids of Kiyomi . known to have high proportions of three - parted A new U.C . study showed that among 272 leaves . Rootstocks and rootstock breeding F1individuals , all but one had significant pro - portions of multifoliolate leaves , but the percen - Rootstocks are of major importance to citrus cul - tages varied from 30 percent in some hybrids to ture in most countries , because they help to im - 100 percent in others . When these hybrids nearly part disease resistance , cold hardiness , high yield Citrus , most of the resulting were backcrossed to and fruit quality , and other favorable character - plants had very few or no multifoliolate leaves , istics to their scions . In California , clones of the yet some showed proportions up to 90 percent . sweet and sour orange were the principal stocks The conclusionwas that the three - leaflet charac - for many years , until the damaging effects of ter is not completely dominant , but that it is rather Phytophthoru fungi and the tristeza virus made simply inherited , probably dependent on only a them unacceptable in many areas . few genes . When tristeza began to destroy orchards of sweet orange on sour orange rootstock , use of a Future prospects substitute rootstock became imperative . As part of Citrus and Citrus rela - comprehensive studies of Advances in breeding and genetics by conven - tional methods will continue to be slow because of 1946 at U.C . , Riversideby tives , conducted since quantitative inheritance of most characters , long W.P . Bitters , many taxa were tested for tristeza generation time , and nucellar embryony . However , The Troyer citrange was outstanding , resistance . because sweet orange scions grown on this root - breeding efforts can be directed at more specific goals than in the past because of new information stock were tolerant to the disease , and also had and plant materials . The range of strictly sexual other favorable qualities . Troyer continues to be parents has been increased , more tetraploids are the most important rootstock for oranges in Cali - available for the production of triploids , and hy - A genetically similar cultivar , the Carrizo , fornia . brids with improved horticultural characters have has also been successful for that purpose . been developed . One parent of Troyer is Poncirus - the trifoliate Identification and recovery of sexual seedlings as a rootstbck also imparts toler - orange - which should continue to improve with the use of avail - ance to tristeza . Clones of this cultivar have im - in vitro techniques . How - able biochemical and portant rootstock value , but under more limited in vitro methods of hybridization ever , progress in conditions than Troyer . Studies were made on has been slow , although fusion of somatic proto - Poncirus , includ - other old , established hybrids of plasts ( cells with cell walls removed ) has been 1448 and 1449 ( hybrids with lemon ) ing citremons achieved . Research on cell selection has been initi - 1452 ( a hybrid with grapefruit ) . The and citrumelo ated . Continued research should provide a choice of methods for transferring and recombining a wider range of genetic material . CR - 6