.168. Background Material For A Study of Rootstocks A. H. Krezdorn The basicelement in any attempt to advanceknowledge is the ability to communicate. Eacharea of study requiresa speciallanguage of definitions and facts that enablethose involved to discussa subjectintelligently. The purposeof the following materialis to presenta readyreference of facts and terminology for usein this course. The materialis presented in brief form to servethe immediateinterests of the participants. Classification and Nomenclatureof Plants Classification. - Scientistshave classified plants, (placedthem into groupsbased on their relationshipto eachother) and then affixed names to these groups according to accepted rules of nomenclature. Classification is inexact becausethere is no way to determine the precise relationship of plants. Thus, plants may be grouped differently by equally able scien- tists who interpret available information differently. Perhapsfortunately, not many scientists are inclined to undertake the tremendous task of classifying large groups of plants. Walter T. Swingle, a U.S. Department of Agricultu re scientist wf10worked in Florida, is the universallyaccepted authority for the largefamily of plants that includesthe genusCitrus. All life is divided into that which is animaland that which is plant and then into groupsdecreasing in sizeand increas- ing in closenessof relationship. The position of citrus in the Plant Kingdomis: Division . Embryophyta Siphonogama Subdivision. Angiospermae Class . Dicotyledoneae Subclass . Archichlamydeae Order. Geraniales Suborder. Geraniineae Family. Rutaceae Subfamily. Aurantioideae The subfamily Aurantioideae is a ramer large category but wormy of mention becauseit contains not only the various kinds of commercial citrus but wild relatives, some which hybridize with citrus and some which have potential as rootstocks. Swingle has divided the subfamily Aurantioideae into tribes, subtribes, subtribal groups and genera as follows: Subfamily - Aurantioideae TribeI - Clauseneae Subtribe 1 . Micromelinae GenusI - Micromelum Subtribe2 - Clauseninae GenusI - Glycosmis GenusII . Clausena GenusIII - Murraya Subtribe3 - Merrillinae GenusI . Merrillia TribeII . Citreae Subtribe1 - Triphasiinae GenusI - Wenzelia GenusII . Monanthocitrus GenusIII . Oxanthera GenusIV . Merope GenusV - Triphasia GenusVI . Pamburus GenusVII - Luvunga .169- GenusVIII. Paramignya Subtribe2 . Citrinae GenusI . Severinia GenusII . Pleiospermum GenusIII . Burkillanthus GenusIV . limnocitrus GenusV - Hesperethusa GenusVI . Citropsis GenusVII. Atalantia GenusVIII - Fortunella GenusIX . Eremocitrus GenusX . Poncirus GenusXI . Microcitrus GenusXII. Citrus Subtribe3 - Balsamocitrinae GenusI - Swinglea GenusII - Aegle GenusIII . Afraegle GenusIV - Aeglopsis GenusV . Balsamocitrus GenusVI - Feronia GenusVII. Feroniella Nomenclature. - The ordinary laymandefines citrus asthe sweetoranges, tangerines, grapefruit, lemonsand lime fruit, commonly found on the grocery shelf under the citrus label. Growers, more knowledgeable of citrus technology, include unpalatable speciesand close relatives such as sour orange, trifoliate oranges, kumquats, and others in their definition of citrus. Scientists use the term citrus in several ways as follows: A. The subtribe C;tr;neaehas been divided into 3 citrus groups. Subtribe - Citrineae 1. Primitive Citrus Fruit Trees GenusI - Severinia GenusII . P/eiospermum GenusIII. Burki//anthus GenusIV. Limnocitros GenusV. Hesperethusa 2. Near-CitrusFruit Trees GenusI. Citropsis GenusII - Ata/antia 3. True Citrus Fruit Trees GenusI - Fortune//a (kumquats) GenusII. Eremocitrus GenusIII. Ponciros(trifoliate orange) GenusIV. C/ymenia GenusV. Microcitrus GenusVI - Citros (includescommon commercialcitrus types) Thus,the scientistmay usethe tenn citrus in a broad senseto meanthe commercialtypes of the genusCitrus, the inedibletypes of Citrusand certain Citrus relatives. B. The term Citrus; ie, citrus spelledwith a capital C and underlinedor italicized indicatesthe genericname of that group. All of the fruit marketedcommercially as citrus is includedin this genus.The kumquat, in the genusFortunella, is a minor exception. .170. Scientists and well-informed growers often use scientific botanical names. such as Citrus sinensis (L.) Osbeck. The .:ientific name can be divided into 3 parts the genus, Citrus, the speciesepithet sinensis and the author citation, (L.) Osbeck. The common name of this species is sweet orange or orange. The advantageof the scientific nameis that eachspecies, the basictaxonomic unit, can haveonly one nameand that nameis formed accordingto specificrules. The unfortunate fact, however,is that thosewho classifyplants may differ asto which plants should belongto a givengroup. This situation borderson the ridiculous in the genusCitrus which Swingledivided into only 16 species.Tyosaburo Tanaka from Japan,on the other hand,divided the samegenus or group of plants (Citrus) into over 145 species!Tanaka's classification is too unwieldy to be functional. Swingle'sis more usable than Tanaka'sbecause it is simple,but it is obviouslyin error in somecases. Scientists and professionalhorticulturists gen- erally useSwingle's system modified at times to include someof Tanaka'ssystem when modificationsappear appropriate. A briefsummary of Swingle'ssystem with occasionalcomparisons with Tanaka'ssystem is presentedbelow: Genus- Citrus 1. SubgenusCitrus, includesthose species with edible fruit and pulp not containingdrops of acrid or bitter oil. This subgenuscontains the following species: a. C. medica(citrons), a speciesvalued primarily for the useof its peel. The 'Etrog' variety or cultivar is used in certain religiousceremonies by Hebrewsand someselections of the 'Etrog', such asArizona 861 and USDA 60-31, arecommonly usedas indicator plantsto detect the exocortisvirus. b. C.limon (lemons),a speciesused by Swingleto include 811lemons, both acid and acidless. His classification is probably too inclusiveand certain exceptionsare commonly made,with C. limon primarily including the acid lemonsof commerce.Tanaka's classification includes the following types commonly usedas rootstocks: (1) C.jambhiri - rough lemon (2) C. limonia . Rangpurlime (3) c. volkameriana - Volkameriana lemon The 'Meyer' and 'Ponderosa'lemons are considered hybrids by Swinglebut separatespecies by Tanaka. c. C. aurantifolia (acid limes), a speciesincluding all acid limesaccording to Swingle. Somecommonly used exceptionsby Tanakaare: (1) C. aurantifolia. the 'Key', 'West Indian', 'Mexican' or 'Gallego'lime. The 'Key' lime is usedas an indicator plant for the tristezavirus. (2) C. latifolia - the 'Tahiti', 'Persian'or 'Bearss'lime. (3) C. macrophylla - the Alemow. Tanakadescribed this speciesas a lime-like fruit. Somefeel it is lemon. like and othersthat it is a hybrid of C. celebica. This specieshas become an important lemon rootstock in recentyears. (4) C. limetta - the limmetas,a group of lemon-likefruit with both acid and acidlessforms. (5) C. limmetoides - the Indian or Palestinesweet lime that is widely usedas a rootstock in someparts of the world. The Columbiansweet lime is indistinguishablefrom the standardPalestine sweet lime. d. C. aurantium (sour orange),a speciesused widely asa rootstock wherethe tristezavirus doesn'tprevent its use. There are numerousforms of sour orangeincluding a low acid type called bittersweet. (1) C. taiwanica. is a sour orange-likefruit with resistanceto certain forms of tristeza. (2) Australian sour orange - is a hybrid of unknown origin that is reputedly tolerant of tristeza;however, this selectionhas few if any characteristicsthat resemblesour orange. Callingthis fruit a sour orangeis misleading. e. C. reticulata (mandarins),a specieswith loose-peeledfruit that includesthe commontangerines and satsumas (Unshiu,Mikan) oranges.Tanaka divides this speciesinto severalas indicatedin the table below. CommonName Swingle ;1.f1;';',,~ Tanaka , c-..' C. unsh;u C. reticulata C. reticulata C. tanger;na C. reticulata C. ret;culata C. reticulata C. reshn; C. reticulata C. clement;na -171- co~t. " , Tanaka CommonName Swingle - C. deliciosa 'Willowleaf C. reticulata 'King' tangor? C. nobilis b , -' tangor? C. temple 'Temple''.-- (;" :' """'c " C. reticulata Vir. 'Calamodin' C. madurensis Busters x C. ichangensis YOlO ~~' Vi' (tangerinex C.ichangensis C.junos ~_..~ ~ (1) The Cleopatramandarin is important asa rootstock throughout the world and the Yuzu is usedto someextent asa rootstock in Japan. (2) Most of the mandarinsare usedfor their fine quality asfresh fruit. (3) The tangerineshave been hybridized with grapefruit to form tangelossuch as 'Orlando', 'Minneola' and 'Nova'. (4) The 'Clementine'and 'Temple' are usedwidely in breedingprograms because they are monembryonic and of high quality. f. C.grandis (pummeloor shaddock),is little usedin the United Statesbut important for its fruit in the Far East. The fruit and tree are similar to grapefruit in many respectsbut the flesh of pummelois sweeterand lessjuicy than that of grapefruit and the peel of pummelois generallythicker. The cultivars in this species are monoembryonicand therefore the pummelois usedin grapefruit breedingprograms. The Cubanshaddock is sometimesmentioned as a rootstock. g. C.paradisi (grapefruit),a speciesvalued more for its fruit in the Westernthan the Easternworld, is usedpri- marily asa fresh fruit Grapefruit hasbeen used occasionally as a rootstock in Florida but without any apparentadvantages. h. C. sinensis(sweet oranges), is the most widely grown speciesof