The Biology of Grafting

The Biology of GraftingThe Biology of Grafting• Natural graftingNatural grafting

– Bracing of limbs in commercial orchards to support Bracing of limbs in commercial orchards to support weight of fruitweight of fruit

– Root grafting in woods is prevalent (CHO’s of upper Root grafting in woods is prevalent (CHO’s of upper canopy trees provide support for understory trees). canopy trees provide support for understory trees). This grafts only occur between trees of the same This grafts only occur between trees of the same speciesspecies

– Problems with root grafting include: transmission of Problems with root grafting include: transmission of fungi, bacteria and viruses between plants (Dutch fungi, bacteria and viruses between plants (Dutch Elm Disease spreads this way)Elm Disease spreads this way)

The Biology of GraftingThe Biology of Grafting• Formation of the graft unionFormation of the graft union

– A “de novo” formed meristematic area must A “de novo” formed meristematic area must develop between scion and rootstock for a develop between scion and rootstock for a successful graft unionsuccessful graft union

• 3 events3 events– 1) adhesion of the rootstock & scion1) adhesion of the rootstock & scion– 2) proliferation of callus at the graft interface = 2) proliferation of callus at the graft interface =

callus bridgecallus bridge– 3) vascular differentiation across the graft 3) vascular differentiation across the graft

interfaceinterface

The Biology of GraftingThe Biology of Grafting• Steps in graft union formationSteps in graft union formation

– 1.) lining up of the vascular cambium of rootstock and 1.) lining up of the vascular cambium of rootstock and scion. Held together with wrap, tape, staples, nails or scion. Held together with wrap, tape, staples, nails or wedged togetherwedged together

– 2.) wound response2.) wound response• Necrotic layer 1 cell deep forms on both scion and stockNecrotic layer 1 cell deep forms on both scion and stock

• Undifferentiated callus tissue is produced from uninjured Undifferentiated callus tissue is produced from uninjured parenchyma cells below the necrotic layerparenchyma cells below the necrotic layer

• Callus forms a wound periderm (outer “bark”) which becomes Callus forms a wound periderm (outer “bark”) which becomes suberized to prevent entry of pathogenssuberized to prevent entry of pathogens

• Necrotic layer dissolvesNecrotic layer dissolves

The Biology of GraftingThe Biology of Grafting

– 3.) callus bridge formation3.) callus bridge formation• Callus proliferates for 1 - 7 daysCallus proliferates for 1 - 7 days

• Callus mostly comes from Callus mostly comes from scionscion (due to basal (due to basal movement of auxins and CHO’s, etc.)movement of auxins and CHO’s, etc.)

• An exception to this is on established rootstock An exception to this is on established rootstock which can develop more callus than that from the which can develop more callus than that from the scion. scion.

• Adhesion of scion and stock cells with a mix of Adhesion of scion and stock cells with a mix of pectins, CHO’s and proteins. Probably secreted by pectins, CHO’s and proteins. Probably secreted by dictyosomes which are part of the Golgi bodies in dictyosomes which are part of the Golgi bodies in cells.cells.

The Biology of GraftingThe Biology of Grafting

– 4.) Wound-repair :• First the xylem and then the phloem is repaired

• Occurs through differentiation of vascular cambium across the callus bridge

• Process takes 2 - 3 weeks in woody plants

– 5.) Production of 2º xylem and phloem from new vascular cambium in the callus bridge

• Important that this stage be completed before much new leaf development on scion or else the leaves will wilt and the scion may die

The Biology of GraftingThe Biology of Grafting

• Some water can be translocated through callus cells but not enough to support leaves

• Cell-to-cell transport via plasmodesmata = symplastic transport (links cells membranes)

• Apoplastic transport is between adhering cells

Factors influencing graft union successFactors influencing graft union success

• IncompatibilityIncompatibility• Plant species and type of graftPlant species and type of graft

– Easy plants = apples, grapes, pears Easy plants = apples, grapes, pears – Difficult plants = hickories, oaks and beechesDifficult plants = hickories, oaks and beeches

– Gymnosperms are usually grafted scionsGymnosperms are usually grafted scions– Angiosperms are usually budded scionsAngiosperms are usually budded scions

Factors influencing graft union successFactors influencing graft union success• Environmental conditions following graftingEnvironmental conditions following grafting

– TemperatureTemperature: effects callus production. : effects callus production. • Depends on plant! (beech calluses better at 45ºF while grape is Depends on plant! (beech calluses better at 45ºF while grape is

best at 75ºF)best at 75ºF)• Easy to control in a greenhouse but difficult in the fieldEasy to control in a greenhouse but difficult in the field

– MoistureMoisture: needed for cell enlargement in the callus bridge: needed for cell enlargement in the callus bridge• Maintain using plastic bags over scionMaintain using plastic bags over scion• Wrap with grafting tape, Parafilm, grafting rubbers and waxWrap with grafting tape, Parafilm, grafting rubbers and wax• Place union in damp peat moss or wood shavings for callusingPlace union in damp peat moss or wood shavings for callusing

Factors influencing graft union successFactors influencing graft union success• Growth activity of the rootstockGrowth activity of the rootstock

– ““T-budding” depends on the bark of the rootstock T-budding” depends on the bark of the rootstock “slipping” meaning the cambial cells are actually “slipping” meaning the cambial cells are actually dividing and separate easily from each otherdividing and separate easily from each other

– ““slipping” usually occurs in late spring or early slipping” usually occurs in late spring or early summersummer

– At certain periods of high growth in spring, plants At certain periods of high growth in spring, plants (like walnut, maple and grape) can have excessive (like walnut, maple and grape) can have excessive root pressure producing sap and “bleeding”, forcing root pressure producing sap and “bleeding”, forcing off the scion and an result in an unsuccessful graftoff the scion and an result in an unsuccessful graft

Factors influencing graft union successFactors influencing graft union success

• Art of grafting (especially with conifers)Art of grafting (especially with conifers)

• Virus contamination, insects and diseaseVirus contamination, insects and disease– Viruses cause delayed incompatibilitiesViruses cause delayed incompatibilities

• Blackline in walnut and brownline in plumBlackline in walnut and brownline in plum

– Bacteria and fungi can enter the wound Bacteria and fungi can enter the wound made during graftingmade during grafting

Factors influencing graft union successFactors influencing graft union success• Plant growth regulators and graft union Plant growth regulators and graft union

formationformation– Exogenous auxins have not proven beneficialExogenous auxins have not proven beneficial– Endogenous auxin is needed in the scion to produce Endogenous auxin is needed in the scion to produce

calluscallus

• Post-graft (bud-forcing) methodsPost-graft (bud-forcing) methods– ““crippling” or “lopping” = cutting halfway through crippling” or “lopping” = cutting halfway through

the rootstock shoot on the side above the bud union the rootstock shoot on the side above the bud union and breaking over the shoot. This “breaks” apical and breaking over the shoot. This “breaks” apical dominance and the scion bud can elongatedominance and the scion bud can elongate

Factors influencing graft union successFactors influencing graft union success

• Polarity in graftingPolarity in grafting– Top-graftingTop-grafting: proximal end of scion inserted : proximal end of scion inserted

into distal end of rootstockinto distal end of rootstock– Root-graftingRoot-grafting: proximal end of scion inserted : proximal end of scion inserted

into proximal end of rootstockinto proximal end of rootstock– Inverse scions in bridge grafts can remain Inverse scions in bridge grafts can remain

alive but will not expand/growalive but will not expand/grow– BuddingBudding: upright orientation of bud should be : upright orientation of bud should be

maintainedmaintained

Factors influencing graft union successFactors influencing graft union success• Genetic limits of graftingGenetic limits of grafting

– Monocots are harder than dicot. Why?Monocots are harder than dicot. Why?• Lack vascular rings and have scattered vascular bundles Lack vascular rings and have scattered vascular bundles

insteadinstead

– General rules:General rules:• The more closely related plants are (botanically), the better The more closely related plants are (botanically), the better

the chances for the graft to be successfulthe chances for the graft to be successful• Grafting within a clone (no problems)Grafting within a clone (no problems)• Grafting between clones within a species (usually no problems)Grafting between clones within a species (usually no problems)

– Problems can occur with Problems can occur with PseudotsugaPseudotsuga (evergreen conifer) and (evergreen conifer) and Acer Acer rubrumrubrum and and Quercus rubraQuercus rubra (deciduous angiosperm plants) (deciduous angiosperm plants)

Factors influencing graft union successFactors influencing graft union success• Genetic limits of graftingGenetic limits of grafting

– General rules:(continued)General rules:(continued)• Grafting between species within a genus (50/50 chance of Grafting between species within a genus (50/50 chance of

success). Reciprocal interspecies grafts are not always success). Reciprocal interspecies grafts are not always successfulsuccessful

• Grafting between genera within the same family (rather Grafting between genera within the same family (rather remote)remote)

– ChamaecyparisChamaecyparis (cypress) on (cypress) on ThujaThuja (arborvitae) (arborvitae)

– CitrusCitrus (citrus) on (citrus) on PoncirusPoncirus (hardy orange) (hardy orange)

– PyrusPyrus (pear) on (pear) on CydoniaCydonia (quince) (quince)

– In the Solanaceae (nightshade family) grafting between genera is not In the Solanaceae (nightshade family) grafting between genera is not a problem! Tomato, tobacco, potato, pepper, petunia, morning glory, a problem! Tomato, tobacco, potato, pepper, petunia, morning glory, etc.etc.

Factors influencing graft union successFactors influencing graft union success

• Genetic limits of graftingGenetic limits of grafting– General rules:(continued)General rules:(continued)

• Grafting between families: Grafting between families: nearlynearly impossible! impossible!

• The first known graft union between two The first known graft union between two different families was published in 2000. The different families was published in 2000. The families were two succulents:families were two succulents:

• Cactaceae and CapparaceaeCactaceae and Capparaceae

Graft IncompatibilityGraft Incompatibility

• CompatibilityCompatibility = ability of two different = ability of two different plants grafted together to produce a plants grafted together to produce a successful union and continue to develop successful union and continue to develop satisfactorilysatisfactorily

• Graft failureGraft failure: caused by anatomical : caused by anatomical mismatching/poor craftmanship, adverse mismatching/poor craftmanship, adverse environment, disease and graft environment, disease and graft incompatibilityincompatibility

Graft IncompatibilityGraft Incompatibility

• Graft incompatibility from:Graft incompatibility from:– Adverse physiological responses between Adverse physiological responses between

grafting partnersgrafting partners– Virus transmissionVirus transmission– Anatomical abnormalities of the vascular Anatomical abnormalities of the vascular

tissue in the callus bridgetissue in the callus bridge

Graft IncompatibilityGraft Incompatibility• External symptoms of incompatibilityExternal symptoms of incompatibility

– Failure of successful graft or bud union in high Failure of successful graft or bud union in high percentagespercentages

– Early yellowing or defoliation in fallEarly yellowing or defoliation in fall– Shoot die-back and ill-healthShoot die-back and ill-health– Premature deathPremature death– Marked differences in growth rate of scion and stockMarked differences in growth rate of scion and stock

• Overgrowth at, above or below the graft unionOvergrowth at, above or below the graft union

• Suckering of rootstockSuckering of rootstock

• Breakage at the graft unionBreakage at the graft union

Graft IncompatibilityGraft Incompatibility

• Anatomical flaws leading to incompatibilityAnatomical flaws leading to incompatibility– Poor vascular differentiationPoor vascular differentiation– Phloem compression and vascular discontinuityPhloem compression and vascular discontinuity– Delayed incompatibility may take 20 years to Delayed incompatibility may take 20 years to

show up (often in conifers and oaks)show up (often in conifers and oaks)

Graft IncompatibilityGraft Incompatibility• Physiological and Pathogen-Induced Physiological and Pathogen-Induced

IncompatibilityIncompatibility– Non-translocatableNon-translocatable = localized. Problem is fixed by = localized. Problem is fixed by

using mutually compatible interstock(no direct using mutually compatible interstock(no direct contact between scion and stock)contact between scion and stock)

– TranslocatableTranslocatable = spreads. Interstock does not solve = spreads. Interstock does not solve the problem. Some mobile chemical causes phloem the problem. Some mobile chemical causes phloem degradation. Ex: cyanogenic glucosides like degradation. Ex: cyanogenic glucosides like prunasin is converted to hydrocyanic acid (from prunasin is converted to hydrocyanic acid (from Quince to pear)Quince to pear)

Graft IncompatibilityGraft Incompatibility

– Pathogen-induced virus of phytoplasma Pathogen-induced virus of phytoplasma inducedinduced

– TristezaTristeza = viral disease of budded sweet = viral disease of budded sweet orange that is grafted onto infected sour orange that is grafted onto infected sour orange rootstockorange rootstock

Graft IncompatibilityGraft Incompatibility• Predicting incompatible combinationsPredicting incompatible combinations

– Electrophoresis test to look for cambial peroxidase Electrophoresis test to look for cambial peroxidase banding (chestnut, oak and maple). Peroxidases banding (chestnut, oak and maple). Peroxidases produce specific lignins and the lignins must be produce specific lignins and the lignins must be similar for both scion and stock for the graft to be similar for both scion and stock for the graft to be successful long-term.successful long-term.

– Stain tissues at the graft union and examine Stain tissues at the graft union and examine microscopicallymicroscopically

– Magnetic resonance imaging (MRI) checks for Magnetic resonance imaging (MRI) checks for vascular discontinuityvascular discontinuity

Graft IncompatibilityGraft Incompatibility

• Correcting incompatible combinationsCorrecting incompatible combinations– Generally not cost-effective. Remove and Generally not cost-effective. Remove and

top-work the rootstocktop-work the rootstock– Bridge graft with a mutually compatible Bridge graft with a mutually compatible

rootstockrootstock– Inarch with a seedling of compatible Inarch with a seedling of compatible

rootstockrootstock

Effects of rootstock on scionEffects of rootstock on scion

• Size and growth habitSize and growth habit– The most significant effectThe most significant effect– Dwarfing rootstock was developed in the 15th Dwarfing rootstock was developed in the 15th

century!century!

• Fruiting increases:Fruiting increases:– Precocity = early maturityPrecocity = early maturity– Bud formation and numbersBud formation and numbers– Fruit set = # of fruits that actually developFruit set = # of fruits that actually develop– Yield = # and weight of fruit at harvestYield = # and weight of fruit at harvest

Effects of rootstock on scionEffects of rootstock on scion

– NoteNote: trees on dwarfing rootstocks are more : trees on dwarfing rootstocks are more fruitful and if closed planted result in a higher fruitful and if closed planted result in a higher yield per acre!yield per acre!

– Dwarf trees have less management costs associated Dwarf trees have less management costs associated with pruning and sprayingwith pruning and spraying

• Size, quality and maturity of fruitSize, quality and maturity of fruit– No transmission of fruit traits from rootstock to No transmission of fruit traits from rootstock to

scionscion– Quality due to mineral nutrient uptake by the Quality due to mineral nutrient uptake by the

rootstock can be improved or decreasedrootstock can be improved or decreased

Effects of rootstock on scionEffects of rootstock on scion• Misc. effects of stock on scionMisc. effects of stock on scion

– Winter-hardiness. Rootstock can effect rate of Winter-hardiness. Rootstock can effect rate of maturity of the scion as it hardens-off in the fallmaturity of the scion as it hardens-off in the fall

– Increase the scion tolerance of adverse edaphic Increase the scion tolerance of adverse edaphic (soil) conditions(soil) conditions

• Ex: heavy, wet, compact, low OEx: heavy, wet, compact, low O22 soils soils– Betula populifolia (Betula populifolia (Japanese white birch) grafted on Japanese white birch) grafted on Betula Betula

nigranigra (River birch) (River birch)

– Increase pest and disease resistance (esp. Increase pest and disease resistance (esp. nematodes). Ex: Citrus, grapes, peachesnematodes). Ex: Citrus, grapes, peaches

Effects of scion on rootstock Effects of scion on rootstock

• Can increase suckering from rootsCan increase suckering from roots