"A perfect union at stock and cion following veneer grafting. The stock is upon the left and the cion upon the right. The united tissue is seen running through the center (x25)." Photomicrograph and caption from LH Bailey, 1922, The Nursery Handbook, Macmillan.

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A. Characteristics of a functional graft union

1. Mechanical strength

Initially some sticking together of scion and stock is achieved by intercellular adhesion at first and intermingling of callus subsequently, but ultimately it is the  interlocking of xylem fibers (wood) that results in a strong, permanent graft union.

2. Translocation

The structural integrity of the graft union not only holds the grafted plant together, but it is the reestablishment of anatomical and functional continuity between xylem and phloem that allows for translocation of water and minerals by the xylem, and conduction of carbohydrates and other organics by the phloem.  

B. Sequence of graft union formation

(Sources: MuCully, 1983, in R. Moore (ed.), Vegetative Compatibility Responses in Plants, Baylor Univ. Press - as cited by Santimore, 1988, Graft Compatibility in Woody Plants: an expanded perspective, J. Environ. Hort. 6(1): 27-32; and Jeffrie and Yoeman, 1983, New Phytol.93: 491)

Note: Initial events are fundamentally the same as the early stages in rooting of a cutting i.e. wound healing.

1. Necrotic plate

The necrotic plate is a layer of desiccated, crushed cell walls at the cut surface of both stock and scion. Suberin (a waxy material) and pectin are deposited within the necrotic plate. The necrotic plate functions to seal off the wounded tissue from pathogens, and to restrict water loss. The pectin deposited between stock and scion cell acts as a "glue" (mechanical).

2. Callus formation

Division of secondary xylem and phloem parenchyma cells occurs in the vicinity of the vascular cambium:

a. Tissue and cellular origin
In most species, cell division (callus) is not from the vascular cambium itself, but rather from the secondary xylem and phloem cells that were most recently formed from division of the vascular cambium.

b. Contribution by stock and scion
When grafting onto an intact stock plant, early callus formation is mainly from the stock, which has more favorable water relations (although the relative contribution of each varies with species).

c. Enlargement
As the new callus increases in volume, it ruptures the necrotic plate, and begins to expand into whatever spaces exist between stock and scion.

3. Intermingling of callus from stock and scion

a. Intermingling callus from stock and scion increases mechanical strength and eventually fills any gaps between stock and scion.

b. Interlocking callus allows limited passage of water and nutrients between stock and scion. There must be a significant amount of passive translocation even without xylem or phloem continuity, since some delayed incompatibilities have survived for years with little or no vascular continuity - just callus.

4. Formation of wound vascular elements within the callus

Random, small, discontinuous xylem cells begin to form but do not yet "reconnect" the water transport system between stock and scion (McCully, 1983)

5. Interconnecting vascular cambium

New vascular cambium differentiates inwards from the vc of the stock and scion. Eventually the two ends meet. Without so called "cambial contact" (reasonable cambial alignment) the two ends don't "find" each other, and vascular continuity is never established.

6. Vascularization

a. Functional vascular cambium

The new vascular cambium cells begin to divide, cutting off cells to the inside (which differentiate into xylem) and to the outside (which differentiate into phloem). In some species like tomato, tobacco, and cotton, new xylem and phloem differentiates directly from callus, and only afterwards does a vascular cambium form between the two.

b. Reestablishment of vascular continuity

Regeneration and bridging of conducting elements (xylem tracheids and vessels, and phloem sieve tubes) allows for translocation across stock and scion; whereas interlocking of new xylem fibers is largely responsible for mechanical strengthening.

• M9, a dwarfing apple rootstock, has short fusiform initial cells of the vascular cambium, which produce short xylem fibers, resulting in minimal interlocking between stock and scion.  Eventually the graft union may snap off during high wind or heavy crop load due to brittle wood.

7. Continued secondary growth eventually results in a more or less normal looking trunk

C. Hormonal Control of Vascularization:

1. Leaves and buds of the scion "induce" vascularization:

a. Experimental removal of leaves and buds from the scions of coleus autografts affect vascularization (xylem vessel formation).

In this experiment by Stoddard and MCCully (1980, Bot. Gaz. 141: 401), coleus stem was cut and regrafted to itself. The effect of leaf and bud removal on formation of xylem vessel elements across the graft union was observed microscopically.

b. What is the stimulus from leaves and buds?

(1) Lilac pith experiments by Wentmore and Shirokin (1955, J. Arnold Arboretum, 36:305).

• A cube of pith (undifferentiated parenchyma similar to callus) was excised from lilac stem. 
• A bud inserted at the top of the pith cube stimulated both xylem and phloem formation in the underlying pith, just as would normally occur during graft union formation in response to scion buds and leaves (described above).
• When the plant hormone auxin was applied at the top of the pith cube instead of a bud, it to stimulated xylem formation.
• Application of sucrose stimulated phloem formation.

  (2) Experimental tissue culture autografting system to study auxin effects by Parkinson and Yoeman (1982, New Phytol. 91:711).

• Experiments involved Nicandra, Nicotiana, Datura (3 genera in the Solanaceae family).
• Split petri dish system - two agar gelled media differing in phytohormones (auxin and others) were poured into opposite sides of a petri dish, with a space between them. Then the petri dish was placed vertically so that there was an upper (apical) and lower (basal) medium, either of which could contain auxin or not depending on experimental treatment.
• An internodal stem segment was cut in half horizontally and placed tightly back into contact, held together by a piece of plastic (Tygon) tubing, to create a scion and stock autograft.
• This "autograft" was placed to bridge the space between the two media, with the far end of the "scion" in contact with the apical medium and the far end of the "stock" in contact with the basal medium.
• The petri plate was turned to a vertical position so that the autograft was oriented vertically with the scion on the apical end and the stock at the basal end.
• Sufficient time was allowed for "graft union" formation to occur between stock and scion, and then the autografts were cleared and stained so that xylem strands bridging the graft union could be counted microscopically.
• The effect of auxin placement, in either the apical medium, the basal medium, in both, or neither, showed that not only was auxin involved in xylem formation across the graft union but that an auxin gradient with concentration greater in the apical than the basal end resulted in the greatest amount of xylem formation.

Do the results of Wentmore and Shirokin (lilac pith experiment) and those of Parkinson and Yoeman (split petri dish experiment) suggest what the stimulus from leaves and buds might have been in the coleus experiment (Stoddard and McCully) and other more practical grafting systems?

2. Practical use of plant growth regulators for grafting.

Even though these experiments suggest that naturally occurring auxin is involved in graft union formation, synthetic auxins (rooting hormone) or other plant hormones are not commonly applied horticulturally, although there are a few encouraging indications like the experiments with apple bud grafting by Jim Cummins, and the use of auxin (IBA) to increase grafting success in spruce (Beeson, 1990) and pecan (Yates, 1992). Both of the latter are described and cited in a review of the practical uses of IBA on the Hortus USA web site.

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