GRAFT OR BUD UNION Author:- S.Hemanth

Physiology of graft (or) bud union formation 

 Article by:- S.Hemanth

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 

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: 

1. 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. 

2. Contribution by rootstock and scion. 

When grafting onto an intact stock plant, early callus formation is mainly from the stock, which has more favourable 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 

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

2. 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. 

5. Interconnecting vascular cambium 

New vascular cambium differentiates inwards from the vascular cambium 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 

1. 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. 

2. 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 MC Cully (1980), 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.