Articles
GROWTH CHARACTERISTICS OF APPLE DWARFING ROOTSTOCKS AS RELATED TO LENTICELS, ROOTS AND THE REGENERATIVE TISSUES WITHIN THE UNION OF STOCK AND SCION
A large percentage of graft unions are successful.
However, incompatibility and failure of the new graft union (resulting in impeded growth) is usually related to the disorientation of horizontal rays in the wood, resulting in breakage of the union after successive years of growth in the orchard.
As the failure of tree growth persists, calcium accumulates in the conducting phloem, particularly in the senescing tissues, rather than being translocated to newly formed growing points.
M.27 and M.9 rootstocks have extensive phloem thickness, with a major portion being classified as non-functioning.
Xylem rays between the vessels are denser and smaller in M.27 than in MM.111.
Starch accumulates contiguous to and within the regenerative tissues of the union between the stock and scion.
Growth initials occurring in the outer portion of the pith are formed from starch-containing cells.
This may be adventitious in nature and will cause the initiation of lateral buds (fruit or leaf), burr knots and lateral rooting.
Starch also accumulates contiguous to the pith area next to the older xylem elements.
It has been found in areas contiguous to the cambial layer within the graft union and is associated with the formation of callus.
This is a protective mechanism for food reserves to be stored in injured tissues, thus enabling support for renewed growth in successive years.
The selection of stock/scion combinations should be evaluated as to growth characteristics.
In a study of 13 apple dwarfing rootstocks, the size of the xylem vessels varied with Budagovsky 490 being the largest and MARK the smallest.
This would indicate the need for varied cultural practices according to the growth differences of the rootstock combination used.
Lenticel variation between rootstocks was evaluated.
It was found that M.27 had extensive cork development as contrasted to MM.111. Lenticels were closer together in M.26 and M.9. Among the Polish series of rootstocks, P.22 induced severe lenticel development associated with a highly senescent growth condition of the scion ‘Starkspur Supreme Delicious’. These symptoms resembled measles, a nutritional disorder.