Effects Of Rootstocks And Reasons For Selection

Rootstocks are used in tree fruit production for a number of reasons. The principle goal is to maintain specific characteristics of a fruit scion, since it does not propagate readily by other methods. A fruit cultivar does not come true from seed or may not form seeds, and cuttings made from the scion may not root readily. Further, if roots form on cuttings, they may not be environmentally adaptable, may have pest susceptibilities, or may confer unfavorable characteristics to the plant. Thus, a rootstock is used for the perpetuation, survival, and growth of the scion.

Rootstocks also may impart other characteristics to a scion cultivar. By virtue of width and depth of root growth extension, seasonal growth pattern, and root-wood fiber strength, rootstocks affect the anchorage and stability of the resultant grafted plant. Rootstocks may vary in tolerance to soil characteristics and thus be selected for adaptability to a specific soil's physical nature (texture, density, depth, and compaction), chemical nature (pH, salt content, cation exchange capacity), or environment (gas content, specifically oxygen and carbon dioxide, and water content). Rootstocks also express resistance or susceptibility to soil temperature extremes. Probably most limiting is cold temperature tolerance; therefore, rootstock hardiness is an important selection criterion in some growing regions. In addition, rootstocks express resistance or susceptibility to insect, nematode, disease, and vertebrate pests. All of these conditions—soils through pests—are considerations when selecting rootstocks.

The genetic expression of growth and adaptive variation to soil environment or pests notwithstanding, rootstocks cause changes in characteristics of the scion cultivar. A rootstock can control the genetic expression of scion growth (particularly the annual periodicity or season of growth), plant size, structural growth (angle of limbs), precocity (time from grafting until flowering), flowering date, and fruit maturity date. The basis for genetic control of a rootstock on a scion has not been clearly elucidated, although scientists have proposed models for phytohormone balance and interactions as well as assimilate (carbohydrate and nitrogen) feedback.

Whether it is a direct genetic expression or an indirect effect, rootstocks can also influence flower and fruit size and color, fruit firmness and flavor, mineral uptake and corresponding foliar and fruit mineral composition, scion cold hardiness, and floral frost tolerance. Moreover, changes in disease susceptibility are sometimes attributed to rootstocks.

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