SPECIFIC ROOTSTOCKS may have desirable effects on commercial varieties of tree crops propagated on them. Peach trees that grow on Nemaguard peach rootstocks are resistant to nematodes. The well-known clonal apple rootstocks of the Malling series bring about various degrees of tree dwarfing and cause earlier bearing. Effects that various specific rootstocks can impart to citrus trees include increased tolerance of soil salinity, better uptake of certain minerals, resistance to certain diseases, greater cold resistance, tree dwarfing, precocity in bearing, increased fruit size and increased yield. The date of fruit maturity may also be affected.
Most pecan nurserymen who have experimented with seed of certain varieties and with seed from certain seedling trees make it a practice to use the seeds that appear most desirable from the standpoint of good germination and vigor, uniformity, disease resistance and freedom from winter injury in the seedlings. However, only one of the parents of the seed is known; the other is the unknown pollen parent. There may be wide differences among the individuals grown from any particular lot of seed. For each rootstock to be inherently alike, it is necessary to propagate a clone vegetative, for instance by cuttings.
The ideal rootstock.
How a pecan tree may be affected by the many different kinds of understock that it is possible to use must be determined by experimentation. For trial, we are growing trees of diverse genetic composition at the U. S. Pecan Field Station, some of which are known crosses between certain pecan varieties and others of which are hybrids between pecan and hickory. Selections from these are to be tested as rootstocks to determine if they have qualities that would make their clonal propagation desirable.
Vegetative propagation of rootstocks.
It is possible to propagate pecans both from root or shoot cuttings. Shoot cuttings must be rooted, whereas root cuttings must develop both a shoot and a supporting root system.
Shoot development on root cuttings.
Success in producing plants from root cuttings depends on the amount of stored food contained, as well as on the method of handling. The taproots of vigorous young trees contain the greatest amount of starch and other food supplies. The lateral roots of such trees rank second, whereas the laterals of larger and older trees are woodier. Apparently the lower the food supply the more susceptible the roots are to decay.
If a cutting is taken from tree's root crown it may have one or more latent buds on it. Such buds usually develop shoots readily when the cutting is placed in the proper environment. If no latent bud is present, as will be true of most root cuttings, an adventitious bud must be differentiated before a shoot can develop;
Pecan root cuttings exhibit strong apical dominance in callus formation. Contrary to what might be expected of a root section, they develop adventitious buds and then shoots from an apical callus much more readily than they initiate new roots at the base. Such buds normally appear in due time if the cutting develops an apical callus and keeps it live.
Typical development of adventitious shoots on taproot cuttings when held in a callusing medium is shown in figure 1. When pecan trees not of excessive size are cut off below the ground the stumps sprout in like manner from either latent or adventitious buds and put up many shoots, figure 2. Although not equal to taproot cuttings lateral roots of nursery-grown trees have shown some promise as cutting material. Apparently it is best to keep any roots to be used in outdoor beds in cold storage until the cutting bed has reached a favorable temperature. A temperature of 75 to 80 degrees has appeared to be near optimum.
Root development on root cuttings. The normal uptake of water and minerals by the pecan tree is through fine roots. These roots are easily killed by unfavor-
*Research horticulturist. Crops, Research Division, Agriculture Research Service, U. S. Department of Agriculture, U. S. Pecan Field Station, Brownwood.
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able soil conditions, such as drought. However, new roots readily develop on older roots providing they are small in diameter; the smaller the root the more freely it initiates new roots when conditions are favorable. For this reason pecan growers generally refer to any roots of a diameter up to some 1/8 inch as "feeder roots." If a cutting has a live feeder root it can develop more new roots with little delay, and from these can grow a functioning root system of the size needed to support the leaf area at the other end of the cutting. A taproot cutting having 1-year-old roots developed from feeders is shown in figure 3. If the cutting does not have a live feeder root it must initiate a new one from within the cortex of a thicker root or from newly developed callus tissue at the base of the cutting. This may require considerable time under natural conditions.
Rooting from the end of a thick cutting can be hastened by applying a root inducing hormone, such as indole butyric acid, figure 4.However, hormones have the effect of promoting the flow of food materials to the point of application and thereby inhibiting shoot formation and growth The better procedure is to delay hormone application until a shoot has developed The type of 1-year-old plant that results when a rooting hormone is applied at the base is shown in figure 5.
If the root section used as a cutting is of large diameter or length, it may be preferable to root one or more shoots grown from it and to then discard the old root section, figure 6. Girdling such shoots results in rooting when they are held in a rooting medium, figure 7. A possible alternative method would be to remove the shoots from the supporting old root section when at a suitable stage of development and then to handle them as softwood cuttings.
Root development on shoots. In past years where woodlands were cleared and pecan saplings were cut off below the plow level, pecan sprouts have persisted for many years even though they usually were cut off every winter and sometimes in the summer. The underground portions of such sprouts root naturally. How soon they root and the amount of rooting depends on soil moisture.
A similar process for rooting apple clones known as "stooling” can be used for propagation of pecan rootstock clones. Treatment of the young shoots, such as girdling, wounding or a hormone application should hasten rooting. Gossard (1) found that tooth picks impregnated with indole butyric acid would produce roots from trench-layered tops of grafted nursery trees when they were inserted in holes drilled radially through stems. The rooted shoots should be removed from the stool and lined out in a greenhouse or nursery, where they can receive prime care to get them well started into growth.
Plants can be from dormant wood pecan cuttings by a method reported by Romberg (3) in which the cutting is grafted or inarched to a small established seedling at a point intermediate between the ends, figure 8.Thus supported, the cutting can be well rooted before detachment from the nurse plant. Romberg
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found that a cutting made by rooting stems from a variety would be in a senescent state. Such rooted cuttings apparently produce senescent roots and behave abnormally as compared to cuttings made from juvenile wood.
Gossard (2) and Sparks and Pokorny (4) have reported success in rooting of cuttings of the current season's growth. The latter were most successful with cuttings taken in June, given a light abrasion and treated with indole butyric acid. The cuttings were placed in sterilized sand under intermittent mist.
Conclusion. The propagation of clonal rootstocks is possible but requires special techniques and equipment. The best procedure and minimum cost per tree remain to be determined. Either of several methods might be preferable at times. Such rootstocks may be desired for experimental purposes but feasibility of their use for commercial planting has yet to be investigated.
- Gossard, Atherton C. Rooting pecan stem tissue by layering. Proc. American Society for Horticultural Science. 38: 213-214. 1941.
- Gossard, Atherton C. The rooting of pecan softwood cuttings under continuous mist. Proc. American Society for Horticultural Science. 44: 251-254. 1944.
- Romberg, L. D. Use of nurse seedlings in propagating the pecan from stem cuttings. Proc. American Society for Horticultural Science. 40: 298-300. 1942.
- Sparks, Darrell and F. A. Pokorny. Investigations into the development of a clonal rootstock of pecans by terminal cuttings. Proc. Southeastern Pecan Grower's Assn. 51-56. 1966.