EFFECTS OF GRAFTING ON UNLIKE STOCKS
It is unquestionably a great shock to a tree when 90% of its top is cut off. If it is healthy and vigorous, the root system will try to recover, using every means possible to do so. If a new top is grafted to it, the stock must either accept and nourish that foreign and sometimes incompatible new part, or give up its struggle for life. Nature and the tree stock usually accept the challenge and the graft begins to grow. In an attempt to continue with its own identity, the stock will bring into activity adventitious buds. These are tiny microscopic buds imbedded in the bark of a tree that are not apparent to the eye but are nature's protection against destruction of the individual plant. But these must be removed by the horticulturist to insure proper nourishment of the grafts.
Because the root system is striving hard to live, and because it is usually the stronger, it may force the top to accept certain of its characteristics. Occasionally, it may assume some qualities of the original top. Such cooperation is necessary if either is to survive. First of all, the grafted scions must accept the vital quality of climatic hardiness, a powerful factor developed through ages spent in a certain climate. To hasten the acclimatization of a tender variety, I cut scionwood from such unions early in the winter, storing it until spring. When these scions are grafted on new root systems, I find that they are much more readily accepted than the first grafts were. The following season, I allow the grafts of this later union to go through their first winter of exposure. Early each spring I continue to cut scions from the most recent unions and graft them to new root systems, so hastening and setting the factor of hardiness through frequent asexual propagation.
Because my observations of the effects of scion on root and vice versa, have not extended over a sufficient period of time, I think it is possible that the changes I have seen may be only transient. In any case, I do know that the phenomenon occurs, for I have seen many examples of it.
One instance in which the stock was apparently affecting the scions, occurred in the case of several varieties of black walnuts which had been grafted on wild butternut stock over a period of sixteen years. The walnut top flourished but tended to outgrow the butternut, so that the caliber of the walnut was greater than that of the stock a few inches below the graft union. I also noticed that, although the graft began to bear about as early as black walnuts do when they are grafted on their own species, the nuts did not mature at all during the first few years of bearing. In 1938, after a favorable season, I found mature nuts on one variety, the Thomas. These nuts varied in size more than they do when grafted on black walnut. The most surprising thing about them, though, was that they did not have the characteristic black walnut flavor. When properly dried and cured, they could have passed as an entirely different nut since they tasted like neither the black walnut, the butternut nor the Persian walnut.
The overgrowth of the Ohio black walnut, grafted on butternut, was even more apparent than that of the Thomas. These nuts were, as I have said, immature the first few years they appeared and they, too, lacked the usual black walnut flavor. In their case, however, the most striking change was in the shape and structure of their shells which were elongated like butternuts, with corrugations typical of those found on butternuts and nearly as deep and sharp. (See Illustration in Chapter 1, Page 5.)
In 1937, I made experimental graftings on native black walnut stocks of the Weschcke No. 4 butternut, a variety I found to be superior to hundreds of other native trees tested. The grafts grew luxuriantly and in 1940, produced about two pounds of nuts. These nuts were approximately 30% larger than those on the parent tree. They cracked well and the kernels were similar to those from the parent tree. They definitely distinguished themselves, however, by being a free-hulling nut, which is not true of the mother tree nor of most butternuts. Soon after the nuts had dropped to the ground and were still green, they were hulled and their hulls peeled off like those of the Persian walnut, leaving the nuts clean and free from remnants. Apparently this phenomenon was a transient one since later crops did not display this free-hulling feature.
I have mentioned, elsewhere, the seedling apricot which came into bearing in St. Paul, and how I obtained grafts before it died during a very cold winter. I have grafted scions of this apricot on both hybrid and wild plum stocks repeatedly and this apricot now exhibits a material gain in hardiness. It overgrows the plum stock, but this does not seem to inhibit its bearing, the fruit growing to greater size than that of the mother tree.
These are some of the instances in which I have seen stock exert a definite, and, mainly a beneficial influence on its grafted top. It may easily be that these are only of a temporary nature and until I have seen them maintained for many more years, I must consider them to be transient effects.