American Rose Annual 49: 159-164 (1964)
Stock-Scion Relationships in Roses*
Griffith J. Buck**
*From Journal Paper No. J-4726 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. Project No. 1453.
**Professor, Department of Horticulture, Iowa State University.
Ed. NOTE: This research in part has been financed by a grant from the American Rose Foundation.
PLANT propagation by grafting is one of the oldest horticultural practices and one that has intrigued the gardening and non-gardening public alike. During the Middle Ages, numerous reasons for justifying the practice came into being, few of which were derived from fact. It was during this period that the advice to graft roses on barberry and holly to produce yellow flowers and evergreen plants was current. Then, too, the early horticulturists were secretive about the mechanics of grafting so that the uninitiated could only surmise the technique and knowledge the practice required. Even now, the practice seems to partake of black magic; for, even though he accepts it as a fact, the layman finds it difficult to understand why the white, small-flowered, coarsely vigorous plant of the multiflora rose can, when grafted, produce the elegance of a 'Peace' or 'Royal Highness.'
The grafted plant is an artificial plant. It does not exist in nature except as the rare result of an accident, nor can it reproduce itself. It is composed of parts of two distinct individuals: one furnishing the root system the understock or simply stock; the other contributing a stem or growing point which becomes the aerial part of the new plant the scion. Roses are usually propagated by budding, which is one of the many grafting techniques.
The propagator is interested in producing grafted plants that grow and perform in a normal fashion. When two plant parts are grafted together, producing a normal plant, they are said to be compatible or congenial. Lack of compatibility is expressed in several ways; such as complete failure of the graft union, faulty graft unions or growth inhibiting effect of the scion or understock on the other member. The most common expressions of lack of compatibility are found in the nursery row: bud-graft failure; failure of the bud to grow even though it remains alive; and faulty graft unions. In this last one, the scion breaks away from the stock because of failure of the graft components to form a tightly knit union. This should not be confused with "blow-outs," the forceful breakage of the immature graft union by the stresses and strains induced by cultivation or wind. While there is little doubt that some of the more subtle expressions of less than complete compatibility can be found in plants which have been growing in gardens for varying periods of time, it is difficult to separate incompatibility symptoms from others induced by culture, environment, or pests; for they are remarkably similar.
Aside from the aberrations from normal growth patterns which can be attributed to some degree of incompatibility, there are many compatible combinations which produce unusual growth effects due to the interaction of stock and scion. Some of these have value and are used commercially; others are detrimental and should be avoided. These growth effects can be divided into two groups: (1) those which show the effect of the understock on the scion and (2) those showing the effect of the scion upon the understock.
The most readily visible influence of the understock upon scion growth is that of vigor. Garden roses vary greatly in their ability to root from cuttings and those plants which do result are slow to mature. There is similar variation, too, in the vigor of the root systems developed on these cutting-grown plants. This is directly related to the degree of vigor of plant growth. Most of the understocks now in use for rose propagation are clones. This means that they are very uniform in all of their plant characteristics, including the type and character of their root system which after the grafting process, assuming a compatible stock-scion combination, retains its vigor and transmits it to the scion. In a less than completely compatible stock-scion combination, the vigor of the understock is not fully transmitted to the scion, and there is consequent reduction in vigor of growth.
There is some indication that earlier and heavier flower production is stimulated by grafting.4 The presence of a graft union, even in the case of completely compatible stock-scion combinations, has a partial blocking effect on the translocation of nutrient materials, including photosynthetic products, leading to increased starch storage in the stems with a consequent increase in flower bud initiation which is dependent upon high starch reserves. Understock studies with other plants (namely, oranges and persimmon) have shown earlier flowering when budded to themselves than when unbudded, although the size of the plants was similar.
In the imperfect graft union or the semi-incompatible stock-scion combination, the movement of nutrients within the plant is impeded producing, usually a dwarfed plant which may or may not flower freely. The dwarfing effect limits the general vigor and vegetative growth of both stock and scion. In a few cases, the early growth of certain stock-scion combinations is typical of the semi-compatible union due to slow regeneration of the tissue systems in the graft union but, with increasing age, this dwarfing effect tends to disappear. This is evident in some of the canina-type understock-scion combinations. It is quite possible that the reputation for superior hardiness which the canina-type understock is supposed to transmit to the scion variety is due chiefly to the lack of complete compatibility between stock and scion with its effect on increasing the starch content of the stems leading to their greater maturity and thus hardiness.
The popular assumption, a carry-over from the Middle Ages when it was assumed that yellow roses could be obtained by grafting the roses on the yellow-flowered barberry, that the floral characteristics of the understock and scion intermingle have no valid basis. The effect of the understock upon flower quality is associated with the understock's effect on scion vigor. The most frequent effects seen are flower malformation and petalage. Both phenomena are directly related to nutrition. Excessive vigor tends to produce such floral malformations as split centers, the "black" or "green center" frequently seen in such roses as 'Souvenir de Claudius Pernet' and 'Kordes' Perfecta,' foliaceous pistils and, in some instances, complete absence of pistils and stamens. Other floral malformations are "bullnose" and "bull-head" buds. (A bull-nose is a flower bud whose petals are dwarf and misshapen, allowing the pistils, which may be foliaceous, to protrude. A bull-head is a bud which is either unable to open or, if it does open, forms a misshapen flower. This condition is accompanied by dwarf, very stiff outer petals which fail to reflex, small overall bud size and, in some cases, by a crooked or partially fascicled peduncle or neck.) All of these floral aberrations may be seen at any time that plant is producing exceptionally vigorous growth. They are nearly always in evidence during the early part of the "maiden" season—the first summer after cutting back the understock to force growth from the scion. After the first flush of excess vigor recedes, normal flower quality is regained. Some varieties are more prone to these types of faulty flowers than others.
Flower petalage varies with the nutritional status of the plant and its resulting vigor. As with most other floral characteristics, the carbohydrate content of the plant affects the number of petals a flower will have and, indirectly, the form of the bud; the more petals, the more ovoid the bud. The well-nourished plant will, other things being equal, realize its petalage potential; the poorly nourished plant produces flowers deficient in petalage. The influence of the understock is clearly defined in its effect on growth stimulation and ability to absorb and translocate nutritive minerals.
In roses the understock does not appreciably affect the hardiness of the scion with the possible exception noted earlier in the case of the canina understocks.1,7 The inherent cold tolerance of both stock and scion are left unchanged by grafting. The apparent effect of increased scion hardiness obtained by grafting to a hardy understock can no more be attributed to understock than to such cultural methods as attention to fertilization, irrigation and foliage retention. The point often overlooked in understock selection is that, in some instances, the understock, notably fortuniana, odorata, and 'Mme. Plantier,' can be more intolerant of low temperatures than the scion variety. Of course these understocks should be avoided in areas where soil temperatures fall below 15°F., for it is quite possible that the scion variety will survive and that the understock will winterkill.
Disease resistance in the understock affects the scion variety in a similar manner. It is unfortunate that the blackspot tolerance of the multiflora stocks is not transmitted to the scion, for cultural methods could be decidedly simplified. The effect of using disease-tolerant understocks is an incidental one: a reduction in reservoir of infection in the nursery area with a resulting decrease in the incidence of the disease on the plants growing there.
The major influence of the scion seems to be concerned with the vigor of the root system. A weak-growing variety budded on a vigorous understock will, within a short time, limit its growth. The reverse has been observed also. A strongly growing variety budded to a less strongly growing understock will stimulate the understock into growing larger than it would if it had not been grafted.5
It has been observed in some plants that the scion will modify the character of the root system. This is true in apple and citrus especially if the understocks are of seedling origin. There is less modification in the clonal understocks for these plants. In the rose, however, the soil type on which the plants are grown has a greater influence on the character of the root system than the scion variety. There is some diffuse evidence that the root system modifies the plant type of the scion, i.e., the multiflora stocks produce erect, columnar plants; 'Ragged Robin,' spreading plants. But this is a quantitative effect and difficult to assess.2
The basic causes of reciprocal stock-scion relationships have yet to be discovered. The explanations which have been offered to explain those observed results are often conflicting and are unsubstantiated.
One of the best substantiated of the proposed theories places the emphasis upon the stem portion of the stock rather than on the absorbing root system.6 This is not to say the root system in unimportant, rather that understock influences are the result of translocation effects rather than of the absorptive ability of the root. This is borne out by various grafting techniques using the stems of understocks in interstem grafts. The expected stock influence, although somewhat diluted, can be observed in the scion variety.
Translocation is concerned not only with the movement of nutritional materials and water but also with the movement of growth regulators hormones which inhibit or stimulate growth. In this respect it has been observed that when the same scion variety of hybrid tea rose is grafted upon different understocks there is definite understock-stem effect upon scion growth. If understocks with chilling requirements of varying length (fortuniana, odorata, manetti, Clarke and Welch multifloras and multiflora seedlings), when budded with a hybrid tea, are allowed to become fully dormant, differing growth responses can be observed when growth is resumed. On those understocks which have little or no chilling requirement as a condition to breaking dormancy seedling multiflora, odorata and fortuniana , the scion variety will promptly begin growth. On the other stocks, the scions begin growth after a delay corresponding to the length of the chilling requirement for each understock. This enforced dormancy, more frequently referred to as rest period or winter rest, is initiated by a complex of growth inhibiting compounds which are dissipated by exposure to cold: the chilling requirement. On its own roots, the typical hybrid tea has a very low chilling requirement comparable to that of such evergreen stocks as odorata and fortuniana. The root systems of plants have no chilling requirement. It is partly for this reason that florists producing potted rose plants for Easter and Mother's Day prefer plants budded upon seedling multiflora roots.