DIMENSIONAL AND ZONATIONAL VARIATION IN DORMANT SHOOT APICES OF PINUS PONDEROSA

TEPPER, H. B. (State U. Coll. of Forestry, Syracuse, New York.) Dimensional and zonational variation in dormant shoot apices of Pinus ponderosa. Amer. Jour. Bot. 50(6): 589-596. Illus. 1963.-The dimensions and zonation of the apices of the dormant leader and terminal branches in 8 ponderosa pine trees are compared. Apices in upper positions in the crown tend to have larger diameters than those in lower positions. While no consistent trend in the variation of the height of the shoot apex could be demonstrated, the height/diameter (H/D) ratio of the apex increases in progressively lower positions in the crown. Correlation analysis of the pooled data from all trees indicates there is no correlation between height and diameter, but that there is a significant correlation between H/D ratio and diameter of the apex. The H/D ratio of apices of small diameter is larger than that in those of large diameter. While the cytohistological zonation of apices in all positions in the crown is similar, apices of large diameter have a more extensive distal zone, rib meristem zone and peripheral zone than those of small diameter. THE POPULATION of shoot apices for a species comprises apices which differ from one another in position (e.g., the terminal and the various orders of lateral branch apices) and in function (e.g., dormant or active and vegetative or floral apices). Collections made for anatomical studies of a single species rarely include all these varied types. In the extreme case, only a small number of apices from a single tree is examined. While sampling is often limited, we commonly assume that descriptions of shoot apices resulting from such samples are characteristic both of the individual and of the species. While these assumptions may hold true in certain cases, nlumerous reports of differences in the dimensions and zonation of apices on a single individual have been reported (see Popham [1960] for a summary). These reports suggested that it would be informative to study the apices in different positions in the crowns of ponderosa pine trees. Terminal branch buds were chosen because they vary widely in size, and it was anticipated that this might be paralleled by variation in the size and structure of their shoot apices. Flurthermore, such a study would add to the information that is already available on the shoot apex of P. ponderosa (Sacher, 1954, 1955; Gifford and Mirov, 1960). MATERIALS AND METHODS-All buds used in this study were collected at the United States Forest Service, Forest Genetics Institute at Placerville, California, on November 23, 1959.3 At that I Received for publication December 24, 1962. 2 This is a portion of a dissertation presented to the Graduate School of the University of California, Davis, California, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. I I am indebted to Dr. N. T. Mirov and Mr. W. C. Cumming for arranging this collecting trip and to Mr. E. J. Carpender and Mr. C. M. Blankensop for climbing and collecting the buds. time all apices were in the condition of winter dormancy. Growing apices were not collected, for in pines growth of branches begins at different times and extends for varying periods, depending on the position of the branch in the crown (Duff and Nolan, 1958). As the size and structure of apices vary in relation to the developmental state of the shoot (Sacher, 1954), it would be difficult to collect morphologically comparable growing apices from different positions in the crown. With winter collections this problem is avoided as all apices are in the dormant state. Two trees were sampled in each of 4 age classes (11-, 22-, 32-, and 70-year-old trees). The 11-and 70-year-old trees were open grown and of good form. The 22-and 32-year-old trees were relatively well-exposed on 3 sides. The leader bud and terminal branch buds in various positions in the crown were collected from each tree. Three buds were collected from each whorl of branches, except when fewer were available due to damage from one cause or another. Severely shaded branches of the 22-and 32-year-old trees were not sampled. Usually terminal buds of branches in the 5th, 10th, 15th and 20th whorls from the leader bud were collected. However, as the 1 1-year-olds were relatively small, the 3rd, 5th and 7th whorls were sampled. In the 70-year-olds, it was convenient to collect the 11th, 21st and lowermost whorls. The latter were of irregular form and terminal branch buds could no longer be recognlized. Therefore, the buds collected were the largest and most vigorous ones present. All apices were killed in Craf III on the day of collection. A tangential cut, exposing 1 side of large buds, was made to facilitate penetration of the killing solution. Buds were subsequently descaled and all but a small portion at the terminus of the bud discarded. A tertiary butanol series