BULLETIN 226 APRIL, 1928 GROWTH STUDIES OF THE PECAN By C. L. ISBELL AGRICULTURAL EXPERIMENT STATION OF THE ALABAMA POLYTECHNIC INSTITUTE M. J. FUNCHESS, Director AUBURN TABLE OF CONTENTS PAGE INTRODUCTION AND STATEMENT OF PROBLEM HISTORICAL SOURCE OF MATERIALS Methods -- 3 3 4 5 5 PART I.-BUD DIFFERENTIATION AND DEVELOPMENT Catkin flower bud differentiation and development Pistillate flower bud differentiation and development Buds with unknown future development PART II.-GROWTH AND FRUITING HABITS Varietal variation in number and abscission of buds Characteristics of pecan shoot growth Influence of kind of shoot growth made one year on the number of flowers produced and nuts set the following spring ................................ . 4-- 5 11 15 19 19 20 Influence of kind of terminal on pistillate flower production the following spring Flower production from terminal buds Influence of second growth on number of flowers produced and nuts set the following spring Influence of second growth on the ultimate weight of the nut PART III.-INFLUENCE OF PRUNING, DEFOLIATION, RINGING, AND DISBUDDING ON NUMBER OF SHOOTS AND FLOWERS PRODUCED Influence of pruning on shoot and nut production Influence of defoliation and ringing on shoot and fruit-bud formation Influence of disbudding on shoot and fruit-bud formation -GENERAL SUMMARY ACKNOWLEDGMENTS LITERATURE CITED DISCUSSION - -33 34 36 36 38 39 39 43 59 63 64 66 67 GROWTH STUDIES OF THE PECAN* By C. L. Isbell INTRODUCTION AND STATEMENT OF PROBLEM NTEREST IN the pecan producing industry of the South has grown very rapidly during the last quartercentury. In 1899 the number of bearing pecan trees, including seedling and budded or grafted, was given as 643,292. Figures indicating the number of trees of nonbearing age at that time are not available. These numbers increased to 1,619,521 for bearing and 1,685,066 for non-bearing trees by 1909; to 2,672,191 and 2,257,288 by 1919; and to 4,618,297 and 5,120,016 by 1924, respectively. The production of nuts in pounds increased from 3,206,850 in 1899 to 9,890,769 in 1909 and to 31,808,649 in 1919; and the value from $971,596.00 for 1909 to $7,792,866.00 for 1919 (2). These great increases have created a demand among growers and prospective growers for information on all phases of pecan growing. The investigation herein reported was started to study the growth habits of the pecan. Three, more or less separate phases of growth, were studied,--namely, bud differentiation and development; growth and fruiting habits; and, influence of pruning, defoliating, ringing, and disbudding on the number of shoots and flowers produced. THE LAST fifty years many contributions have been made to our knowledge of the time and nature of bud formation in deciduous fruits and the growing habits associated with this function of trees. The literature on this subject indicates that in general for each kind of fruit there is a fairly definite period when fruit bud differentiation takes place, and that the initiation of the process depends on the existence of certain nutritive conditions within the tissues at or near the particular points and time in question. Moderately vigorous vegetative growth in deciduous fruit trees is essential for maximum fruit-bud differentition and maximum fruitfulness. In some instances * Also presented to the faculty of the Michigan State College of Agriculture as a thesis in partial fulfillment of requirements for the degree of Doctor of Philosophy. WITHIN HISTORICAL pruning, defoliating, ringing, fertilizing and other treatments have exerted little or no influence on the number of fruit buds formed; in others they have resulted in increased numbers and in still others in deceased numbers. Wellington (15) and Wiggans (16) have given rather complete summaries and bibliographies of experimental work bearing on fruit-bud formation. For that reason in this publication reference to other literature will be made only where it seems to have some definite bearing on the data being reported, and then only in connection with the specific topic under consideration. When this investigation was started in June, 1922, apparently no study of similar nature had been made on the pecan or any other nut bearing tree with similar growth and fruiting habits. SOURCE OF MATERIALS OST OF the materials used in this investigation were obtained from a variety planting of pecans set in 1914 and from a seedling tree planted about 1900*. The trees of the variety planting were set 40 feet apart each way and peach trees were used as fillers until the first year the experiment started. The seedling tree is located on the college campus. The soil in which the trees are growing is sandy, underlaid with clay, and its natural fertility is below that required for best growth of the pecan. During the experiment the trees grew under lawn-sod mulch consisting of Bermuda and lespedeza during each summer and hairy vetch and bur clover during each fall and winter. From year to year the young trees received sufficient complete fertilizer to maintain vigorous growth. These applications were made in the spring just about the time the nuts were apparently set. It was found early in the experiment that if the shoots of most varieties fail to produce pistillate blossoms they generally abscise the terminal bud and subsequent growth is made from lateral buds near the apical end of the shoot. It seemed advisable, therefore, to make a special study of these subterminal buds-that is, those axillary or extra-axillary buds just below nuts or below the point where a terminal bud or a terminal part of the shoot had abscised.** * The seedling produces a good nut and would come in the early blooming group according to Stuckey's (13) classification. This tree has been given the variety name Earl, for Prof. Earl who planted it. It is referred to in this paper under that name. ** The lateral bud referred to here is usually the uppermost of the subterminal node remaining after the terminal bud or the terminal part of the shoot has On some varieties it is an axillary bud; on others it is an extra-axillary. abscised. 4 PART I.-BUD DIFFERENTIATION AND DEVELOPMENT Methods. Shoots from which buds were taken, as well as those labeled for study, were distributed over the entire tree. The first samples of buds taken for microscopic examination were killed in Gilson's killing solution and infiltrated with paraffin, as outlined by Chamberlain (3). The nature of the bud scales and the close folding of the young leaves prevented thorough infiltration, except in buds that were in very active growth, and the material broke in sectioning. More satisfactory results were secured by removing the bud scales, aspirating for one hour and then infiltrating with celloidin. In subsequent collections the scales were removed immediately and chromeacetic acid was used for killing. Sections were cut to a thickness of about thirty microns, stained with Delafield's haematoxylin, destained with acid alcohol, washed, dehydrated with alcohol, cleared with xylol and mounted in balsam. Clove oil was used for clearing a few sections. Double staining with eosin and haematoxylin was used with a few sections. Catkin Flower Bud Differentiation and Development HE PECAN differs from many other monoecious plants in that the staminate catkin buds and the vegetative growing point which later may differentiate the pistillate flower buds are each enclosed in a separate bud scale or scales, within a common outer scale covering. When the rudimentary bud formed in the axil of the leaf (either before or after the leaf unfolds) starts rapid development it forms a mixed bud consisting of three or more buds under a common bud scale with each bud enclosed in a separate scale. All of these except the middle bud are destined to give rise to catkin buds. Figures 1 to 13 inclusive are arranged to show the time at which the catkin differentiation occurs and the progressive stages in its development. T -N f/ , . Fi,6.. [ I\ I I .- 1 l I -d - - li l .Ian F,,- LEGENDS Fig. 1.-Stuart bud taken May 8, 1926, from basal part of shodt just after pistillate flowers appeared at the top of the shoot. On the right and left catkin flower buds forming. In center vegetative bud. Fig. 2.-Earl bud taken June 19, 1922, from a node near the developing nut. On the left is a catkin already well developed. Fig. 3.-Earl bud taken December 15, 1922, from a node near the nut scar. Catkin on the left well developed. Fig. 4.-Earl bud taken February 15, 1923, from a shoot that bore nuts in 1922. It shows on the left the extent of the development of the catkin and its hairy condition. Fig. 5.-Earl bud taken March 5, 1923, from a shoot that fruited in 1922. It shows on the left a catkin rather well developed not long before the buds would have unfolded in the spring. Fig. 6.-Earl bud taken January 18, 1923, from a shoot that fruited in 1922, showing one catkin bud with catkins enclosed and part of another. The vegetative part of the composite bud is not shown. Fig. 7.-Delmas bud taken June 30, 1922, from a node near where terminal bud abscised between June 23 and June 30. Catkin on left and right not far advanced. rt, . li (4! I i.- 1211 .rlJul\v 1i)2'. 12, !). -Eail .Jul IJ 1, 1922. 10. -hLa l -h'eI t.1, 192:;. I r 11L- -Stuart Apriil 8,19i2. Fig0.. N* wt~ il, ,1 Fi. 1"ii~ 12.- FEail-Apil , 192:3. -Scey-'Junej 2:, 1 '2'. rt~ ~ LEGENDS Fig. 8.-Earl bud taken July 12, 1922, from a node near the developing nut. It shows a well developed catkin on the right and a vegetative bud in the center. 9.-Earl bud taken July 4, 1922, from a node on the basal part of a fruiting shoot. It shows the development of the catkin buds on the right and left, and the vegetative bud in the center. Fig. Fig. 10.-Earl bud taken Feburay 1, 1923, from the basal part of a shoot that fruited in 1922. It shows the relative development of catkins and vegetative parts of the composite bud. Fig. 11.-Stuart bud taken April 3, 1926, from a shoot that fruited in 1925. Enlarged for comparison of catkin and vegetative parts of strong buds near the terminal part of the shoot, just as growth was starting and bud scales were being lost in the spring. Parts of some of the individual staminate flowers were broken off in sectioning. Fig. 12.-Earl bud taken April 4, 1923, from a shoot that fruited in 1922. Enlarged for comparison of catkin and vegetative parts of buds near the basal parts of a shoot just as growth was starting in the spring. Fig. 13.-Schley bud taken June 23, 1922, from the base of a second growth shoot, showing a catkin bud being abscised at A. DISCUSSION IT WILL BE SEEN from Figure 1 that catkin flower buds begin to form and staminate flowers to differentiate in the buds along the base of the new shoot soon after growth begins in the spring. The rapidity with which the catkins develop soon after they are differentiated and the continuation of their development until a short time before blossoming the following spring is shown in Figures 2 to 5 inclusive. Many of the well developed mixed buds contain three or four catkin buds by the end of the growing season. Figure 6 is an illustration of such a bud showing an entire catkin bud and a portion of another. As the growing season advances buds formed at newly developed nodes on either first or secondary shoots differentiate catkin buds, as is shown in Figure 7. As might be expected, due to their differentiation very early in the growing season, the catkin buds on the basal portion of the shoot are more developed than those in buds toward the terminal part of the shoot. This difference, however, does not continue throughout the development of the catkin; in fact catkins in buds located near the terminal part of the shoot finally develop to a much greater size, as will be seen by contrasting Figure 8 with 9, 3 with 10, and 11 with 12 taken from buds toward the terminal and basal parts of the shoot respectively. These differences would appear greater were Figures 9, 10 and 12 not magnified more than 8, 3 and 11 with which they are contrasted. When second growth takes place-that is, when lateral branches develop from mixed buds of the current season -- the embryo catkins that were located in the buds are usually abscised, as shown in Figure 13. They may, however, remain on the base of the new shoot and produce catkins the following spring. Although at the end of the growing season catkins in buds toward the base of the shoot are usually smaller than those in more terminal buds, they are larger in proportion to the vegetative bud with which they are associated. This is clearly shown by comparing the catkins with the vegetative parts in Figures 10 and 11. When growth starts in the spring the vegetative part of well developed buds located near the terminal part of the shoot appears to develop more rapidly than the catkins with which it is associated. The more basal buds, however, either remain dormant or unfold and produce 10 catkins and very weak vegetative growth, the vegetative parts usually abscising when catkins fall. Such a shoot is show at point A in Figure 31. From the foregoing it is shown that the catkins are differentiated in the composite or mixed lateral buds of the pecan almost as soon as the buds themselves are formed and before the leaves subtending them have attained full size. This differentiation is more or less of a continuous process, taking place as new shoot growth is made and new lateral buds are formed. There is a second period of catkin differentiation corresponding with the laying down of new buds on the second growth. Considerable development of these catkins takes place as the growing season advances and is coincident with the increase in size of the buds*. Pistillate Flower Bud Differentiation and Development AS ALREADY STATED, the vegetative center of the composite or mixed lateral bud of the pecan remains vegetative from the time it is formed until the beginning of the growing period the following spring. With the appearance of conditions favorable for growth, both the vegetative center of the bud and the rudimentary catkins continue their development, the vegetative center developing more rapidly. The first stage of its growth appears in longitudinal section as an elongation of the central axis and a change in the shape of its crown from that of a broad to a rather narrow cone: Immediately, there appears at or just below and to the side of the growing point an enlargement which tends to give the growing point a shouldered or twisted appearance. Others appear as growth advances. These protuberances mark the initiation of individual pistillate flowers which develop rapidly. While pistillate flowers are being differentiated on the terminal parts of the rudimentary shoot, leaves, nodes and internodes are developing rapidly and rudimentary buds in the axils of the leaves are being differentiated and developed. Figures 14 to 22 inclusive are arranged to show the appearance of the terminal growing point at different stages before and during the process of pistillate flower differentiation. * This is in line with preliminary reports on this question made by the writer before the Horticultural Section of the Southern Agricultural Workers in February, 1923, and at the annual meetings of Georgia-Florida Pecan Growers' It is and the National Pecan Growers Associations, based on earlier studies. further corroborated by the investigation of Woodroof (17). 11 I 11-I 1 *r*Stu~it tuart Ilecu 5, 1125 .Jan). I.I1 c7 IL). IT 1i 7.-SItuarI I'd. 20), 1') . 12 Fes i9 IL . t!!l7IN kjb' v, A tu"t - :r pi'1 , lU2G . 1 .'1~ (. -Suw-t-April 14 1!2 . Swul '. n\ \ -Apil 14, 1;)25. - 1' nl12, 192. LEGENDS Fig. 14.-A mixed bud of the Stuart taken Dec. 5, 1925, from a node near the terminal part of a shoot that fruited in 1925. This bud shows the early winter stage of development of the vegetative part of the bud. Fig. 15.-Stuart bud taken Jan. 11, 1926, from a node near the terminal on a shoot that fruited in 1925. This bud shows the midwinter development of the bud. Fig. 16.-Stuart bud taken Feb. 9, 1926, from a node near the terminal on a shoot that fruited in 1925, showing catkin on the right and elongating crown of the vegetative bud on the left. Fig. 17.-Stuart bud taken Feb. 20, 1926, from a node near the terminal on a shoot that fruited in 1925, showing the vegetative bud with internodes elongating and crown of the growing point also becoming elongated preceding pistillate flower differentiation. Catkin buds were broken off in sectioning. Fig. 18.-Stuart bud taken April 3, 1926, from a node near the terminal on a shoot that fruited in 1925, showing an enlarged longitudinal view through the growing point just as it was starting rapid growth. Fig. 19.-The terminal of a developing Stuart shoot taken April 10, 1926, showing a stage of pistillate flower differentiation further advanced. The bud scales had been lost and the young leaves were beginning to grow rapidly. Fig. 20.-The terminal of a developing Stuart shoot taken April 14, 1926, showing the pistillate flowers further developed than in any previous figure. The vegetative shoot has made considerable growth; so have the leaves, but the young leaves were still folded over the cluster of pistillate flowers so that it could not have been seen without sectioning. Fig. 21.-The terminal of a developing Sawyer shoot taken April 12, 1925, just before the pistillate flowers were large enough to be seen without magnification. Fig. 22.-Terminal of a Sawyer shoot taken April 12, 1925, showing pistillate flower bud developed to the point where the ovule was formed. 14 DISCUSSION FIGURES 14 to 17 inclusive show the development of the vegetative parts of the bud during winter and early spring. It will be noted that there appears to be a slight change in the crown of the bud in that it becomes more pointed as the time for very active spring growth There is no evidence, however, in these approaches. figures that pistillate flower differentiation has begun. Figure 18 shows rather clearly how the vegetative part of the mixed bud in the pecan appears as it changes from a vegetative to a pistillate flower structure. The crown of the bud first elongates then shows a slight protuberance which is a pistillate flower primodium. That the young pistillate flowers differentiate and develop rather rapidly once they have started is well shown by contrasting the extent of the differentiation and development of pistillate flowers in Figures 18, 19, and 20. Although Figures 21 and 22 are of a different variety and represent conditions existing in the spring of 1925 rather than 1926 they show that pistillate flowers are developed rather rapidly. A partly developed ovule is shown at "A" in Figure 22. The above studies, which covered a period of five years and included different varieties, indicate that in east central Alabama pistillate flower bud differentiation in the pecan takes place in early spring just as bud scales are dropped and rapid growth is starting and then proShuhart (14), and Woodroof and ceeds rather rapidly. Woodroof (18) found pistillate flower bud differentiation taking place at about the same stage of spring development of the tree, as was also suggested by the writer (7). Buds With Unknown Future Development N THE STUDY of the differentiation and development of staminate and pistillate flowers a number of miscellaneous observations were made which are of much Photographs, some of which are shown in interest. Figures 23 to 29 inclusive, were made to record these findings. 1 15 aind a1 cltkin it I show ing ai vegetaltive hull H the cetee each~ iflt. '11he V(2(tiatVt llEllt was cllI ,atln itie ruale. an seli out ofthe X ttii endi . Fi'. 2L1. A mired hud of tIC Earil talker NX.2, 192'2, Ilulil nar. the tetinall onl a liioot that friteid inl 1921. Outer hud1( scalt hal fuillen. lis is I ty Hel if huii that lose thle Ilutei Jfig 25.- mi\ hlullltdof the ;)tlllit titkii l)ee. 1', 1925, 1ronth nodle nearl thlt terilil part of aI sholithiat ritell In 192, shoing ii the deeitopmleil o the hat,. X while (ei ill tile hid. The eron ofiU the Xem tati Xe hu tis ranised. II XXeX er, it doll inot lookl etaeti like thet lii( crow 01'X ai buti d that x\iii souoi dlifferenitiate pistillate flIXe t. Fig (Ill lintl ltit o bud then tFeIi~~ltal122 Iln thatrue ot frul m IIt a11node ii tie near he terhu X e' IX tit like th dat parnt ' l stui, 11 . l iX tII[H whe 'ith ) ? 28 uoig. eogtedui anul t t[Iin al thsofei \Iuli F2 hg,. l-Sugut'udtul a FhXeb.llGiti 92G, the m tnot nl thelriphtnt of the rundiet eileave ttid myimHudiferentiate flwr. pistillute tlt wn uh: an oint i. er- i l12 e nea ul the~ theiri ia ils Onr' mtuch Fig. varieties they abscise hefore growth starts the spring following their formatiun or jutst as rapidl spring growth starts 29).-Longittitinal sectioin through the growing end ouf a laiteral shoot arisiug from an axillar hu itdtiurinug- the stunner. 16 l'iu. 2:) .June 22', 1I'?22. 17 DISCUSSION nuts, proOME AXILLARY buds, usually just below duce lateral growth the summer immediately following their formation. Such buds would produce catkins and might differentiate pistillate flowers the following spring if they did not produce this vegetative growth. Figure 23 illustrates one of these buds coming into growth. It will be seen from the abscission layer at the base of the catkin flowers that they are about to abscise. The vegetative part of the bud is very different in appearance from an axillary bud that differentiates pistillate flowers when it comes into growth, as was illustrated in Figures 18 and 19. If the axillary buds are influenced to produce second or lateral growth due to defoliation by storms, caterpillars, drought or other causes, the catkins are not likely to be abscised, but appear in the form of blossoms along with the appearance of the second growth of the vegetative shoot. During late summer, fall, winter, and early spring some of the largest and apparently best developed axillary buds located toward the terminal parts of the shoot may lose the bud scales. Just what will be the fate of such buds is uncertain; usually some of them drop; others may grow. Figure 24 is a good illustration of these buds. There is another class of buds, probably not very numerous, that reach quite a development by early winter. These buds are interesting because of the apparent development of the crown of the bud as if pistillate flowers might differentiate. This class is represented in Figure 25. Woodroof (17) believes such buds to be winter-resting buds, while Shuhart (14) classifies them as pistillate buds in winter stage. There are also two other types of strong buds that are interesting in their winter stage, but their future is also uncertain. They are shown in Figures 26 and 27-the former with internodes apparently elongated and the growing point, judging from the dark stain it takes, somewhat active; the latter with axillary buds well formed and with a somewhat unusual type of growing point. It would be interesting to know whether or not such rudimentary axillary buds contain primordia for catkin flowers fourteen months before they appear, but the histological technique used in this investigation did not make possible its determination. S 18 As has been stated, most varieties of the pecan abscise the terminal bud. It will be shown later that terminal buds that do not abscise are not likely to develop catkin flowers. -Figure 28 shows rudimentary catkins that will probably abscise and a terminal bud that may or may not differentiate pistillate flowers. That the end of a growing shoot as seen in longitudinal section is in appearance much like that of a true terminal just before growth is resumed in the spring is evident from a comparison of Figures 28 and 29. PART II.-GROWTH AND FRUITING HABITS Varietal Variation in Number and Abscission of Buds OST VARIETIES tend to form several buds at a node, the number depending somewhat on the vigor of the shoot and the location of the node. The Stuart variety sometimes forms as many as six buds at a single node. The size of the buds at a node usually decreases from the uppermost to the basal one. In general the buds are successively larger from the basal to the distal end of the shoot, as shown in Fig 30 A and A', B and B'. When exceptions occur they are generally found where a vegetative shoot slowed down in growth, produced several short internodes, and grew more rapidly later that season. A temporary exception may occur at nodes located about the middle of rapidly-growing-vegetative shoots. In the former, buds located at nodes in short internode areas are small, as shown at D, E. and F in Fig 38. In the latter, buds at nodes near the middle of the shoot are largest. Other varieties, of which the Success is an example, tend to form few buds at each node and appear to lose many of the uppermost buds of the nodal group by abscission. The distal bud at every node may drop from some shoots, the dropping taking place in late summer, fall, or winter. There seems to be a tendency, however, for the buds at the subterminal nodes to be retained for growth the following spring. When the uppermost bud at a node abscises, the second bud usually increased in size and functions in its place. 19 Characteristics of Pecan Shoot Growth made of the growing habits of time growth started in spring until it started the following spring. These field observations showed that the pecan may-and usually doeshave a great number of different kinds of shoots. Some of the most common and most important of these types are: long and short shoots that fruited the year of their formation; long and short shoots that remained vegetative but dropped the terminal bud before the winter following their formation; and long and short shoots that remained vegetative and retained their terminal buds the year of their formation. All of these types are of much interest because of their comparative fruitfulness the year following their formation, and because a careful study of these types and how to cause the formation of a large number of the most desirable should give the These shoots are illuspecan grower better returns. trated in Figure 30. Very short weak shoots that arise out of buds located toward the middle or basal portion of shoots of the previous season's growth are of interest, because they usually abscise when catkins fall without making much vegetative growth or developing pistillate flowers. Such a shoot is illustrated at A in Figure 31. Weak shoots that abscise the terminal or growing part of the shoot, including undeveloped leaves, just about the time catkins are falling, and more vigorous shoots showing pistillate flowers, are interesting because they usually go through the remainder of the current growing season without producing further leaves or linear growth. This type of shoot is illustrated at A in Figure 32. Shoots that reach medium length or above and are vigorous but fail to produce pistillate flowers, as do other shoots of similar length and apparent vigor, attract the pecan grower's attention because of their failure to produce nuts. Such a shoot is shown in Figure 31 at B. Shoots that reach medium length or above, produce pistillate flowers, and set nuts while short shoots and shoots of similar length and apparent vigor remain vegetative are also of special interest. Such a shoot is illustrated at B in Figure 32. A the pecan from was FIELD STUDY the 20 Weak shoots that differentiate pistillate flowers which abscise just before or just after reaching sufficient development to be seen without microscopic examination are of special interest to pecan growers because they represent one of the critical steps between large and small yields. It is reasonable to believe that orchard management practices could be modified so that such pistillate blossoms would set and produce nuts. Shoots that behave this way are illustrated in Figure 33. As has been stated, most shoots of most varieties of pecans drop the terminal bud and make further development out of other buds; but, as many terminal buds are retained on some varieties and give rise to nut-producing shoots, such shoots are of importance. Figures 34 to 36 inclusive illustrate three ways in which a shoot may dispose of its terminal bud. Shoots that develop a comparatively small number of strong buds near the terminal are usually light bearers. Even when nuts are produced, the number in the cluster is likely to be small. This type of shoot is illustrated at A in Figure 37. Shoots that develop many strong buds toward the terminal are usually heavy bearers. Such a shoot is shown at A' in Figure 37. Shoots that produce a second growth while carrying nuts are of much interest because of the influence of this second growth on the location and number of flowers they produce the following spring. The first and second growth in shoots of this type are illustrated at 1 and C respectively in Figure 37. Very long shoots are not likely to fruit the year following their development, especially if they make any form of second growth. There are several different types of these long shoots which make some form of second growth. They make long vegetative shoots, drop the terminal end, then make additional growth that season; or they make long vegetative growth which slows down due to unfavorable growing conditions then make additional vegetative growth, without dropping the terminal bud. These shoots are illustrated in Figure 38. 21 A an A'.-Long, and short shoots that \\ (ite an madeliv til moreI~I~ Fig. 8,1. A.-Very wcak shoot that prodluced very little vegetative growth. Such shoots abscise when the catkins fall. B.- Vegetative shoot that was apparently vigorous enough to produce pistillate flowers, hut failed to do so. 23 Fia. .Itt -At11 A iros shot lik .homt ties that produce a larhoot on wXhich the Wuoiol cause i 1) lr ilgitig almost healed. The \vegetativ e growth and catkin flowers apypeared above and below the ring and pistillate flowers only above the ring. [ig. 41 .- A P81)t s~hoot onl which the ring healed entirely. The catkin flowers app~eared1 below and abov\e the ring while the x egetative gr'owxth and istillate cluasters appeared only above the ring. Foliage r'emovedl to shaw p~istillate flowers( on young shoot. II YahsiI.t shoot where0 the ringt partly Fig. 49., N~t tiset bo0th above a ml(below theri. Iealed. hox e atured, wthile those ,Iht hlobO the rig droppe~d bc1'oe maturitF. 4 adIX ttitiuIj OF rt t IN bud od. two VQU eU afttl eIntI rimed(( trVV is I)\ abuII w \ire. The vait UViN SaI >UN\ N eI 1(11 Ith VUar Iil'. 5t1. Shots that wxere ( A) and werte noat (11) de(foliatedl. (itherwxisi these shootsxwee apparieintly ailike. They wxere oni (le sanae hranciih. Niote that Bi pi'iditedl mrie catkins than A tiiii i aii proi duiced ai clutei of pistillaite floe rs wlhile A didi not. 1 and May 13, 1925, the treatedl shoots were examined1 turther to determine the numlii of branches that had arisen both abov e and below the rig: also the x igor of these new shoots and the number of pistillate clusters andi staminate tloxx:ers pr1oduced. A youinng shoot that made little growth and carried small, 1ight colored leav es wvas termedl v ery weak; one that mnadle a long, wel (eveloped growxth and carried large. cd deep) gr een l eav es simnil ar to the biest arising from buods ion the u ntreated shoots was terme \ery v( vigoro~us. The teims wxeak, medlium, and xigor()us relpresent gradlatiotns between these twxo extremes. Figures indicating relative vigor of the treated shoots as rceflectedl by xigor of you ng shoots arising from th em Nvere o~btainled byV assigning to eery xvi gorous, vigorous, medliumn igorous, x-weak and v ery weak shoots values oii 10, 8, 6. 4 and 2 resp~ectively and m oltipllying the numher oif shoots occurring in each grou p by the assigned valuie and adding to secure total v igor, then div iding by numbler of shoots treated1 to get axverages. By this method of estimating vigor of the treatedl shoots may be as lowv as zero or greater than ten. The rco irds that were obtained are shoxvn in Table 9. 57 -B 1'TW\EEN ATAY Table 9.-Influence of Defoliation, Ringing, Ringing and Defoliation during summer of 1924 on Shoot Growth and Fruit-Bud Formation-4 to 6 trees were used in all cases and from 5 to 77 shoots were treated Treatment Variety Average No. of shoots produced per treated shoot spring following treatment Above girdle Ringed and defoliated to base. Ring partly or entirely healed Ringed and defoliated to V Av. vigor per Av. no. of blossoms produced per treated shoot as treated shoot spring following indicated by treatment vigor of shootsproduced spring Catkins* Pistillate clusters following treatment r Above Below Below Above Below Above Below girdle girdle girdle girdle girdle girdle L1(girdle ,-. r( Delmas Pabst Delmas Pabst Delmas Pabst Delmas Pabst Delmas Delmas Pabst vv NI1~ Delmas Pabst Delmas Pabst 1.4 1.66 0.43 0.14 1.63 1.60 0.78 0.12 1.72 1.55 0.58 0.08 1.80 2.20 1.72 2.17 1.0 0.55 1.26 1.96 1.09 0.60 1.46 1.78 1.50 1.44 2.00 1.66 8.93 11.77 2.08 0.42 10.18 9.60 4.07 0.36 17.44 9.11 2.34 0.33 6.8 3.22 10.52 13.57 7.81 - 3.60 - 13.4 17.22 0.00 0.25 c,,,c,,, 12.54 12.00 0.0 0.15 4.33 4.55 4.56 5.43 2.72 3.00 4.82 12.72 3.11 5.0 5.30 5.79 0.0 0.66 0.00 0.07 0.0 0.60 0.0 0.0 0.14 0.33 0.0 0.04 0.06 0.40 0.50 0.75 - 0.0 0.11 0.00 0.03 0.0 0.0 0.03 0.18 no part of ring healed base; Ringed and defoliated to ring. Ring partly or entirely healed Ringed and defoliated to ring; no part of ring healed Ringed, ring partly or entirely 13.85 12.72 19.44 7.77 1 2.00 12.16 Pabst healed yC11 L~I~Y CIIL~IL~I~Y VI LI~CC~r~U Ringed, no part of ring healed Defoliated Check *In I 16.88 16..00 0.0 1.12 18.37 32.1.8 21.77 14.34 1, 0.05 0.33 0.03 0.33 11.74 14.61 13.08 19.79 case of Pabst a storm removed some of catkins before count was made. 58 7 T IS OBVIOUS that data in Table 9 were obtained from a comparatively small number of shoots but there seems to be sufficient evidence to show that: (1) The number of young shoots produced above the ring, where it was partly or entirely healed, was in all cases greater than the number below the ring. (2) The number of young shoots produced above the ring, when no part of the ring healed, was less in all cases than the number produced below the ring. (3) The vigor of the shoots producedabove and below the ring depended on the extent to which the ring had healed. Where it had healed the vigor of shoots was greater above the ring than where it did not heal. (4) Defoliation tended to increase the number of shoots produced and lower the vigor of shoots produced in both varieties. (5) Defoliation materially cut down the number of clusters of pistillate flowers in both varieties. (6) Defoliation cut down catkin flower production in the Delmas and apparently increased it in the Pabst variety. The increase in case of the Pabst may have been due to possible error in estimating and figuring catkins, since it is possible that only the weak shoots of the check held catkins after the storm. (7) Ringing and preventing the ring from healing reduced catkin and pistillate cluster formation in both varieties more than did defoliation. (8) Ringing and allowing the ring partly or entirely to heal resulted in a slight increase in the number of catkins in Pabst and Delmas and decreased the number of pistillate clusters in both varieties. The decrease was considerable in case of Delmas. (9) Ringing and defoliating to the base of the shoot and then not allowing the ring to heal resulted in reduction of catkins and in the complete prevention of nut cluster formation in Delmas and in reduction in case of Pabst. (10) Ringing and defoliating to the base of the shoot and then allowing the ring to partly or entirely heal resulted in an increase in number of catkins and pistillate clusters in Pabst, and to a considerable reduction in catkins and prevention of pistillate cluster formation in case of Delmas. (11) In general preventing the healing of the ring reduces flower production. Influence of Disbudding on Shoot and Fruit-Bud Formation T- O OBTAIN definite data on the degree of uniformity or similarity between the several buds at a single node, as measured by the kind of shoots to which they give rise, a series of disbudding experiments was started on Stuart and Pabst trees. Shoots that carried nuts in 1925 were labeled and the first, first and second; first, second and third buds at the first six nodes below the nut scar were removed on different trees at weekly intervals from March 24 to April 14, 1926, and their subsequent records checked against those of similar shoots from which no buds were removed. The response to disbudding for the two varieties was very similar. Data for Stuart are given in summarized form in Table 10 and illustrated in Figure 52. 59 Table 1 O.-Influence of Disbudding on Production of Shoots and Flowers (variety.Stuart) Position (1) On node Below terminal 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Total Check 76* 74 46 12 5 3 1 1 No. of shoots produced 1 bud re- . moved 23 19 23 15 13 10 53 41 19 6 3 2 1 1 228 64 3.6 2 buds re moved 2 2 5 2 2 61 59 34 15 15 5 3 buds re moved 3 3 Check 77 77 76 71 73 68 60 No. of catkins produced 1 bud 2 buds 3 buds rereremoved moved moved 20 20 23 26 24 24 56 No. of pistillate clusters Check 71 41 9 1 1 1 1 bud 2 buds 3 buds rerere moved moved moved 11 11 8 3 3 3 34 54 1 1 1 1 2 1 61 67 67 58 46 19 15 8 2 1 222 69 3.0 inclusive. 1 53 43 28 18 9 4 657 77 8. 5 60 61 55 33 28 17 10 5 61 55 49 36 26 19 5 65 66 57 47 28 17 10 1 16 4 1 39 8 1 1 46, 52 15 2 3 1 ________________ 218 shoots 202 65 3.1 402 64 6.3 319 65 4.8 357 69 5.2 126 77 1.6 94 64 1.5 103 65 1.6 137 69 1.9 *No. used 77 2.8 Av. per shoot (1) Disbudding was practiced on nodes 1-6 V t he 1926 shoot vrou th arising, from 1!x25 shoots that w tre disbudd ed The shout on the right w~as not dis budded and serves as a check. F"rom the second, third and fourth shoots froam the right ant, two and three butds respectiv ely wxere removxedl at each af the first six nodes helowx the nut Scar. to differen t extents. Fig: 5~2.-This figzure s hows DISCUSSION [T WILL BE seen from Table 10 that normally most of the vegetative growth and pistillate-flower-producing shoots arise from buds located at nodes near the terminal part of the shoot. Similarly the majority of the catkins are normally developed from buds located at nodes near the terminal part of the shoot. However, some of the catkins are normally produced from buds extending almost to the base of the shoot. The second bud, as well as the first, at a given node may produce either pistillate or staminate flowers, or vegetative growth, or all three, while the third bud arising at a given node apparently does not differentiate flowers and produces very few catkins when the buds above it are removed as late as March 24. The removal of more than one bud at nodes near the terminal shifts fruiting possibilities to more basal buds with a tendency to increase yields and lower catkin production, but where only one bud is removed from nodes near the terminal it tends to scatter fruiting and vegetative growth over the entire shoot. It also has the effect of lowering production from 1.6 to 1.5 nuts per shoot, as shown in Table 10, while the number of catkins is reduced from 8.5 to 6.3 per treated shoot. Disbudding as late as April 14 caused buds to fruit that otherwise would have remained latent or produced only catkins and short vegetative shoots that would have abscised when the catkins fell. T HESE DISBUDDING experiments indicate that the condition existing in the pecan is more closely comparable to that existing in the grape, raspberry, and probably other bramble fruits than to that found in the apple, peach, and most other tree fruits whose fruitihg characteristics have been carefully studied. In other words, a comparatively large percentage of the over-wintering buds may be potentially pistillate flower buds. Under normal circumstances only a few of these will produce pistillate flowering shoots. The rest remain dormant or are abscised or perhaps give rise to weak vegetative parts that abscise along with the falling of the catkins. They function only when those that normally give rise to pistillate flowers are removed by 62 pruning or other means. This is in marked contrast to the condition existing in the peach, pear, etc., where the flower parts are differentiated in the bud during the growing season of the year before which they open. GENERAL DISCUSSION T HE DIFFERENTIATION of the staminate flower cluster in the bud is a process that extends over a comparatively long period, beginning in early spring in the first formed buds of the season and occurring in mid or late summer in those buds laid down in the axils of late formed leaves on secondary shoots. The extreme earliness with which these first catkins are differentiated (associated as it is in time with very slight leaf development) leads to the surmise that it is probably more closely related to and dependent on food storage conditions in the parent twig or branch, in the old wood: and, therefore, on late summer and fall activities in the tree than on spring growing conditions. However, catkin differentiation in the bud is so abundant and takes place under such a wide range of environmental, nutritive, and growth conditions that obviously it seldom, if ever, becomes a limiting factor; and, therefore, for all practical purposes may be ignored. Pistillate-flower cluster differentiation, on the other hand, is not all-summer in duration but occurs within a relatively short period as growth is starting in the spring. The time of its occurrence is probably in itself sufficient evidence that-to the extent that it is a response to nutritive conditions within the plant-it is due to or associated with winter storage of food materials. This means that it is determined by what goes on in the tree during the summer and fall before. In other words, the pistillate flower crop of the following year is apparently being determined while the nuts of the current year are filling and maturing, and it is then that cultural and fertilizing practices are very important if they are to function in increasing yields. Further evidence on this point is supplied by the data from the defoliation and ringing experiments. HE DISBUDDING and pruning (heading back) experiments indicate a rather marked degree of flexibility in the pecan. It obviously has the ability to adapt itself to circumstances, in regard to fruiting, by developing 63 a fruit crop from buds that would never have opened had the buds that normally open been uninjured or unremoved. This probably means that some types of pruning could be employed without materially interfering with crop production. However, it is doubtful if the evidence available warrants the interference that pecan yields may be increased practically or profitably by pruning. There is a marked correlation between type and amount of new shoot growth made and the tendency to form pistillate-flower producing shoots. With some varieties this correlation is close (i. e. the range in growth associated with pistillate bud production is narrow) with others it is not so close. The real problem of the grower is to handle his trees in such a manner that each year a comparatively large percentage of their shoot growth will be as nearly the optimum as possible for the variety in question. This means that control over production is possible largely through the soil and, incidentally, particularly important is it that cultural operations and fertilization practices be sufficient to assure good vegetative growth. SUMMARY 1. Buds located at the more basal nodes of the new shoots rapidly differentiate into three, sometimes four, and occasionally five growing points under a common bud scale. Each of these growing points is surrounded by a separate bud scale. 2. The growing point occupying the central part of the compound bud retains its vegetative nature throughout the growing season, while the others rapidly differentiate catkins. 3. As the growing season continues buds formed at other nodes undergo similar changes. However, toward the end of the season buds on the terminal part of the shoot develop more rapidly than those near its base. Ultimately the more terminal buds come to have more and larger catkins than those at more basal nodes. 4. Catkin flower clusters develop more rapidly in the buds of those varieties that are heavy catkin producers and that put out catkin flowers early in the spring than in the case of varieties that are light catkin producers. 5. Shoots that terminate spring growth with a cluster of nuts or by dropping the terminal bud and that later make a second growth, differentiate catkin flowers on the "second growth" part of the shoot (that is, on its laterals or secondaries) very much as they did on first growth. These constitute the majority of the catkins appearing as blossoms on such shoots the following spring. The basal catkin buds on second growth shoots are the apical catkin buds of the first shoot growth. Such buds may drop when the second growth shoot pushes out, or remain on the new shoot until the following spring, at which time they may abort or produce flowers. 6. The true terminal bud in most varieties does not usually differentiate and develop catkin flowers. 7.' If a tree is prematurely defoliated because of insect attack, 64 drought, storms, or other causes, and the tree makes a second growth, catkin flowers may appear along with the new growth. 8. The part of the compound bud that remains vegetative while catkin flower buds are differentiating continues its development by forming nodes, internodes, leaves and rudimentary buds in the axils of the leaves. Buds toward the terminal part of the shoot become very much larger than those toward the basal part of the shoot. In general the apical bud at each node becomes larger than the one immediately subtending it at the same node and the second bud at a node larger than the one just below it, ard so on with all buds occurring at any given node. 9. Just about the time buds begin to swell in the spring following their formation some of the hitherto vegetative buds begin to differentiate pistillate primordia. These continue their development as the internodes of the vegetative shoot elongate until about ten or more leaves have unfolded, at which time the pistillate flowers become visible on the terminal part of the young shoot. 10. When second growth occurs on any shoot the buds formed toward its terminal are the ones that produce the majority of the nuts on that shoot the following year. 11. Normally the terminal bud of most varieties of the pecan abscises and the pistillate flowers are differentiated in larger apical buds at the nodes near the terminal. However, all varieties studied may hold at least a few terminal buds and differentiate pistillate flowers in them, and some varieties may form and hold many terminal buds that differentiate pistillate flowers. 12. The shoots of some varieties, Success for example, may lose almost all buds formed at all nodes at some time of the year. There is a tendency in such cases for the strong buds at nodes near the terminal part of the shoot to be retained for flower and fruit production. 13. The pecan produces a number of different kinds and lengths of shoots which behave somewhat differently in the several varieties. Such variations suggest that orchard practices should be adapted to the variety in order that maximum production may be obtained. 14. In general very short and very long shoots carry very few nuts. 15. Each variety produces the maximum number of nuts on shoots of rather definite length. 16. Long shoots that fruit produce more nuts than similar short shoots on a given variety. 17. Many shoots, especially those that are weak, abscise their cluster of pistillate flowers. With some this abscission occurs before the cluster of flowers is visible, with other it occurs after it is visible, but before being receptive to pollen. 18. Prunine shifts vegetative growth, catkin flower development and pistillate flower differentiation to buds at more basal nodes. 19. Defoliation reduces catkin flower development and pistillate flower bud differentiation. 20. Ringing and allowing the ring partly or entirely to heal scatters catkin flower development and pistillate flower bud differentiation over the treated shoot. 21. Ringing and preventing the healing of the ring stons catkin flower development and prevents pistillate flower bud differentiation above the ring. 22. Removing first and second apical buds at nodes near the terminal parts of a shoot in general shifts catkin flower development and pistillate flower differentiation to buds at more basal nodes, showing that the third apical bud at a node does not usually have pistillate flower producing possibilities. 65 ACKNOWLEDGMENTS wishes to express his thanks to the auT HE WRITER the Alabama Polytechnic Institute and thorities of Michigan State College for making this investigation possible through their cooperation; to the several members of the Botanical and Horticultural Departments of both institutions who have furnished helpful suggestions and criticism; to Prof. V. R. Gardner especially for suggesting the problem and furnishing general guidance; to Prof. F. C. Bradford and Dr. J. W. Crist who have furnished special guidance in obtaining and classifying these data. 66 LITERATURE CITED 1. Bradford, F. C. Nitrogen-carrying Fertilizers and the Bearing Mature Apple Trees. Mich. Exp. Sta. Spec. Bul. 127. 1927. 2. 3. Census Reports U. S. from 1899-1919 inclusive. Chamberlain, Charles J. Methods in Plant Chicago, 1915. Histology. Third Revised Habits of Edition. 4. Gourley, J. H. Studies in Fruit Bud Formation. Tech. Bul. 9. 1915. N. H. Agri. Exp. Sta. 5. Harvey, E. M., and A. E. Murneek The Relatiorn of Carbohydrates and Nitrogen to the Behavior of Apple Spurs. Ore. Agr. Exp. Sta. Bul. 176. 1921. Heinicke, Arthur J. Factors Influencing the Abscission of Flowers and Partially Developed Fruits of the Apple. (Pylus malus L.) Cornell Univ. Agr. Exp. Sta. Bul. 393. 1917. Isbell, C. L. Studies in Fruit Bud Formation of Pecan and the Growing Habits Associated with it. Thirty-fifth Annual Report Alabama Agricultural Experiment Station. 1924. Studies in Fruit Bud Formation of Pecan and the Growing Habits Associated with It. Thirty-seventh Annual Report Alabama Agricultural Experiment Station. 1927. 6. 7. 8. 9. Lewis, C. I., and R. W. Allen The Influence of Nitrogen upon the Vigor and Production of Devitalized Apple Trees. Ore. Sta. Rept. Hood River Branch Sta. 1915. Partridge, N. L. The Fruiting Habits and Pruning of the Concord Grape. Agr. Exp. Sta. Tech. Bul. 69. 1925. 10. Mich. 11. Petri, L. An Experiment Conducted to Determine the Nitrogen Nutrition of the Olive. Atti. R. Accad. Econ. Agr. Georg. Frienze, 5. Ser. 13. 1916. (Abs. in Exp. Sta. Rec. 35: 839. 1916). Roberts, R. H. Effect of Defoliation upon Blossom Bud Formation. Wis. Agr. Exp. Sta. Res. Bul. 56. 1923. Stuckey, H. P. The Two Groups of Varieties of Hicoria Pecan and Their Relation to Self-sterility. Ga. Agr. Exp. Sta. Bul. 1916. 67 12. 13. 14. Shuhart, D. V. The Morphological Differentiation flowers of the Pecan.-Jour. Agr. 1927. of the pistillate Res. 34: 687-696. 15. Wellington, J. W. Station Investigations on Fruit Bud Formation. Work and Expenditures of the Agricultural Experiment Stations, 1922. U. S. Dept. of Agriculture. Wiggans, C. C. Some Factors Favoring or Opposing Fruitfulness Apples. Mo. Agr. Exp. Sta. Res. Bul. 32. 1918. Woodroof, The tive Ga. in 16. 17. J. G. Development of the Pecan Buds and the QuantitaProduction of Pollen. Exp. Sta. Bul. No. 144. 1924. 18. -----------and N. C. Woodroof. Fruit Bud Differentiation and Subsequent Development of the Flowers in the Hicoria Pecan. Jour. Agr. Research. 33; 7: 677. 1926. 68