Univ. of Missouri Agricultural Experiment Station Research Bulletin 47: 3-19 (Sept 1921)
Localization of the Factors Determining Fruit Bud Formation
H. D. Hooker Jr. and F. C. Bradford

Belief in a relationship between slow growth and a fruitful condition in apple and pear trees has come down to the present with the approval of many generations of growers. Said John Lawrence1, in 1717, concerning the pear: “***** but yet for the sake of that noble Fruit which some Kinds produce by the Help of a Wall, it is worth while to humble him and keep him in Order. For which purpose**** I sometimes plash the most vigorous Branches, cutting them near the place from whence they shoot, more than half through, which effectually checks its Vigour, and consequently renders it more disposed to make weaker Shoots, and form bearing Buds.”

The chief concern of the older writers on fruit bud formation seems to have been the prevention of excessive growth. This was natural, since they were dealing chiefly with fruit gardens, manured and cultivated and consequently with trees growing luxuriantly. When fruit growing spread to the orchard the literary heritage from the garden survived and though there was an undoubted realization of the unfruitfulness of greatly weakened trees it is but recently that there has been a crystallization into definite phrases of this feeling that a certain amount of growth is necessary for fruit bud formation and that, within limits, fruitfulness and vegetative development are associated phenomena.

SOME OF THE FACTORS INVOLVED

The work of Klebs2, of Fisher3, of Kraus and Kraybill4, and of Hooker6 has given some conception of the internal chemical factors connected with the initiation of fruit bud differentiation. Briefly stated, this seems to be associated primarily with carbohydrate accumulation and in apple spurs, with starch storage in particular. However, even though carbohydrate accumulation occur, fruit and differentiation does not take place if there be a limiting factor which seriously retards or altogether stops vegetative growth. The inference seems warranted, therefore, that the supply of water, of heat6, of nitrates or of any other essential nutrient may so check growth and carbohydrate utilization that carbohydrate accumulation results, if conditions be favorable for carbohydrate manufacture; any one of these factors may become limiting and prevent fruitfulness, though under field conditions the nitrogen supply seems to be the factor most frequently operative in this direction. When the nitrogen supply is plentiful, carbohydrate is usually found in the plant in small amounts, because it has been utilized in growth; when the nitrogen supply is low, carbohydrate is usually found accumulated in relatively large amounts. In the two year cycle involving fruit bud differentiation one year and fruit formation the next, through which most apple spurs on fruitful trees usually pass, starch accumulation is associated with a relatively low nitrogen content during the period of fruit bud differentiation the one year and a practical absence of starch is associated with an exceptionally high nitrogen content during the period of fruit setting the other year.

It should be pointed out that this inverse correlation between nitrogen and carbohydrate (particularly starch) content does not represent a relationship of fundamental importance for fruit bud differentiation. It is, in a sense, accidental, though it is common because nitrogen supply is most often the limiting factor determining carbohydrate accumulation. In case some other factor, for example water supply, were operative in checking growth, it is clear that carbohydrate accumulation might take place even in the presence of abundant nitrogen. In fact some such situation must obtain in those spurs on certain apple varieties which form fruit buds regularly every year. If fruit setting depend on the presence of a relatively large amount of nitrogen in the spur as Harvey and Murneek7 suggest and if fruit bud differentiation depend on starch accumulation, then large amounts of nitrogen and of carbohydrates must be present almost simultaneously in these spurs. This situation has been observed in spurs of Payne’s Late Keeper, a local variety in which a large percentage of the spurs are characterized by successive fruit bud formation. A sample of bearing spurs collected July 3, 1920, had 1.236 per cent nitrogen and 3.16 per cent starch. Comparison of these figures with the data published in Research Bulletin 40 of this Station shows that this nitrogen content is of the order found in spurs of other varieties during the spring of their bearing year and that the starch content is equal in amount to that found in those spurs of these varieties that are differentiating fruit buds.

THE RELATION OF GROWTH TO PERFORMANCE

The fact that very little growth and very vigorous vegetative development are alike unfavorable for fruit bud differentiation suggests relationship between spur growth and spur performance in the apple. Roberts8 found that in Wealthy and other apple varieties under certain conditions spurs of certain length growths showed the highest percentage of fruit bud differentiation and that both longer and shorter spurs showed lower percentages.

To establish, for closer selection of samples for chemical study, the value of such an index to the probable performance of the individual spurs under conditions obtaining in the trees growing in the University orchard at Columbia studies of spurs of several varieties were undertaken. Measurements totaling around 13,000 were made, work proceeding with each variety till several successive series showed no change in the results obtained. For purposes of this investigation no spur was considered which had not blossomed at least once; growth in one year of over 10 cm. was arbitrarily considered to remove the twig from the spur class to the shoot class. The massed results are shown in Table 1, in terms of percentage of spurs in each class forming fruit buds. The figures presented here are from purely vegetative growths, i.e., no measurements of growth of any spur in its blossoming season are included. It may be stated, however, that inclusion of the growths during a season of fruiting made no material difference except in the percentage of fruit buds formed; the relative positions of the various classes remained the same.

These figures show, in each variety, an increase in the percentage of fruit buds formed, with an increase in the growth of the current year. In some, however, the rise from the lowest class to the next higher is much more abrupt than in others, in Gano from 17.5 to 45.7 as compared with a rise from 39.3 to 53.8 per cent in Jonathan. Viewing it in another way: in the lowest growth class Gano formed less than half the percentage of fruit buds that Jonathan spurs making the same growth formed. This difference could not be due to the Jonathan growths averaging nearer the upper limit of the class than did the Gano, for actually conditions were reversed. It seems quite evident that Jonathan will form a greater percentage of fruit buds on very short growths than will Gano.

Were spurs defined as growths up to 3 cm. only, there would be, in all four varieties, a maximum percentage of fruit bud formation in those growths between 1.6 and 2.0 cm. However, in each case, except Devonshire Duke, the percentage of fruit buds formed on growths between 3.1 and 10.0 cm. is higher than in any of the smaller classes. In other words, there is a strong tendency, in the trees examined, for a continuing increase in percentage of fruit bud formation with increase in growth.


pp. 17-18

Hartig9 found that previous to the seed year in the beech and oak large amounts of starch were stored in the medullary rays of the wood. In non-bearing years the starch stored in the two youngest annual ring was reduced to about half after the middle of June, but the supply was replenished in October; in the seed bearing year the starch content of the entire wood was reduced to a minimum, consuming the accumulation of eight years; furthermore nearly all the nitrogen disappeared from both wood and bark. Hartig concluded that the food supply of the buds was derived from local accumulations, that the activity of the cambium utilized only a very small amount of the starch stored in the wood, but that the accumulation of surplus reserves over a period of eight years was used in seed production. “In many trees, for example elms and fruit trees,” he states, “a seed year usually follows a year of rest in which surpluses are accumulated; in other kinds of trees seed years recur only after three, five or even ten years.”

These findings are important since they indicate that in the beech and oak, as well as the alternate bearing apple, the tree acts more or less as a unit; they suggest that the starch content of apple wood may be significant and they imply a rather direct relation of the reserve starch in the trunk and branches to the fruitful condition of the tree as a whole. If Hartig's surmise that the starch accumulations in the trunk are used for seed production be correct, the question of the passage of carbohydrates up the trunk must be studied from a new point of view. In the beech, for example, no significant upward translocation of carbohydrates would occur eight years out of nine. Very materially different results might be obtained from investigating trees in the bearing year than in the off year and a consideration of the condition of the material used for experimental purposes might go far to reconcile the conflicting reports on the upward translocation of carbohydrates.

REFERENCES

  1. Lawrence, J. The Clergy-Man's Recreation, p. 49. 5th. ed. London, 1717. [The same passage is in the 1st edition of 1714]
  2. Klebs, G. Proc. Roy. Soc. London 82: 547-558. 1910.
  3. Fisher, H. Gartenflora 65: 232-237. 1916.
  4. Kraus, E. J. and Kraybill, H. R. Vegetation and reproduction with special reference to the tomato. Oreg. Agr. Exp. Sta. Bul. 149. 1918.
  5. Hooker, H. D. Jr. Seasonal changes in the chemical composition of apple spurs. Mo. Agr. Exp. Sta. Res. Bul. 40, 1920.
  6. Walster, H. L. Bot. Gaz. 69: 97-125, 1920.
  7. Harvey, E. M. and Murneek, A. E. The relation of carbohydrates and nitrogen to the behavior of apple spurs. A. E. Oreg. Agr. Exp. Sta. Bul. 176. 1921.
  8. Roberts, R. H. Off-Year Apple Bearing and Apple Spur Growth. Wis. Agr. Exp. Sta. Bul. 317, 1920.
  9. Hartig, R. Anatomie und Physiologie der Pflanzen, pp. 251-253. Berlin, 1891.
  10. McCue, C. A. Del. Agr. Exp. Sta. Bul. 126, 1920. [pp. 1822.A study of determinations obtained by analysis of the leaves of peach trees receiving Various fertilizer treatments leads to the conclusion that greater variation in nitrogen content exists between trees of like fertility treatment and those of unlike treatment.
        Annual and biennial bearing trees were observed within a single apple variety. The author, separating biennial bearing Staymans into two classes, even and Odd year producers, concludes that there is a close correlation between available plant food and bearing habit. In alternate bearing trees, the year of production seemed to succeed a year of relatively great increase in trunk increment.]

Girdling, Ringing, Binding