PRODUCTIVITY OF PROPAGATED APPLE AND PEAR SEEDLINGS AS DEPENDENT ON THE
T. VISSER and D. P. DE VRIES
Institute for Horticultural Plant Breeding (I.V.T.), Wageningen, The Netherlands
Received 13 December 1969
Adult apple and pear seedlings varying in juvenile period were propagated on rootstocks. They were subsequently found to be more precocious and more productive when the juvenile period had been shorter. In view of the significant relation between vigour measured by stem diameter) and precocity of both seedlings and propagated trees, it is possible to pre-select for the potentially more precocious seedlings on the basis of vigour.
The juvenile period of apple seedlings has been shown to be directly related to the length of the unproductive period (from propagation on a rootstock to first flowering) of their parents (VISSER, 1965; VONDRACEK, 1967). VISSER (1967) also observed that adult seedlings after having been budded on a rootstock flowered sooner when their juvenile period had been shorter. Further data on the performance of these seedlings are presented in the following.
The experimental set-up (see also VISSER, 1967) consisted of budding adult apple and pear seedlings (11-14 years old) from several progenies on a rootstock in August 1964. Within a progeny a representative sample of seedlings was chosen covering the various juvenile periods found. The juvenile periods varied between 4 and 10 years for apple, and between 7 and 12 years for pear. In all 80 apple seedlings were budded on MIX, while each of the 87 pear seedlings was budded on three stocks, viz. on quince A with or without an interstock of Beurré Hardy or Passe Crassane. They were planted in the orchard early in 1966. A fair number of the apple trees already flowered during the spring of that year, in contrast to the pear trees of which few blossomed. As no interaction was apparent between the pear rootstocks, the data have been averaged for the three stocks.
As appears from Fig. 1, showing the situation in 1967 and 1969, the inverse relationship between the duration of the preceding juvenile period and the percentage of trees flowering in 1967 (left) is qualitatively - and even quantitatively - the same for apple and pear: fewer trees flowered when the preceding juvenile period had been longer. Although in 1969 (right) more pears flowered, the trend is similar to that in 1967; as yet not many trees with the longest juvenile period had flowered. For apple the situation changed in that all but a few trees flowered, irrespective of the length of the juvenile period (as was already the case in 1968).
|Fig. 1. Relation between juvenile period and the percentage of trees flowering two and four growing seasons following propagation.|
The productivity of the trees was expressed by the flowering or yield rate, estimated in 9 classes: 1 = no flowering or yield, 2 = very little flowering or yield, ... 9 = abundant flowering or yield. Class 1 was not relevant here as only flowering trees were considered.
Fig. 2, which presents the flowering or yield rate for 1968 and 1969, shows that the juvenile period of the seedlings determines their initial productivity on a rootstock. The figure also suggests that the effect of the juvenile period on productivity decreases with time. This may appear from the tangents of the regressions for pear, the one for 1968 being higher than that for 1969; the tangent of the latter is about the same as that of the regressions for apple in either year, when nearly all trees had flowered at least once.
It is of importance that a significant inverse relation between stem diameter and juvenile period as was found for seedlings on their own roots (VISSER, 1964), also existed for the trees on a rootstock, in casu between stem diameter and unproductive period (measured by the percentage flowering). This is illustrated for pear by Fig. 3 (left).
Fig. 3 also shows a similar relation for the rate of flowering and in addition the influence of the juvenile period which was larger when the period had been shorter. The extent to which these relationships are genetical, is indicated by e.g. the (highly significant) correlation of 0.49 (n = 82) between the stem diameters of the pear trees before (1963) and after (1967) propagation and by the correlations of 0.48-0.60 (n = 80) between the stem diameters of the pear trees on the three stocks. These correlations are relatively high, taking into account environmental influences during the years before and after propagation and the possible occurrence of a scion/rootstock interaction.
|Fig. 2. Relation between the previous juvenile period of propagated trees and their flowering or yield rate in two successive years.|
|Fig. 3. Dependence of the percentage and rate of flowering of pear on the stem diameter of the propagated trees for two categories of juvenile periods.|
It is evident from the foregoing that there is an inherent relation between the length of the juvenile period of apple and pear seedlings on their own roots and the length of the unproductive period and initial productivity following propagation on a rootstock. Obviously, the productivity in a given year is partly dependent on when the trees became generative. It is therefore possible that the differences in productivity become smaller as the trees become older.
As early and high productivity are essential requirements of a modern fruit variety, the selection of seedlings with these properties in an early stage has been a challenge to fruit breeders. The relation between vigour and juvenile period offers the possibility to do so by discarding the weaker-growing (potentially later-generative) seedlings already in the nursery. It is realized that this relation is to some extent determined by environment. As such, a direct test of the efficiency of selection cannot be made until further experimentation. Nevertheless, the method is recommended because the limiting factors in fruit breeding are time and space in the orchard rather than the number of seedlings in the nursery. Reduction of the number of seedlings in the nursery is only justified if it is expected to lead to improved quality of the trees planted in the orchard.
Our thanks are due to Mr. J. J. Verhaegh for his assistance in obtaining the data presented.