1. The problem.
In propagation for summer planting the plant material generally will not be available in time to the growers. As the cause was not known, the question arose whether the reproducibility of the varieties was sufficient.
2. Experimental procedure.
The factors for reproducibility were ascertained. To this end an analysis was made of the factors determining the number of salable plants. This investigation consisted of three parts, which can be summarized as follows.3. Number of salable plants per mother plant.
As the number of rooted runners increases, more runner plants per mother plant are formed (Tables I, p. 14, and 3, p. 16).
The optimum temperature for the growth of the sprout of a runner plant is higher than the temperature at which the propagation is done in the Netherlands (Fig. 3, p. 21, and 4, p. 22). The optimum temperature for the development of the root initials falls within this range (Table 5, p. 25). As for the establishment of strawberry plantations preference is given to plants with a not too heavy sprout, the temperature in the Netherlands is generally favourable for a rapid production of salable plants. Under good cultural conditions more salable plants are formed as the total number of runner plants formed becomes larger (Fig. 6, p. 26)
The number of rooted runners increases as the total number of runners formed is increased (Fig. 7, p. 28).
Therefore the number of salable plants formed by a mother plant is determined by the total number of runners formed, on the basis of the following considerations:
1) number of runners ->number of rooted runners
2) number of rooted runners ->number of runner plants
3) number of runner plants ->number of salable plants.
An exception is the variety Senga Sengana. It forms many runners, but the number of salable plants per mother plant is comparatively low, because only few plants per runner are formed (Table 3, p. 16). This is caused by:a.
The phenomenon that in many runners at the base of the second or third runner plant a side crown is formed instead of a runner, giving rise to a runner plant consisting of two crowns (Fig. I, p. 17), which only starts to form runners after a certain lapse of time. The occurrence of this Phenomenon is associated with the rooting of the runner plants (Fig. 2, p. 18) and the growth of the mother plant.b.
The low number of rooted runners compared with the total number of runners during the occurrence of the phenomenon mentioned above (Fig. 7, second observation, p. 28).
By increasing the number of mother plants, the production of runners and consequently of salable plants per surface unit could be accelerated, permitting to lift the plants earlier. With Talisman this was the case both when the number of rows and the number of plants in the row were increased, with Cambridge Vigour only when the number of rows was increased (Table 6, p. 31).
On the propagation fields plant spacings are too wide for the rapid production of a large number of salable plants per surface unit. This is one of the causes that the plant material will not be available in time to the growers.
4. Number of runners per mother plant.
Vigorous growth of the mother plant is an essential condition for the formation of a large number of runners (Tables 7, p. 34, 8, p. 35, 9, p. 36, and 10, p. 37). The higher the growth rate of a variety, the higher the number of runners formed. Hence, if many runners are to be formed, the mother plants have to be heavy in the spring (Tables 12, p. 41, and 13, p. 42). From the start heavy mother plants produce more runner plants (Table 14, p. 43). At the end of the propagation period the number of salable plants produced by heavy mother plants is considerably higher than that produced by light mother plants (Tables 15, p. 43, and 16, p. 44). Vigorous vegetative growth of the mother plant forms the basis for good reproducibility.
As selection for productivity actually involves selection for vigour, this also lays the foundation for good reproducibility. The selection for reproducibility should therefore be directed towards continuous growth of the runners.
On the propagation fields the mother plants are not heavy enough in the spring. As a result, the number of runners and consequently the number of salable plants formed per time unit is too small. In addition to the spacing being too wide, this accounts for the fact that sufficient salable plants will not be available in time on the propagation fields.5. Conditions for runner firmation.
A strawberry plant starts to form runners when the daylength and the temperature have exceeded a certain value. Under a daylength of 16 hours there are varietal differences in the temperature at which runners are formed (Table 17, p. 50). These differences are not related to the distribution of the varieties. On the other hand, for northern varieties the daylength should be longer or the temperature higher than for southern varieties (Tables 18 and 19, p. 54/55).
For successful cultivation of a variety in a certain region both the period of runner formation and that of flower initiation must have a certain duration. The length of both periods will mainly depend on the moment at which the transition from runner formation to flower initiation takes place. As this transition is determined by the daylength and the temperature and the varieties differ in their reaction to these factors a variety can only be grown successfully in regions where this transition occurs at such a time that the periods of runner formation and flower initiation are both long enough. Therefore, the distribution of a variety is limited.
As the duration of runner formation within the geographic range of a variety is constant, the growth rate of the mother plant is the only factor which determines the number of runners formed.
6. Final conclusion. The fact that the plant material does not become available in time is not the result of insufficient reproducibility of the varieties but of imperfections in the propagation technique.