CHANGES IN THE FRUIT RETENTION STRENGTH OF THE RED RASPBERRY (RUBUS IDAEUS L.) DURING RIPENING AND THEIR RELEVANCE TO THE SELECTION OF RASPBERRY CLONES SUITABLE FOR MECHANICAL HARVESTING
The plant breeder can, of course, shake each plant by hand to estimate the relative ease of berry abscission for each seedling (Fejer and Spangelo, 1973) but this is a very subjective approach and the results may not be an accurate indication of a seedling's possible behaviour when harvested by machine.
Ideally one should simulate the action of the harvester by shaking the berries until they separate from their receptacles (plugs). This is possible with an electro-dynamic vibrator but it takes a long time to collect sufficient data to compare cultivars and it would be impractical to use the machine in the field. An alternative is to simulate hand picking by pulling the berry whilst the pedicel is firmly held and measuring the force required to remove the berry from its plug. In the laboratory an Instron Tensile Testing Machine (Hindman and Burr, 1948) has been used to measure raspberry fruit retention strength (Mason, 1974a; Jung, Bilanski and Evans, 1974) but like the electro-dynamic vibrator this instrument is not suitable for use in the field and data cannot be obtained rapidly. To overcome these problems hand held tension gauges have been used for measuring the retention strength of berries on the plant or immediately after severing the pedicel from the lateral (Nyborg and Coulthard, 1969; Fejer and Spangelo, 1973; Anon, 1973; Mason, 1974b; Jolliffe, 1975a, b).
Fruit retention strength (FRS) usually decreases as the berries ripen (Jolliffe, 1975a, b; Jung, Bilanski and Evans, 1974; Mason, 1974b) and this is probably the result of the histological changes which occur in the stipes connecting the drupelets to the torus (Wilson, 1976). Attempts have also been made to identify the changes in berry composition