- In the Pacific Northwest some prune cultivars often exhibit inconsistent
fruit set. Several factors have been associated with this characteristic.
Temperature, genotype and boron content are among the most frequently
Studies were conducted to determine the effect of fall ethephon
application on flower bud growth and ovule longevity in two Prunus domestica L.
cultivars, 'Italian' and 'Brooks'. Ethephon was applied at the 60% leaf drop stage
in the years 1988 and 1989. Growth of terminal buds, as well as tissue mineral
content was determined from 50 days prior to bloom until immediately prior to
bloom. Ovule longevity was determined by UV fluorescence microscopy after
staining ovules with aniline blue. Temperature effects on ovule longevity were
determined in flowers of excised twigs held for 18 days after full bloom in growth
chambers at 5, 10, 15 and 20°C.
Ethephon application reduced the growth rate of buds for the cultivars by
having an effect on both fresh and dry weight. Mineral content of buds was
markedly affected in the case of 'Italian'. Higher concentration of Ca and lower
concentration of P, as well as higher B content were present in the ethephon
treated buds of this cultivar, when compared to the untreated control buds. No
difference was seen for N or K. 'Brooks' seemed not to be affected by ethephon
application in terms of the mineral content of its buds.
Under field conditions 'Brooks' showed a higher ovule viability than
'Italian'. Eighty percent of the 'Brooks' flowers had at least one viable ovule at 20
days after full bloom (DAFB). On the same date, only 40% of the flowers of
'Italian' had viable ovules, and 60% showed total ovule senescence. Ethephon
application altered these genotypic differences. Flowers of ethephon-treated
'Italian' trees started with less viable ovules at full bloom, but these remained
viable for a longer time. At 20 DAFB, ethephon-treated trees, in 80% of the
cases, had at least one viable ovule. Ethephon did not change the pattern of
ovule longevity of 'Brooks'.
Increasing temperatures significantly reduced the viability of the ovules
across the days, in both cultivars. At 5°C, both cultivars showed a low rate of
ovule senescence and differences between 'Brooks' and 'Italian' were explained by
the initial lower viability of 'Italian' at bloom time. As temperature increased,
ovule senescence was faster in 'Italian'. At 15°C, only one ovule per flower
remained viable by 8 DAFB. At 20°C, total ovule senescence in some flowers
had already occurred by 2 DAFB, four days earlier than in 'Brooks'.
Ethephon application markedly affected the rate of ovule senescence,
depending on the temperature treatment and the genotype. In 'Italian', a clear
effect on delayed ovule senescence was seen at 15°C and 20°C. At both
temperatures, total ovule senescence was delayed at least by four days. As the
temperature decreased, there appeared to be little difference between the
ethephon-treated flowers and their controls. For 'Brooks' only a slight effect on
ovule longevity was observed at 20°C.