Abstract:
Foliar B sprays have increased fruit set in 'Italian' prune
(Prunus domestica L.) by 2 fold in some trials and had no effect
in others. This work was conducted to identify reasons for this
inconsistent response. Since B levels in flowers are associated
with the amount of fruit set, factors affecting B supply to
flowers were also studied.
Foliar B sprays (500 ppm) were applied to 'Italian' prune
trees in the fall. Results from 2 trials suggest that the
response to B is influenced by the overall fruit set and B levels
in flowers in a given year. Boron sprays had no effect on set
during a warm spring when overall fruit set was high (12.2%), but
increased set by 32% during a cool year when the overall set was
low (3.2%).
The B content of flowers forced on excised branches was
similar to that of flowers on intact trees, indicating that
flowers derive most of their B from reserves in nearby branch
tissues. Simulated rain and relative humidity treatments
indicated that B in flowers is not readily leached by rain and
that transpiration rate has little effect on B accumulation in
flowers.
Boron accumulated slowly in buds prior to bud swelling (5
weeks before bloom), but rapidly from then until bloom. Xylem
differentiated in the axes of buds as they began to swell. Prior
to this, the discontinuous xylem vessel connection may impede B
movement into buds. The observation that the concentrations of P,
K, Ca, and S in the xylem exudate increased markedly 5 weeks
before bloom, while B concentrations remained constant, suggests
that B in branches is not as readily remobilized as other
elements. The amount of B supplied to buds via the xylem was
estimated from the accumulations of B in buds, the B concentration
in xylem exudate, and the calculated transpiration from buds.
Only a fourth of the B entering buds during the 2 months prior to
bloom was supplied by the xylem.
