Agriculture Reference
In-Depth Information
their heat loss by radiation. On a small scale, use of smoke or above-canopy
material may impede heat loss and provide effective frost protection. Drawing
down warmer air with 'wind machines' from above a 'frost inversion' where
the coldest air is nearest to the ground, is widely used in appropriate circum-
stances. Other frost control measures depend on heat inputs to the buds either
by use of orchard heaters or by water sprinkling so that water freezing on the
buds releases latent heat of fusion. Bud heat loss, hence the heat requirement,
is a function of orchard net radiation and windspeed (Landsberg
et al.
,
). It
also depends on the heat transfer properties of the buds, which are influenced
by bud size
per se
and the effective changes in this as ice accumulates on the
buds during sprinkling. Hamer (
) has published data on bud heat
transfer coefficients and their use to estimate the water requirements for frost
protection by water sprinkling.
An alternative approach to frost problems is to spray the buds, blossoms
or fruitlets with gibberellic acid or mixtures of hormones. These treatments
substitute for the hormones normally produced by seeds and lead to the de-
velopment of fleshy tissues to form seedless fruits. This is discussed in detail in
the section on fruit set and fruit growth.
,
Pollination
At the time of blossoming the anthers open and pollen grains escape, this event
being referred to as anthesis. The embryo sacs in the ovules usually mature
at the same time or a few days later. In order to accomplish fertilization the
pollen grain must be transferred to the surface of a stigma and germinate. The
pollen tube must then grow down the style, the generation nucleus dividing
mitotically into two sperm nuclei, and the ovule must be fertilized before it
degenerates. Most apple and pear cultivars are not self-fertile or have only a
limited degree of self-fertility. In the typical orchard cross-pollination between
different cultivars is the norm.
Pollen production
If pollination is to occur naturally it is very important that the flowering per-
iods of the cross-pollinating cultivars are matched. These are established by
long-term observation in each fruit-growing area. It cannot be assumed that
the relationships will hold good over areas of widely differing climate: the
different cultivars may well have different winter-chilling and thermal time
requirements for budbreak. Even within a single area the relative dates of
flowering may vary from year to year to such an extent that two or more pollen-
izer cultivars may be needed for a main commercial cultivar (Church and
