Horticulture Principles and Practices

5.3.5 TRANSLOCATION
Translocation is the long-distance transport of organic solutes through the plant. Photosynthetic
products are moved out of the leaves and into the assimilate stream from the
sources (especially leaves but also storage tissue) to where they are used or stored (sinks).
The primary translocation source (leaves) is located above the primary sink (root). However,
the movement of organic solutes is not unidirectional or fixed.
During vegetative growth, assimilates are distributed from leaves to growing parts in
upward and downward directions. However, when the plant enters the reproductive phase
of growth, developing fruits require large amounts of assimilates and hence there is redistribution
so that most of the flow from neighboring sources and even from distant ones are
redirected to the fruits. Movement in the assimilate stream occurs via the phloem vessels
as sap, a fluid consisting mainly of sugar and nitrogenous substances (Figure 5–11).
Phloem transport is believed to occur by the mechanism of pressure flow. This
hypothesis suggests that assimilates are moved from translocation sources to sinks along
a gradient of hydrostatic pressure (turgor pressure) of osmotic origin. Sugar is asserted to
be transported in the phloem from adjacent cells in the leaf by an energy-dependent active
process called phloem loading. The effect of this process is a decrease in water potential
in the phloem sieve tube, which in turn causes water entering the leaf in the transportation
stream to move into the sieve tube under osmotic pressure. The water then acts as a vehicle
for the passive transport of the sugars to sinks, where they are unloaded, or removed,
for use or storage. The water is recirculated in the transpiration stream because of the
increased water potential or the sink resulting from the phloem unloading.
5.4 DEVELOPMENTAL STAGES OF GROWTH
A growing plant seedling goes through a number of developmental changes:
embryonic → juvenile → transitional → maturity → senescence
5.4.1 EMBRYONIC STAGE
The seed consists of an embryo or miniature plant. Until the conditions for germination
are right, the embryo remains dormant. Seed dormancy is said to occur when a viable
seed fails to germinate under favorable environmental conditions. This biological mechanism
is especially advantageous when plants are growing in the wild. It ensures that
seeds will germinate only when adequate moisture and other necessary environmental
FIGURE 5–11 Accumulation
of carbohydrates and sugars
above the position of girdling or
stem that interrupts the continuity
of the phloem.
Accumulation of
photosynthates
Bare stem
170 Chapter 5 Plant Physiology