Agriculture Reference
In-Depth Information
flow is always from the root to the leaves, sugars
can flow in the phloem sieve tube cells both up and
down the plant moving to the plant organs where
it is needed, such as growing points, shoots, roots,
flowers, fruits or storage organs. Moving the sugars
in the phloem, like the uptake of minerals in the
roots, is an
active process
requiring energy, as
the concentration of sugars in the phloem is much
greater than in the leaf cells. The
companion cells
which accompany each
sieve tube cell
are thought to
control this process. The flow can be interrupted by
the presence of disease organisms such as club root
(see p. 256).
Active transport
is the movement of a substance
into a cell across the cell membrane against a
concentration gradient. It requires energy and
substances are taken up selectively.
Mineral nutrients are taken up predominantly by the
extensive network of fine roots that grow in the top
layers of the soil (Figure 10.1). Damage to the roots
near the soil surface by cultivation should be avoided
because it can significantly reduce the plant's ability to
extract nutrients and water. Care should be taken to
ensure that trees and shrubs are planted so their roots
are not buried too deeply, and many advocate that the
horizontally growing roots should be set virtually at the
surface to give the best conditions for establishment.
Over-enthusiastic hoeing of weeds can damage crop
roots near the soil surface and also cause increased
loss of water from the soil by bringing more moisture
to the surface.
Having crossed the roots, mineral nutrients are
transported up the xylem to the leaves and are also
redistributed in the phloem to other plant organs such
as flowers and fruits.
Phloem tissue
transports sugars made in
photosynthesis from the leaves to other plant
organs where it is used in respiration to release
energy or stored as starch for later use.
Further reading
Capon, B. (2005)
Botany for Gardeners.
Timber Press.
Hodge, G. (2013)
RHS Botany for Gardeners
. Mitchell
Beazley.
Ingram, D.S., Vince-Price, D. and Gregory, P.J. (2008)
Science and the Garden.
Blackwell Science.
Lack, A.J. and Evans, D.E. (2005)
Instant Notes in
Plant Biology.
Taylor & Francis.
Stern, K.R., Bidlack, J.E. and Jansky, S.H. (2011)
Stern's Introductory Plant Biology
. McGraw Hill
Movement of sugars in the plant
Phloem tissue
(see Chapter 6) is responsible for
transporting sucrose from the leaves as a food
supply for the production of energy through respiration
in plant cells. Unlike xylem, where the direction of
10
Please visit the companion website for further information:
www.routledge.com/cw/adams
