Agriculture Reference
In-Depth Information
Water relations
Introduction
Most of the water taken up by fruit trees is in response to evaporative loss
(transpiration) through the leaf pores or stomata. This is a consequence of the
need for stomata to be open to admit CO
entry for photosynthesis.
Over any given period the fruit tree usually takes up more than
times
as much water as it produces dry matter (Lenz,
). A cropping orchard in
Washington State, USA requires up to
m of irrigation per year (Evans,
).
tha
−
as compared with about
This is
.
t of water in the fruits of
tha
−
crop. The close correlation between water use and crop yield that
is generally observed is not a result of water use in the actual production of
the crop. It is primarily a consequence of the close correlation between CO
assimilation and water loss, as a result of the dependence of both of these on
leaf area and stomatal behaviour.
However, the ability to keep stomata open for CO
assimilation and to
avoid consequent desiccation depends on the adequacy of the supply of water
as well as mechanisms for controlling water stress. To this extent water supply
is a truly limiting factor to crop production.
The water fluxthrough the soil-tree-air system is under tension, i.e. nega-
tive pressure. Tree tissues and cells equilibrate with this tension and different
aspects of growth, development and function respond to this in different ways.
Smaller water flows in the tree are the results of gradients in osmotic
potential due to solutes moving into conducting elements. This is espe-
cially important in the transport of water and solutes in the xylem to de-
veloping tissues in spring, before there is any major transpirational flow.
Osmotic forces also determine the mass flow of water and solutes in the
phloem.
Many aspects of water relations, e.g. uptake by roots and effects of water
availability on growth, fruiting and nutrition have been discussed earlier. This
chapter emphasizes the factors controlling water availability and requirements
a
