Agriculture Reference
In-Depth Information
◦
C with a distinct peak at around
◦
C (Higgins
et al.
,
)or
-
;
some data of Watson
et al.
,
). These optima are lower than many cited in
earlier literature. Apple photosynthesis at
◦
Cis
% of the maximum at
optimal temperature and is very much higher at low temperatures than that of
peach, grape and some other fruit trees. The decline with temperature from
◦
Cto
-
◦
C, where it is near zero, is fairly consistent and is at least partly
due to concomitant effects on stomatal aperture with change in leaf-to-air
vapour pressure differences.
Kriedemann and Canterford (
) found net photosynthesis of pear leaves
to show a broad temperature optimum of
◦
C, at which it was about
-
mg CO
dm
−
h
−
, with lower rates at
◦
C(
c
.
mg CO
dm
−
h
−
)
mg CO
dm
−
h
−
) under laboratory conditions
of saturating light and high relative humidity.
◦
C and
◦
C(
c
and at
.
Effects of water stress and water potential
Waterstressisusedasthedescriptivetermforanimbalancebetweenthesupply
of and the demand for water. It is accompanied by changes in plant water
potential which may or may not have deleterious effects on plant processes.
Lakso (
) found that net photosynthesis can occur at very low apple leaf
water potentials and that substantial reduction of photosynthesis may not
occur until the water potential (
ψ
) falls below
−
bar. Kriedemann and
Canterford (
) found that pear leaf photosynthesis did not decline until
leaf water potential had fallen to below
bar although a history of water
shortage reduced the daily maximum rate of photosynthesis. Photosynthesis
in apples and pears can withstand much lower water potentials than is the
case in many annual crops and grapes. There is also some evidence of lower
photosynthetic rates at very high as well as very low water potentials (Flore
and Lakso,
−
). It may be that photosynthesis declines above an optimum
cell volume.
Lakso (
) showed a high correlation between net photosynthesis and
stomatal conductance in leaves from field and potted apple trees with vary-
ing water stresses imposed. Lakso (
) has suggested that the close coupling
between photosynthesis and stomatal behaviour may indicate that photosyn-
thesis controls stomatal behaviour as well as vice versa.
Effects of flooding
Flooding results in dramatic reductions in leaf photosynthesis of apple.
Fernandez
et al.
(
) found that flooding of 'Jonnee' apples on a range of
rootstocks reduced net photosynthesis to less than
% of that of control trees,
some trees showing deleterious effects within
days.
