Agriculture Reference
In-Depth Information
Fig 4.24.
The relationship between the light-saturated rate of photosynthesis (A
sat
)
at the time of bulbing and mean growth temperature from transplanting to bulbing
for plants grown at elevated (
) or normal (
) CO
2
concentrations (532 and 374
mol/mol CO
2
(from Wheeler
et al
., 2004. Courtesy of
Annals of Applied Biology
).
mol/mol CO
2
, respectively) and measured at 560 (
) or 350 (
)
cotton plants in the same situation increase their leaf osmotic potential to
compensate fully for increases in the root medium, and their turgor is not
reduced (Gale
et al.
, 1967). The decrease in leaf turgor induced in onion
reduces the photosynthetic rate of the leaves, the effect being larger the higher
the evapotranspirative demand on the leaves. These changes in turgor and
photosynthetic rate are quickly reversed by transferring the plants back to
non-saline solution.
In summary, onion plants have less ability to compensate for decreases in
the water potential of the root medium by parallel decreases in the leaf water
potential than do other crop species. This causes leaf turgor pressure to decline
rapidly as the water potential of the root medium declines and this, in turn,
decreases leaf expansion rate, photosynthetic rate and, ultimately, crop growth
rate. Studies in saline solutions indicated a 50% lowering of growth by a
sodium chloride solution of osmotic pressure 0.125 MPa, whereas for cabbage,
lettuce and beans an equivalent growth reduction required an osmotic
pressure of 0.4 MPa (Bernstein and Hayward, 1958). In addition, the root
system of onions is rather shallow, sparse and lacking root hairs (see Chapter 2,
'The Root System'). Consequently, water extraction is confined mainly to the
top 25 cm of soil (Goltz
et al.
, 1971).
These physiological and morphological traits indicate that the photo-
synthetic rate and the growth rate of onion are more sensitive to water stress
than the majority of crops. On the other hand, observations on plants growing
in arid conditions show that they can survive long periods of water stress,
making no growth, but ultimately recovering when water becomes available
(Levy
et al.
, 1981). Furthermore, onion roots can remain alive in the absence of
