Agriculture Reference
In-Depth Information
diminished which in turn attracts water into the cell by tending to main-
tain turgor pressure. Earlier research indicate that compatible solutes like
sugars, glycerol, amino acids such as proline or glycinebetaine, polyols,
sugar alcohols (like mannitol and other low molecular weight metabolites)
would also contribute to this process. In addition, Hessini et al. (2009)
argued that these compounds benefit stressed cells in two ways: (1) by act-
ing as cytoplasmic osmolytes, thereby facilitating water uptake and reten-
tion, and (2) by protecting and stabilizing macromolecules and structures
(i.e., proteins, membranes, chloroplasts, and liposomes) from damage in-
duced by stress conditions. Osmotic adjustment allows the cell to decrease
osmotic potential and, as a consequence, increases the gradient for water
influx and maintenance of turgor. Osmotic adjustment has been assessed
as a capacity factor (rate of change in solute potential (Ψ
s
) with RWC), as
described by Kumar et al. (1984). Physiological indices such as leaf water
potential (Ψ
leaf
), solute potential (Ψ
s
), relative water content, turgor poten-
tial (Ψ
p
), osmotic adjustment, leaf diffusive conductance (Kl),
l
), difference
between canopy and air temperature (T
c
-Ta) and water loss from excised
leaves can be used as a screening tool. A study conducted by Kumar and
Singh (1998) on
Brassica
genotypes revealed that higher osmotic adjust-
ment extracted relatively more water from the deep soil layer (90-180 cm)
than genotypes with lower osmotic adjustment (ranging from 50 mm to
69 mm). High-osmotic adjustment genotypes maintained full turgor down
to a Ψ
leaf
of −2.4 MPa, but turgor potential (Ψ
p
) fell more rapidly with de-
creasing Ψ
leaf
in genotypes showing low osmotic adjustment. The decrease
in Ψ
leaf
with RWC was smaller in low than high-osmotic adjustment geno-
types of
Brassica
species. Osmotic adjustment was linearly, but negative-
ly, related to water loss from leaves and positively related to Kl
l
and T
c
-T
a
.
Plants with higher osmotic adjustment transpired more water (greater Kl)
l
)
and therefore, had cooler canopies (lower canopy temperature and greater
T
c
-T
a
difference) than the plants with lower osmotic adjustment (Kumar
and Singh, 1998). Ψ
s
in low-osmotic adjustment plants fell linearly and
more rapidly with decrease in Ψ
leaf
, whereas it was not related to Ψ
s
in
high osmotic adjustment genotypes (Kumar et al., 1984). The relationship
between Ψ
s
and RWC revealed that high-osmotic adjustment genotypes
maintained higher RWC as water deficits increased, with a greater de-
crease in Ψ
s
. Even at low water potential the leaves maintained greater
turgor and this may have contributed to the maintenance of higher Kl
l
and
photosynthetic activity. High-osmotic adjustment genotypes maintained
