Geoscience Reference
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in source-sink relationships resulting in a larger proportion of
photosynthates partitioned to the roots.
Role of lipids in
drought stress
conditions
Along with proteins, lipids are the most abundant compo-
nent of membranes and they play a role in the resistance of
plant cells to environmental stresses (Kuiper 1980; Suss and
Yordanov 1986). Strong water deficit leads to a disturbance of
the association between membrane lipids and proteins as well
as to a decrease in the enzyme activity and transport capac-
ity of the bilayer (Caldwell and Whitman 1987). Poulson et al.
(2002) established that for
Arabidopsis
, polyunsaturated tri-
enoic fatty acids may be an important determinant of responses
of photosynthesis and stomatal conductance to environmental
stresses such as vapour pressure deficit. When
Vigna unguicu-
lata
plants were submitted to drought the enzymatic degrada-
tion of galacto- and phospholipids increased. The stimulation
of lipolytic activities was greater in the drought-sensitive than
in drought-tolerant cultivars (cvs) (Sahsah et al. 1998).
Drought stress provoked considerable changes in lipid
metabolism in rape (
Brassica napus
) plants (Benhassaine-Kesri
et al. 2002). The decline in leaf polar lipid was mainly due to
a decrease in MGDG (monogalactosyldiacylglycerol) content.
Determination of molecular species in phosphatidylcholine
and MGDG indicated that the prokaryotic molecular species of
MGDG (C18/C16) decreased after DS while eukaryotic molec-
ular species (C18/C18) remain stable. It was suggested that the
prokaryotic pathway leading to MGDG synthesis was strongly
affected by DS while the eukaryotic pathway was not. Strong
WD results in a profound overall drop in MGDG, the major leaf
glycolipid. In drought-sensitive seedlings of
Lotus corniculatus
the ratio of MGDG/DGDG declined threefold, while the rela-
tive part of MGDG was 12-fold lower.
Mechanisms of
acclimation to
water deficit
and stress
tolerance
Many plant systems can survive dehydration, but to a different
extent. According to Hoekstra et al. (2001) on the basis of the
critical water level, two types of tolerance are distinguished:
drought tolerance can be considered as the tolerance of moder-
ate dehydration, down to moisture content, below which there
is no bulk cytoplasmic water present—about 0.3 g H
2
O g
−1
DW.
Desiccation tolerance refers to the tolerance of further dehy-
dration, when the hydration shell of the molecules is gradually
lost. Desiccation tolerance also includes the ability of cells to
rehydrate successfully.
Major alterations in patterns of gene expression are known to
occur at the early stages of stresses. Some of these changes are
