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PLANT STRESSES: SALT
INTRODUCTION
• When irrigation water contains a high concentration of solutes and when there is no opportunity to flush out accumulated salts to a drainage system, salts can quickly reach levels that are injurious to salt-sensitive species.
• It is estimated that about one-third of the irrigated land on Earth is affected by salt.
CLASSIFICATION OF PLANTS
• Halophytes are native to saline soils and complete their life cycles in that environment
• Glycophytes (literally “sweet plants”), or nonhalophytes, are not able to resist salts to the same degree as halophytes.
CLASSIFICATION OF PLANTS
• Among crops,
• maize, onion, citrus and bean are highly
sensitive to salt;
• cotton and barley are moderately tolerant;
• sugar beet and date palms are highly tolerant .
Effects
• Dissolved solutes in the rooting zone generate
a low (more negative) osmotic potential that
lowers the soil water potential.
• The general water balance of plants is thus
affected
• most plants can adjust osmotically when
growing in saline soils.
• Such adjustment prevents loss of turgor
(which would slow cell growth)
Specific ion toxicity effects• Injurious concentrations of ions—particularly
Na+, Cl–, or SO42–— accumulate in cells.
• An abnormally high ratio of Na+ to K+ and
high concentrations of total salts inactivate
enzymes and inhibit protein synthesis.
• Photosynthesis is inhibited when high concentrations of Na+ and/or Cl– accumulate in chloroplasts.
• photophosphorylation may be affected.
• Plants minimize salt injury by excluding salt from meristems, particularly in the shoot, and from leaves that are actively expanding and photosynthesizing.
• In plants that are salt sensitive, resistance to moderate levels of salinity
• in the soil depends in part on the ability of the roots to prevent potentially harmful ions from reaching the shoots.
• Sodium ions enter roots passively (by moving
down an electrochemical-potential gradient,
so root cells must use energy to extrude Na+
actively back to the outside solution.
• have salt glands at the surface of the leaves.
• The ions are transported to these glands,
where the salt crystallizes and is no longer
harmful
• Salt effects:
– Plant Growth
– Production
– Survival
Salts:• - Humid areas – NaCl dominant• - Dry areas: sulfates – CaSO4
• Na2SO4
• MgSO4
• - alkaline (pH = 8 – 10)
• Ion toxicity – disruption of enzyme activity at high salinities
• Ion imbalance: High Cl- concentrations - NO3- uptake inhibited
• High Na+ replace Ca2+ in root cell membranes – loss of K+ from roots
• Mechanisms to deal with salinity:• - anatomical, morphological, physiological and
biochemical • - adaptations - two categories:• - exclusion• - inclusion
Exclusion• Trait of most glycophytes – maintain low salt
levels in roots:• - filtration – salts filtered out at plasmalemma
of root parenchyma cells• - high levels of phospatidyl choline in
plasmalemma restrict Cl- uptake – • excretion – high energy cost – decreased
growth
Inclusion• Once salt enters root – mechanisms to resist
or tolerate salinity – missing in glycophytes:• Transport prevention – salts kept in roots –
stopped from entering xylem• active reabsorption of salts from xylem to
older root cells• high energy (ATP) cost