salt stress in higher plants

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