Question

Explain mechanism of tolerance to ion stress.

Answer 1

I. Avoidance: Avoidance is the process of keeping the salt ions away from the parts of the plant where they are harmful, by-

• Salt exclusion: The ability to exclude salts occurs through filtration at the surface of the root. Root membranes prevent salt from entering while allowing the water to pass through. The red mangrove is an example of a salt-excluding species.

• Salt excretion/extrusion: Salt excreters remove salt through glands or bladders or cuticle located on each leaf.

 Salt bladders - eg) Atriplex , Mesembryanthemum crystallinum L.

 Salt glands - active process, selective for sodium and chloride. eg. Black and white mangroves. Salt glands are dump sites for the excess salt absorbed in water from the soil; help plants adapt to life in saline environments.

 Secretion through cuticle – eg. Tamarix

• Salt Dilution: By dilution of ions in the tissue of the plant by maintaining succulence. Plants achieve this by increasing their storage volume by developing thick, fleshy, succulent structures Succulence is mainly a result of vacuoles of mesophyll cells filling with water and increasing in size. This mechanism is limited by the dilution capacity of plant tissues

• Compartmentation of ions:

 Organ level - high salts only in roots compared to shoots especially leaves.

 Cellular level- high salts in vacuoles than cytoplasm thus protecting enzymes.

II. Tolerance

• Osmotic adjustment- a biochemical mechanism that helps plants acclimates to dry and saline conditions.

• Hormone synthesis - ABA stress hormone hardens plants against excess salts.