Types and Drought Resistance Mechanism in plants

The ability of a crop species or variety to grow and yield satisfactorily in areas subjected to periodic water deficits is termed as drought resistance,

The ability of a plant to complete the life cycle before serious soil and plant water deficits develop.

The ability of the plant to endure periods of drought whilst maintaining a high plant water stress. This is also referred to as drought avoidance (Levitt, 1972).

The ability of the plant to endure periods without significant rainfall and to endure low tissue water potential.

Table – Mechanisms of drought resistance (after Jones et al. 1981)

1. Drought escape

(a) Rapid phenological development

(b) Developmental plasticity

2. Drought tolerance with low tissue water potential

(a) Maintenance of turgor

(i) Solute accumulation

(ii) Increase in elasticity

(b) Desiccation tolerance

(i) Protoplasmic resistance

3. Drought tolerance with high tissue water potential

(a) Maintenance of water uptake

(i) Increased rooting

(ii) Increased hydraulic conductance

(b) Reduction of water loss

(i) Reduction in epidermal conductance

(ii) Reduction in absorbed radiation

(iii) Reduction in evaporative surface.

Two features of desert ephemerals that are important in drought resistance are-

I)                    Rapid phonological development.

II)                  Developmental plasticity.

Ability to produce flowers with a minimum of vegetative structure enables them to produce seeds on a limited water supply.

This feature enable the plants to produce an abundance of vegetative growth, flowers and seeds in seasons of abundant rain, enables the desert ephemerals to both escape drought and survive long periods without rain.

In crop plants, the greatest advance in breeding for water limited environment is achieved by a shortening of life cycle, thereby allowing the crops to escape drought. Therefore, there is a strong consistent negative correlation between grain yield and days to first ear emergence and 40-90% variation in wheat yield under drought condition was accounted for by earliness. In wheat it was observed that drought resistance is greater in early lines than late ones even at the same intensity of drought. However, under adequate water supply, yield is often positively correlated with maturity date in determinate annual crops such as maize, sorghum and sunflower.

An important aspect of developmental plasticity is the ability of plants to transfer assimilates accumulated prior to seed-filling to the grain during the seed filling stage. It was also suggested that when water supply is adequate only a small proportion of grain dry weight comes from the store of prior assimilate in the stems and roots, but when stress occurs in the seed filling stage, an increased proportion of the prior assimilate is transferred to the seed.

To achieve the developmental plasticity,-plants frequently have an indeterminate habit. This is an important survival mechanism in that it enables the large amounts of seed produced in wet years to carry the species through prolonged drought periods.

Selection of rapid phonological development is the most rewarding approach in breeding for thought resistance in crops. In cereals, drought resistance varieties of wheat and barley flowered early than the others. However earliness is often negatively correlated with yield in year of adequate rainfall.

Ability of the plant to endure periods of drought by maintaining high tissue water potential. This mechanism is also called as drought avoidance.

To maintain a high water status during a period of high evaporative demand / or increasing soil water deficit, the plant has two options. It must either reduce the water loss or maintain its supply of water.

i)                    Increased pubescence and

ii)                   Increased leaf waxiness

i) Deeper root system.

ii) Hydraulic conductance of plants (increasing either the diameter Of-xylem vessels or their numbers).

It is the ability of the plant to endure periods of drought and endure low tissue water potentials.