Let us make an in-depth study of salt stress and salt resistance in higher land plants.
Under natural conditions, higher land plants growing near the seashore and estuaries encounter higher salt concentrations in the soil. Natural salt seepage from geological marine deposits makes adjoining inland areas unfit for agriculture. However, accumulation of salts in the soil from irrigation water is much more problematic in agriculture. Millions of acres of land have gone out of production on account of saline soils.
According to an estimate, about one third of irrigated land on Earth is affected by salt stress. Na+, CI–, HCO3– , K+, Ca2+, Mg2+ and SO42+ ions usually contribute to the soil salinity. Higher concentrations of sodium, chloride and carbonate ions are potentially toxic to salt sensitive plants. Presence of high salt concentrations in the soil is a common and important stress factor in deserts.
On the basis of their response to high concentration of salts in the soil, the plants are grouped into two categories:
(i) Halophytes and
(ii) Non-halophytes or glycophytes.
Halophytes are native to saline soils where they can grow satisfactorily, compete with other species in the same habitat and complete their life cycle. Non-halophytes or glycophytes are plants which cannot resist salts to the same degree as halophytes and exhibit signs of growth inhibition, leaf discoloration and loss of dry weight above a threshold concentration of salts in the soil. The halophytes which cap resist a wide range of salt concentrations are called as euryhaline and those with narrow range of resistance, are called as stenohaline.
Injuries to salt affected plants involve both osmotic effects and specific ions effects and are manifested in various ways. Higher concentration of salts in the cell sap may affect water relations and metabolism of plants. Salt accumulation appears to reduce activities of some enzymes leading to decreased metabolic rate.
Accumulation of chloride ions reduces water absorption and transpiration and gives rise to succulence. Direct injuries to plants may also be observed when salts affect structure and function of cytoplasm such as leaf edge burns, necrotic spots on the leaves and chlorophyll bleach.
The plants cope with salt stress or salinity in various ways:
(i) Some plants can avoid salinity
(ii) Some evade salinity and
(iii) Few others tolerate it.
This is usually accomplished by:
(i) Limiting germination,
(ii) Growth and reproduction to specific seasons during the year,
(iii) By growing roots into non-saline regions and
(iv) By limiting uptake of salts.
This can usually be achieved by accumulation of salts in specific cells of the plant or by secretion of excess salts from the plant. In some halophytes such as desert plant Tamarix pentandra, heavy incrustations of salts can be observed on the surface of the leaves as a result of excretion of excess salts. In salt bush Atriplex spongiosa, special salt glands (sometimes consisting of only 2 cells, a stalk and a terminal bladder) are found on the surface of the leaves (Fig. 23.2). The ions are transported to these glands where crystallization of salts occurs and they become un-harmful. Often, these glands fall from the leaf surface or burst, leaving rest of the leaf tissue with low salt content.
Ahmad (1968) observed accumulation of granular substances in some cells of leaves and pith of shoots of two halophytic species, Suaeda fruticosa and Haloxylon recurvum of West Pakistan. Spectrographical analysis showed these granular substances to be made up of Na, K, Ca, Mg and quartz. Thus, as in case of vesiculated hairs (salt glands) of Atriplex, crystallization of salts in certain loci of these plants also prevented them from harmful effects of salts.
This is attained only in those plants where protoplasm can endure high salt content without apparent damage and functions normally. Salt tolerance varies among different organs of the same plant, among tissues and among different stages of development of plant. Salt tolerance also varies with concentration and composition of salts. Among crop plants, maize, peas, beans, onion, citrus, lettuce etc. are sensitive to salts. Cotton and barley are moderately salt tolerant while sugar-beet and date-palms are highly salt tolerant species.
In many halophytes, osmotic potential of the cells is maintained by accumulation of ions in the vacuole and synthesis of compatible solutes (compatible osmotica) especially proline (Fig. 23.3). Some other amino acids such as glycinebetaine and sugar alcohol sorbitol also accumulate in the cytosol without damaging the salt sensitive enzymes.