M. A. Shahid, Dr. M. A. Pervez, Dr. Rashad M. Balal*, Tahira Abbas, Dr. M. Afzal*
Institute of Horticultural Sciences, University of Agriculture, Faisalabad (Punjab) Pakistan
University College of Agriculture, University of Sargodha (Punjab) Pakistan
Abiotic stresses reduce crop productivity and play a major role in determining the distribution of plant species. Plants experience several distinct abiotic stresses either concurrently or at different stages of plant growth and development. Some common abiotic stresses include water, heat, salt and light stress which ultimately affect the plant growth and yield. Among these stresses, salinity is the most deterimental which limits the plant growth and productivity. To cope with these stresses plants modify themselves by different mechanisms including change in morphological, physiological and biochemical processes.
Salinity is the build up of soluble salts by which saline soils are formed. High salinity causes both hyperionic and hyperosmotic stress and can lead to plant death. Salinity may be due to many reasons but some of the adverse effects of salinity have been attributed to increase in chloride and sodium ions in different plants hence these ions produce the critical conditions for plant survival by intercepting different plant mechanisms. Salinity in a given land area depends upon various factors like amount of evaporation which leads to increase in salt concentration and amount of rain that leads to decrease in salts. The general effect of salinity is decrease in the growth and development resulting in smaller leaves, shorter stature and sometimes fewer leaves. The primary effect of salinity especially at high salt concentrations is due to osmotic variations. The osmotic effects of salinity lead to slow growth rate and developmental characteristics such as root/shoot ratio and maturity rate. It is reported that plants growing under saline conditions are affected in three ways: reduced water potential in root zone causing water deficit, phytotoxicity of ions such as Na+and Cl–and nutrient imbalance depressing uptake and transport of nutrients. Na+competes with K+for binding sites essential for cellular functions. Earth is a salty planet because its water is enriched with 30 g of sodium chloride per liter, so this salt solution affects the land as well as crops. Salt stress causes altered K+/Na+ratios and Na+and Cl–ion concentrations that are harmful to plants. Sodium and chloride which are the major ions produce many physiological disorders in plants but chloride is the most dangerous because NaCl liberates almost 60% more ions into the soil solution than Na2SO4. Excess of these salts also enhances the osmotic potential of soil matrix as a result of which water intake by plants is restricted. Due to the accumulation of Cl–, relative salt tolerance has been linked to plant growth water use efficiency and transpiration. Inspite of upper plant parts, salinity also effects roots growth and physiology and ultimately their function of nutrient uptake.
Salinity is a major constraint to crop production which is often observed under natural environmental conditions. Increased salinization of arable land is expected to have devasting global effects, resulting in 30% land loss within next 25 years and upto 50% by the middle of 21thcentury. It is a problem of great importance, because many agricultural areas previously fertile became saline due to irrigation with unfit water. Salinization of underground water resource is another major problem affecting the agricultural productivity. It is very important to sustain the soil fertility and quality of water resources to fulfill the food, feed and fiber demand of ever-growing population of the world. Salinity limits the production of nearly over 6% of the world’s land and 20% of the irrigated land (15% of total cultivated areas) and negatively impacts agricultural yield throughout the world. In Pakistan, a loss of about 20 billion rupees (350.88 million US$) annually has been estimated from the salt-affected irrigated areas of the Indus Basin on account of decrease in crop yield. Salinization of good arable land in Pakistan is creating a problem with immense socio-economic losses. The loss of excellent natural resource is another problem, because the population depends for its livelihood on these lands, which are gradually dwindling through the spread of salinity.
Salt stress causes multifarious drastic effects in plants and among these factors production of reactive oxygen species (ROS) is a common phenomenon. These ROS are highly reactive because they can interact with a number of cellular molecules and metabolites and ultimately leads to cellular damage. In saline environment, plant growth is affected by complex interaction of hormones, osmotic effects, specific ion effects and nutritional imbalances, probably all occur simultaneously. In pea, it affects the leaf growth, photosynthesis, mineral nutrition, stomatal conductance, transpiration, water and ion transport and increases sugars, amino acids and different ions along with acute effects on yield and quality. Salinity induced symptoms such as nonspecific chlorosis, stunted leaf size and impaired shoot growth. They however, under salt stress adopt different mechanisms to adjust the osmotic and ionic stresses caused by salinity. These mechanisms include osmotic adjustment by accumulation of compatible solutes such as Proline, Glycine-betaine and Polyols. Proline accumulation is known as the adaptation of salinity and Glycine-betaine is one of the most abundant quaternary ammonium compounds produced in higher plants due to salinity. Plants have various mechanisms to reduce the drastic effects of salt and these mechanisms take place at three levels of organization i-e. whole plant, cellular and molecular level. Various scientists have described the physiological mechanisms at whole plant level. Salt tolerance of plant depends upon its ability to check the movement of salt at five sites i-e. selectivity of uptake by root cells, loading of xylem, removal of salt from xylem in the upper part of the roots, stem and petiole, loading of the phloem and excretion through salt glands. All halophytes have well developed mechanisms to monitor the uptake, transport and excretion of salt. Glycophytes use first three mechanisms. As pea is not a halophyte so the above mechanisms are not well established in it. There are many other features that work to maintain low rates of salt accumulation in leaves. High shoot/root ratios and high intrinsic growth rates and absence of apoplastic pathway in roots all serve to reduce the rate at which salt enters the transpiration stream and deposit in the shoot. As these features are not efficient in salt sensitive plants like pea so there is a need to enhance their salt tolerance by exogenous application of beneficial compounds and growth regulators. These compounds comprise of proline, glycinebetaine, salicylic acid, brassinosteroids, silicates etc. These chemicals play a vital role in osmotic adjustment under saline condition and create the hindrance in the way of ion toxicity .
