Salt induced changes in leaf phenology of wheat plants are regulated by accumulation and distribution pattern on Na+ ion

Salinity is one of the major factors reducing agricultural productivity and natural resources worldwide. In Pakistan about 40 per cent of the irrigated cropped land, which produces around 90% of the total agricultural output of the country, has come under salinity. Field losses in yield of wheat cultivated in moderately salt affected areas of Pakistan are estimated about 64%. This study assesses changes in bread wheat (Triticum aestivum L.) genotypes that are likely to affect leaf growth and phenology under salt stress, specifically focusing on accumulation and distribution of sodium (Na+), potassium (K+) and Chloride (Cl-) ions. Four bread wheat genotypes were grown in nutrient solution at two treatment levels (control and 150 mM NaCl). The bread wheat genotypes responded differently at higher salt stress especially at the 2nd phase of plant growth. Salt stress not only reduced leaf elongation and leaf length but also caused reduction in plastochron index, rate of leaf formation and its development. Accumulation of Na+ and Cl- ions increased during periods of intense leaf growth, while proline accumulation was quite low in the salt sensitive genotype. In contrast, tolerant genotypes had lower Na+ and Cl- concentration in leaves along with higher proline contents throughout salt stress. An increase in proline content was also observed in the older and more injured leaves which occurred at later stages of leaf growth. No direct relationship was found between leaf injury and inorganic ions (Na+ and Cl-) concentrations. However, intensity of injury was related to the duration of salt stress as the size of injured zones on leaves increased while inhibition of leaf elongation and toxic ions content decreased with leaf age. This suggests that salt tolerance could also be associated with plant's capacity to regulate the rate of ion transport or accumulation in leaf tissues and it is concluded that whole-plant response system should not be overlooked when breeding wheat for salt tolerance to improve salt tolerance in plants.