Effect of NaCl, Na2SO4, and mannitol on utilization of storage starch and formation of plastids in the cotyledons and roots of alfalfa seedlings

Abstract

Utilization of storage starch in the cells of cotyledon mesophyll and root meristem in the course of alfalfa (Medicago sativa L.) seed germination on the solutions of NaCl, Na2SO4, and mannitol at different concentrations and identical osmotic pressure was investigated using the method of transmission electron microscopy. Ultrastructural analysis showed changes in the number of starch grains and deceleration of chloroplast development depending on the osmotic component of salt influence. At low concentrations corresponding to osmotic pressure of 202.6 kPa, Na2SO4 did not affect the formation of the photosynthetic machinery and utilization of starch inclusions; mannitol contributed to the preservation of considerable reserve of starch without disturbing the development of chloroplasts; NaCl did not inhibit the development of the photosynthetic machinery and induced an increase in the number of starch grains presumably at the expense of newly produced starch. When the concentration of the investigated substances increased up to the values corresponding to the osmotic pressure of 607.8 kPa, NaCl did not suppress transformation of amyloplasts into chloroplasts and utilization of starch; Na2SO4 inhibited the development of chloroplasts and starch utilization; mannitol decelerated transformation of amyloplasts and inhibited mobilization of starch grains. The obtained results make it possible to propose a method of preliminary estimation of tolerance of dicotyledons to abiotic stresses based on the cytological analysis of utilization of starch grains and formation of photosynthetic compartments of chloroplasts in the mesophyll of cotyledons.