Comparison Of NaCl And PEG Stresses To Different Genotypes Of Maize Seedlings

Osmotic stress exists under drought or salinity circumstances , which caused reduction of plant growth. This thesis aimed to compare osmotic stress arising from the two different circumstances, for so far it is still not clear whether they has differences on physiological metabolism. Two genotypes of maize seedlings, Denghai 11 (salt-resistant) and Suyu 16 (salt-sensitive) were treated with isotonic solutions of NaCl and PEG (Polyethylene glycol) at five osmotic potential levels, from -0.3 MPa to -0.8 MPa, for 72 hours. Plant growth, some physiological aspects and plant responses to drought and salinity stress were studied. The results were summarized as follows:(1) Comparison of NaCl and PEG stresses on Denghai (salt-resistant maize) seedlings.NaCl and PEG treatment decreased biomass, plant height, leaf relative water content, chlorophyll content and K+ content in shoots and roots, and increased electrolyte leakage rate, contents of free proline and soluble sugar in leaves. Moreover, the changes were aggravated by increased concentrations of NaCl and PEG. Biomass, plant height and leave relative water content with NaCl treatments were higher than those with isotonic PEG treatments. Na+ accumulation in shoots increased with the increasing PEG concentration, while that in roots decreased. With the decreased osmotic potential caused by NaCl and PEG, chlorophyll content in leaves and root activity first increased and then decreased. When osmotic potentials in culture solution were -0.5Mpa and -0.6Mpa, root vitality was higher under NaCl stress than under isotonic PEG stress, while when osmotic potentials were -0.7Mpa and -0.8Mpa, it was exactly contrary. Chlorophyll content in leaves seemed lower under PEG stress than under isotonic NaCl stress, but the difference between them did not reach to significance. Free proline and soluble sugar, as important organic osmolytes present higher contents in leaves under PEG stress than under isotonic NaCl stress. Osmotic stress arising from PEG caused greater influence on root growth than osmotic stress arising from NaCl.(2) Comparison of NaCl and PEG stresses on Suyu (salt-sensitive maize) seedlings.leaf relative water content and plant height decreased with the decreased osmotic potential, but the difference between two osmotic stresses respectively from NaCl and PEG was not significant. Biomass affected by iso-osmotic NaCl and PEG and the amplitude of variation fluctuated, Fresh weight and dry weight of shoots were higher under PEG stress than under isotonic NaCl stress with osmotic potential -0.5Mpa and -0.6Mpa, while lower with osmotic potential -0.7Mpa and -0.8Mpa. Fresh weight and dry weight of roots were higher under NaCl stress than under isotonic PEG stress. Chlorophyll content in leaves first decreased and then increased under NaCl stress with decreasing osmotic potentials, while presented a completely opposite tendency under isotonic PEG stress. The cell membrane permeability of leaf increased when the decreasing osmotic potentials, and higher under PEG stress than under isotonic NaCl stress. More free proline and soluble sugar accumulated in leaves with the decreased osmotic potentials. The content of soluble sugar was higher under PEG stress than under isotonic NaCl stress, but there was no significant discrepancy between them. Root growth were affected by NaCl and PEG stress. The effects of NaCl stress on length, surface area and volume of roots were higher than that of PEG stress, while both had little effect on root average diameter. Root vitality increased when osmotic potential changed from -0.3Mpa (control) to -0.5Mpa, though the increase for PEG treatment was little, and then decreased when osmotic potential changed from -0.5Mpa to -0.8Mpa Moreover,root vitality was higher under NaCl stress than under PEG stress. Except for osmotic potential -0.5Mpa, K~+ content in leaves was lower than control. K accumulation decreased with the decreasing osmotic potential and higher under NaCl stress than under isotonic PEG stress.Above results indicated that not only under the anisotonic conditions but also under the isotonic conditions, there were evident differences in the physiological responses of experimental maize varieties to drought stress simulated by PEG and salinity stress given with NaCl. Salt-resistant maize (Denghai 11) was more affected by low osmotic potential arising from PEG than by that arising from NaCl, but salt-sensitive maize (Suyu 16) presented a contrary tendency. Therefore, Physiological effects caused by low osmotic potential levels under salinity stress differed from that under drought stress.