Abstract:

Among many kinds of stresses, drought and salinity are the most serious ones that limit plant growth and crop productivity in agriculture with their damage exceeding the sum of that attributed to all other natural disasters. Plant responses to drought and salinity have much in common: Water stress in its broadest sense encompasses both drought and salt stress; Salt stress occurs from both osmotic stress due to low water potentials and salt-specific effects. However, studies on the comparative physiology of plants to water and salt stress are few. Aloe is a typical xerophyte with important economic and social value, but is not a halophyte. Little information was available on its response to salt and its mechanisms of tolerance. In this paper, dry matter accumulation of seedlings, growth rates, water content, electrolytic leakage of leaves and ionic absorption and distribution of organ and tissue levels in six-leaf Aloe vera seedlings exposed to iso-osmotic conditions of -0.44 Mpa and -0.88 Mpa using NaCl, and compared to exposure to polyethylene glycol 6000 (PEG 6000) concentrations to understand the responses of plants to water and salt stress. The results showed that leaf elongation was inhibited, plant dry matter accumulation rates decreased significantly, water content in leaves decreased, and leaf electrolytic leakage increased 10 days after treatments using NaCl and PEG. Growth inhibition of salt-treated Aloe seedlings greatly exceeded that of PEG-treated seedlings. Ion content analysis in different organs and X-ray microanalysis of root and leaf cross sections indicated that Na+、Cl－ content increased significantly and K+、Ca2+ absorption in the roots and transport to leaves were inhibited in seedlings exposed to NaCl stress. Maintenance of better ion homeostasis under the PEG treatment was a primary reason for the greater adaptation to water stress than to iso-osmotic salt stress in Aloe. However, Aloe seedlings also showed some specific adaptations to -0.44--0.88 NaCl stress in Aloe seedlings: 1) Ionic selective absorption and transport were high in Aloe roots under salt stress and increased with increasing salt stress; 2) Salt accumulation in aqueous tissues was significantly higher than that in the other tissues of Aloe leaves. Also, as a CAM (Crassulacean acid metabolism) plant, the transpiration rate in Aloe seedlings was very low and the rate of salt accumulation in the shoots was also slow.