EFFECTS OF ISO-OSMOTIC SALT AND WATER STRESSES ON GROWTH AND IONIC DISTRIBUTION IN ALOE SEEDLINGS

Abstract

Among many kinds of stresses, drought and salinity are the most se rious ones that limit plant growth and crop productivity in agriculture with the ir damage exceeding the sum of that attributed to all other natural disasters. P lant responses to drought and salinity have much in common: Water stress in its broadest sense encompasses both drought and salt stress; Salt stress occu rs from both osmotic stress due to low water potentials and salt-specific effect s. However, studies on the comparative physiology of plants to water and salt st ress are few. Aloe is a typical xerophyte with important economic and social val ue, but is not a halophyte. Little information was available on its response to s alt and its mechanisms of tolerance. In this paper, dry matter accumulation of s eedlings, 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 Na Cl, a nd 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 accum ulation rates decreased significantly, water content in leaves decreased, and le af electrolytic leakage increased 10 days after treatments using NaCl and PEG. G rowth inhibition of salt-treated Aloe seedlings greatly exceeded that of PEG -treated seedlings. Ion content analysis in different organs and X-ray microanal ysis of root and leaf cross sections indicated that Na +、Cl - content increa sed s ignificantly and K +、Ca 2+ absorption in the roots and transport to leave s were i nhibited in seedlings exposed to NaCl stress. Maintenance of better ion homeosta sis under the PEG treatment was a primary reason for the greater adaptation to w ater stress than to iso-osmotic salt stress in Aloe. However, Aloe seedl ings als o showed some specific adaptations to -0.44--0.88 NaCl stress in Aloe seed ling s: 1) Ionic selective absorption and transport were high in Aloe roots under salt stress and increased with increasing salt stress; 2) Salt accumulation i n aqueous tissues was significantly higher than that in the other tissues of A loe lea ves. 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 sho ots was also slow.