Mechanism Of Alfalfa(Medicago Sativa L.) Seedlings Response To Spatially Heterogeneous Salinity

Soil salinity is spatially heterogeneous, but relatively little is known about non-halophyte physiology under non-uniform salinity with one part or both roots half exposed to salinity stress. In this study, two-part root system were used to address the following four contents:(1) alfalfa physiology response to non-uniform salinity; (2) relationship between agronomic or physiological traits and low salinity, high salinity, mean salinity, root-weighted mean salinity or water uptake-weighted mean salinity; (3) roots morphological plasticity to adapt to non-uniform salinity; (4) role of the low salinity root under non-uniform salinity in enhancing plant growth. The results are as follows:Compared with uniform high salinity, non-uniform salinity significantly increased alfalfa shoot dry mass and stem extension rate when low salinity root under 0 or 75 mM NaCl. but did not increased those when low salinity root under 150 mM NaCl. Increasing leaf water potential and compensatory water uptake by low-salt side root were found in non-uniform salinity which low salinity root under 0 or 75 mM NaCl, however, compensatory root growth in the low-salt side, increasing in whole plant water uptake, and decreasing in leaf Na+ concentration were only founded in one part root under salinity. Correlation analysis between stem extension rate and physiological characteristics, showed that the correlation coefficient were highest between stem extension rate and low-salt root water uptake.Correlation analysis between stem extension rate and physiological characteristics with lowest, highest, mean, root-weighted mean or water-uptake mean salinity in the root zone, respectively showed that physiological characteristics exception to net photosynthetic and leaf K+ concentration had significantly correlation to the five types salinity. The combined data showed that growth and gas exchange parameters responded most closely to the water-uptaked mean salinity. Root dry mass and predawn leaf water potential responded most closely to the root-weighted mean salinity, leaf Na+ concentration were decided by mean salinity in the root zone.Salinity patterns had no effect on root total root length and total surface area but not salinity concentration. Salinity patterns and salinity concentration and both interaction significantly effect the root length and root surface area in the non-saline part. Salinity decreased ratio of root length which diameter less than 0.3mm, however, non-uniform increased ratio of root length which diameter less than 0.3mm.When low salinity root under K+ deficit, drought stress, amiloride and sodium orthovanadate, the result showed that stem extension rate and water uptake in drought stress and sodium orthovanadate were similar to those in uniform salinity, Na+ concentration in low salinity root were increased by sodium orthovanadate treatment, However, stem extension rate, water uptake and leaf water potential of K+ deficit in low salinity root were similar to non-uniform treatment.In conclusion, non-uniform salinity enhance plant growth when low salt root under non- or mild salinity, with low-salt root salinity increasing plant growth was inhibited by high-salt root salinity. The mechanism of non-uniform salinity improve plant growth were different between low salinity root under 0 and 75 mM NaCl. Growth of alfalfa seedling is determined by the water uptake-weighted mean salinity of the root zone. Low salinity root were contributed to plant growth under non-uniform salinity for water uptake regulation and Na+ exclusion. Little contribute to plant improve growth under short time non-uniform salinity of the low salinity root for K+ uptake.