Comparative Study of Alkaline, Saline, and Mixed Saline-Alkaline Stresses with Regard to Their Effects on Growth, Nutrient Accumulation, and Root Morphology of Lotus tenuis

Both saline and alkaline conditions frequently coexist in nature; however, little is known about the effects of alkaline and salt-alkaline stresses on plants. We performed pot experiments with four treatments, control without salt addition and three stress conditions-neutral, alkaline, and mixed salt-alkaline-to determine their effects on growth, nutrient accumulation and root architecture in the glycophytic species Lotus tenuis. Neutral and alkaline salts produced a similar detrimental effect on L. tenuis growth, whereas the effect of their combination was synergistic. Neutral salt addition, alone or mixed with NaHCO3, led to significant leaf Na+ build up and reduced K+ concentration. In contrast, in plants treated with NaHCO3 only, Na+ levels and the Na+/K+ ratio remained relatively unchanged. Proline accumulation was not affected by the high pH in the absence of NaCl, but it was raised by the neutral salt and mixed treatments. The total root length was reduced by the addition of NaCl alone, whereas it was not affected by alkalinity, regardless of the presence of NaCl. The topological trend showed that alkalinity alone or mixed with NaCl turned the root more herringbone compared with control roots, whereas no significant change in this index was observed in the treatment with the neutral salt only. The pattern of morphological changes in L. tenuis root architecture after the alkaline treatment (in the absence of NaCl) was similar to that found in the mixed salt-alkaline treatment and different from that observed in neutral salt. A unique root morphological response to the mixed salt-alkaline stress was the reduction in the ratio between xylem vessels and root cross-sectional areas.