Dr. Hideyasu Fujiyama is working as a Professor and Faculty of Agriculture at Tottori University, Japan. His major research interest is Chemical analysis of the interaction between the environment and bioresources.
Sodium (Na) is one of the most harmful elements to plants. The problem of Na accumulation in soil is serious in arid land agriculture. A high concentration of Na in the medium decreases plant growth by three reasons. First is that all Na salts are water-soluble and decrease osmotic potential of soil solution and inhibit water absorption by plants. Second is that Na competitively inhibit the absorption of macronutrients such as potassium (K), calcium (Ca) and magnesium (Mg) by plants and brings their deficiencies. Third is that Na increases soil pH and inhibits the absorption of micronutrients such as iron (Fe) and manganese (Mn) and brings their deficiencies. The latter two disturb nutritional homeostasis in plants. However, there are plant species that need Na for satisfactory growth. Most of them belong to Chenopodiaceae. We call them “Na loving plants”. Their response to Na is completely different from that of highly Na tolerant plants. One hundred % relative growth in most plant species is obtained at Na free condition even if they are highly Na tolerant. On the other hand, the growth of Na loving plant species increases with increasing Na concentration in the medium and reach maximum growth and decreases with increasing Na concentration. There is a difference in Na concentration in the medium at maximum growth between Na loving plant species. Salicornia bigelovii whose optimal Na concentration is 200 mol kg-1 (2/5 that of sea water) cannot survive under Na free condition. We found in Na loving Swiss chard that Na enhanced absorption and transport of nitrate (NO3-) and that Na increased NO3- reductase (NR) activity. Moreover, we found that Na had a role in stomatal opening in Swiss chard. In most plant species, K plays a role in these functions. Therefore, we propose hypotheses that Na plays a role of K in Na loving plant species and that the difference in Na dependence between them is due to the ability of them to substitute Na for K.