Influence of selenium (Se) on carbohydrate metabolism, nodulation and growth in alfalfa (Medicago sativa L.)

Selenium (Se) is an essential micronutrient for humans and animals, but its role in plants remains unclear. Selenium enters the food chain via crops, and thus, plants constitute an essential source of Se in human nutrition. As a N-2-fixing plant of high nutritive value, alfalfa (Medicago sativa L.) is an important forage legume for sustainable agriculture. This study investigated the effects of Se on carbohydrate metabolism, nodulation and growth in alfalfa. In addition, the impact of Se on fructose 1,6-bisphosphatase (F1,6-BPase), a key enzyme in carbohydrate metabolism, as well as on nitrogenase activity in N-metabolism was examined. Alfalfa was grown either in perlite or nutrient solution at different Se (0, 1, 5, 10 and 15 mu mol L-1 Na2SeO4) and N (2 and 10 mmol L-1) concentrations. Plants in perlite were inoculated with Sinorhizobium meliloti and used for studies on nodulation, growth and nitrogenase activity. Plants grown in nutrient solution were used for studies on carbohydrate metabolism. Selenium applications (5 and 15 mu mol L-1) increased soluble sugars (SS) in the leaves, on average, by 44 % in both adequate-N and low-N-plants respectively. At the low-N level, Se (10 and 15 mu mol L-1) increased SS in the stems and roots, on average, by 29 % and 45 % respectively. In adequate-N-plants, Se increased SS in the stems, on average, by 46 % but had no effect in the roots. Selenium (10 and 15 mu mol L-1) enhanced starch accumulation in the leaves about 55 % in low-N-plants. At the adequate-N level, Se (15 mu mol L-1) increased starch accumulation about 36 %. However, the starch concentrations in the roots were inconsistent. Selenium also increased F1,6-BPase activity in the upper leaflets. In addition, in low-N-plants, the low Se (1 mu mol L-1 and 5 mu mol L-1) applications increased nodule number (NN) about 40 % and 62 % respectively, but NN decreased with plant growth. In symbiotic plants, Se did not significantly affect nodule fresh weight (NFW), nitrogenase activity and N concentrations. Selenium also had a slightly negative effect on dry matter accumulation in shoots and roots of alfalfa. The results indicate that, Se up-regulates carbohydrate metabolism via altered redox potential which may have some stimulatory effects on nodulation. These effects were, however, dependent on the Se concentration and the developmental stage of the plant. More detailed studies are needed to fully understand the role of Se in N-2 fixation.