Inoculation of Glomus intraradices BEG141 to increase transfer and redistribution of nitrogen to tuber of sweet potato

Objectives Effects of inoculation with arbuscular mycorrhiza (AM) fungi on nitrogen transfer and allocation in sweet potato (Ipomoea batatas L.) were studied in this experiment, to understand the rules controlling the process of nitrogen metabolism promoted by AM fungi.

Methods A pot experiment was conducted using sweet potato as test material. AM strain of Glomus intraradices BEG141 was prepared and inoculated into soil in concentration of 100 g strain in 8 kg of soil (+AM), taking no inoculating AM stain as control (–AM). After 30, 60 and 90 days of transplanting of seedlings, 20 μL of 99% (¹⁵NH₄)₂SO₄ solution were injected into the xylem below the top fifth open leaf, with a total ¹⁵N injection of 199.5 μg/plant. Sweet potato samples were collected three days after each ¹⁵N injection, and divided into vine, leaf, fibrogen and tuber, the dry biomass, root colonization rate, ¹⁵N abundance and N metabolism related enzyme activities were determined.

Results The +AM treatment significantly increased the root colonization rate and the abundance of vesicle, hyphae and arbuscular of sweet potato. With the growth of sweet potato, the infection rates were increased significantly and reached up to 67%. The biomass and nitrogen uptake of sweet potato between +AM and –AM treatments were not significantly different at the 30 days, but became significant at the 60 days and 90 days. At the same growth days, the +AM treatment significantly increased the activities of GDH, GS and GOGAT in sweet potato leaves. Two-factor analysis showed that AM and growth days had significant positive interaction effects on increasing biomass dry weight, nitrogen accumulation and activities of GDH and GS (P < 0.05). At the 30 day and 60 days period, +AM treatment promoted the ¹⁵N accumulation and allocation rates in vines and leaves of sweet potato, which were significantly higher than those in –AM treatment. At the 90 days after transplanting, however, the accumulation of ¹⁵N in vines and leaves was significantly decreased compared to –AM, and the transfer and allocation of ¹⁵N to tuber roots were increased significantly by +AM.

Conclusions Inoculation of AM fungi stimulates the activities of GDH, GOGAT and GS, which promotes the transformation of inorganic nitrogen to organic nitrogen. Infection of AM fungi improves the regulation of absorbed N. The N will be allocated into vegetative organs for the plant growth in early stage, and stimulate its transfer to underground, thus increase the accumulation of dry matter in storage organ.