An experiment was carried out at Bangladesh Agricultural University Farm to find out the time of nodule initiation, nodulation pattern and their size distribution. Five mungbean varieties viz. Barimung-2, Barimung-4, Barimung-5, Binamung-2 and Barisal local, and rhizobial inoculum (
Bradyrhizobium strain BAUR-604) were used in this experiment. The unit plot size was 5 m × 4 m. The experiment was designed in a randomized complete block design having four replications of each treatment. Each variety was tested with and without inoculation. For recording nodule initiation date, five randomly selected plants from each plot were uprooted at alternating days starting from 7 days after sowing (DAS). For recording nodule size, all the nodules were collected from roots. Nodules were grouped by grading them on the basis of their size in diameter (<1, 1-2, 2-3, 3-4 and >4 mm sizes). The nodules were first visible at 9 DAS, when only a few small nodules were observed. The number of nodules increased progressively through the increasing growth period and reached their peak at 42 DAS (i.e., at 50% flowering stage). The number of nodules (1-2, 2-3 mm) started to decline after 42 DAS sharply and in case of <1 mm size nodules and the declining was noticeable after 21 DAS, while the bigger nodules (3-4 and >4 mm) increased up to 56 DAS. No effective nodule of any size was found at 77 DAS. The results suggest that nodule initiation in the roots of mungbean varieties start at 9 days after sowing seeds (DAS), reached their peak at 42 DAS and thereafter started reducing in numbers until 70 DAS due to spontaneous degeneration. A higher number of nodules in different sizes were observed in Barimung-2 at different DAS. At 14 DAS, smaller size nodules were higher than the bigger nodules in all the varieties. At 42 DAS, the highest number of nodules 0.3 (<1 mm), 7.4 (1-2 mm), 4.4 (2-3 mm), 2.7 (3-4 mm) and 0.3 (>4 mm) per plant were found in Barimung-2, while the other varieties produced the nodules in the range of 0.2-0.3 (<1 mm), 5.6-7.0 (1-2 mm), 3.3-4.2 (2-3 mm), 2.0-2.5 (3-4 mm) and 0.2-0.3 (>4 mm) per plant. This indicates that Barimung-2 always produces larger size nodules at different sampling dates. Smallest size (<1 mm) nodules increased up to 21 DAS, whereas medium (1-2 and 2-3 mm) size nodules continued to increase up to 42 DAS and the larger (3-4 mm) to the largest (>4 mm) up to 56 DAS and then decreased. At the same DAS,
Bradyrhizobium inoculation produced 0.3 (<1 mm), 7.8 (1-2 mm), 4.7 (2-3 mm), 2.8 (3-4 mm) and 0.3 (>4 mm) nodules per plant, while uninoculated plants produced 0.2 (<1 mm), 5.4 (1-2 mm), 3.2 (2-3 mm), 1.9 (3-4 mm) and 0.2 (>4 mm) nodules per plant. Inoculated Barimung-2 always produced a greater number and size of nodules at different sampling dates for uninoculated plants.
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