Nitrogen Effect On Tuberization For Yield Production In Sweet Potato

The major limiting factor for increasing in storage root production of sweet potato(Ipomoea batatas(L.) Lam.; Convolvulaceae) is that the imbalance of matter distribution in the late-mid period(from top cover stage to harvest), which is attributed to the top excessive development and repressed the storage root swelling of sweet potato. It would played a crucial role to prevent top excessive development in the coordination between top and total root after top cover stage that enhanced root system development during early stages and promoting tuberization to produce larger numbers of uniform. Thereby positively impacting storage root yield at harvest, but now there few appropriate management practices in field. Therefore, in this study two varieties with different valid storage root number were subjected to two N forms(ammonium-N vs. amide-N) under two N rates(60 Kg / hm2 vs. 180 Kg / hm2),, plus a check treatment received no N fertilizer supply to further elucidate the effects of N on tuberization and its functionary mechanism throughout the early growth stage to achieve a better understanding of the ammonium-N influence on sweet potato early growth as well as to test and verify the final storage root yield and its components in the field. The results obtained were listed as followings: 1 Yield components at top cover stage and yield performance at harvest time under different nitrogen treatmentIt was clear that there were significant N-rate effect and N form effect on valid storage root number per plant in two sweet potato varieties at top cover stage, and the N application was unfavorable to form valid storage root, but the low-N rate treatment and the ammonium-N treatment were easier to form more valid storage roots per plant relative to the high-N rate treatment and the amid-N treatment respectively. The number of young storage root(0.5 cm ≤ RD < 5.0 cm)was higher and accounted for the most of valid storage roots components per plant in low ammonium-N treatment, which attributed to form more storage roots in comparison with control plants.The resulting harvest under the two seasons appear similar, and low ammonium rate-N treatment showed best yield performance with the highest fresh yield of storage root and the higher valid storage root number in two sweet potato genotypes among the N application combinations. The high-N rate treatment and the amid-N treatment with less storage root number achieved the poorer yield performance. 2 Biomass accumulation, allocation pattern and root architecture attributes under different nitrogen treatment during early stagesIt was observed that plants with N fertilizer treatments showed significantly more top growth performance. It was clear that there were significant N form effect on top growth performance, and the ammonium-N treatment behavior better than amide-N. At the same time, ammonium-N treatment under each N rate with more stem branches and the maximum leaf area index.While the N-treatment altered root development significantly more than above ground, and there were significant N-rate effect and N form effect on it. with the ammonium-N treatment and the low rate-N behavior better than amide-N and the high rate-N. In addition, the low ammonium rate-N treatment possessed the higher root biomass, high root activity, more AR number and even an increase in thick AR numberDuring tuberization period, the least T/TR ratio in the least ammonium-N treatment, but at top cover stage the T/storage root ratio was less or similar to control. 3 Developmental and physiological mechanism of tuberization under different nitrogen treatment 3.1 Characteristic expression of genes related to tuberization under different nitrogen treatmentThe expression of SporaminA, SporaminB and IbEXP1 gene of low ammonium rate-N treatment in two sweet potato varieties were up-regulated higher at initial phase of tuberization(14 days after planting). But on the other hand, there were change on the expression of these genes, the SporaminA and SporaminB genes expression were similar or higher slightly than check treatment received no N fertilizer, while the IbEXP1 gene down-regulated significantly, was lower than other treatments at later stage of tuberization. 3.2 Physiological mechanism of IAA and its related enzyme activity under different nitrogen treatmentThe variety S19 with more valid storage root number possessed lower level activities of IAAO, PPO, POD and higher level IAA than J23 during tuberization(14, 21 and 28 days after planting).There were significant N form effect on the activities of IAAO, PPO and POD of root in two sweet potato varieties with the ammonium-N treatment behavior better than amide-N throughout the early growth stage. In addition, the low ammonium rate-N treatment possessed the lowest level activities of IAAO, PPO, POD and highest IAA level. 3.3 ZR, IAA and their balanced relations of root under different nitrogen treatment during early stagesThe ZR and IAA metabolism in root regulated by nitrogen nutrition throughout the early growth stage. At the same time, the change of ZR/IAA ratio was more regularity than the change in the rules of absolute contents of hormones of ZR and IAA.The variety S19 with more valid storage root number possessed the similar ZR/IAA ratio to J23 at initial phase of tuberization(14 days and 21 days after planting), otherwise, possessed the smaller ratio than J23 at later phase of tuberization(14 days and 21 days after planting). There were significant N form effect on the ZR/IAA ratio in two sweet potato varieties with the ammonium-N treatment lower than amide-N throughout the early growth stage.In addition, the low ammonium rate-N treatment in two sweet potato varieties possessed the lower or similar ZR/IAA ratio than no nitrogen. 3.4 Anatomy and development mechanism of tuberization under different nitrogen treatmentThere were distinctive changes of anatomy in young root of the low ammonium rate-N treatment in two sweet potato varieties. It showed that the primary xylem made up of multilayer thick walled cells, and it possessed maximum primary xylem vessels and lignified parenchyma cells in stele at 7 days after planting. Whilemean, the primary xylem increased by the wall meristem activity with the more secondary xylem at 14 days after planting.Higher degree of lignification in the column under nitrogen treatment, and the low ammonium rate-N treatment possessed more secondary xylem, less multilayer thick walled cells and the higher ratio of column accounted for the root cross section at 21 days after planting. 3.5 Sucrose, starch metabolism and its related enzyme activity under different nitrogen treatmentThe variety S19 with more valid storage root number possessed lower sucrose content in root and higher activity level CWI and SNI than J23 at initial and later phase of tuberization(14 and 28 days after planting). There were significant N rate effect on the CWI and SNI activity of root in two sweet potato varieties with the high rate N treatment lower than low rate N throughout the early growth stage. In addition, the low ammonium rate-N treatment in two sweet potato varieties possessed the lowest sucrose content in root and higher activity level CWI and SNI in root.The variety S19 with more valid storage root number possessed similar AGPase behavior in root and lower activity level SuSy and starch content than J23 at initial and later phase of tuberization(14 and 28 days after planting). The low ammonium rate-N treatment in two sweet potato varieties possessed the lowest level starch content, SuSy and AGPase activity in root.