Taproots of mature Medicago sativa L. plants store abundant starch that cycles with defoliation. Starch is a key component that provides energy for regrowth of alfalfa plants post defoliation (PD). Starch is located mostly in the parenchyma cells of wood medullary ray and bark tissues. The objectives of this study were to investigate the influence of defoliation on the distribution of starch granules in cells within bark and wood tissues and changes to starch granule structure. Plants were grown to maturity in the greenhouse, defoliated, and root samples were analyzed by SEM from plants sampled on different days PD. TEM was used to study the structure of starch granules on Days 0, 12, and 28 PD and 12 day non-defoliated plants. A one-way ANOVA revealed a significant difference in the distribution of starch granules between the two tissues over the 35 day PD interval. The distribution of starch granules declined significantly on Days 10 and 12 when compared with Days 1 to 8 PD. Starch granules increased between Days 14 and 35 in bark cells, and in medullary ray cells the starch grain increase between Days 14 and 35 showed variations. There was a preponderance of smooth, clearly defined, and continuous surface starch granules in cells on Day 28 PD. In contrast, the surfaces of most starch granules appeared rough, irregular, and had various size crevices on Day 12 PD and non-defoliated plants. On rare occasions, intact amyloplasts were seen during times of net starch synthesis, but never during times of net starch degradation. This study revealed the post defoliation timeline when the lowest number of starch granules and major structural changes in amyloplasts occur. These results indicate when to target genes regulating reserve starch degradation in alfalfa with potential to contribute to alfalfa productivity.
Citation: Mendrano D, Gana JA, Williams A,Tubuo E, Maselli A et al. (2014) Defoliation Influences Starch Granule Distribution and Structure in Medicago sativa L. Taproots as Revealed by SEM and TEM. J Plant Biochem Physiol 2:134. doi: 10.4172/2329-9029.1000134