Starch Grain Distribution in Taproots of Defoliated Medicago sativa L.

Defoliation of alfalfa (Medicago sativa L.) results in a cyclic pattern of starch degradation followed by reaccumulation in taproots. Characterization of changes in anatomical distribution of starch grains in taproots will aid our understanding of biochemical and physiological mechanisms involved in starch metabolism in taproots of this species. Our objectives were to determine the influence of defoliation on starch grain distribution and size variation in taproots of two alfalfa lines selected for contrasting concentrations of taproot starch. In addition, we used electron microscopy to examine the cellular environment of starch grains, and computer-based image optical analysis to determine how cross-sectional area of tissues influenced starch accumulation. Taproots of field-grown plants were sampled at defoliation and weekly thereafter over a 28-day period. Taproot segments were fixed in glutaraldehyde and prepared for either light or electron microscopy. Transverse sections were examined for number and size of starch grains and tissue areas were measured. Starch grains were located throughout bark tissues, but were confined primarily to ray parenchyma cells in wood tissues. During the first week of foliar regrowth after defoliation, starch grains in ray cells near the cambium disappeared first, while degradation of those near the center of the taproot was delayed. During the third and fourth weeks of regrowth, there was a uniform increase in number of starch grains per cell profile across the rays, but by 28 days after defoliation there were more starch grains in ray cells near the cambium than in cells near the center of the taproot (low starch line only). Bark tissues from both lines showed synchronous degradation and synthesis of starch grains that was not influenced greatly by cell location. Diameter of starch grains varied with cell location in medullary rays during rapid starch degradation, but was not influenced by cell position in bark tissues. Therefore, during foliar regrowth there is a spatial separation in starch degradation and synthesis in alfalfa taproots. Amyloplasts from alfalfa taproots contained numerous starch grains, prolamellar-, and electron-dense bodies. The high starch line had 23% more cross-sectional area as ray cells in wood tissues when compared to the low starch line, which may explain part of the difference in starch accumulation between these alfalfa lines.