Starch grain distribution and amylolytic activity in alfalfa taproots
Abstract
The overall objective of this study was to examine starch metabolism in taproots of alfalfa (Medicago sativa L.) after defoliation. The first objective was to determine how starch grain distribution in taproots was influenced by defoliation. Transverse sections of bark and wood tissues of two alfalfa lines were examined with a light microscope at various times after defoliation for number and size of starch grains. During the first two weeks of regrowth after defoliation, starch grains in ray cells near the cambium were degraded first, and dissolution of those near the center of the taproot was delayed. Bark tissues of both lines showed synchronous degradation of starch grains. Amyloplasts from taproots contained numerous starch grains, prolamellar-, and electron-dense bodies as determined by transmission electron microscopy. I conclude that after defoliation there is a spatial separation is starch degradation, and to a lesser extent starch deposition, in alfalfa taproots. The second objective was to determine how nuclear ploidy level influences starch degradation and amylolytic activity in taproots of diploid and tetraploid genetically comparable populations, and in a commonly grown tetraploid cultivar (Hi-Phy) during regrowth after defoliation. Taproots were analyzed for starch concentrations, and activities of $\alpha$-amylase and $\beta$-amylases. In addition, an electrophoretic blotting technique was used to qualitatively characterize amylases. There were differences in starch degradation and rates of shoot regrowth between the three populations after defoliation. Alpha- and $\beta$-amylase activities, expressed on a tissue basis, were greater in Hi-Phy compared to 2X and 4X, which may account for the greater decline in starch concentration of this cultivar. During the first eight days of regrowth, 4X showed a greater increase in $\alpha$-amylase activity and starch degradation relative to 2X. Although $\beta$-amylase activity was at least 500-fold greater than $\alpha$-amylase activity in all populations, $\alpha$-amylase activity was more closely associated with starch degradation than was $\beta$-amylase activity. Multiple forms of $\beta$- and $\alpha$-amylase were observed in taproots of all populations. I conclude that there are ploidy differences in starch degradation and amylase activity after defoliation, and that $\alpha$-amylase may play a key regulatory role in starch degradation in alfalfa taproots.
Degree
Ph.D.
Advisors
Volenec, Purdue University.
Subject Area
Agronomy|Botany
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