Starch-granule distribution between cell types of alfalfa nodules as affected by symbiotic development

Abstract

The distribution of starch granules between infected and uninfected cells in the central and cortical regions of alfalfa (Medicago sativa L.) nodules was investigated by morphometry of light micrographs of semithin sections of resin-embedded specimens. Nodules were collected from greenhouse-grown plants at early anthesis. The influences of symbiotic development on starch granule distribution were also investigated. In the early infection zone, infected cells had 1.7 times more starch granules and 1.5 times more starch per cell than uninfected cells. In the middle and late infection zones no significant differences in granule numbers or starch per cell occurred between cell types. Uninfected cells had greater starch density and smaller cell size than infected cells. Starch quantity and density in both cell types decreased from early infection to onset of senescence, with infected cells losing starch more rapidly. Comparisons between nodules from plants placed in darkness for 48 h and nodules from plants grown in light–dark cycles showed that starch depletion (starch area per cell and starch density) occurs more rapidly in infected cells than in uninfected cells. We conclude (i) that starch metabolism does not occur exclusively or even primarily in one cell type and (ii) that during prolonged periods of reduced photosynthesis infected cells deplete starch reserves faster than uninfected cells.