Progress 10/01/03 to 09/30/04
Outputs
Observations using bright field microscopy and scanning electron microscopy reveal there is no abscission zone associated with the nodes on the rachis of Elymus elymoides. Nevertheless, there appears to be an abscission zone at the base of individual seeds. Disarticulation of the rachis can occur under field conditions at any node (1-4 typical) with the second node being the most common (anecdotal observations). There has been no observed anatomical or morphological differences using stereo, bright field, or scanning electron microscopy between the tissues of the first four nodes on the rachis of the seedhead. All observations of the nodal disarticulation region reveal tearing and not enzymatic dissolution of the cell walls. The disarticulation typically begins with a crack near the base of the node on the side opposite the attachment of the rachilla and progresses around and through the node to achieve separation. Disarticulation has been observed as early as Zadok
scale 70, but frequently can occur much later in fully hydrated greenhouse specimens. Each internode is traversed by numerous axial vascular strands. By contrast, the nodal region appears structurally weaker due to the branching of vascular strands into the rachilla. It is hypothesized that as the rachis ages and the seed head becomes heavier the nodal region becomes increasingly weaker, further exacerbated by ground cell deterioration. Under typical field conditions, as the entire plant matures it becomes more dehydrated further weakening the rachis, which leads to an earlier shattering than is observed with fully hydrated greenhouse samples. The greater observed disarticulation at the second node appears to be due to its location on the rachis, with the weight of the seed head placing the greatest stress on this node and making it vulnerable to mechanical disturbances.
Impacts
Preventing disarticulation of seedheads from squirreltail will permit growers to obtain seeds for rangeland restoration. Presently, disarticulation
Publications
- No publications reported this period
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Progress 10/01/02 to 09/30/03
Outputs
The onset of this project was delayed until summer 2003, because seedheads were not available. Once developing seedheads were available, then further refinement of the histological procedures was required including more extensive aspiration of tissue segments during fixation to remove intercellular air that prevented proper infiltration of the plastic embedding medium. The second and third nodes of developing seedheads from squirreltail representing Zadok scale 58 (some anther extrusion) to 88 (dough stage of grains in florets) have been histologically prepared for examination using brightfield and scanning electron microscopy. Observations with SEM show a groove forming around the rachis on the side away from the point of attachment of the floret at the node. The groove may represent a collapse of underlying cells rather than a genetically programmed incipient abscission zone leading to a separation layer. A separation layer appears as evidenced by a crack in this
grove as the rachis matures. Separation progresses around and through the rachis tissues. The cracking has been observed as early as Zadok scale 70. Further observations are in progress to determine if this separation is associated with dissolution of cell walls characteristic of abscission in dicot species. Like dicots species, there is a change in the number of collenchyma cells under the epidermis. Additionally, the changes seen in the collenchyma layer are similar to those observed with cotton. Histological observations of longitudinal sections through this region are in progress to determine cellular changes including possible dissolution of cell walls prior to onset of separation that eventually leads to disarticulation of the entire upper part of the seedhead from the basal first or second node of the rachis. Previous observations (T. Chastain, personal communication) noted that typically abscission or disarticulation occurred at the second node of the rachis. Although we can
not confirm that the second node is the primary site of initial disarticulation, separation rarely occurs at the first node. At this stage in this study, it appears that disarticulation is due to a combination of factors possibly including endogenous hormone levels, position on the rachis, overall weight of the maturing seedhead, and mechanical disturbances such as wind.
Impacts
Preventing disarticulation of seedheads from squirreltail will permit growers to obtain seeds for rangeland restoration. Presently, disarticulation or shattering does not permit seeds to be harvested in sufficient quantities for this purpose. Combining results from this study with additional physiological experiments could lead to a significant reduction in seed shattering.
Publications
- No publications reported this period
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