Ovule development in Iris tenax

Research from Carol A. Wilson, 2001.  Floral stages, ovule development, and ovule and fruit success in iris tenax, focusing on var. gormanii, a taxon with low seed set.  American Journal of Botany 88: 2221-2231.

Goals

To determine if abnormalities in early ovule (immature seed) development might be responsible for the low reproductive output in Iris tenax var. gormanii that was reported by Piper (1924), to examine the timing of ovule development in relation to flower development and to reexamine ovule development in I. t. var. tenax.

  

Data

Whole buds and flowers of I. tenax var. tenax and I. t. var. gormanii were collected at eight stages.  Line drawings were produced from scanned images of each stage.  Ovaries (immature fruits that contain many ovules) were collected at each of the eight floral stages and after pollination when the flower begins to twist and collapse.  Ovaries were embedded in paraffin or plastic, sectioned, placed on slides and stained.

Important Findings

LINK: Stages of ovule development  in relation to flower opening

Reexamination of Ovule Development in I. tenax var. tenax


Smith and Clarkson (1956) reported that synergids persisted until fertilization and if an ovule was not fertilized these two cells were the last cells to degenerate.  This was an unusual finding because in most flowering plants the synergids begin to break down prior to fertilization.  In my study all of the mature ovules (male phase) had synergids that were degenerated or degenerating which is the pattern found in the majority of angiosperms.

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Smith and Clarkson (1956) reported that the membrane surrounding the ovule disintegrated at about the two-nucleate stage and nuclei from the surrounding cells moved into the ovule and then disintegrated.  I found that nuclei of the surrounding cells tend to be appressed to the ovule but are clearly separated from it by cell walls.  It is my interpretation that these large cells expand and define the space that the ovule will soon inhabit and then are crushed and break down as the ovule takes up this space.

   

Two glandular areas were identified in the tissues surrounding the ovule (at arrows).  One is in the cells surrounding the ovule in the region of the micropyle (opening where the pollen tube grows to the ovule) and the other is adjacent to the micropyle along the base of the funiculus (stalk where the ovule connects to the ovary).  It is likely that both of these tissues help direct the pollen tube to the ovule.

Conclusions

Ovule development in I. tenax var. gormanii is normal and did not deviate from development in I. t. var. tenax.  Most of the stages of ovule development occur while the flower is still in bud with final maturation occurring during the male phase of flowering.  My data did not support the reports by Smith and Clarkson (1956) that nuclei migrated from surrounding cells into the ovule and that synergids persisted until after fertilization.  Two new glandular areas were identified for Iris that probably aid in directing the pollen tube toward the ovule.

References

Piper, C. V.  1924.  New flowering plants of the Pacific Coast.  Proceedings of the Biological Society of Washington 37: 91-96.
Smith, F. H., and Q. D. Clarkson.  1956.  Cytological studies of interspecific hybridization in Iris, subsection Californicae.  American Journal of Botany 43: 582-588.