Catherine M. Vollmer

Changes in the cotyledons ofCucurbita maxima during germination

SummaryThe ultrastructural changes occurring in cotyledon protein bodies of germinating squash seeds were investigated using mainly the freeze-etching technique. The appearance and subsequent disappearance of soft globoid-like material in the proteinaceous matrix was documented as the first major ultrastructural change. The globoid crystal was digested either by the formation of internal pitted regions or through external digestion. During later stages of breakdown the protein bodies became a series of largely aqueous vacuoles containing irregularly shaped pieces of protein material. This protein material eventually disappeared and the vacuoles fused to form the central vacuole of the mesophyll cells. This fusion of vacuoles was followed in living cells using Nomarski optics.

Changes in the cotyledons of Cucurbita maxima during germination. V. The nuclear envelope

The frequency and distribution of nuclear pores in cotyledon mesophyll cells of Cucurbita maxima were studied from early stages of germination until cotyledon senescence. Changes in nuclear volumes and nuclear surface areas were also studied. The number of nuclear pores per square micron of nuclear surface area increased significantly when the cotyledon changed from a storage organ to a photosynthetic leaf and then decreased with the onset of senescence. Temperature of seedling growth influenced the number of nuclear pores per square micron of nuclear envelope. Plants grown under different temperatures to about the same morphological stage of development had significantly different numbers of nuclear pores per square micron of nuclear envelope. Plants grown under cooler temperatures had more nuclear pores per unit area than did plants grown under warmer conditions, even where nuclear surface areas were not significantly different between plants grown at the two different temperatures.

Temperature-induced changes in the nuclear envelope of Euglena and Scenedesmus

Euglena and Scenedesmus cells were grown at two different temperatures prior to measurement of the number of nuclear pores per square micron of nuclear surface areas and nuclear volumes. Euglena cells grown at 15°C had a significantly larger nuclear volume, a significantly larger nuclear surface area, and a significantly greater number of pores per square micron of nuclear envelope surface than did Euglena cells grown at 30°C. An estimate of nuclear pores per nucleus showed that on average there were several thousand more pores per nucleus in cells grown at 15°C than in cells grown at 30°C. In Scenedesmus the mean number of nuclear pores per square micron of nuclear surface area was significantly greater in cells grown at 23°C than in cells grown at 15°C. The mean nuclear volumes and surface areas were, however, significantly greater in cells grown at 15°C than in cells grown at 23°C. An estimate of nuclear pores per nucleus indicated no difference between the two growth temperatures for Scenedesmus . The results of these experiments provide further evidence that the nuclear envelope does change in response to growth temperature.