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Title: LOW TEMPERATURE SCANNING ELECTRON MICROSCOPIC OBSERVATIONS ON ENDOSPERM IN FROZEN HYDRATED LETTUCE SEEDS DURING IMBIBITION AND GERMINATION

Author
item NIJSSE, J - WAGENINGEN AGRIC. UNIV.
item ERBE, ERIC
item BRANTJES, N - WAGENINGEN AGRIC. UNIV.
item SCHEL, J - WAGENINGEN AGRIC. UNIV.
item WERGIN, WILLIAM

Submitted to: Seed Science Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Lettuce seeds have been studied by numerous research scientists because, unlike most other commercial crops, germination of their seeds is complex, requires light and can be unpredictable. Previous studies indicate that either of two events may be required for germination: 1) the embryonic root within the seed may elongate and cause pressure that bursts through the seed coat, or 2) the seed coat may be chemically weakened and thereby allow the root to penetrate. To understand this problem, scientists have used a high-powered microscope, called a scanning electron microscope, to examine the structure of dry, water-soaked and germinating seeds. However, these studies required them to treat the seeds with chemicals, remove all water and then completely dry the seeds. When these procedures were repeated in our laboratory, we found that they caused considerable shrinkage of cells so that the explanation for germination could not be resolved. Consequently, we used an alternative preparation procedure that freezes the seeds without causing shrinkage so that they could be observed during germination. The results indicate that normal germination is not associated with a chemically weakened seed coat but rather with cell wall disruption resulting from the elongation of the root.

Technical Abstract: Frozen, hydrated lettuce (Lactuca sativa L.) seeds were observed using low temperature scanning electron microscopy (LTSEM). The structure of normally imbibing and germinating seeds was characterized and compared to that of seeds in which these processes were inhibited by high temperature, far-red light or polyethylene glycol. The results indicate that normal germination nis not associated with general breakdown or digestion of the entire endosperm at the micropylar region but rather with localized cell wall separation that may be associated with pectolytic activity. Seeds imbibed in polyethylene glycol, which inhibited germination, exhibited embryos that appeared to lack turgor and were not able to emerge from the endosperm. Thermal or far-red inhibitory treatments were associated with the appearance of unique tubular structures in cells of the endosperm and the embryo.