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John D. Williamson

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W. Roland Leatherwood, D. Mason Pharr, Lisa O. Dean and John D. Williamson

Sugars and sugar alcohols have well-documented roles in salt tolerance of whole plants and maturing seeds. Less is known, however, about possible effects of these compounds during germination. Seeds from mannitol-accumulating salt-tolerant celery [Apium graveolens L. var. dulce (P. Mill.) DC], non-mannitol-accumulating salt-tolerant cabbage (Brassica oleracea L. var. capitata L. ‘Golden Acre’), and salt-sensitive non-mannitol-accumulating tobacco (Nicotiana tabacum L.) and arabidopsis [Arabidopsis thaliana (L.) Heynh.] were placed on vertical Phytagel plates containing 0 to 300 mm NaCl. Germination percentage, root elongation, and carbohydrate content of seeds and seedlings were assessed. With the exception of cabbage, there was no positive relationship between ability to germinate in NaCl and the reported species salt tolerance of the mature plant. For instance, while cabbage seeds germinated in 300 mm NaCl, germination of two celery cultivars was inhibited completely by 150 mm NaCl. In contrast, seeds from salt-sensitive tobacco and arabidopsis germinated in 200 mm NaCl. There was also no obvious relationship between the observed salt tolerance and total soluble carbohydrates in either non-imbibed seeds or in seedlings germinated in salt. For example, the most-salt tolerant species in these studies, cabbage, had the third highest seed and seedling carbohydrate concentration, while the next most tolerant, arabidopsis, had the lowest. However, both species contained significant amounts of the osmoprotective oligosaccharides raffinose or stachyose. In addition, although celery seedling mannitol concentration initially increased at low NaCl concentrations (50 mm), germination and mannitol concentration decreased at higher NaCl concentrations (100 mm). Finally, the broadest response observed was a large increase in seedling sucrose at the lowest salt concentration that significantly inhibited germination. Although most seeds, with the notable exception of cabbage, did not germinate at 150 mm NaCl, they were still metabolically active because the sucrose content was two to eight times higher than in non-imbibed seeds, suggesting a possible role for sucrose in salt-stressed germinating seeds. These results not only suggest that mechanisms providing salt tolerance in mature plants are different from those in germinating seeds, but also that, even when the same mechanisms are employed, they may be less effective in seeds.

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John D. Williamson, Dianne B. Jennings, Wei-Wen Guo, D. Mason Pharr and Marilyn Ehrenshaft

The traditional use of polyols as osmotica in plant culture media is based on the assumption that polyols are not taken up or metabolized by cells. In reality, polyols are significant photosynthetic products and efficiently utilized metabolites in a large number of plants. In addition to these metabolic roles, initial interest in polyols focused primarily on their function as osmoprotectants. This was hypothesized to be due to their ability to act as compatible solutes. More recent research, however, indicates much broader roles for polyols in stress responses based on their significant antioxidant capacity. These include protection against salt and photooxidative stress as well as a potential role in plant pathogen interactions.