SummarySeveral lines of evidence suggest a prominent role of abscisic acid in controlling the length of the dormancy period of potato tubers. The ABA content was measured in tubers of six different potato varieties during storage. Consistent with previous publications a continuous decline in ABA content during storage was evident. However, this decline of ABA did not correlate with the sprouting behaviour of the tubers, ruling out that ABA is responsible for the break of dormancy. Activities of starch degrading and glycolytic enzymes were deterriained to investigate whether changes in their activities would correlate with the break of dormancy. Most of the enzymes investigated did not change significantly during storage. After the onset of sprouting activity of beta-amylase increased two-fold and alpha-amylase activity was stimulated by approximately 30%. The increase in beta-amylase activity was due to transcriptional activation, as shown by northern analysis. Since no change of starch degrading enzymes could be detected prior visible sprout growth, starch degradation is not a prerequisite for the initiation of sprouting.

Non-mycorrhizal Norway spruce seedlings (Picea abies Karst.) and Norway spruce seedlings colonized with Paxillus involutus Fr. were grown in an axenic silica sand culture system. After successful mycorrhizal colonization, the seedlings were exposed to 200 or 800 micro M AlCl(3) for 10 weeks. In both non-mycorrhizal and mycorrhizal seedlings, exposure to Al significantly reduced root growth and the uptake of Mg and Ca. After 5 weeks of exposure to 800 micro M Al, the mycorrhizal seedlings had significantly higher chlorophyll concentrations than the non-mycorrhizal seedlings, although no difference in Mg nutrition was apparent. After 10 weeks of exposure to Al, both non-mycorrhizal and mycorrhizal seedlings exhibited needle chlorosis and reduced photosynthetic activity. However, the aluminum-induced reduction in shoot growth was largely ameliorated by colonization with P. involutus. We conclude that mycorrhizal colonization modifies the phytotoxic effects of Al in Norway spruce seedlings. However, differences in physiological responses to Al between mycorrhizal and non-mycorrhizal seedlings may be largely reduced in the long term as a result of impaired mineral nutrient uptake.

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