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PLANT PHYSIOLOGY , Vol 113, Issue 4 1243-1252, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Relationship of Endo-[beta]-D-Mannanase Activity and Cell Wall Hydrolysis in Tomato Endosperm to Germination Rates
P. Dahal, D. J. Nevins and K. J. Bradford
Department of Vegetable Crops, University of California, Davis, California 95616-8631
The endosperm tissue enclosing the radicle tip (endosperm cap) governs
radicle emergence in tomato (Lycopersicon esculentum Mill.) seeds.
Weakening of the endosperm cap has been attributed to hydrolysis of its
mannan-rich cell walls by endo-[beta]-D-mannanase. To test this hypothesis,
we measured mannanase activity in tomato endosperm caps from seeds allowed
to imbibe under conditions of varying germination rates. Over a range of
suboptimal temperatures, mannanase activity prior to radicle emergence
increased in accordance with accumulated thermal time. Reduced water
potential delayed or prevented radicle emergence but enhanced mannanase
activity in the endosperm caps. Abscisic acid did not prevent the initial
increase in mannanase activity, although radicle emergence was markedly
delayed. Sugar composition and percent mannose (Man) content of endosperm
cap cell walls did not change prior to radicle emergence under any
condition. Man, glucose, and other sugars were released into the incubation
solution by endosperm caps isolated from intact seeds during imbibition.
Pregerminative release of Man was suppressed and the release of glucose was
enhanced when seeds were incubated in osmoticum or abscisic acid; the
opposite occurred in the presence of gibberellin. Thus, whereas sugar
release patterns were sensitive to environmental and hormonal factors
affecting germination, neither assayable endo-[beta]-D-mannanase activity
nor changes in cell wall sugar composition of endosperm caps correlated
well with tomato seed germination rates under all conditions.
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