Priming relieves dormancy in lettuce seeds independently of changes in osmotic constituents

Weges, Roelf; Koot-Gronsveld, E.A.M.; Karssen, C.M.

doi:10.1034/j.1399-3054.1991.810413.x

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“…Current evidence indicates that the embryo growth potential changes relatively little prior to radicle emergence (Haigh and Barlow, 1987;Bradford, 1990;Welbaum and Bradford, 1990;Weges et al, 1991;Ni and Bradford, 1992). Thus, the restraint imposed by the endosperm tissues surrounding the radicle tip (the endosperm cap) is the major determinant of $b(g) (Ni and Bradford, 1993), consistent with a decrease in $b(g) of approximately 1.0 MPa when the endosperm cap was excised from tomato seeds (Dahal and Bradford, 1990).…”

Section: ~ ~~~~mentioning

confidence: 87%

Relationship of Endo-β-D-Mannanase Activity and Cell Wall Hydrolysis in Tomato Endosperm to Germination Rates

1997

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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-p-o-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 i n accordance with accumulated thermal time. Reduced water potentia1 delayed or prevented radicle emergence but enhanced mannanase activity i n 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 i n osmoticum or abscisic acid; the opposite occurred i n the presence of gibberellin. Thus, whereas sugar release patterns were sensitive to environmental and hormonal factors affecting germination, neither assayable endo-p-D-mannanase activity nor changes in cell wall sugar composition of endosperm caps correlated well with tomato seed germination rates under all conditions. ~ ~~~~In many seeds, the embryo is surrounded by endosperm or perisperm tissues through which the radicle must emerge during germination. Degradation and weakening of the endosperm (or perisperm) tissue adjacent to the radicle tip precedes radicle protrusion in seeds of pepper (Capsicum annuum L.) (Watkins et al., 1985), Datura species (Sánchez et al., 1990;Mella et al., 1995), muskmelon (Cucumis melo L.) (Welbaum et al., 1995), and wild-type (Haigh, 1988; Dahal and Bradford, 1990;Leviatov et al., 1995;Nomaguchi et al., 1995) and GA-deficient (gib-2) mutant tomato (Lycopersicon esculentum Mill.) (Groot and Karssen, 1987;Groot et al., 1988). Weakening has also been proposed to occur in the micropylar endosperm of tobacco seeds based on the localized expression of P-1,3-glucanase prior to radicle emergence (Vogeli-Lange et al., 1994;Leubner-Metzger et al., 1995. Analysis of the salt tolerance of tomato seed germination indicated that addi- tive endosperm effects accounted for most of the genetic variance in mean germination rates (Foolad and Jones, 1991), in agreement with evidence that the endosperm tissue, rather than the testa, is the major barrier for radicle protrusion in tomato seeds (Groot and Karssen, 1987;Leviatov et al., 1994).Mathematical models and extensive empirical data have demonstrated that the timing of germination is related to specific $ threshold values for radicle emergence, which vary among individual seeds (reviewed in Bradford, 1995).The $b(g) is dete...

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