Potato (Solanum tuberosum L. cv Russet Burbank) microtubers generated in vitro from single-node explants contained substantial amounts (approximately 250 pmol/g fresh weight) of free abscisic acid (ABA) and were completely dormant for a minimum of 12 weeks. Microtubers that developed in the presence of 10 NM fluridone (FLD) contained considerably reduced amounts (approximately 5-25 pmol/g fresh weight) of free ABA and exhibited a precocious loss of dormancy. lnclusion of exogenous racemic ABA in the FLD-containing medium suppressed the premature sprouting of these microtubers in a dose-dependent manner. At a concentration of 50 p~, exogenous ABA restored internal ABA levels to control values and completely inhibited FLD-induced precocious sprouting. Exogenous jasmonic acid was ineffective in suppressing FLD-induced sprouting. Application of FLD to preformed, fully dormant microtubers also resulted in a reduction in internal ABA content and precocious sprouting. These results indicate that endogenous ABA is essential for the induction and maintenance of potato microtuber dormancy.
Certain N-substituted phthalimides (NSPs) have gibberellin (GA)-like activity in a number of GA bioassays. The interaction between representative NSPs and a protein fraction from cucumber (Cucumis sativus L.) hypocotyls that has GA-binding characteristics consistent with those expected of GA receptors was studied. Analysis of in vitro equilibrium saturation data indi-
The speed of ethylene-induced leaf abscission in cotton (Gossypium hirsutum L. cv LG-102) seedlings is dependent on leaf position (i.e. physiological age). Fumigation of intact seedlings for 18 hours with 10 microliters per liter Qf ethylene resulted in 40% abscission of the still-expanding third true (30) leaves but had no effect on the fully expanded first true (10) leaves. After 42 hours of fumigation with 50 microliters per liter of ethylene, total abscission of the 30 leaves occurred while <50% abscission of the 10 leaves was observed. On a leaf basis, endogenous levels of free IAA in 10 leaves were approximately twice those of 30 leaves. Free IAA levels were reduced equally (approximately 55%) in both leaf types after 18 hours of ethylene (10 microliters per liter) treatment. Ethylene treatment of intact seedlings inhibited the basipetal movement of [14C]IAA in petiole segments isolated from both leaf types in a dose-dependent manner. The auxin transport inhibitor N-1-naphthylphthalamic acid increased the rate and extent of ethylene-induced leaf abscission at both leaf positions but did not alter the relative pattern of abscission. Abscission-zone explants prepared from 30 leaves abscised faster than 10 leaf explants when exposed to ethylene. Ethyleneinduced abscission of 30 explants was not appreciably inhibited by exogenous IAA while 10 explants exhibited a pronounced and protracted inhibition. The synthetic auxins 2,4-D and 1-naphthaleneacetic acid completely inhibited ethylene-induced abscission of both 10 and 30 explants for 40 hours. It is proposed that the differential abscission response of cotton seedling leaves is primarily a result of the limited abscission-inhibiting effects of IAA in the abscission zone of the younger leaves.
Cotton (Cossypium hirsutum L. cv LG102) seedlings raised from seeds exposed to 100 WM norflurazon (NFZ) during imbibition contained reduced levels of free abscisic acid (ABA) and were visibly achlorophyllous. Exposure of untreated cotton seedlings to ethylene concentrations >1 pL/L for 24 h resulted in cotyledon abscission. In contrast, exposure of NFZ-treated seedlings to concentrations of ethylene 550 wL/L elicited no cotyledon abscission. Application of ABA, an ABA analog, or jasmonic acid to NFZtreated seedlings restored ethylene-induced abscission. lsolated cotyledonary node explants prepared from NFZ-treated seedlings exhibited an altered dose-response pattern of ethylene-induced petiole abscission. Endogenous levels of free IAA were unaltered in NFZ-treated seedlings. Ethylene treatment (50 bL/L, 24 h) had no effect on free indoleacetic acid (IAA) levels in either control or NFZ-treated seedlings. Levels of conjugated (ester plus amide) IAA were substantially increased in NFZ-treated seedlings regardless of ethylene treatment. These results indicate that endogenous ABA plays an essential, but physiologically undefined, role in ethyleneinduced cotyledon abscission in cotton.
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