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Abscisic acid (ABA) levels in 3-mm apical root segments of slowly droughted sunflower plants (Helianthus annaus L. cv Russian Giant) were analyzed as the methyl ester by selected ion monitoring gas chromatogaphy-mass spectrometry using characteristic ions. An internal standard, hexadeuterated ABA (d6ABA) was used for quantitative analysis. Sunflower seedlings, grown in aeroponic chambers, were slowly droughted over a 7-day period. Drought stress increased ABA levels in the root tips at 24, 72, and 168 hour sample times. Control plants had 57 to 106 nanograms per gram ABA dry weight in the root tips (leaf water potential, -0.35 to -OA2 megapascals). The greatest increase in ABA, about 20-fold, was found after 72 hours of drought (leaf water potential, -1.34 to -1A7 megapascals). Levels of ABA also increased (about 7-to 54-fold) in 3-mm apical root segments which were excised and then allowed to dessicate for 1 hour at room temperature.Apart from its widely recognized role as an agent of stomatal closure, ABA may have other roles to play in the adaptation of plants to drought stress (8,17). The observations that ABA levels increase in the roots of water-stressed plants (7,11,12,19,22) and that this increase does not depend on transport from the shoot (19,22) are particularly provocative. Here, any action of ABA would not directly involve stomatal closure.There is evidence for the involvement of ABA in some drought-associated developmental responses; several studies (5, 9, 18, 23) have reported that the effects of exogenously applied ABA on plant development show similarities to the effects of water stress. Barlow and Pilet (1) have shown that exogenously applied ABA reduces cell division and DNA synthesis in the root apical meristem of corn. Ciamprova and Luxova (3) noted structural changes in the root apices of water-stressed maize plants. Earlier studies with sunflower seedlings found that drought stress inhibited root growth (5) and increased ABA levels in the root tissue (5,7). Changes in the anatomy of the root apices accompanied reduced growth in the droughted seedlings; moreover, the application of ABA (10 Mm) also inhibited root growth and induced anatomical changes at the root apices similar to those induced by drought (5).These observations suggest that the increases in ABA levels in droughted sunflower roots (7) may directly affect growth and development at the root apex where the developmental pattern by Sylvania Gro-Lux lamps (F72T12-GRO-WS-VHO) for 16 h each day at 25°C, followed by a dark period of 8 h at 16°C. Drought Regime. A time clock was attached to the electric motor which drives the spinner on one of the aeroponics chambers (7). Drought was imposed by withholding water for timed intervals; the seedlings were misted for 120 s h-' in two 60-s bursts for the first 24 h of treatment, then for 90 s h-' in one 60-s and one 30-s burst, thereafter. This droughting regime provided for a slow increase in water stress without subjecting the seedlings to a great initial shock at the beginning o...

1995.Changes in abscisic acid and indole-3-acetic acid in axillary buds of Elytrigia repem released from apical dominance [110][111][112][113][114][115][116] Growth of axillary buds on the rhizomes of Elytrigia repens (L.) Nevski is strongly dominated by the rhizome apex, by mechanisms which may involve endogenous hormones. We determined the distribution of indo!e-3-acetic acid (IAA) and abscisic acid (ABA) in rhizomes and measured (by gas-chromatography-mass spectrometry) their content in axiliary buds after rhizomes were decapitated. The same measurements were also made in buds induced to sprout by removing their subtending scale leaves. The ABA content tended to be higher in the apical bud and in the axiliary buds than in the adjacent intemodes, and tended to decline ba.sipetaily in the intemodes and scale leaves. IAA was similarly distributed, except that there was iess difference between the buds and other liiizome parts. After rhizomes were decapitated, the ABA content of the firet axillary bud declined to 20% of that of control values within 24 h, while the IAA content showed no marked tendency to change. The ABA content also declined within 12 h in the first axillary bud after rhizomes were denuded, while the content of IAA tended to iticrease after 6 h. These changes occurred before the length of the first aidllary bud increased 24-48 h after rhizomes were decapitated or denuded. We conclude that the release of axillary buds from apical dominance in E. repens does not require IAA content to be reduced, but is associated with reduced ABA content.

During overwintering in a northern climate, winter wheat goes through a hardening process, followed by dehardening in late winter – early spring. This sequence of events may be partially controlled by changes in endogenous hormone levels. Crowns and leaf tissue from field grown winter wheat (Triticum aestivum L. cv. Norstar) seeded at the beginning of September were collected and freeze-dried at monthly intervals during the winters of 1985–1986 and 1986–1987. Material was also sampled and freeze-dried from seedlings grown in a growth chamber under hardening conditions (21 °C for 2 weeks plus 3 °C for 6 weeks) or nonhardening conditions (3 weeks at 21 °C). The tissues were analysed for cytokinins and abscisic acid. Cytokinin levels, measured with the soybean hypocotyl section assay, declined from October onwards and then rose to a peak in late winter (January and February, winter 1986–1987; February and March, winter 1985–1986), subsequently declining again. Abscisic acid, quantitated as the methyl ester by gas chromatography with an electron capture detector, increased in level from October to December, then decreased to a relatively low level between January and March. Hardened seedlings from the growth chamber contained significantly higher abscisic acid levels and significantly lower cytokinin levels than did the nonhardened seedlings. Key words: abscisic acid, cytokinins, hardening, Triticum aestivum, winter wheat.