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of N limitation, leaves will abscise and reintroduction of N will then cause development of new leaves from terminal meristems and axillary buds. Plant responses to N deficiency are mediated by plant hormones, especially by cytokinins ( Argueso et al

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Abstract

The levels of auxin, gibberellin, and cytokinin were measured using bioassay methods in pruned and unpruned apple trees (Malus domestica Borkh. cv. McIntosh). Vigorous shoot growth following heavy dormant pruning was accompanied by an increase in cytokinin concentration in the tissues in the early spring. As cell division and cell expansion progressed, the levels of auxin and gibberellin increased. The gibberellin activity in samples from pruned trees was 3 times higher than in samples from unpruned trees. Pruning diminished the midsummer level of cytokinins in the annual shoots.

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This study was designed to examine whether shoot injury induced by high root-zone temperature is associated with changes in shoot detoxifying metabolism and to determine the level and duration of high root-zone temperatures that would induce physiological changes in two cultivars of creeping bentgrass (Agrostis stolonifera var. palustris Huds) differing in heat tolerance. Plants of `Penn A-4' (heat tolerant) and `Putter' (heat susceptible) were grown in sand and exposed to root-zone temperatures of 20 (control), 21, 22, 23, 25, 27, 31, and 35 °C in water baths while air temperature was maintained at 20 °C in a growth chamber. Turf quality, leaf cytokinin content, and antioxidant enzyme activities declined at increased soil temperatures and the duration of treatment for both cultivars. A decline in turf quality occurred following 40 days of exposure to 35 °C for `Penn A-4' and 26 days of exposure to 31 °C for `Putter'. The root-zone temperature causing the decline of isopentenyl adenosine and zeatin cytokinins was 25 °C at 37 d for `Putter' and 27 °C at 47 days for `Penn A-4'. The temperature causing the decline of superoxide dismutase and catalase activities was 25 °C and 27 °C at 33 days for `Putter' and 27 °C and 31 °C at 43 days for Penn A-4, respectively. Malondialdehyde content increased at 27 °C for `Putter' and 31 °C for `Penn A-4' at 43 days of treatment. The decline in cytokinin content and antioxidant enzyme activity occurred at a lower soil temperature and earlier during the treatment than the decline in turf quality, possibly contributing to turf quality decline. The root-zone temperatures causing the decline in turf quality, cytokinin content, and oxidative damage were higher in the heat-tolerant cultivar than heat-susceptible cultivar.

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Abstract

Cytokinins were extracted from whole root tissue of aeroponically cultured Chrysanthemum morifolium Ramat., purified by cation exchange, paper and Sephadex LH-20 column chromatography and bioassayed with tobacco callus. Compounds with chromatographic properties similar to those of zeatin and zeatin riboside were found to be the major cytokinin components of the roots.

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damage by enhancing thermotolerance ( Park et al., 1996 ; Sun et al., 2002 ). Different approaches have been examined in alleviating heat stress injury, including exogenous application of plant hormones, such as cytokinins. Cytokinins regulate many

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budbreak is correlated with an increased concentration of cytokinins in the xylem sap just before budbreak ( Tromp and Ovaa, 1990 ; Van Staden and Davey, 1979 ). Furthermore, exogenous application of cytokinins can promote budbreak during late dormancy

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of PGRs in the culture medium; relatively high levels of cytokinin promote shoot formation, whereas high levels of auxin promote rooting, and intermediate levels induce callus formation ( Thorpe, 2007 ). In CDM, adventitious shoots are likely to be

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. Thidiazuron, a substituted phenylurea with its cytokinin- and auxin-like effects, is the most recently studied of the cytokinins ( Debnath and Teixeira da Silva, 2007 ). There are many reports of improved regeneration capacity with TDZ compared with other

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Abstract

Flower quality in Leucospermum can be improved by increasing the diameter and length of the reproductive shoot and by increasing floret initiation and capitulum size. During flower induction, endogenous cytokinin levels were low, with mainly “storage” forms of cytokinins being present. However, during flower differentiation, endogenous cytokinin levels increased and most of the activity was in the form of “free” cytokinins. One application of benzyladenine, if applied during the flower induction phase, increased both reproductive shoot diameter and the number of florets per capitulum.

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liquid culture. Subculturing was done after every 4 weeks on the same medium. Black parts in the culture were removed before subculturing. Multiplication was carried out up to the third subculture to evaluate the type of cytokinin, its concentration, and

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