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James W. Rushing

Broccoli (Brassica oleracea L. var. italica `Citation') florets were treated postharvest with either benzyladenine or trans-zeatin at either 10 or 50 ppm before packaging in perforated polyethylene bags and storage at 16C. The most pronounced effects were observed with benzyladenine at 50 ppm. Compared to controls, respiration rate was reduced 50% and ethylene production increased 40% throughout the first 4 days of storage. Total chlorophyll content had dropped 60% in controls, but was unchanged in cytokinin-treated florets, which had a 90% longer shelf life than controls. These effects depended on the amount of cytokinin applied and were of greater magnitude with benzyladenine than with zeatin.

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J.G.M. Cutting, D.K. Strydom, G. Jacobs, D.U. Bellstedt, K.J. Van Der Merwe, and E.W. Weiler

Xylem sap was vacuum-extracted weekly from 1-year-old apple shoots from trees treated with dinitro-o-cresol (DNOC) oil, hydrogen cyanamide, or untreated controls. Sampling began 1 week before treatment and continued until 2 weeks after budbreak had occurred in the control trees. Sorbitol, calcium, magnesium, potassium, and zeatin-type cytokinin concentrations were determined by enzymatic, atomic absorption, and immunoassay methods, respectively. The rest-breaking treatments resulted in earlier and more intense budbreak. Xylem sap cytokinin concentrations increased rapidly in response to the rest-breaking chemicals and peaked just before or at budbreak. The rapid increase in cytokinin was closely followed by increases in calcium and magnesium concentrations in the sap. Potassium concentration appeared to be unaffected by rest-breaking treatment. Sorbitol levels dropped rapidly as a result of the rest-breaking treatments and appeared to be used rapidly in budbreak and early bud growth.

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Stuart R. Reitz and John T. Trumble

We examined two aspects of treating plants with a cytokinin-containing seaweed extract (SWE). In the first series of experiments, we tested the hypothesis that immature lima bean (Phaseolus lunatus L.) and tomato (Lycopersicon esculentum Mill.) plants provided with exogenous cytokinins could recover from defoliation by a generalist insect herbivore, Spodoptera exigua (Hübner), more rapidly than plants without cytokinin supplements. However, the SWE inhibited growth of lima beans at all levels of herbivore damage. The SWE neither inhibited nor stimulated growth of tomatoes following defoliation. Because SWE effects largely were neutral for tomato growth, we conducted a second series of experiments to test the hypothesis that SWE treatments alter the attractiveness of tomato foliage to S. exigua larvae. In these experiments, we determined consumption of, and preference for, SWE-treated tomato foliage by S. exigua larvae. Repeated root applications of SWE led to increased consumption and preference by S. exigua. Repeated foliar applications did not alter consumption or preference compared with controls. Spodoptera exigua larvae gained significantly more mass when feeding on SWE-treated foliage compared with controls. While these data indicate that plant responses to exogenous cytokinin-containing materials depend on taxa and application method, the practical uses of SWE appear limited given the negative effects on plant growth and increased attractiveness of treated foliage to herbivores.

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Servet Kefi, M.M. Meagher, P.E. Read, and A.D. Pavlista

The effects of different cytokinin-like compounds on invertase activities at different tuberization stages of potato (Solanum tuberosum L. `Atlantic') were examined. Single nodal segments were cultured on MS medium plus 6% sucrose and supplemented with either 2 mg kinetin/L, 0.1 mg thidiazuron (TDZ)/L, 1.0 mg AC 243,654/L, 0.1 mg AC 239,604/L, or no cytokinin. Tissue samples for determining invertase activity were taken at three stages of tuberization: stage 1, the “hook stage”; stage 2, the “swelling stage”; and stage 3, “tuber initials.” Invertase activity was significantly affected by the interaction between cytokinin-like compounds and tuberization between cytokinin-like compounds and tuberization stages. The highest invertase activities in the stolons at stage 1 were found in kinetin and TDZ treatments. Invertase activity in the stolons on the control medium significantly increased from stage 1 to 2 and decreased at stage 3. Invertase might play a role in either stolon elongation or carbohydrate utilization by increasing the pool of reducing sugars.

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Wen-Shaw Chen

The changes in cytokinins and gibberellins in xylem sap of lychee (Litchi chinensis Sonn. cv. Heh yeh) trees were investigated at the stages of leaf expansion, dormant bud (when apical leaves are dropped), 30 days before flower bud formation, flower bud formation, and full bloom of grafted field-grown lychee trees. Also; the diffusible IAA and ABA in diffusate from shoot tips were examined at the successive stages of development. High gibberellin was found in the xylem sap at the stage of leaf expansion. A constant level of IAA was maintained through the five growth stages. At 30 days before flower bud formation, ABA increased dramatically. Concurrently, total cytokinin content increased in the xylem sap, reaching a maximum during flower bud formation and full bloom. Gibberellin content in the xylem sap was at a low level 30 days before flower bud formation and through the stage of flower bud formation.

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David G. Clark, John W. Kelly, and H. Brent Pemberton

Six cultivars of potted rose (Rosa ×hybrida L.) plants were evaluated for shipping stress-induced leaf chlorosis during holding at 8, 16, or 28C for 2, 4, or 6 days. `Meijikatar' showed more leaf chlorosis than the similar `Meirutral' at the higher simulated shipping temperatures and longer durations. Plants of `Meijikatar' were treated before simulated shipping with BA, TZ, or Promalin at 0, 25, 50, or 100 mg cytokinin/liter each, then paper-sleeved and stored in the dark in fiberboard boxes at 16C for 5 days. Plant quality 5 days after removal from storage was better with BA at 50 or 100 than at 0 mg·liter–1. All cytokinin-treated plants showed less leaf chlorosis than controls. Benzyladenine at 50 or 100 mg·liter–1 reduced leaf chlorosis when compared to all TZ treatments. There were no differences among treatments in the number of etiolated shoots per plant. Chemical names used: N-(phenylmethyl)-1H-purin-6-amine (benzyladenine, BA); trans-zeatin (TZ); gibberellic acid (GA4+7) + BA (Promalin).

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Saila T. Karhu

Microshoots of `McIntosh' apple (Malus domestica Borkh.) were grown on Murashige-Skoog (MS) nutrient medium supplemented with either sucrose or sorbitol or with sucrose and an elevated level of cytokinin. Shoot growth was recorded and concentrations of fructose, glucose, sorbitol, and sucrose were analyzed in nutrient media and shoots during a 6-week subculture period. Axillary branching was stimulated by high cytokinin and sorbitol media, with increased biomass production and carbohydrate use on the high-BA medium only. The sucrose in the nutrient medium was hydrolyzed to fructose and glucose, which were equally taken up by shoots. Sorbitol was taken up somewhat less effectively. The elevated level of BA decreased sucrose hydrolysis in the nutrient medium. There were high concentrations of sorbitol in shoots grown on the sorbitol medium, and sorbitol also accumulated at the end of the culture period in shoots grown on sucrose. The amount of sucrose was low, and glucose was more abundant than fructose in microshoots. The starch content of leaves was not affected by treatments or sampling time. Chemical names used: N6-benzyladenine (BA).

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Servet Kefi, Paul E. Read, Alexander D. Pavlista, and Stephen D. Kachman

The role of sucrose alone and in combination with different cytokinin-like compounds on the microtuberization of potato, Solanum tuberosum `Atlantic', was investigated. Single nodal segments were placed in Magenta boxes containing Murashige & Skoog medium supplemented with one of 15 treatments in a 3 × 5 factorial. Treatment factors were sucrose at 3%, 6%, or 9%, and cytokinin-like compounds at five levels [cytokinin-free; 2 mg kinetin/L; 0.1 mg thidiazuron (TDZ)/L; 1.0 mg AC 243,654/L; 0.1 mg AC 239,604/L]. Except in a few cases in kinetin and TDZ treatments, nearly all cytokinin treatments failed to induce tuberization at the 3% sucrose, noninductive level. However, all cytokinin treatments induced tuberization in the presence of 6% sucrose. By raising the sucrose level from 6% to 9%, more and larger microtubers were obtained in the cytokinin-free medium. At the 9% sucrose level, even though more tubers per box were produced by TDZ and AC 243,654 treatments, less total fresh weight of tubers per box resulted from kinetin, TDZ and AC 243,654 treatments because tubers formed were smaller. Higher sucrose concentrations (9%) favored tuberization in the cytokinin-free medium, whereas 6% sucrose was optimum for the medium containing cytokinins. Sucrose might produce a strong tuberization signal that might either change endogenous hormone levels affecting tuberization or activate a number of genes coding tuber proteins and enzymes related to starch synthesis.

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Abba Upadhyaya, Tim D. Davis, Daksha Sankhla, and N. Sankhla

Both kinetin and BA promoted in vitro shoot formation from hypocotyl explants of Lupinus texensis Hook. placed on Murashige and Skoog (MS) medium. With either cytokinin, shoot formation was best at ≈4.5 μm. Adventitious root formation was observed only on tissue culture-derived shoots placed in MS media containing 5.4 to 54 μM NAA. IAA and IBA, at concentrations ranging from 5 to 55 μm, failed to stimulate rooting. Even at the optimal concentration of NAA, only 14% of the shoots produced roots. Thus, although hypocotyl explants readily produced shoots, adventitious root formation on these shoots occurred with relatively low frequency. Chemical names used: 6-benzylaminopnrine (BA); indole-3-acetic acid (IAA); indole-3-butyric acid (IBA); 6-furfurylaminopurine (kinetin); 1-naphthaleneacetic acid (NAA).

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M.H. Aboul-Nasr, S.Z. Elagamy, and A.M. Kassim

Three strawberry cultivars were used in this study. Runners from the three cultivars were grown on Murashige and Skoog medium supplemented with benzylaminpurine or kinetin at four concentrations (2, 1, 0.5, and 0.25 mg·L-1 in addition to the control treatment). The use of cytokinin, regardless of type and concentration, promoted the explant proliferation, shoot height, number of leaves, and plantlet fresh weight. Callus formation was enhanced by BA application, especially with `Pajaro'. However, `Chandler' did not form at all using cytokinin. Produced plantlets had lower nutrient constituents (N, P, K, Ca, and Na) when grown on media containing cytokinin compared to the control. Based on these results, cytokinin is recommended to get more shoots per explant. Furthermore, using the lowest concentration (0.25 mg·L-1) will produce height proliferation, greater shoot length, and more leaves.