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Susan S.C. Liou and William B. Miller

During transportation and storage of Dutch tulip bulbs, potential ethylene exposure could lead to flowering abnormalities, including accelerated flowering, shortened plant heights, and in the most extreme case, flower abortion. Sources of ethylene include Fusarium-infected bulbs, deteriorating tissues, and combustion engines. Treatment with 1-MCP (1-methylcyclopropene) may prevent ethylene action as 1-MCP occupies ethylene specific receptors on target tissue. Two aspects of this problem were quantified using four tulip cultivars: duration of ethylene exposure necessary to induce damage as well as the effective period of protection by 1-MCP against ethylene. Flower abortion appeared in susceptible cultivars after ≥9 days of ethylene exposure (10 ppm) and was only found in mature bulbs (late November). The effective protection period of 1-MCP against ethylene (2-week exposure of 10 ppm) was determined, using flowering percentages, to be as long as 4–7 days in young bulbs and 28 days for older bulbs. Effects of ethylene on other flower attributes and implications of these findings in industry practices will be discussed.

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Susan S.C. Liou* and William B. Miller

Tulip bulbs are produced in the Netherlands and are shipped to United States during the months of July and August in temperature-controlled shipping containers. Each shipment is often composed of a mixture of many cultivars. Mechanical failure of temperature controls may result in high temperatures that ultimately may reduce forcing quality of the bulbs. When such accidents occur, an immediate decision must be made about whether to invest more time and money on these potentially damaged bulbs. Such a decision is not easy because symptoms of heat damage are often delayed until months later. Research on a single cultivar, `Apeldoorn', has shown that heat stress can cause flower abortion and other abnormalities. However, cultivars undoubtedly vary in their response to heat stress. Thus in the 2002 and 2004 forcing seasons, ≈45 cultivars were screened for response to a standard heat stress of 4 days at 35 °C. Prior to the heat stress, bulbs were held at 17 °C or 9 °C for 4 weeks, mimicking conditions used for late and early forced bulbs, respectively. Flower and leaf height, percent flower abortion, and flowering date were evaluated. Heat stress caused flower abortion and reduced plant height in sensitive cultivars. Across all cultivars, cold storage prior to the heat stress significantly increased bulb's sensitivity to heat stress. Using percent flower abortion, cultivars were grouped into three categories: resistant, moderate, and susceptible. With this information, we hope that damage assessment may become easier and fewer bulbs wasted.

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Susan S. Liou, Chris B. Watkins, and William B. Miller

During transport and the subsequent storage of tulip bulbs, inadvertent failure in ventilation and/or high contamination of Fusarium-infected bulbs may expose healthy bulbs to high concentrations of ethylene. Ethylene is known to cause many detrimental effects on forcing quality, including gummosis, increased respiration, flower bud abortion, bulb splitting and poor rooting. In this work, exposure duration and timing as well as the post-stress storage temperatures were evaluated for their potential effects on ethylene sensitivity in bulbs of four tulip cultivars. Degree of damage in sensitive cultivars `Apeldoorn' and `World's Favourite' increased with days at about 10 ppm ethylene starting at 9 and 16 days respectively. This effect strongly depended on timing of ethylene stress, as late treated bulbs showed more severe responses to ethylene treatment than early treated bulbs. Additionally, bulbs that were cooled immediately after ethylene stress, compared with those stored at 17 °C after stress, have significantly higher flowering quality in all attributes measured. This response was also strongly dependent on timing of ethylene stress and cultivar. Implications of the potential cold reversal of ethylene damage as well as effects of ethylene exposure duration and timing of stress on shipping and storage recommendations will be discussed.

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William B. Miller*, Martijn Verlouw, Susan S. Liou, Holly O. Cirri, Karen Snover-Clift, and Chris Watkins

Ethylene evolution is a consequence of Fusarium infection of tulip bulbs, yet little is known about the bulb-pathogen interactions involved in the induction or time course of ethylene synthesis. The resulting ethylene can affect adjacent, non-infected bulbs, and results in a variety of disorders, most notably flower abortion. Earlier work indicates that cultivars vary in their sensitivity to ethylene, but there are few data on ethylene production by cultivar. In this experiment, we assessed Fusarium-induced ethylene production in 36 tulip cultivars. Bulbs were wounded, inoculated with a liquid Fusarium suspension (isolated from infected bulbs) and held at 25 °C. Control bulbs were wounded, but not inoculated. Ethylene production was monitored by headspace analysis and gas chromatography. Ethylene increased rapidly after a lag phase of at least 8 days, but there were large differences in ethylene production among cultivars. Of the cultivars tested, `Furand' evolved more than 340 μL/kg/fwt/hr (≈250 μL/L/bulb/day) on the 11th day after infection, a rate ≈440-fold greater than in non-inoculated bulbs. Inoculated cultivars producing ethylene at rates exceeding 50 μLL/kg/hr included `Mary Belle', `Libretto', `Nashville', `Yonina', `Friso', and `Prominence'. About 25% of the cultivars produced ethylene at rates >10 μL/kg/hr, and ≈40% of cultivars produced less than this rate on day 11. High-ethylene producing tulips could be stored separately from other cultivars, or be given increased ventilation during storage or transportation. Knowledge of cultivar variation might also be useful in breeding programs. Further questions concerning the specific tissue responsible for ethylene synthesis (bulb, fungus, or both?) also arise.