Preventing Premature Bud Drop By Susan Han

In recent years, sales of impatiens — Impatienswallerana, double impatiens and New Guineaimpatiens — have been phenomenal. However, one problem with impatiens ispremature flower and bud drop, which can be induced by ethylene, low lightconditions, water stress and other conditions. Furthermore, the abscised budsand flowers are sources for infection by disease and infestation of insects ingreenhouses and retail stores, and the reduced bud count decreases theaesthetic value of the plants.

1-methylcyclopropene (MCP) is a newly registered plant growthregulator that is an organic gas with anti-ethylene effects, currentlyavailable commercially under the trade name EthylBloc (Floralife, Inc.). Thecompound does not leave residue on the plants. MCP has been evaluated in manyflowering plants and has been shown to protect against ethylene-induced petaland flower drop in these and other crops. In impatiens, 1-MCP has been shown tocompletely overcome flower abscission induced by exogenous ethylene. While itis very exciting to know that this new compound can protect plants againstexogenous ethylene, ethylene can be excluded from display areas and thus doesnot pose a major threat in the retail outlet. In impatiens, however, growersoften encounter premature bud and flower drop induced by environmental orcultural conditions, and it would thus be critical to evaluate the possible useof MCP for protection under these circumstances.

Grower Implications

Results of this study demonstrate that the water stressoften imposed on impatiens in the greenhouse and retail outlet caused bud dropin double impatiens and not in Impatiens wallerana. Prolonged holding of plants (all three types ofimpatiens) in an interior environment and boxing plants for transportationinduced bud drop. The stress-induced premature bud drop was reduced bypre-treating plants with MCP.

Results and discussion

Impatiens walleranawere propagated by seed, whereas New Guinea impatiens and double impatiens wereobtained from Color Links. Plants were planted in 4-inch containers filled withPro-Mix (Premier Horticulture) and were grown in the glasshouse at theUniversity of Massachusetts until flowering, at which time they were subjectedto either light and/or water stress. The low-light condition was set up tosimulate plants moved from a greenhouse to an interior retail outlet, whereasthe water stress was set up to mirror the wilting of plants often observed ingreenhouses as well as in retail stores. A low-light condition was obtained bymoving flowering plants into a 70º F laboratory illuminated 12 hours a daywith cool white fluorescent lamps. Water stress was provided by allowing plantsto wilt in the greenhouse before watering them again. Half of the plants werepre-treated with MCP prior to being exposed to the stress whereas the controlplants were not treated with any chemicals. The degree of wilting was monitoredby measuring the soil moisture content. The number of buds abscised from eachplant was monitored. Furthermore, simulated long-distance shipment wasconducted by placing flowering plants in boxes for different durations (zero,three or five days) followed by evaluation in the interior environmentmentioned above. Half of the plants were pre-treated with MCP prior to placingthem in the box whereas the controls were left untreated. The number of flowersthat opened and buds that prematurely abscised from each plant were monitored.

Impatiens wallerana

Low light condition.Premature bud drop occurred when plants were held in an interior environmentfor more than five days (see Figure 1, left). Bud drop increased rapidly duringthe second week. Pre-treatment with MCP reduced bud drop but the benefits ofthe MCP treatment was not realized until the plants had been held in thelow-light condition for more than one week. In addition, the size of the openflowers were significantly Á smaller when plants were held in thelow-light environment, presumably due to limited carbohydrates required for thedevelopment and opening of the flowers.

Water stress.Wilting of plants in the greenhouse (soil moisture content was approximatelyequal to 45 percent at the time of wilting) did not induce significant buddrop. In all three cultivars evaluated, bud drop on each plant was less than 10with most of the plants losing less than three buds during a three-week periodfollowing the water stress (see Figure 3, page 59). However, growth of thewilted plants in the Dazzler series (PanAmerican Seed) was affected by thewater stress and the plants weighed less than the non-wilted plants four weeksafter wilting, although visually, the differences were not detectable.Treatment with MCP had no effect in the water-stress-induced growth reduction.

Simulated transportation. Placing plants in cardboard boxes for three or five dayssignificantly induced premature bud drop. Pre-treatment with MCP significantlyreduced bud drop and should be used in long distance transportation of pottedimpatiens.

Double impatiens

Low light condition.Low light induced rapid and large numbers of bud drop in double impatiens. Thirtyto 50 percent of the buds had dropped by day five in the interior environment.Pre-treatment with MCP delayed the onset of bud drop, and no bud drop occurreduntil after holding the plants in the low-light condition for a week. If plantsare to be held longer than one week, a second application of MCP would furtherdelay the onset of bud drop. A third treatment with MCP would not be necessary,as additional benefits from that treatment were not observed.

Water stress.Wilting of double impatiens yielded variable results. In one study, up to 25percent of the buds abscised within three weeks after the wilting of ‘FiestaPink Ruffles’ (Ball Floraplant) whereas in another study on ‘Fiesta OleCherry’, less than 4 percent of the buds abscised. Comparison of the soilmoisture content showed that the soil moisture content in the former was lowerto 46 percent before water was applied whereas the soil moisture content in thelatter was 56 percent. Visually, both cultivars appeared to wilt to the samelevel, although the available water content was quite different. We suspectthat the variable result on bud drop was a result of the differences in thesoil moisture content at the time of wilting. Data on bud abscission indicatedthat bud drop continues many days after the wilting (see Figure 4, page 59). Inaddition, wilting is associated with an increase in plant death and should beavoided.

The effects of MCP were unclear. In the study wheresignificant bud drop occurred following wilting, MCP significantly reduced buddrop. Whereas, in the other study in which wilting did not induce bud drop, MCPhad no additional benefits (see Figure 4, page 59) investigation on thebeneficial effects of MCP on double impatiens should be investigated prior tomaking a sound conclusion.

Simulated transportation. A 3- or 5-day storage in cardboard boxes induced massive bud drop indouble impatiens with 20 and 60 percent of the buds abscised, respectively,within four days after removing plants from the boxes (see Figure 2, page 58).Pretreatment with MCP significantly reduced bud drop suggesting that all doubleimpatiens intended for boxing would benefit from a pretreatment with MCP andemphasizing the importance of removing plants from the boxes as soon as theyreach their destination.

New Guinea impatiens

Low light condition.Regardless of cultivar, bud abscission began 4-5 days after placing the plantsin an interior environment. Treatment with MCP before placing the plants in thelow-light environment significantly delayed and reduced bud abscission.

Simulated transportation. Storage in boxes greater than three days induced bud abscission withapproximately 20 percent buds abscised within five days after removal from thebox. Boxing for three days induced some bud abscission. MCP treatment beforestorage reduced abscission indicating that plants intended for long distancemarketing should be pre-treated with MCP.

Summary

Generally speaking, double impatiens are more susceptible tostress-induced bud drop than Impatiens wallerana and New Guinea impatiens. Low light (typical in an interiorenvironment) and prolonged boxing induced premature bud drop, which could bereduced by pretreating the plants with MCP. Wilting of plants in thegreenhouse, a common cultural practice either to restrict growth of the plantsor by negligence, resulted in massive bud drop in double impatiens when soilcontent dropped to approximately equal to 45 percent. Treatment with MCP helpreduce bud drop but to a limited extent. In contrast, Impatienswallerana tolerated wilting well and didnot drop large numbers of buds. Data taken four weeks after wilting, however,showed that wilting reduced the subsequent growth rate of plants.

This article was reprinted with permission of FloricultureIndustry Research and Scholarship Trust.

Susan Han

Susan Han is associate professor at the University of Massachusetts, Amherst, Mass. She can be reached by E-mail at susanh@umext.umass.edu



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