Factors Affecting Postharvest Quality of Cut Lillies
Production and sales of cut lilies have increased dramatically in recent years. According to the latest published USDA Census in 1998, over 76 million stems of cut lilies were produced in the United States, with a value exceeding $54 million. Many growers find the crop to be very profitable and relatively easy to schedule for year-round production. In addition, many florists prefer or demand locally grown lilies.
While it is true, in general, that shipping cut flowers long distances reduces quality, it is particularly noticeable in cut lilies. Cold storage of cut lilies, a standard practice to preserve freshness is, however, inevitable, and with the increase in global transportation, the duration and temperature of cold storage can vary greatly. The effects of cold storage on buds are not evident when plants are removed from the cold room, thus giving buyers little indication of cut stem quality.
At the University of Massachusetts, we have been investigating factors that affect postharvest leaf and flower quality of cut Asiatic and Oriental lilies. Unless otherwise stated, most of the experiments were conducted on ‘Stargazer’ Oriental lily harvested from a local commercial greenhouse. Our studies focused on means to improve overall quality of the cut stems, taking into consideration the quality of both the leaves and the flowers.
Sensitivity to ethylene
It is generally accepted that cut lilies are sensitive to ethylene and that treatment with silverthiosulfacte (STS) increases longevity. In fact, all cut lilies processed through the Dutch and New Zealand auctions have to be pretreated with STS. The benefit of treating cut lilies with STS, however, is debatable.
Van der Meulen-Muisers and van Oeveren studied 16 cultivars of Asiatic lilies and concluded that “the possibilities (of STS) for extending flower longevity are limited.” I concur with that conclusion because exposure of Á freshly cut Stargazer stems, when exposed to 1 ppm and 10 ppm ethylene gas for 24 hours, showed no effect on leaf or flower quality. Pretreatment of those stems with STS induced early development of leaf yellowing, without any beneficial effect on the flowers. Furthermore, treatment with Ethylbloc (1-methylcyclopropene), another ethylene inhibitor, did not improve postharvest quality.
Our results indicate that Stargazer lilies are not sensitive to ethylene. On the other hand, when Stargazer was treated with EthylBloc prior to a 2-week cold storage, we saw a significant drop in cold storage-induced bud abortion, suggesting that pretreatment with an ethylene inhibitor might be necessary for stems that need to be cold-stored.
There are two possible explanations for the differences in ethylene response between lilies that have been cold-stored and those that are freshly harvested. First, it is possible that the sensitivity of the flowers to ethylene increases during cold storage. Second, it is possible that an ethylene receptor protein, which must be present in order for the ethylene to function as a trigger for a senescence process, was not present in freshly cut stems, a fact that has been shown in Arabidopsis. In Arabidopsis, ethylene receptors do not exist in “presenescent” or “late senescent” flowers but are found in “early senescent” flowers, and it is possible that ethylene receptors in lily buds do not exist until they have been stressed from cold storage and are undergoing premature bud senescence.
Sugar in vase solution and defoliation
Leaf yellowing is one of the most serious postharvest problems in cut lilies because it is aesthetically unpleasing. Therefore, to minimize leaf yellowing, the customary practice among florists is to either remove most, if not all, of the leaves or to place the cut stems in a sugarless solution.
Leaves however, are an important part of cut flowers, and in many Á species, blemish-free foliage is an important factor in determining the quality of the cut stems. In many cut flowers, leaves also serve as a carbohydrate source for the developing buds, and their removal may affect the quality of flowers.
We conducted a series of experiments to determine the role of sugar in the preservative solution and in the leaves on postharvest-quality cut lilies. We found that for Asiatic lily ‘Vivaldi’ and ‘Geneva’ and Oriental lily ‘Acapulco’ and Stargazer, lower leaves began to turn yellow between 1 and 2 weeks on freshly harvested stems, and the yellowing continued to progress upward until the end of the vase life. Addition of 2 percent sugar in the vase solution induced earlier development of leaf yellowing (Figure 2), thus supporting the claims from retail florists that stems should be placed in a solution without sugar. However, we know that spraying the leaves with a solution containing 50-100 ppm each of BA and GA4+7 (currently registered as Fascination) completely prevents leaf yellowing and significantly improves the quality of the buds.
In terms of flower buds, it is known that both sugar in the solution and carbohydrates stored in the leaves play an important role in the quality of the open flowers. Neither the addition of sugar nor defoliation affected the longevity or the size of Stargazer flowers, but addition of sugar and keeping the leaves intact significantly enhanced the intensity of petal color. In defoliated stems, sugar can also be used to counteract the light-color petals, so the addition of sugar to the preservative is even more critical to those stems.
Overall, for best petal coloration and leaf quality, cut stems should be placed in a solution containing 2 percent sugar and the leaves should be left intact and sprayed with a solution containing BA and GA4+7.
The quality of leaves and flowers of cut lilies suffers after cold storage. Two cultivars of cut Asiatic lily, Geneve and Vivaldi, and two cultivars of Oriental lily, Stargazer and Acapulco, were used in our study. The study indicates that storage in a 38¡ F cold room for more than one week significantly increased bud abortion, reduced longevity of flowers and reduced the vase life of cut stems in all cultivars. There are, however, differences in the susceptibility of cultivars to cold storage-induced bud abortion.
In addition, we conducted an in-depth study on the effects of cold storage on Stargazer lilies, the cultivar most susceptible to cold storage-induced disorders. We found that after cold storage, buds often do not fully develop into open flowers (they were either opened partially or aborted), are much smaller in size and are paler in color. Overall, the quality of the open flowers of Stargazer lilies cold-stored for two weeks is such that they would not be considered acceptable by the industry. However, we found that the harvesting stage of the stems is a critical factor, as there was a dramatic increase in cold storage-induced bud disorders for stems harvested at a slightly earlier stage. In addition, we found that placing the cut stems in a sugarless solution during cold storage significantly reduces cold storage-induced bud disorders when compared to those sleeved and stored dry.
Furthermore, studies conducted on potted Stargazer showed that storage temperature is another important factor to consider. Storage at 33¡ F for two weeks resulted in 100 percent bud abortion in potted Stargazer, whereas storage temperatures of 39¡ F or higher reduced bud abortion to approximately 30 percent. Adding light or spraying leaves with a solution containing 100 ppm each of BA and GA4+7 further reduced bud abortion. While providing sufficient light to leaves during cold storage might not be a viable option for cut lilies, since they are typically bundled and sleeved prior to that point, this study suggests that lilies should be stored in a cooler with a somewhat higher than usual temperature.
Conventionally, cut flowers intended for long-term storage or long-distance transportation are harvested at a tighter bud stage than those intended for the local market, and are stored dry in a 33¡ F cooler. Based on our information, handling of cut lilies, specifically Stargazer, should be different. For cut lilies, stems should be harvested at a more mature stage and stored wet in a cooler set at approximately 40¡ F to minimize the damage caused by cold storage. The major drawback of handling lilies this way is that buds will continue to develop during the cold storage and some might even open while in the cooler.
In addition to the effects of cold storage on flower quality, cold storage also worsened leaf disorders (Figure 2). The longer the duration of cold storage, Á the earlier the development of leaf yellowing and the higher the percentage of leaves that were chlorotic. In our study, we used a 38¡ F cooler for 1-2 weeks, a span that is not uncommon with cut flowers, considering that growers, shippers, wholesalers and retailers each keep flowers in the cooler for a few days.
The good news is that spraying leaves with Fascination significantly reduced cold storage-induced leaf yellowing. Furthermore, the growth regulator treatment improves bud quality by reducing bud blasting and increasing vase life, suggesting that, commercially, it would be best to treat all cut lilies with the growth regulator solution. It is interesting that the timing of the Fascination application is not critical. The solution can be sprayed before or after cold storage with the same results. Therefore, cut lilies can be sprayed with growth regulators by handlers at any level, including growers, wholesalers or florists. This is great information for florists who often bear the postharvest loss associated with prolonged cold storage.
In conclusion, there are several means that can be employed to minimize, but not completely overcome, the detrimental effects of cold storage on cut lilies: 1) select cultivars that are less susceptible to cold storage-induced disorders; 2) make certain that cut stems are at the optimum stage of harvesting, which varies between cultivars; 3) spray all leaves with Fascination either before or after cold storage; 4) place cut stems in a sugarless solution during cold storage and then place them in a preservative solution containing sugar after the cold storage; 5) set cooler temperature at approximately 40¡ F; and 6) minimize the duration of cold storage time.