Return to: CFREC Home Page
Return to: CFREC Research Index
University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report RH-92-2
R.J. Henny, R.H. Stamps and W.C. Fooshee*
Postharvest wilt of leatherleaf fern fronds is a continuing problem for commercial producers in Florida. Two types of postharvest decline of leatherleaf fern fronds are discussed in the literature: 1) wilt; and 2) yellowing. Fern wilt is characterized by partial or complete folding of green pinna (leaflets) along the midvein and/or loss of overall rigidity of the frond while yellowing of fronds begins along the midveins of pinna and spreads toward the margins (11).
Fern wilt has also been called frond 'curl' (7) to distinguish it from various types of wilt diseases; however, in this paper 'wilt' will be used exclusively since it is the term commonly used in industry to define premature postharvest desiccation of fronds. Yellowing has been attributed to natural frond senescence and is considered a separate phenomenon from wilt (5).
Both field and laboratory studies have been employed in the study of fern wilt. Production conditions studied include type of growing structure (shadehouses or natural oak hammocks), shade and fertilization levels, watering rates and intervals, effects of herbicides, frond age, harvest time and conditions, season of harvest, handling procedure before and during packing and conditions during storage and shipment. Laboratory studies have involved gathering of data in climate controlled rooms including measurements of water usage, fresh weight change, and vase life of individual fronds. These studies have tested various types of floral preservatives, holding solutions, and chemical pulses and their effects on water use and vase life.
Experiments on fern wilt usually encounter a relatively high degree of variability within treatments. Vase life of fronds harvested from the same growing areas within a few days of each other may be different, whereas water uptake and vase life of individual fronds of the same weight, size, appearance and placed in the same holding solutions can be very dissimilar. To date, there is no single explanation for these inconsistencies. This paper will focus on published results from studies involving postharvest vase life of leatherleaf fern and point out areas where data are consistent and what implications these data provide.
Production Conditions:
Direct linkage of field production conditions to vase life of leatherleaf fern is difficult because of many interacting factors. Still there have been consistent trends in many of the reports. In one study, fronds harvested from shadehouses had a higher incidence of wilt than those from an oak hammock (5,4). The amount of fern wilt has also been reported to vary between producers. Marousky (4) found that 50% of fern from one commercial source wilted while fronds from another grower had no wilt. In a 3 year study of 3 commercial ferneries, the overall incidence of wilt 12 days postharvest varied from 23% for one grower to 8% for another - a three-fold difference (5). In another study, the effects of light levels varied depending on the location. There was no difference in wilt of fern harvested from 63% or 73% shade from one commercial fernery, but fern produced under 63% shade from a different fernery exhibited almost twice as much wilt as fern produced under the 73 % shade level (11). Finally, fern harvested from 63% or 73% shade provided by black polypropylene fabric showed no difference in vase life (11).
Little is known about the effects of production fertilization practices on vase life. Conover et al. (1) suggested that growers avoid urea as a source of nitrogen in favor of a source that provides 40-60% of nitrogen in the nitrate form. In the same report, a comparison of fern receiving 900 lbs N/A/yr versus 450 lbs N/A/yr seemed to indicate that fertilization level was not a factor. In another experiment, fronds produced using 750 lbs N/A/yr and 1500 lbs/N/A/yr had the same vase life (15).
Fern wilt has been shown to be seasonal with the highest occurrences during hot periods. Mathur et al. (5) found that most wilt occurred from July through October. In a similar study (9), it was found that fronds developed from fiddleheads emerging in June, July and August had the shortest vase life. These findings were verified by a third study (11) in which it was suggested that high temperature stress during these months may contribute to wilt. Similar declines in frond vase life for fronds produced during hot weather compared to those produced during cooler weather have been reported (15). Effects of production temperature on vase life have also been studied using growth chambers. One study involved high (86°F day/77°F night) and low (68°F day/59°F night) temperature regimes both with the same light levels; vase life of fronds grown at the high temperature regime was 28 to 66% less than those grown at the low temperature regime (13). This data supports the observation of shorter vase life for fronds harvested during the summer months. Stamps (21) conducted another similar study using growth chambers and found that 81% of fronds produced at the high (95/75°F) temperature regime exhibited desiccation symptoms while none of those from the low (75/55°F) temperature regime did.
Other environmental factors need to be considered during wilt season; these include increases in photoperiod and total irradiation, high night temperatures and amount of water the fern beds receive (either rainfall or irrigation). Of those four factors, only irrigation levels have been mentioned in the literature. Conover et al. (1) mentioned that overwatering of fern was a factor that seemed to aggravate wilt. This would correspond with the higher rain levels occurring during the summer months. A reduction of irrigation level by approximately 50% resulted in increased vase life on 3 of 7 dates in one study (6). In another study, fern from beds that had been watered the day before harvest did not last as long as fern from beds that had not been watered for a week (10).
Pesticides are used routinely during the production of leatherleaf fern, especially during the summer months when fern wilt is most prevalent. Several studies have been conducted to determine if pesticides affect fern vase life. The first study (22) examined possible effects of selective herbicides on vase life. None of the herbicide treatments reduced vase life compared to the untreated control. In fact, vase life of oxadiazon treated fronds was greater than the control for one harvest. A subsequent study of four herbicides applied in 15 rates and combinations, again, found no herbicide effect on vase life (22). A short term study of the effects of two fungicides (benomyl and mancozeb) commonly used on leatherleaf fern showed no effect on vase life (17).
Frond age at the time of harvest has been shown to be a significant factor in vase life. Marousky (4) showed that fronds with mature pinnae had a greater degree of wilt than fronds with immature pinnae. In that study, mature fronds were dark green, fully expanded and commercially acceptable while an experienced fern cutter judged immature fronds to be 2.5 weeks younger than mature fronds. Another extensive study involved tagging newly emerged fiddleheads every 4 weeks for 1 year. Fronds were harvested 6 weeks after tagging and again at 4-week intervals until 6 harvests had been completed. Fronds harvested 6 weeks after emergence usually had longer vase life than older fern although results were inconsistent (9).
Postharvest Conditions:
Since harvested fronds are commonly dipped in various solutions before being packed and stored, these factors have been investigated to test for effects on vase life (14). Data indicate that leatherleaf fern fronds are tolerant of storage. Two studies found that storage of fronds did not affect vase life (16, 19); however, a storage temperature of 40F for 10, 21 or 31 days was reported to be better than 75°F (16). It was also found that prestorage dipping of fronds in the fungicides benomyl or iprodione was not detrimental to subsequent vase life (16).
One consistent characteristic of fern wilt is the involvement of an insufficient water supply to, or excessive water loss from the frond. This has been verified using two different approaches. In one report, wilted fronds weighed on the day wilt was first observed showed an average decrease in fresh weight of 22.9% compared to initial fresh weights (3). In another study, average frond discard weights in treatments that wilted were 79-87% of initial weights, but in non-wilting treatments they were 95-100% of initial weights (21). In the second approach, wilt symptoms were reversed by cutting off a 1 cm section (recutting) from the base of the stipe (2). Recutting resulted in a large increase in water uptake and frond rehydration within 24 hours following recutting (except when fronds had reached the permanent wilting point). These rehydrated fronds showed an average gain of 31.8% in fresh weight during this period. In addition to implicating water deficits, such data indicate that factors in the lower 1 centimeter of the stipe are significantly involved in the occurrence of wilt. Characterizing the nature of these factors and their effect on frond water relations is a key step in understanding wilt.
The water uptake curve of individual fronds is another consistent postharvest response of leatherleaf fern fronds. Water uptake during the first 24 hour period exceeds that taken up during the second 24 hour period. The rate of subsequent water uptake continues to decline over the course of 5-7 days and then remains fairly constant, though at a much lower rate than during the first day. There may be differences in total water uptake between experiments, but the slope of the uptake curves remains similar. This decline in water uptake has been shown to be logarithmic/exponential (13, 21).
Possible explanations for the observed decline in water uptake include: the gradual closing of stomates retarding the amount of water loss by the frond; development of blockage in the vascular system reducing the ability of the stipe to transport water; or, a combination of stomatal closure and stipe blockage.
If stomatal closure is a significant factor in reducing water uptake, they apparently close gradually over several days or water uptake would be less than observed during the first 24 hour period; however, this would conflict with reports that stomates close within a few hours after cutting (7). In that study, frond water potentials, as measured by a diffusive resistance porometer, indicated that stomata began to close within 30 minutes of harvest. Attempts to check for stomatal closure by microscopic examination have not been conclusive because of difficulty in determining if stomates are open or closed by visual examination alone (Henny and Fooshee, unpublished). Stomatal impressions taken from fronds at different times or conditions appear similar.
The occurrence of some type of vascular blockage has been suggested in the literature (2, 7, 3). Unfortunately, evidence is still indirect and based on water uptake data. The only attempt to locate blockage using a scanning electron microscope did not show any vascular blockage (7); however, it is possible that the blockage was lost during specimen preparation. It is interesting to note that frond. grown at higher temperatures in growth chamber studies had narrower stipes with fewer vascular bundles and smaller tracheid lumen areas compared to fronds grown at lower temperatures (13). Fronds with reduced vascular systems took up less water, lost weight more rapidly and had reduced vase life compared to fronds with larger stipes. Since frond surface areas and stomatal densities were not different, the reduced water uptake was probably due to the restricted vascular system. In fact, stipe water conductivities measured at the time the fronds were placed in water were lower for the narrower stipes.
Water uptake data strongly indicate that some form of vascular blockage is located in the basal 1 cm of the stipe. Total water uptake in detached fronds increased when 1 cm was removed (i.e. recutting) from the stipe base on a daily basis (3). The resulting water uptake curve retained a similar slope as uncut fronds (i.e. decreasing over time), but the total amount of water uptake approximately doubled that of fronds that were not recut. Possible explanations for these results are that either stomates close gradually over several days following harvest reducing water loss and hence a need for uptake, or blockage, initially present only in the base of the stipe, progresses throughout the entire stipe. One argument against progressive blockage is that rehydration of fronds, that did not wilt until several days after harvest, can be achieved by recutting the basal l cm of the stipe. Had blockage progressed throughout the stipe during this time, recutting only the base would have little effect.
Discussion of stomates, vascular blockage and water uptake leads to an apparent contradiction: chemically extended vase life of fronds has not been correlated with higher amounts of water uptake (Stamps, 1983). In fact, postharvest treatments involving holding solutions or pulses that increase solution uptake were inversely related to vase life. This is based on data from tests using holding solutions consisting of low pH, 200 ppm 8-hydroxyquinoline citrate or 200 ppm citric acid (19). In each case, solution uptake was greater than for fronds held in deionized water, but vase life was reduced. Similarly, fronds that were put under water stress following harvest took up more water, but had shorter vase life than nonstressed fronds that took up less water (7). Stamps and Johnson (18) also reported a negative correlation between vase life and water uptake. The only exception is the case of daily recutting of fronds; this treatment increases both daily water uptake and vase life (3). It should be noted that recutting involves only the basal one centimeter of the frond and not the physiology of the entire frond as do chemical treatments.
An alternative approach to increasing water uptake to maintain water balance in harvested fern fronds has been the use of antitranspirants to reduce water loss (8). Antitranspirants were applied as field sprays 3-28 days before harvest or as postharvest dips. Field application did not affect postharvest water use whereas postharvest dips reduced water use during the first 24 hours; however, the incidence of wilt was not affected by either treatment method. In addition, sticky residues from pinolene-based antitranspirants were a problem on treated fronds.
Stamps and Nell (20) showed that pulses containing ethylene inhibitors did not extend vase life, indicating that sensitivity to ethylene was probably not a factor in wilt. Pulsing for 10-15 minutes with 800 or 1000 ppm 8-hydroxyquinoline citrate increased vase life compared to water controls in two different reports (19, 20).
An intriguing aspect of fern wilt is that 2 fronds can be similar in appearance and size, harvested from the same bed, with similar postharvest water-uptake curves and yet one will wilt within a few days while the other may last for weeks. Such results may point to physiological conditions predisposing fern to wilting under certain conditions. This concept has been mentioned in the literature (7) and determining the nature of this condition may be another key step in finding methods of preventing wilt.
*Professor, Plant Geneticist; Associate Professor, Cut Foliage Specialist; Senior Biological Scientist, Central Florida Research and Education Center, 2807 Binion Road, Apopka, FL 32703-8504, respectively.
Literature Cited
1. Conover, C.A., R.T. Poole and L.L. Loadholtz. 1979. Update on leatherleaf fern wilt. ARC-Apopka Research Report RH-79-l.
2. Henny, R.J. 1982. Reversing postharvest wilt of leatherleaf fern. ARC-Apopka Research Report RH-82-23.
3. Henny, R.J. and W.C. Fooshee. 1984. Daily recutting of stipe affects postharvest vaselife, water uptake and fresh weight change of leatherleaf fern fronds. AREC-Apopka Research Report RH-84-25.
4. Marousky, F.B. 1983. Premature wilt of leatherleaf fern with different pinnae maturities from various growing environments. Proc. Fla. State Hort. Soc. 96:270272.
5. Mathur, D.D., R.H. Stamps and C.A. Conover. 1982. Postharvest wilt and yellowing of leatherleaf fern. Proc. Fla. State Hort. Soc. 95: 142-143.
6. Mathur, D.D., R.H. Stamps and C.A. Conover. 1983. Response of Rumohra adiantiformis to water application level and nitrogen form. HortScience 18(5):759760.
7. Nell, T.A., J.E. Barrett and R.H. Stamps. 1983. Water relations and frond curl of cut leatherleaf fern. J. Amer. Soc. Hort. Sci. 108(4):516-519.
8. Nell, T.A., C.A. Conover, J.E. Barrett and R.T. Poole. 1985. Effects of pre- and postharvest anti-transpirant applications on vase life of leatherleaf fern. Scientia Horticulturae 26:225-230.
9. Poole, R.T., C.A. Conover and R.H. Stamps. 1984. Vase life of leatherleaf fern harvested at various times of the year and at various frond ages. Proc. Fla. State Hort. Soc. 97:266-269.
10. Poole, R.T., R.H. Stamps and C.A. Conover. 1985. Fern wilt: The continuing quest for the culprit. Univ. of Fla. Commercial Fern Grower 8(7): 1-2.
11. Stamps, R.H. 1981. Effects of production shade level on postharvest decline of leatherleaf fern. ARC-Apopka Research Report RH-81-16.
12. Stamps, R.H. 1983. Effects of harvest time, postharvest dips, storage and frond physical characteristics on postharvest decline and water uptake of leatherleaf fern. HortScience 18(4):563. (Abstract).
13. Stamps, R.H. 1984. Production temperature effects on anatomy, morphology, physiology and postharvest longevity of leatherleaf fern (Rumohra adiantiformis Forst. Ching). PhD Dissertation, University of Florida, Gainesville. 122 pp.
14. Stamps, R.H. 1987. Harvest time and postharvest immersion affect water uptake and postharvest decline of leatherleaf fern. Proc. Intern. Soc. Trop. Hort. 31:42-50.
15. Stamps, R.H. 1989. Biostimulant and high fertilizer rates do not affect leatherleaf fern frond development, yield or vase life. Proc. Fla. State Hort. Soc. 102:274-276.
16. Stamps, R.H. and A.R. Chase. 1984. Fungal inoculation, fungicide treatments, and storage affect postharvest decay and vase-life of leatherleaf fern fronds. HortScience 19(2):292-293.
17. Stamps, R.H. and A.R. Chase. 1987. Foliar applications of benomyl and mancozeb do not affect leatherleaf fern carbon assimilation, transpiration, light compensation point or vase life. Proc. Fla. State Hort. Soc. 100:362-364.
18. Stamps. R.H. and C.R. Johnson. 1984. Vesicular-arbuscular mycorrhizal inoculation and fertilizer level affect yield, morphology, chlorophyll content, water uptake and vase life of leatherleaf fern fronds. Proc. Fla. State Hort. Soc. 97:264-266.
19. Stamps, R.H. and T.A. Nell. 1983. Storage, pulsing, holding solutions and holding solution pH affect solution uptake, weight change and vase life of cut leatherleaf fern fronds. Proc. Fla. State Hort. Soc. 96:304-306.
20. Stamps, R.H. and T.A. Nell. 1986. Pre- and postharvest treatment of cut leatherleaf fern fronds with floral preservatives. Proc. Fla. State Hort. Soc. 99:260-263.
21. Stamps, R.H., T.A. Nell and D.J. Cantliffe. 1989. Production temperature affects leatherleaf fern postharvest desiccation. HortScience 24(2):325-327.
22. Stamps, R.H. and R.T. Poole. 1987. Herbicide effects during leatherleaf fern bed establishment. HortScience 22:261-264.