Response of Detached Leatherleaf Fern Fronds to Silver Nitrate Pulses

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University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report, RH-92-19

R.J. Henny and W.C. Fooshee*

Postharvest wilt of detached leatherleaf fern fronds involves both reduced water uptake by the stipe, and excessive water loss by the pinnae, which leads to frond desiccation. Wilting fronds show a loss of fresh weight that may drop to one third of their original harvest weight 1, 4). Fronds in the early stages of wilt, but not yet having reached the permanent wilting point, can be rehydrated by cutting off a 1 cm (0.4 inch) segment of the stipe base (4). Rehydrated fronds averaged a 32% increase in fresh weight during the first 24 hour period following recutting while fronds allowed to reach the permanent wilting point (i.e. physiologically dead) could not be revived. These results indicate that blockage of water flow in the lower stipe contributes to wilt. However, in studies using a scanning electron microscope to examine the cut end of the stipe, no physical blockage of the xylem vessels was found (6, 7).

The role of stipe blockage in wilt has been brought into question by published results showing that chemically enhanced solution uptake in detached fronds is not correlated to increased vase life (7). In fact, postharvest treatments involving holding solutions with pH of 2.0 or 3.0, 800 ppm 8-hydroxyquinoline citrate, 200 ppm citric acid or chemical pulses that increase water uptake were inversely related to vase life. The only exception was a 15 minute pulse with 800 ppm 8-hydroxyquinoline citrate that increased water uptake and also resulted in a longer vase life (7). Daily recutting of the stipe also increased water uptake and vase life but did not involve chemical additives (4, 5).

A similar postharvest wilt problem has been reported with detached maidenhair fern fronds that survived an average of only 2-3 days in deionized water (2). Studies of wilt in maidenhair fern established that a short vase life was due to inhibition of water flow through the rachis (2). Adding 25 ppm silver nitrate to the holding solution increased average vase life of maidenhair fronds from 2 to 14 days.

In contrast, pulsing solutions containing silver did not increase vase life of leatherleaf fern (8). Silver nitrate was not among the silver-containing compounds tested, therefore the following experiments were conducted to evaluate pulsing with silver nitrate solutions and subsequent effects on postharvest water uptake and vase life of leatherleaf fern.

Materials and Methods

On March 25, 1991 dark green, mature fronds of similar size were harvested from fern growing in ground beds at CFREC-Apopka in a polypropylene shadehouse with 73% light exclusion. Fronds were taken to an air-conditioned interior room maintained at 80 + 2F with 12 hours of light (100 foot-candles intensity at frond height) from fluorescent lights. The stipes were cut with a new single edge razor blade to 15 cm (6 inches) in length and placed into beakers containing deionized water (DI) until treatment with silver nitrate solutions.

Treatments consisted of pulsing fronds in silver nitrate at three concentrations (25, 50 or 100 ppm) for three intervals (20, 40 or 60 minutes). In addition, an untreated control was maintained in DI. After pulsing, individual fronds were weighed and placed into 100 ml graduated cylinders containing approximately 90 ml DI. The final volume was then adjusted to 100 ml with the frond in place. Data consisted of recording total water uptake for 14 days - the length of the experiment. Water volume was adjusted to 100 ml each day that data were taken. Wilt or yellowing was recorded daily. The experimental design was randomized block with each treatment replicated 10 times for a total of 100 fronds in the test. This experiment was repeated on May 6, 1991.

Results and Discussion

In these experiments, fronds pulsed with silver nitrate at different rates and durations showed no significant difference in postharvest vase life (Tables 1 & 2). However, the treated fronds used greater amounts of water than untreated controls. In both experiments water uptake by treated fronds was close to double that for controls over the 14 days. In experiment 1, a 100 ppm pulse for 60 min resulted in 98% greater uptake than control while in experiment 2 the maximum uptake followed a 25 ppm pulse for 40 min and was 124% greater than untreated. This is the second report in which vase life of chemically treated leatherleaf fern fronds was not reduced while water uptake was increased.

These results concur with conclusions that increased water flow through detached leatherleaf fern fronds is not essential for prolonged vase-life since fronds that used twice as much water as controls did not have a greater average vase life. In the second experiment, no wilt was observed and shortened vase life was due to yellowing of the fronds. It is possible that the large increase in water uptake in response to pulsing with silver nitrate accelerated natural frond senescence.

Stamps and Nell (8) tested four different silver-containing compounds on leatherleaf fern but none of them had any effect on vase life even though they increased holding solution uptake. Fujino and Reid (2, 3) in their study on maidenhair fern, initially selected silver nitrate and tested it in conjunction with several other compounds with bactericidal properties because it was theorized that blockage may have been due to bacteria in the holding solution or within the rachis itself. Because none of the other bactericides increased maidenhair fern vase life, the authors concluded that the blockage was a physiological response to wounding and not bacteria. A-physiological response to wounding would implicate the presence of ethylene; however, an experiment using ethylene inhibitors on leatherleaf fern failed to increase vase life (8). Such information makes it difficult to explain the actual mode of action of silver nitrate in these two experiments.

Table 1. The effect of silver nitrate (AgNO₃) pulses on water uptake and vase life of detached fronds of leatherleaf fern. Experiment 1.

^zMaximum value possible was 14 days.
^yNS = nonsignificant and ** = significant at 1% level. There were no significant interactions.

Table 2. Total water uptake, fresh weight change and average vase life of leatherleaf fern fronds as affected by pulses of silver nitrate (AgNO₃). Experiment 2.

^ZMaximum value possible was 14 days.
^YNS = nonsignificant, ** = significant at 1 % level. There were no significant interactions.

*Professor, Environmental Horticulture and Senior Biologist, Central Florida Research and Education Center - Apopka, 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-79-1.

2. Fujino, D.W. and M.S. Reid. 1983. Factors affecting the vase life of fronds of maidenhair fern. Scientia Horticulturae 21: 181-188.

3. Fujino, D.W., M.S. Reid and H.C. Kohl. 1983. The water relations of maidenhair fronds treated with silver nitrate. Scientia Horticulturae 19:349-355.

4. Henny, R.J. 1982. Reversing postharvest wilt of leatherleaf fern. ARC-Apopka Research Report RH-82-23.

5. Henny, R.J. and W.C. Fooshee. 1984. Daily recutting of stipe affects postharvest vase-life, water uptake and fresh weight change of leatherleaf fern fronds. AREC-Apopka Research Report RH-84-25.

6. 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.

7. 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.

8. 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.