Response of Hardy Ferns to Drought Stress ^†

Abstract

Drought-tolerant perennials are intensively sought-after plants for decorative purposes. Perennial ferns are cold-hardy cultivars that are great for shady to partially shaded areas of the garden, but their response to drought stress is unknown. In this study, we examined how reduced watering altered the growth of Athyrium niponicum ‘Red Beauty’, Cyrtomium fortunei ‘Clivicola’, Dryopteris atrata, Dryopteris erythrosora, Dryoperis filix-mas, Dryoperis filix-mas ‘Linearis Polydactylon’, and Polystichum setiferum ‘Proliferum’ in comparison with optimal watering conditions. From among the seven fern taxa, Athyrium niponicum ‘Red Beauty’ and Dryoperis filix-mas turned out to be the most sensitive to reduced watering, and they demonstrated leaf browning and drying. Little visual leaf damage was observed in Cyrtomium fortunei ‘Clivicola’, Dryopteris erythrosora, and Polystichum setiferum ‘Proliferum’.

1. Introduction

Despite taking up the least space, growing ornamental plants is the most profitable branch of plant production [1]. Owing to the rising financial capability of buyers and the increasing popularity of urban gardening, the sectors of bedding plants and perennials are developing particularly quickly [2,3]. Unfortunately, the market of ornamental plants has deteriorated because of the global pandemic and related restrictions [4,5]. The crisis spurred a search for new ways of increasing demand, and one of them is introducing new plants that may seem interesting to customers. The species and cultivars of hardy garden ferns are still relatively poorly known, but they can be attractive ornamental plants grown on balconies, in parks, and in green areas [6]. They comprise numerous taxa differing in height, habit, and leaf shape and color. Additionally, many garden ferns retain their decorative foliage in the winter. Apart from their decorative value, many fern species are edible and medicinal plants [7].

The global climate change that is taking place today often results in water shortages and droughts [8]. Water becomes an increasingly scarce resource, especially in large urban agglomerations, and this requires changes in the care of urban green areas [9]. For this reason, attention is paid to the criteria of selecting ornamental plants that retain their decorative value despite drought stress [10]. Plant response to water shortage varies between species and even cultivars and depends on the intensity and duration of the stress [11]. Therefore, many experiments have been carried out to identify ornamental plants featuring increased resistance to unfavorable urban conditions [11,12].

In general, ferns grown in the soil as ornamental plants thrive in shaded and moist sites. However, some species and cultivars seem tolerant to water shortage and are capable of adapting to adverse environmental conditions [13]. This makes studies on the sensitivity of different hardy garden fern genotypes to abiotic stress highly justified, as they should allow for pinpointing the most tolerant species and cultivars. The aim of this study was to compare the growth and decorative value of seven taxa of ground ferns cultivated under optimal conditions and under drought stress. We also determined the long-term effect of drought on plant quality by assessing their condition after winter.

2. Materials and Methods

This study was carried out in a plastic tunnel set up by the Department of Horticulture at the premises of the West Pomeranian University of Technology in Szczecin, and it involved the following taxa of hardy ferns from the Dryopteridaceae family: Athyrium niponicum ‘Red Beauty’, Cyrtomium fortunei ‘Clivicola’, Dryopteris atrata, Dryopteris erythrosora, Dryoperis filix-mas, Dryoperis filix-mas ‘Linearis Polydactylon’, and Polystichum setiferum ‘Proliferum’. The plants were propagated in vitro. The plants grew in pots of 1.7 L capacity filled with peat substrate TS1 (pH 6.0) supplemented with PG Mix fertilizer at a dose of 1.0 kg m⁻³. For 30 days, the plants were cultivated in the substrate with a variable soil water content (SWC): 80% SWC (control) and 30% SWC (drought). On the last day of drought, we measured plant height from the ground level to its highest point, plant width at its widest point, and leaf greenness index in SPAD (Soil Plant Analysis Development) units with the SPAD-502 Chlorophyll Meter optical apparatus (Minolta, Japan). After the drought period ended, all plants were watered in the same way (80% SWC). The long-term impact of drought on plant quality was assessed 181 days after the stress cessation. The bonitation score from 1 to 5 was assigned by three independent researchers. Each variant comprised a total of four plants and four repetitions.

3. Results and Discussion

We found that plant response to drought stress largely depended on the species and cultivar. All plants exposed to drought were lower than those growing under optimal conditions. The greatest growth reduction caused by water shortage was noted in Dryoperis filix-mas (48%), and the smallest in Dryopteris erythrosora (25%). The order of the response intensity was as follows: Dryoperis filix-mas > Athyrium niponicum ‘Red Beauty’ > Dryopteris filix-mas ‘Linearis Polydactylon’ > Polystichum setiferum ‘Proliferum’ > Cyrtomium fortunei ‘Clivicola’ > Dryopteris atrata > Dryopteris erythrosora. The plants exposed to drought had a smaller diameter than those grown under optimal conditions (80% SWC). The difference in comparison with the control was the greatest (37%) for Dryoperis filix-mas, and the smallest (11%) for Polystichum setiferum ‘Proliferum’. The order of plant width differences from the control was as follows: Dryoperis filix-mas > Dryopteris atrata > Athyrium niponicum ‘Red Beauty’ > Dryopteris erythrosora > Cyrtomium fortunei ‘Clivicola’ > Dryopteris filix-mas ‘Linearis Polydactylon’ > Polystichum setiferum ‘Proliferum’. Drought stress clearly decreased the leaf greenness index, and the drop was the greatest (82%) in Dryopteris filix-mas ‘Linearis Polydactylon’, and the smallest (51%) in Polystichum setiferum ‘Proliferum’. For all investigated plants, the order of leaf greenness drop was as follows: Dryopteris filix-mas ‘Linearis Polydactylon’ > Athyrium niponicum ‘Red Beauty’ > Dryoperis filix-mas > Dryopteris erythrosora > Dryopteris atrata > Cyrtomium fortunei ‘Clivicola’ > Polystichum setiferum ‘Proliferum’.

Of the examined fern taxa, the one most sensitive to drought was Dryoperis filix-mas. Two taxa, Cyrtomium fortunei ‘Clivicola’ and Dryopteris erythrosora, demonstrated relative tolerance to water scarcity, as, despite limited watering, they retained their turgor and did not dry. Cyrtomium fortunei ‘Clivicola’ and Dryopteris erythrosora may be better adapted to drought stress due to delayed action of the stress factor in the cells and/or tolerance to the stress factor [14]. Drought tolerance consisting in avoidance of dehydration may result from specific adaptations of their leaves, which show succulent traits [15]. Leaf blades of Cyrtomium fortunei ‘Clivicola’ and Dryopteris erythrosora are stiffer and thicker than those of the other investigated plants. More extensive research on water storage capacity in fern rhizomes and leaves and the assessment of their gas exchange parameters may allow for better understanding of the resistance and tolerance mechanisms the ferns employ when challenged with water stress [7,13,16].

The visual score assessment after winter revealed that even 181 days after treatment, the ferns still showed visible signs of summer drought (Figure 1). Among ferns experiencing water shortage, the lowest bonitation score (2.0) was reached by Dryoperis filix-mas, and the highest (4.3) by Polystichum setiferum ‘Proliferum’ (

Table 1. Long-term effects of drought (assessed 181 days after the stress cessation) on visual score of seven fern taxa.

Fern Taxa	Visual Score
Athyrium niponicum ‘Red Beauty’	3.7
Cyrtomium fortunei ‘Clivicola’	4.0
Dryopteris atrata	3.5
Dryopteris erythrosora	3.8
Dryoperis filix-mas	2.0
Dryoperis filix-mas ‘Linearis Polydactylon’	3.3
Polystichum setiferum ‘Proliferum’	4.3

). The bonitation score increased in the following order: Dryoperis filix-mas > Dryopteris filix-mas ‘Linearis Polydactylon’ > Dryopteris atrata > Athyrium niponicum ‘Red Beauty’ > Dryopteris erythrosora > Cyrtomium fortunei ‘Clivicola’ > Polystichum setiferum ‘Proliferum’.

4. Conclusions

We demonstrated different responses of individual hardy fern taxa to water shortage. The stress negatively affected plant height and width, the leaf greening index, and decorative value after winter as assessed with the bonitation scale. Athyrium niponicum ‘Red Beauty’ and Dryopteris filix-mas were found to be the most sensitive to drought, while Cyrtomium fortunei ‘Clivicola’, Dryopteris erythrosora, and Polystichum setiferum ‘Proliferum’ showed moderate resistance to this stress. The results of our study may be helpful in selecting hardy ferns showing tolerance or resistance to drought.