Abstracts

Materials and Methods

Two Episcia cultivars, Lemon Aide (green leaves) and Choco Brown Soldier (brown leaves), were used in our study. Uniform, rooted stem cuttings, each with six expanded leaves, were transplanted into 9-cm-diameter plastic pots containing a substrate of two parts peatmoss (Base Substrate; Klasmann-Deilmann, Geeste, Germany), one part perlite (2–8 mm; Garden Castle Co., Changhua, Taiwan), and one part vermiculite (2–4 mm; Garden Castle Co.).

On 30 Nov 2024, plants were transferred to a growth room with the air temperature maintained at 25/20 °C (12 h/12 h), averaging 20.9 °C, as measured and recorded every 30 min with a HOBO data logger (Onset Computer Co., Bourne, MA, USA). Plants were grown under 8-h short-day (SD) or 16-h long-day (LD) photoperiods with photosynthetic photon flux densities (PPFDs) of 50, 100, 150, or 200 μmol·m^–2·s^–1 from light-emitting diode lamps (ZX-PC58120; Kao’s Lighting Co., New Taipei City, Taiwan), resulting in 1.4, 2.9, 4.3, and 5.8 mol·m^–2·d^–1 DLIs for SD and 2.9, 5.8, 8.6, and 11.5 mol·m^–2·d^–1 DLIs for LD treatments. Plants were repositioned randomly within the growth room biweekly. Leaf temperature of the most recently expanded leaves averaged 21.6 ± 1.6 °C as measured using an infrared sensor (Fluke 572; Fluke Co., Everett, WA, USA). Each plant received 3 g of controlled-release fertilizer (13N–11P–12K; Taihe Horticulture Co., Taipei, Taiwan) at the beginning of the experiment. A 0.5 g·L^–1 solution of water-soluble fertilizer (20N–8.6P–16.6K; Peters 20–20–20; Scotts Co., Marysville, WA, USA) was applied biweekly. Irrigation was applied when the substrate surface became dry.

At 143 d after treatment began, the most recent fully expanded leaf from each plant was sampled to measure chlorophyll fluorescence parameters using a portable chlorophyll fluorometer (Mini-PAM; Heinz Walz GmbH, Effeltrich, Germany). Minimal fluorescence (Fo) was measured after 30 min of dark adaptation; maximal fluorescence (Fm) was measured after a saturation pulse (∼18,000 μmol·m^–2·s^–1) at 25 °C. The maximum quantum efficiency of PSII (Fv/Fm), where Fv = Fm – Fo, which represents variable fluorescence, was then calculated. Effective quantum yield of photosystem II (PSII) photochemistry (ΦPSII) was also measured. The relative chlorophyll concentration of the most recent fully expanded leaves was determined using a chlorophyll meter (SPAD-502; Minolta Camera Co., Tokyo, Japan). Leaf thickness was measured at the center of the same leaf using a thickness gauge (SM-112; Teclock Co., Nagano, Japan). Freehand cross sections of leaves from the 2.9 and 11.5 mol·m^–2·d^–1 DLI treatments were examined via light microscopy (Eclipse E600; Nikon, Tokyo, Japan), and images were captured using a digital camera (MicroFire; Olympus America, Melville, NY, USA). At 145 d after treatment initiation, all leaves (> 1 cm on the mother plant and stolons) were removed, then the number and area of leaves, as well as the number and average length of primary and secondary stolons, were recorded. Total leaf area was photographed and calculated using ImageJ software version 1.54g (National Institutes of Health, Bethesda, MD, USA). The percentage of plant flowering and the number of flowers, including flower buds (> 0.5 cm), were recorded daily. Shoots were oven-dried at 70 °C to a constant weight for dry-weight determination.

The experiment was arranged in a completely randomized design with five replicated plants. Regression analysis was used to describe relationships between the DLI and growth, chlorophyll fluorescence parameters, and flowering response. Data were analyzed using CoStat version 6.4 (CoHort Software, Monterey, CA, USA), and graphs and regression analyses were generated using SigmaPlot version 10.0 (Systat Software Inc., Chicago, IL, USA).

Results

Leaf number in ‘Lemon Aide’ increased with increasing DLIs from 1.4 to 11.5 mol·m^–2·d^–1, resulting from increased light intensity under LD conditions (Table 1; Figs. 1 and 2A). In contrast, leaf number in ‘Choco Brown Soldier’ showed no further increase beyond 5.8 mol·m^–2·d^–1. Total leaf area increased with increasing DLI up to 5.8 mol·m^–2·d^–1 in both cultivars, but declined at higher DLI levels (Table 1; Figs. 1 and 2B). Average leaf area, calculated as total leaf area divided by leaf number, decreased as the DLI increased (Fig. 2C). Leaf thickness increased with the DLI, resulting from increasing light intensity and LD conditions (Fig. 2D). Leaf soil plant analysis development (SPAD) values declined with increasing DLI in both cultivars, and the decline was more pronounced in ‘Lemon Aide’, which exhibited leaf yellowing under high DLI treatments (Table 1; Figs. 1 and 2E).

Table 1. Analysis of variance for the effects of photoperiod (P) and light intensity (I) on leaf number, total leaf area, leaf thickness, leaf soil plant analysis development (SPAD) value, stolon number, average stolon length, and shoot dry weight of Episcia ‘Lemon Aide’ and ‘Choco Brown Soldier’.

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Stolon number increased with increasing DLI, peaking at 5.8 mol·m^–2·d^–1 in ‘Choco Brown Soldier’ and 8.6 mol·m^–2·d^–1 in ‘Lemon Aide’ (Table 1; Figs. 1 and 2F). At 5.8 mol·m^–2·d^–1, plants grown under SD conditions produced more stolons than those under LD conditions, whereas at 2.9 mol·m^–2·d^–1, stolon number did not differ significantly between SD and LD treatments.

Average stolon length increased with increasing DLI, peaking at 5.8 mol·m^–2·d^–1 before declining at higher DLI levels. ‘Lemon Aide’ consistently produced longer stolons than ‘Choco Brown Soldier’ at 5.8 mol·m^–2·d^–1 and higher DLIs (Table 1; Figs. 1 and 2G). Shoot dry weight increased with DLI, peaked at 8.6 mol·m^–2·d^–1 in ‘Lemon Aide’ and 5.8 mol·m^–2·d^–1 in ‘Choco Brown Soldier’, then declined with higher DLI levels (Table 1; Figs. 1 and 2H).

Freehand sections revealed that leaves were thinner at 2.9 mol·m^–2·d^–1 compared with those at 11.5 mol·m^–2·d^–1, which had more mesophyll cell layers in both cultivars, along with greater anthocyanin accumulation in ‘Choco Brown Soldier’ (Fig. 3).

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Fo, Fm, and ΦPSII values in both cultivars decreased with increasing DLI, with ‘Lemon Aide’ exhibiting a more pronounced decline (Fig. 4A–C). The Fv/Fm also decreased as DLI increased. It remained between 0.75 and 0.80 in ‘Choco Brown Soldier’, but dropped to less than 0.75 in ‘Lemon Aide’ at 8.7 and 11.6 mol·m^–2·d^–1 (Fig. 4D).

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No Episcia ‘Lemon Aide’ plants flowered by the end of the experiment. In contrast, 20% of ‘Choco Brown Soldier’ plants flowered at a DLI of 1.4 mol·m^–2·d^–1, and all plants flowered at 2.9 mol·m^–2·d^–1 under LD conditions or higher DLIs (> 4.3 mol·m^–2·d^–1), regardless of SD or LD treatments (Fig. 5A). Flower number increased with increasing DLIs up to 8.7 to 11.5 mol·m^–2·d^–1 (Fig. 5B). At a DLI of 5.8 mol·m^–2·d^–1, plants grown under LD conditions produced more flowers than those under SD conditions. However, at 2.9 mol·m^–2·d^–1, flower numbers did not differ significantly between SD and LD treatments.

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Discussion

Both Episcia cultivars grew poorly under the low DLI of 1.4 mol·m^–2·d^–1, with minimal leaf number, stolon formation, and shoot dry weight (Figs. 1, 2A, and 2F–H). This is consistent with the results of Harbaugh et al. (1981), who found that Episcia ‘Acajou’ and ‘Frosty’ exhibited reduced fresh weight and ornamental value under 90% to 95% shade (3–5 klx, ∼45–100 μmol·m^–2·s^–1). Low light also limits leaf development in African violets, likely the result of a reduced photosynthate supply (Faust and Heins 1993). However, under low PPFDs or DLIs, Episcia cultivars exhibited shade-adapting mechanisms, such as increased individual leaf area (Fig. 2C), reduced leaf thickness (Fig. 2D), and greater SPAD values (Fig. 2E). Similar adaptation strategies have been observed in other foliage plants, including Adiantum raddianum (Yeh and Wang 2000), Alocasia macrorrhiza (Sims and Pearcy 1992), Ficus benjamina (Fails et al. 1982), and Hedera helix (Yeh and Hsu 2004). These findings align with those of Norcini et al. (1991), who reported that shade enhances light-harvesting capacity through increased chlorophyll concentration and the development of broader, thinner leaves. In another Gesneriaceae ornamental, an Achimenes cultivar exhibited the lowest relative growth rate, attributed to small, thick leaves (Vlahos et al. 1991).

Both Episcia cultivars showed increases in leaf number, total leaf area (Fig. 2A and 2B), stolon number and length (Fig. 2F and 2G), and shoot dry weight (Fig. 2H) as DLI increased from 1.4 to 5.8 mol·m^–2·d^–1, with peak values at 5.8 mol·m^–2·d^–1 for Episcia ‘Choco Brown Soldier’. The best plant performance and ornamental quality were observed at 5.8 mol·m^–2·d^–1 for both Episcia cultivars (Fig. 1), which aligns closely with the optimal DLI range for African violet (Faust and Heins 1994; Hildrum and Kristoffersen 1969). Compared with other foliage plant species, this DLI is similar to the optimal range of 2.9 to 4.6 mol·m^–2·d^–1 for Peperomia ferreyrae (Kang and Lopez 2024), but less than the optimal DLI for sword ferns (Nephrolepis sp.) at 10 to 12 mol·m^–2·d^–1 (Seltsam and Owen 2022). Both Episcia cultivars exhibited reduced ornamental quality at 8.7 to 11.5 mol·m^–2·d^–1, as evidenced by decreases in both total leaf area and leaf SPAD values (Figs. 1, 2B, and 2E). Notably, ‘Lemon Aide’ exhibited yellow, mottled leaves (Fig. 1), a symptom similar to that observed in gloxinia under excessive light (Strømme 1985).

Stolon formation in Episcia not only aids in propagation but also enhances its ornamental value, particularly in hanging baskets. This process appeared to be influenced largely by the DLI, with both Episcia cultivars producing more stolons under SD conditions than LD conditions at 5.8 mol·m^–2·d^–1, whereas photoperiod had no effect at 2.9 mol·m^–2·d^–1 (Fig. 2F). In Achimenes, stolon number was influenced by photoperiod, with effects varying by cultivar (Vlahos 1990). In C. comosum ‘Vittatum’, stolon formation was accelerated by photoperiods ≤ 12 h, whereas the all-green form showed enhanced stolon development under NIL conditions (Hammer 1976; Heins and Wilkins 1978).

Leaf thickness increased with rising DLI in both cultivars (Fig. 3), a response commonly observed in foliage and ornamental plants under high light conditions as part of a photoprotective strategy. Similar trends have been reported in Alocasia macrorrhiza (Sims and Pearcy 1992), Ficus benjamina (Fails et al. 1982), and Guzmania ‘Cherry’ (Kuo and Yeh 2006), in which leaves developed greater thickness under higher light levels. This thickening is attributed primarily to an increase in both the size and number of mesophyll cells, which enhances chloroplast density and photosynthetic capacity (Sims and Pearcy 1992).

For both Episcia cultivars, Fo, Fm, and ΦPSII values declined as the DLI increased from 1.4 to 14.4 mol·m^–2·d^–1 (Fig. 4A–C). Although a loss of PSII reaction centers is typically associated with an increase in Fo (Demmig-Adams and Adams 1992; Maxwell and Johnson 2000), the Fo value is also influenced by chlorophyll content. Fu et al. (2012) demonstrated that increased light intensity reduced Fo values in lettuce (Lactuca sativa), likely as a result of a decrease in chlorophyll content. In our study, decreased chlorophyll levels, as indicated by SPAD values under high DLIs in both Episcia cultivars (Fig. 2E), explain in part the observed decline in Fo. Excessive irradiance often results in reductions in Fm, ΦPSII, and the Fv/Fm (Demmig-Adams and Adams 1992; Maxwell and Johnson 2000). Episcia ‘Choco Brown Soldier’ consistently exhibited higher values of Fo, Fm, and ΦPSII compared with ‘Lemon Aide’, particularly at DLI levels > 8.6 mol·m^–2·d^–1 (Fig. 4A–C). The Fv/Fm of ‘Choco Brown Soldier’ remained between 0.75 and 0.80 (Fig. 4D), within the typical range of 0.75 to 0.85 for nonstressed plants (Bolhàr-Nordenkampf et al. 1989). In contrast, the Fv/Fm of ‘Lemon Aide’ dropped to < 0.75 at DLI levels of 8.7 and 11.6 mol·m^–2·d^–1 (Fig. 4D). These results suggest that ‘Choco Brown Soldier’ experienced less photoinhibition and maintained better PSII integrity, likely a result of increased anthocyanin accumulation under high DLIs (Fig. 3), supporting the role of anthocyanins in mitigating photodamage (Merzlyak et al. 2008). Similarly, bronze-leaf wax begonias exhibit greater Fv/Fm under high light compared with their green-leaf counterparts (Zhang et al. 2010).

Episcia ‘Lemon Aide’ did not produce any flowers after 145 d of treatments. In addition to being more susceptible to photoinhibition, ‘Lemon Aide’ may have allocated more photoassimilates to leaf and stolon formation (Figs. 1, 2A, and 2F) rather than to flower production. Commercial growers have also reported delayed flowering in this cultivar. In contrast, a DLI of 1.4 mol·m^–2·d^–1 was insufficient to induce 100% flowering in Episcia ‘Choco Brown Soldier’ (Fig. 5A), which aligns with previous findings that low light intensities reduce flowering in Episcia (Harbaugh et al. 1981) and African violet (Stinson and Laurie 1954). However, when the DLI increased to 5.8 mol·m^–2·d^–1, ‘Choco Brown Soldier’ produced more flower buds under LD than SD conditions (Fig. 5B). This result is consistent with the work by Stamps and Henny (1986), who reported that NIL increased flower production in Episcia by 30% to 43% compared with plants without NIL.

Conclusion

Episcia cultivars showed optimal growth at a DLI of 5.8 mol·m^–2·d^–1, with improved leaf production, stolon formation, and flower number. Plants of ‘Choco Brown Soldier’ performed better under high DLIs, likely as a result of increased anthocyanin accumulation that helped mitigate photodamage, whereas green-leaf ‘Lemon Aide’ exhibited stress symptoms such as leaf yellowing and greater reductions in Fo, Fm, ΦPSII, and Fv/Fm. Both cultivars showed decreased ornamental quality at higher DLIs (> 8.6 mol·m^–2·d^–1). These results highlight the importance of controlling light conditions to optimize growth and visual quality in Episcia plants.