Sensitivity Consequences of Ethylene in Determining the Vase Life of Eremurus spectabilis and E. persicus

Abstract

Foxtail lily (Eremurus), as a medicinal-ornamental geophyte, has recently emerged in the cut flower market as a novel, commercially significant specialty cut flower (SCF). However, there is limited information about the sensitivity to ethylene of foxtail lily species for managing the ethylene-mediated senescence to prolong the vase life and maintain the ornamental quality of this flower. The purpose of the current study was to compare the ethylene production rates and patterns, as well as the responses to exogenous ethylene and ethylene inhibitors, between two species, E. spectabilis and E. persicus, to better understand the role of ethylene in Eremurus inflorescence senescence. The results revealed that exogenous ethylene (10 μL L⁻¹), as a pulsing or continuous method, dramatically accelerated petal wilting in E. spectabilis and petal abscission in E. persicus. Furthermore, the rate and patterns of endogenous ethylene production varied significantly among the two investigated species. Interestingly, E. persicus exhibited a higher rate of ethylene production than E. spectabilis on the first day after harvesting, but the reverse was true at the end of the vase life (Day 4 of the vase period). The results revealed that the treatments with ethylene inhibitors considerably improved the water relations and vase longevity of both foxtail lily species. The vase life of E. spectabilis was dramatically enhanced by silver thiosulfate complex (STS) treatment (0.2 mM pulse for 24 h) from 5 d (control) to 7 d. Furthermore, 1-methylcyclopropene (1-MCP) at 0.5 and 1.0 μL L⁻¹ markedly improved water uptake, relative fresh weight, and water balance and extended the vase life of cut inflorescences by ~2 d in E. spectabilis and E. persicus, compared with those of control cut inflorescences, respectively. This research revealed that ethylene is involved in controlling the senescence of foxtail lily flowers, and two tested species exhibited distinct forms of ethylene sensitivity, including abscission type in E. persicus and wilting-type in E. spectabilis. Collectively, these findings suggest that ethylene is involved in the senescence of cut foxtail lily inflorescence and that ethylene inhibitors can prolong vase life.

1. Introduction

The cut flower industry, as a sector of agriculture, is of economic importance worldwide via an increasingly globalized market. To help enhance product sales, this industry looks for innovation, new niches, and emerging trends [1]. Cut flowers are used for decorative purposes and include both traditional cut flowers (TCFs) and specialty cut flowers (SCFs). The importance of SCF cultivation is associated with its eco-friendly profile, environmental legislation, and low production costs. New SCFs are often introduced from the endemic flora, and their production has increased in the past two decades [1]. Many species of Eremurus (foxtail lily, desert lily, or desert candle), a genus of the Asphodelaceae family, are new commercially valuable SCFs that are becoming accepted in the international flower market [2,3,4]. In addition to being widely widespread throughout Central and Western Asia, including Iran, Iraq, Afghanistan, Syria, Tajikistan, Lebanon, Pakistan, Turkey, China, and India, the genus Eremurus has its center of diversity in Central Asia [4,5]. The Eremurus genus has about 50 species, seven of which are native to Iran [6]. Among them, E. spectabilis and E. persicus, as spring ornamental geophytes, are abundant species in Iran with narrow and elongated leaves and white florets on an interesting raceme inflorescence (Figure 1A,D), which may be grown outdoors in this region [2,6].

A crucial factor in commercializing cut flowers is postharvest performance [7]. The vase longevity of numerous cut flowers, a key factor influencing the consumer’s decision to repurchase those [8], is determined by their sensitivity to ethylene [7]. Ethylene is involved in the factors causing flower senescence processes, including flower opening to floral organ wilting or abscission [7]. Although ethylene concentrations in the natural environment are typically quite low, it is significantly high in certain confined spaces such as greenhouses or storage areas with concentrated exogenous ethylene sources such as plant materials, microbial activity, and combustion and industrial processes and is sufficiently capable of accelerating the senescence process in nearby ethylene-sensitive cut flowers. In contrast, some flowers emit ethylene spontaneously (endogenously), especially after pollination [9]. The senescence of the flowers results from regulating the expression of the ethylene-responsive genes by some biochemical processes that follow the detection of ethylene by particular receptors [10]. This process can reduce the vase longevity of a wide range of cut flowers [7]. The 1-aminocyclopropane-1-carboxylic acid synthase (ACS) enzyme converts S-adenosyl methionine (SAM) to 1-aminocyclopropane-1-carboxylic acid (ACC), and the ACC oxidase (ACO) enzyme then converts ACC to ethylene, which are the two rate-limiting steps in the ethylene biosynthesis pathway [11,12]. Many cut flowers, including orchids [13,14,15], carnations (Diathus caryophyllus) [16,17,18], snapdragons (Antirrhinum majus) [17,19,20], roses (Rosa hybrid) [17,21], and dahlias (Dahlia variabilis) [22,23], have significantly reduced vase life due to ethylene sensitivity. However, the sensitivity to ethylene varies between species and cultivars [21,23]. Thus, for a species or variety of cut flowers, understanding ethylene sensitivity and how to control it are crucial to preserving flower quality across the floral supply chain.

Treatment with ethylene inhibitors, such as silver thiosulfate complex (STS) and 1-methylcyclopropene (1-MCP), can increase the commercial vase life of cut flowers that are thought to be ethylene sensitive [7,15,17,23,24]. 1-MCP and STS are two commonly used postharvest treatments to extend the vase longevity of cut flowers by inhibiting or slowing down the natural process of senescence that occurs after being cut from the plant [7,15,25,26]. 1-MCP is a synthetic gas that binds to ethylene receptors on the surface of flower cells, thus inhibiting ethylene action by blocking the ethylene signaling pathway [27]. As a result, flowers treated with 1-MCP have an increased vase life, delayed senescence, and reduced petal drop [7,16,25,26]. Furthermore, STS is a chemical compound that inhibits ethylene synthesis by blocking the action of the ACS enzyme, which is responsible for the production of ethylene [24,26]. STS treatment inhibits ethylene action, thereby reducing the damage caused by exogenous or endogenous ethylene to cut flowers, resulting in a longer vase life, improved flower quality, and reduced senescence symptoms [7,15,19,23]. Both 1-MCP and STS are effective at prolonging the vase life of cut flowers, but further information is required about the relative advantages of 1-MCP and STS for specific cut flowers.

Cut inflorescences of foxtail lily have become popular in local markets of Iran in recent years. However, they have a short vase life of about 3–5 days without preservative treatment at room temperature [2]. Foxtail lily cut inflorescences seem sensitive to ethylene [28] since STS was markedly effective in extending their vase life [2]. However, little information of the ethylene sensitivity and response to ethylene inhibitors of Iranian native species of Eremurus genus is available; hence, it is essential to know the ethylene production rate in Eremurus species and their response to ethylene for managing their postharvest performance.

This study’s objectives were to investigate the ethylene production in foxtail lily cut inflorescences, the impacts of exogenous ethylene on inflorescence senescence, and the influence of STS and 1-MCP on the vase life of cut inflorescences of two foxtail lily species, E. spectabilis and E. persicus.

2. Materials and Methods

2.1. Plant Materials

Cut inflorescences of E. spectabilis and E. persicus, as endemic species of foxtail lily in the central zone of Iran [6], were harvested at the tight and swollen bud stages (0 florets open) [3] (Figure 1B,E, Stage 4) during spring in the early morning from the yasuj (Location: Iran. Latitude: 30°38′12″ north. Longitude: 51°41′19″ east. Elevation: 2519 m) and Semiron (Location: Iran. Latitude: 31°50′54″ north. Longitude: 51°48′18″ east. Elevation: 1990 m) regions, respectively. Within 4 h, the harvested inflorescences were transported to the Isfahan University of Technology after being cold stored in water for a few hours. The cut inflorescences were immediately taken to the laboratory in plastic buckets filled with distilled water. The cut inflorescences of both species were sorted on the basis of the thickness and length of the stem. Prior to treatment, they were recut about 45 cm under distilled water to eliminate the likelihood of emboli in the xylem conduits.

2.2. Ethylene Production

The native ethylene production rate of whole inflorescences was determined by a gas chromatography-flame ionization detector method that involved collecting a sample of the air surrounding the flowers and analyzing it using gas chromatography. For this, the whole inflorescence was enclosed in a 2 L airtight glass container. The gas samples were taken from each container after 24 h on a replicate at Days 1, 2, 4, 6, and 8 of the vase life. Three replicates for each species were analyzed at each time point. Gas samples were collected with a 20 mL gas-type syringe, and their ethylene content was determined and analyzed by a Model HP 5890 Series II gas chromatography (Hewlett-Packard, CA, USA). The gas chromatograph set was equipped with an HP-PLOT/Al2O3 column (0.32 mm i.d., 30 m length), a flame ionization detector (FID), and Helium as the carrier gas. The rate of ethylene production is expressed as nmol g⁻¹ h⁻¹ on a fresh weight of cut inflorescence.

2.3. Ethylene Treatment

For each of the two species, 32 cut inflorescences in vase solution were placed into gas-tight 50 L glass boxes and exposed to the four following treatments: continuous ethylene treatment at a 10 μL L⁻¹ in the box, pulsing ethylene (10 μL L⁻¹) for 24 h, Control 1 (keeping untreated inflorescences inside the box throughout the experiment), and Control 2 (untreated inflorescences maintained outside of the box during the experiment). The ethylene concentration was in accordance with Williamson et al. [26], Azuma et al. [23], Liu et al. [25], and Wongjunta et al. [15]. These boxes of cut inflorescences were kept at a temperature of 20 ± 1 °C under 12 h day/12 h night with 10 μmol m⁻² s⁻¹ irradiance, which was provided using cool-white fluorescent lamps throughout the experimental period. The percentage of relative humidity (RH%) inside each box was measured using a hygrometer (Analogue hygrometer, Germany). After 24 h, each box was opened for 5 min to expel the ethylene or air and then closed again while reintroducing ethylene at the same concentration. This method was repeated daily until abscission or wilting was observed in florets or petals. The deterioration and opening status of florets on inflorescences were classified into five stages (Figure 1C,F): Stage 1 (flower bud), Stage 2 (half open), Stage 3 (fully open), Stage 4 (beginning of abscission or wilting), and Stage 5 (full wilting or abscission) [2]. The vase life was judged as ended when either 50% of florets in an inflorescence were at Stages 4 and 5 or all florets of the inflorescence had experienced abscission or wilting without reaching the spent opening floret stages. Ethylene sensitivity was recorded and evaluated as the duration of time that had passed since the start of the ethylene treatment by observing either more than 50% of floret/petal rolling, necrosis, browning, and wilting or all of the inflorescence’s florets had experienced abscission or wilting without spent opening floret stages, which was confirmed by daily photos taken using a fixed camera technique (Figure 2) [29]. Further, eight cut inflorescences of each species were maintained outside the boxes (Control 2) at 20 ± 1 under the same light conditions as above and a relative humidity of 65% until floret/petal abscission or wilting was exhibited, as described above.

2.4. Ethylene Inhibitor (1-MCP and STS) Treatments

To evaluate the effectiveness of the inhibitors of ethylene on the vase longevity and postharvest performance of cut inflorescences of two foxtail lily species, the inflorescences of both species were treated either with 1-MCP (0.5 μL L⁻¹ and 1.0 μL L⁻¹) or STS (0.2 and 0.4 mM). For the 1-MCP treatment, cut inflorescences of each species in vase solution were placed in 50 L gas-tight chambers and treated with EthylBloc^TM (0.014% w:w 1-MCP, AgroFresh, Inc., PA, USA) for 24 h according to the protocols from the manufacturer. The STS concentration and time were in accordance with Williamson et al. [26] and Azuma et al. [23]. For this, the cut ends of inflorescences of both species were pulse treated for 4 h with 0.20 or 0.40 mM of STS solution, which was made by mixing equal volumes of sodium thiosulfate (Na₂S₂O₃) and silver nitrate (AgNO₃) at an 8:1 ratio of molarity [23]. Control inflorescences were placed in distilled water as the vase solution. After treatment, the inflorescences were transferred to an Erlenmeyer containing 200 mL of distilled water. The experiments were carried out in three replications with two cut inflorescences in each replicate. Throughout the experimental period, cut inflorescences of each species in vase solution were kept in a room with a controlled relative humidity of 65% and a temperature of 20 ± 1 °C under the 12 h night/12 h day regime with 10 μmol m⁻² s⁻¹ of quantum irradiance (cool-white fluorescent lamps).

2.4.1. Measurements

Vase Performance Assessments

The vase life was defined from when the cut foxtail lily inflorescences were placed in the vase after treatments until a senescence symptom was observed. The vase life assessment of the cut inflorescences was conducted daily as described in the previous section (Section 2.3). VL was considered to be the number of open florets in relation to their total number, gravitropism, and petal browning, enrolling, necrosis, or wilting. The vase life of an inflorescence was terminated when it exhibited at least one of the following senescence symptoms: more than 50% of the florets in an inflorescence were at Stages 4 (about 30%) and 5 (about 20%) of the florets’ progress development stage (Figure 1C,F), the cut inflorescence had bent, and the petal or floret had experienced abscission.

Measurement of Water Relations

The weight of vases with and without cut inflorescences was recorded on Day 0 and at two-day intervals throughout the vase evaluation period. Water loss (WL) (transpiration) and water uptake (WU) for the inflorescences were calculated by the formulae and expressed in grams/cut inflorescence for that period [2]:

Water uptake (g/cut stem) = VS_d−2 − VS_d

where VS_d is the weight of the vase solution (in g) at d = Days 2, 4, 6, 8, and 10 and VS_d−2 is the weight of the vase solution (g) on the two previous days.

The loss of water (transpiration) was calculated from measurements of the combined weights of the cut inflorescences and vase solution during the evaluation period by using the following formula:

WL (g.cut stem ⁻¹) = (VS + FW)_d−2 − (VS + FW)_d

where VS + FW is the weight of the VS plus the weight of the cut inflorescence (in g) at d = Days 0, 2, 4, 6, 8, and 10 of the vase life.

The water balance (WB) of the cut inflorescences was calculated by subtracting WU from transpiration [2].

Relative Fresh Weight

The relative fresh weight (RFW) is expressed as the percentage of the fresh weight to the initial weight of the cut inflorescence and was calculated using the following equation:

$$\mathrm{R}\mathrm{F}\mathrm{W}\left(\%\right)=\left(\raisebox{1ex}{${\mathrm{F}\mathrm{W}}_{\mathrm{t}}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{F}\mathrm{W}}_0$}\right.\right)\times 100$$

where FW_t is the fresh weight of inflorescences (g) at t days (2, 4, 6, etc.) during the vase-life evaluation period; FW₀ is the fresh weight of inflorescences (g) at t = Day 0. Fresh weight was established at 100% on Day 0.

2.5. Statistical Analysis

The obtained data from different experiments were subjected to analysis of variance (ANOVA) or Student’s t-test using the SAS software (Version 9.1, SAS Inst., Inc., Cary, NC, USA), and means were separated at the 5% level of probability using Fisher’s least significant difference (LSD). The principal components analysis (PCA) was made by means of the Statgraphics centurion XVI software for associations between traits and treatments, and two first principal components (PC1 and PC2) were plotted for the polygon view of the biplot and the average tester coordination.

3. Results

3.1. Effects of Exogenous Ethylene on the Appearance and Vase Lives of Two Eremurus Species

The vase life of E. spectabilis was 5.5 d (control outside the box), which was not significantly different from that of E. persicus (6.5 d). However, as with the pulsing or continuous method, treatment with 10 μL L⁻¹ ethylene significantly shortened the vase life in both species compared to the air-treated controls (Figure 3). In addition, cut inflorescences of E. spectabilis inside the boxes had a shorter time to senescence than those maintained outside the boxes, whereas there were no differences in vase life between the air-treated inflorescences of E. persicus inside and outside the boxes (Figure 3A).

The vase life of cut inflorescences of E. spectabilis was generally terminated by petals enrolling and wilting from an acropetal pattern of inflorescence. In E. persicus, petal wilting and the abscission of florets were observed as symptoms of senescence (Figure 1). However, a sharp floret drop was observed in cut inflorescences of E. persicus when exposed to continuous ethylene treatment in the boxes (Figure 3C). In addition, florets wilting without progressing senescence steps in the petals were observed in E. spectabilis under continuous ethylene treatment with 10 μL L⁻¹ (Figure 3B). Similarly, petal wilting with no visible petal drop was observed in the cut inflorescences of E. spectabilis when they were treated with pulsing ethylene treatment maintained outside the boxes, although the florets of these inflorescences had progressed through the floral development stages (Figure 3B), suggesting that their petals were enrolled, wilted, and browned. At the 10 μL L⁻¹ ethylene pulsing treatment, the majority of the petals of E. persicus exhibited browning of the edges and abscission, suggesting that petal senescence had progressed in this species (Figure 3C).

3.2. Endogenous Ethylene Production Rates and Patterns

The amount of ethylene production (Figure 4) varied significantly among the two foxtail lily species at Days 1 and 4 of the vase life. E. persicus produced a detectable high value of ethylene on Day 1 after harvest, which remained relatively consistent with only about 0.001 nM g ⁻¹ h⁻¹ throughout the 8 d test period. In contrast, E. spectablis inflorescences exhibited a small ethylene production peak on Day 4 (Figure 4), which was significantly higher than the rates produced by E. persicus inflorescences at this time. However, a similar pattern of ethylene production with a relatively low level of ethylene was detected on Days 2 and 6 of the vase life for both species (Figure 4).

3.3. Effects of Ethylene Inhibitors on the Vase Lives and Water Relation of Cut Inflorescences of the Two Test Species

3.3.1. Water Uptake

Data recorded in Figure 5 exhibit that ethylene inhibitor treatments were more effective in maintaining the water uptake of both species than the control. There was a significant impact of treatments (p < 0.01) on the water uptake of cut inflorescences of E. persicus during 2 and 4 days of the vase life. The maximum quantity of vase water was absorbed by 1-MCP-treated inflorescences on Days 2 and 4 of the vase life. However, no significant differences were observed between treated inflorescences and the control at 6 and 8 days of the vase life for E. persicus (Figure 5E). Similarly, there was a significant variation among treatments in the water uptake of E. spectabilis at 4 (p < 0.01) and 6 (p = 0.038) days of the vase life (Figure 5A). The amounts of vase water uptake of the 1-MCP-treated inflorescences of E. spectabilis were higher than those of the untreated inflorescences at Day 4 of the vase life. Generally, the results revealed a slight and sustained reduction in the rate of water uptake by the treated and untreated inflorescences of both species throughout the vase life period (Figure 5A).

3.3.2. Water Loss (Transpiration)

The effects of ethylene inhibitor treatments on water loss during the vase life of cut foxtail lily inflorescences are shown in Figure 6. The results revealed that water loss from the cut inflorescences of E. spectabilis significantly varied between different treatments at Days 2 (p < 0.01) and 6 (p = 0.044) of the vase life (Figure 5B). Similar to the water uptake pattern, the maximum quantity of vase water was loss by 1-MCP-treated inflorescences on Day 4 of the vase life (Figure 6B). In E. persicus, the differences in water loss rates (p < 0.01) among the treated and untreated inflorescences were significant on Days 2 and 4 of the vase life (Figure 5F). The results revealed a sharp decline in the rate of water loss by the inflorescences of E. persicus in all treated inflorescences between Days 4 and 6, which remained almost steady after that, exhibiting a slight and sustained decline throughout the vase life period (Figure 5F).

3.3.3. Water Balance

The water balance value is strongly related to the vase life of cut flowers. The data illustrated in Figure 6 revealed that changes in the WB of both species showed similar trends in all treatments. Cut inflorescences of both species, which were treated with 1-MCP or STS, showed better water balance than untreated cut inflorescences, and the decline in the WB during the vase life was faster for the control (Figure 5C,G). Accordingly, E. spectabilis treated by the 0.5 µL L⁻¹ of 1-MCP exhibited the most-favorable water balance until Day 6 of the vase life (Figure 6C), whereas for E. presicus treated inflorescences showed better water balances compared to the control during the vase evaluation period (Figure 5G).

3.3.4. Relative Fresh Weight

Changes in the RFW of cut inflorescences exhibited similar patterns in both species over the vase period (Figure 5D,H). In E. spectabilis, the differences in the RFW among treatments were significant (p < 0.05) on Day 4 of the vase life (Figure 5D). The results presented in Figure 5D revealed that the highest RFW (105.42%) was observed in the 0.5 µL L⁻¹ 1-MCP-treated inflorescences of E. spectabilis at Day 4 of the vase life. For E. persicus, the values of the RFW of the untreated inflorescences were lower than the values of the treated inflorescences at Days 2 and 4 of the vase life (Figure 5H). In addition, the 1.0 µL L⁻¹ 1-MCP-treated inflorescences had the highest RFW on Days 2 and 4, which decreased sharply afterward. However, the 0.5 µL L⁻¹ 1-MCP-treated inflorescences had the highest relative RFW after Day 6 of the vase life (Figure 5H).

3.3.5. Vase Life

The data analysis revealed that the vase life of cut inflorescences in both species was significantly (p < 0.01) affected by different anti-ethylene treatments (Figure 6A,B). The average longevity of cut E. spectabilis inflorescences kept in water was 4.4 d. In this species, lower concentrations of 1-MCP (0.5 µL L⁻¹) and STS (0.2 mM) significantly prolonged the vase life of the cut inflorescences, by ~2 days, compared to the control (Figure 6A). The average vase life of the cut E. persicus inflorescence kept in water was 6.6 d (Figure 6B). Higher concentrations of anti-ethylene compounds significantly improved the vase life of E. persicus inflorescences compared with that of the control, especially with 1.0 µL L⁻¹ of 1-MCP (Figure 6B). For this species, the longest longevity, 8.4 days, was observed with 1.0 µL L⁻¹ of 1-MCP, followed by 0.4 mM (7.8 days) of STS (Figure 6B).

3.3.6. Associations between Traits

The biplot analysis for the anti-ethylene experiment data revealed that the first and second components displayed 50.60 and 27.51% of the total variation, respectively (Figure 7). The figure of the biplot exhibited that the treated inflorescences of E. persicus by 1-MCP or STS had high values of the relative fresh weight and vase life. On the other hand, the anti-ethylene-treated inflorescences of E. spectabilis had better water relations (high values of the WU, WL, and WB) (Figure 7). The results in Figure 7 also illustrate that the vase life had a positive and high association (an acute angle) with the relative fresh weight. However, there was a negative association (an obtuse angle) among WU plus WL and the vase life. These results were highly coordinated with the numerical Pearson correlation coefficients (data not shown).

4. Discussion

4.1. Effects of Exogenous Ethylene on the Appearance and Vase Lives of Two Eremurus Species

A consequence of the senescence process in cut flowers is the loss of their external quality criteria, including appearance, color, and uniformity [7]. The senescence process of a cut flower cannot be eliminated, but it can be delayed or reduced by various postharvest treatments, ultimately to extend their decorative values [30]. The ethylene produced will accelerate the natural senescence process of the flowers, causing them to wilt and deteriorate [7,17]. In addition, many cut flowers exhibit senescence symptoms when exposed to ethylene [9,17], and this can reduce the vase life of the cut flowers. In this study, the ethylene sensitivity and effectiveness of the ethylene inhibitors, STS and 1-MCP, were evaluated on two Iranian native cut inflorescence species, E. spectabilis and E. persicus. The results revealed that the vase life of these species was prevented by continuous exposure to ethylene and shortened by pulsing treatment with ethylene, indicating that the senescence process of both species was accelerated by ethylene. However, the response to exogenous ethylene varied between the two species. In agreement with these results, variations in the sensitivity of species or cultivars have been reported in important cut flowers such as orchids [15], carnations [18,29,31], tulips [17], roses [21,32], petunias [33], and dahlias [23]. These findings suggested that the extent of ethylene production and its effect on cut flowers vary widely according to species or cultivars, indicating that sensitivity varies among different cut flowers.

In this study, continuous exposure to 10 μL L⁻¹ of ethylene induced a sharp floret/petal drop and reduced the time to floret abscission to 2.5 days in E. persicus (Figure 3). In line with these results, this ethylene-sensitive behavior of E. persicus is exhibited by some other TCF and SCF species, such as Spartium junceum [34], Boronia heterophylla [35], snapdragons [19], some cultivars of tulip [17], and dahlias [23]. van Doorn [17] revealed that petal abscission without prior wilting after exposure to exogenous ethylene generally indicates a high sensitivity to ethylene. Thus, the results of this study suggested that E. precicus exhibited relatively high ethylene sensitivity. Furthermore, petal-abscission-induced exogenous ethylene has previously been reported, which was associated with an increase in the endogenous production of ethylene [9,17,26]. Therefore, the sharp petal/floret abscission that occurred in E. persicus by exogenous ethylene may be related to high ethylene production at Day 1 of the vase life. Petal abscission is attributed to an increase in the activities of enzymes that degrade the cell wall [36]. Furthermore, the expressions of some genes were increased during the petal abscission process, including pectate lyase [37] and xyloglucan endotransglycosylase/hydrolase [38], which encode proteins involving cell wall modification. Thus, petal/floret abscission in E. persicus suggests that these species may have more induction of these genes compared to E. spectabilis, the details of which require more investigation in future studies.

In E. spectabilis, petal wilting was hastened by pulsing ethylene, but florets were translucent at the bud stage without progressing through the developmental process with continuous ethylene treatment (Figure 3). These results are consistent with previous findings for ethylene-sensitive petal wilting in some other species, such as Hyacinthus orientalis, Tulipa gesneriana, Campanula garganica [17], campanulas [39], and some cultivars of dahlia [23]. However, van Doorn [3] suggested that Eremurus hybrid flowers were ethylene insensitive and pulsing 1-MCP treatment as an ethylene inhibitor at 1.0 µL L⁻¹ for 24 h had no effect on the vase life of Eremurus hybrids. Conversely, the results of this study suggested that exposure to exogenous ethylene accelerated petal wilting or abscission in the Eremurus species tested, exhibiting different ethylene-sensitive behaviors, indicating that sensitivity to ethylene varies among different Eremurus species.

In this study, keeping the inflorescences inside the boxes (control boxes) had a negative impact on the vase life of both species. However, there were no statistically significant differences between being inside (Control 1) and outside (Control 2) the box for E. persicus (Figure 3A). It was discovered that the relative humidity within the boxes was 90%, significantly greater than the outside humidity of 35%. Humidity prolongs the vase life of cut flowers by preventing them from drying out too rapidly and by slowing down the water loss process through the petals and leaves. Accordingly, Azuma et al. [23] found that less relative humidity encouraged water evaporation in the petals of dahlia cultivars, resulting in a loss in relative fresh weight and petal turgidity and, ultimately, a reduction in the vase life of dahlia cut flowers. However, in this study, keeping the flowers in the box did not affect how long they lasted in the vase. This may be due to an accumulation of ethylene released by the flowers or the negative impact of saturated humidity on cut foxtail lily inflorescences, a theory that needs more research in the future.

4.2. Endogenous Ethylene Production Rates and Patterns

The monitoring of ethylene production rates during the vase life (Figure 4) indicated that the rate of endogenous ethylene varied significantly among the two foxtail lily species at Days 1 and 4 of the vase life. Interestingly, E. persicus exhibited a higher rate of ethylene production compared to E. spectabilis on Day 1 after harvest, but the reverse was true on Day 4 of the vase life (Figure 4). Furthermore, there was a different behavior in terms of sensitivity to exogenous ethylene between the two tested species. These results suggested that Eremurus species could be categorized by the combined differences in their sensitivity to exogenous ethylene and endogenous ethylene levels, according to the following two patterns: (1) abscission-type in E. persicus with high endogenous ethylene at the first day after harvesting; (2) wilting-type in E. spectabilis with a peak of endogenous ethylene at the end of the vase life.

In this study, regardless of the treatments, the inflorescences of E. persicus (control flowers) exhibited higher vase longevity compared to E. spectabilis cut inflorescences, suggesting that a higher production rate and sensitivity to ethylene may not always be a lower natural vase life in Eremurus species. This is consistent with the results of Ebrahimzadeh et al. [31] and Azuma et al. [23] in carnations and dahlias, respectively. Thus, it could be concluded that the differences in the ethylene production rates and patterns, as well as ethylene sensitivity may be attributed to genetic variation among the tested species.

4.3. Effects of Ethylene Inhibitors on the Vase Lives and Water Relation of Cut Inflorescences of the Two Test Species

To reduce the negative effects of endogenous ethylene on cut inflorescence performance, two species of Eremurus were treated with ethylene biosynthesis and perception inhibitors. It was in our interest to find out whether ethylene inhibitors may prolong the vase life of Eremurus cut inflorescences by preventing senescence. For this, various performance criteria of the two native Eremurus species, E. spectabilis with a short vase life and E. persicus with a long vase life, were studied for the impacts of two ethylene inhibitors, STS and 1-MCP. This study revealed that the two ethylene inhibitors significantly and effectively prolonged the vase life of both species (Figure 6). Furthermore, the water relations of cut inflorescences of both foxtail lily species were positively affected by the STS and 1-MCP treatments (Figure 6 and Figure 7). In agreement with these results, it has been reported that ethylene inhibitor treatments improve the water status of cut flowers, resulting in a longer vase life for ethylene-sensitive species/cultivars, as exhibited for Mokara orchid cut flowers [15], Boronia heterophylla [35], rose cultivars [21], Antirrhinum majus, and Celosia argentea [19,40,41,42]. Similarly, the STS and 1-MCP treatments delayed the fresh weight loss of the cut flowers and negated the ethephon effects, resulting in a significantly extended vase life of Dendrobium cv. “Red Sonia” [42] and Vanda orchid cv. “Sansai blue” [43]. Accordingly, these results suggested that ethylene inhibitor treatments mitigated the acceleration effects of endogenous ethylene on the flower senescence process by interfering with the biosynthesis and perception of ethylene, thereby delaying flower senescence and extending vase life. Additionally, the ethylene inhibitors may downregulate ethylene biosynthesis genes (RhACS2 and RhACO1) or receptor isoforms (RhETR2, RhETR3, and RhETR5) [21] in cut flowers to reduce the ACO and ACS activities and ACC content [15], limiting the production and action of ethylene in cut flowers. However, there were several variations in the responses of the two species to the ethylene inhibitors, which most likely reflected variations in their endogenous ethylene production patterns or original vase-life longevity. Thus, high levels of ethylene inhibitors significantly improved the water relations and vase life of E. persicus (Figure 6B). In contrast, low levels of them were more effective in improving the water relations and extending the vase life of E. spectabilis as compared to high levels (Figure 6A), suggesting the application of an optimal concentration of ethylene inhibitors is critical for the particular species. Overall, the results of the current study suggested that more data regarding the underlying mechanism of ethylene inhibitor treatments against endogenous ethylene in foxtail lily cut flowers are needed.

5. Conclusions

The present study was the first to report the sensitivity to ethylene of E. spectabilis and E. persicus as new SCFs. The results of this study revealed that ethylene is involved in the senescence processes of foxtail lily flowers, which determines the vase life and postharvest performance of the cut inflorescences of the two species tested. Exposure to exogenous ethylene accelerated petal/floret wilting or abscission in the foxtail lily species tested, with different species exhibiting different senescence symptoms. Thus, the two species tested exhibited sensitivity to ethylene, and ethylene inhibitors, STS and 1-MCP, significantly affected the water relations and prolonged the vase life of the cut inflorescences of both species. However, notable differences in endogenous ethylene production patterns and the response to exogenous ethylene were observed between the two investigated species. Based on these findings, ethylene inhibitors can serve as effective postharvest tools to preserve the quality of the foxtail lily cut inflorescences and extend their vase life. However, further research is needed to investigate the physiological and biochemical responses of foxtail lily to exogenous ethylene, as well as ethylene inhibitors to optimize commercial applications of these compounds in cut foxtail lily inflorescences. Collectively, according to the findings of this study, ethylene appears to be the primary factor determining the vase life of the Eremurus species, and using ethylene control strategies such as ethylene inhibitors and ethylene removal technologies are recommended for maintaining cut Eremurus longevity.