Introduction

Cut roses have a short vase life of around 7-10 d depending on the cultivar (In et al. 2017;Macnish et al. 2010). The vase life and physiological status of cut roses are often affected by postharvest conditions such as temperature, light intensity, and relative humidity, as well as by preharvest growth conditions (Fanourakis et al. 2013). Recently, considerable efforts have been applied to improve the postharvest quality and vase life of cut roses by controlling postharvest environmental conditions (Ha et al. 2020a;Horibe et al. 2020;van Meeteren and van Gelder 2000). However, the effect of light intensity on the vase life of cut roses has largely been overlooked. Light acts as an environmental signal and drives photosynthesis in plants. Photosynthesis physiology of plants such as net photosynthesis rate and stomatal conductance is affected by light intensity (Ma et al. 2015). Light also influences some aspects of cut flower traits such as scent, petal color, and vase life (Horibe 2020). To date, the effects of different light intensities on the postharvest performance of cut rose flowers have not been studied in detail.

Photosynthesis is the main process providing energy for plant growth and development. Previous studies have shown that light spectrum and intensity affect photosynthesis rate, photosynthetic electron transport, and plants' maximum quantum efficiency and quantum yield (Aalifar et al. 2020;Moosavi-Nezhad et al. 2021). Chlorophyll fluorescence (CF) is related to the redox state of the electron acceptor plastoquinone A (Q_A) and is used as a probe to study photosynthetic activity in plants. The ratio of F_v/F_m, which is calculated from minimum fluorescence (F₀) and maximum fluorescence (F_m) in a dark-adapted state, is closely related to the quantum yield of net photosynthesis of intact leaves (Gonzalez-Mendoza et al. 2007Krause and Weis 1991). The rise from the basic dark-adapted fluorescence yield to the maximum (OJIP transient) of CF was determined to understand about quantum efficiency of photosystem II photochemistry in plants (Strasser et al. 2000). The OJIP method is used to examine parameters related to electron transfer in photosystem II such as an average absorbed photon flux per cross-section of leaf sample, the flux of electrons from Q_A^- into the electron transport chain, and an energy flux trapped by photosystem II of a photosynthesizing sample cross-section (Strasser et al. 2000). The OJIP test has been successfully used to study the photosynthesis capacity of plants under specific stress conditions (Rapacz et al. 2015;Kalaji et al. 2016). Previous studies have shown the effects of different light spectra on plant photosynthesis characterized by evaluating the photosynthesis rate and maximum quantum efficiency of photosystem II (Aalifar et al. 2020;Moosavi-Nezhad et al. 2021). However, knowledge about the effects of light intensity on the photosynthesis of cut rose flowers and the subsequent importance of photosynthetic performance on vase life is still unknown.

Cut flowers assimilate little carbon by photosynthesis because the light intensity for them is often below the compensation point of photosynthesis. Consequently, their carbohydrate reserves are depleted. This causes cut flowers to yield short vase life (Halevy and Mayak 1981;Pun et al. 2016). Sucrose is known to be related to the photosynthetic capacity of plants. Sucrose can enhance or repress the photosynthesis apparatus by regulating transcription of the genes involved (Lobo et al. 2015;Pego et al. 2000). Therefore, in the cut flower industry, the application of external sucrose in the vase solution perhaps influences the photosynthesis capacity of cut flowers and this impacts senescence in cut flowers (Doi and Reid 1995;Zeng et al. 2023). However, the relationship between exogenous sucrose, photosynthesis rate of leaves, and the longevity of cut flowers is less understood so far. Thus, in this study we determined changes in photosynthesis rate and internal sucrose of leaves in response to different light intensities and exogenous sucrose to examine the influence of sucrose level produced by the leaves on vase life of cut roses. As a part of the study, we monitored the leaf net photosynthesis and CF parameters throughout the vase life period to determine whether the CF can be used for predicting the leaf photosynthesis in cut roses.

Materials and Methods

Plant materials

The standard rose ‘Barista’ (Rosa hybrida L.) flowers were grown in a greenhouse in Gokseong-gun, Jeollanam-do, Republic of Korea. Rose flowers were collected disease symptomless in the greenhouse and harvested at the half-open stage with outer petals bent out. Cut flowers were immediately placed in buckets containing tap water and transported to the laboratory within 4 h. The flower stems were recut to a length of 45 cm, including three upper leaves. The cut rose flowers were then held in distilled water and a controlled environment room at 25 ± 1℃ and relative humidity of 50 ± 2% before conducting treatments.

Sucrose and light intensity treatments

Cut rose flowers were held under different light intensities 10 (L10) or 50 μmol‧m^-2‧s^-1 (L50) supplied by fluorescent tubes and treated with 3% sucrose (L10+SUC and L50+SUC). The cuvette of the photosynthesis meter used in the experiment has a sensor attached to it to measure the light intensity. The measurement location of the light intensity was measured directly above the leaf using a PAR sensor inside the cuvette. The intensities of light were set up to evaluate and compare to indoor conditions (20 μmol‧m^-2‧s^-1) in South Korea and Japan (In et al. 2016). All cut flowers were treated with 10 μL‧L^-1 of nano silver (NS) to inhibit the bacterial growth in vase solution. Concentrations of sucrose and NS were selected based on previous studies (Ha et al. 2017;Ha et al. 2022). The effectiveness of the treatments was compared with control flower groups, which were held under different light intensities 10 (L10) or 50 μmol‧m^-2‧s^-1 (L50) and without sucrose treatment. All cut flowers were maintained at the temperature (25 ± 1℃) and relative humidity of 50 ± 2% for vase life and postharvest quality evaluation.

Photosynthesis rate and stomatal conductance measurement

The net photosynthesis rate and stomatal conductance were measured at days 1, 3, 5, 7, 9, and 11 of the vase periods using a portable photosynthesis system CIRAS-3 (PP Systems, Amesbury, MA, USA). It was equipped with a clamp-on leaf cuvette that exposed 4.5 cm² of leaf area. The measurements were conducted in the uppermost leaf of cut rose flowers. The conditions for photosynthesis measurement were set as follows: the amount of light in the leaf chamber was photosynthetic photon flux density (PPFD) 10 and 50 μmol‧m^-2‧s^-1 respectively, leaf chamber block temperature was 20 ℃, humidity 50%, cuvette flow was 250 cc‧min^-1, the CO₂ concentration was 380 μmol‧mol^-1, and the H₂O value was 50%. To analyze the light response curve in the photosynthesis capacity of cut roses, the amount of light in the leaf chamber was set for PPFD 0, 10, 50, 100, 300, 500, 1000, 1500, and 2000 μmol‧m^-2‧s^-1 with adaptation time for each measurement was 5 min. The light was provided by red, green, and blue light-emitting diodes, set for 38% red, 37% green, and 25% blue, as the closest approximation to sunlight. The measurements were conducted on three randomly selected cut rose flowers of each treatment.

The default zero value of photosynthesis measurement equipment is changes during the measurement. Therefore, the net photosynthesis rate (P_n) was calculated as follows:

where, P_n is the net photosynthesis rate; A₀ is the zero value of the rate of net photosynthesis before measurement; and A_m is the rate of net photosynthesis after measurement.

Measurement of chlorophyll fluorescence parameters

The measurement of chlorophyll fluorescence (CF) parameters was carried out using a portable CF instrument (Fluorpen FC 100, Photon Systems Instruments, Drasov, Czech Republic). The measurement of CF was based on the OJIP transient method. The selected rose leaves were subjected to a 15 min period of adaptation to darkness, enough for completion oxidation of the reaction centers. A detailed description of measured parameters by CF OJIP transient method was shown in Table 1.

Measurement of flower opening, soluble solids content, fresh weight, and water balance

To evaluate the effectiveness of the treatment on cut flowers, changes in flower opening, soluble solids content (SSC), and solution uptake were measured daily at 9:30 am. The flower diameter of cut flowers was evaluated using digital calipers (CD-20APX, Mitutoyo Corporation, Kanagawa, Japan), and the largest diameter of an individual cut flower and the diameter perpendicular to it was recorded for analysis. The SSC (%) was measured in the uppermost leaves of cut flowers. Briefly, 100 mg of leaf tissues were ground with 500 μL of distilled water using a clean mortar and pestle. The SSC of leaves was measured via a digital hand-held pocket refractometer (Atago Co. Ltd., Tokyo, Japan). The fresh weight and solution uptake of cut flowers were measured by weighting during vase life, and the solution uptake and transpiration were calculated. The water balance of cut flowers was calculated as described previously (Ha et al. 2020b).

Vase life and senescence evaluation

The number of days from when the cut flowers were placed in vase solutions and different light intensities to when their vase life ended were recorded as the vase life. Vase life was evaluated daily by the assessment criteria for Rosa (VBN 2014). The vase life of cut roses ended when 50% of the petals wilted or bluing or when pedicel bending was observed.

Experimental design and data analysis

Nine cut flowers were included in each treatment group. The experiments on water relations, flower opening, and vase life were designed with six replicates (one flower per replicate). The remaining three flowers were used for SSC analysis. The photosynthesis, stomatal conductance, and chlorophyll fluorescence analysis were conducted with three biological replicates. Data are summarized as the mean ± standard error (SE). The data were analysed by two-way analysis of variance (ANOVA) or subjected to regression analysis using SPSS version 22.0 (IBM Corp., Armonk, NY, USA). When significant differences were detected, the group means were analysed with post-hoc pairwise comparisons using Duncan’s multiple range test (DMRT). Significance was determined at p = 0.05.

Results

Senescence and vase life of cut flowers

Senescence symptoms of L10 flowers occurred earlier than those of L50 flowers. L10 showed early bent neck and petal wilting compared with those of other treatments (Fig. 1A and B). The sucrose treatment delayed the symptoms of senescence and increased the vase life of all cut flowers (Fig. 1). L10+SUC and L50+SUC exhibited the longest vase life (12 and 12.2 d). Whereas, the shortest vase life was observed in L10 flowers (Fig. 1C). These results indicate that higher light intensity and adding exogenous sucrose lead to retardation of flower senescence in cut roses.

The soluble solids content, flowering, fresh weight, and water balance of cut flowers

To evaluate the effectiveness of light intensity and sucrose on the carbohydrate status of the leaves, flower opening, fresh weight, and water balance of cut roses, the SSC, flower diameter, and changes in fresh weight and water uptake of cut flowers were measured. The SSC of leaves was highest in the L10 flowers (Fig. 2A). However, the flower diameter of cut roses was similar among the treatments (Fig. 2B). Sucrose did not affect carbohydrate content and flower opening of cut roses.

The relative fresh weight and water balance of cut flowers are closely related to their vase life. The number of days that cut flowers retained water balance was the shortest for L10 flowers (Fig. 2D). Treatment with sucrose significantly increased the time that cut flowers retained the initial fresh weight and water balance, compared with control flowers (L10 and L50 flowers) (Fig. 2C and D).

Changes in chlorophyll fluorescence parameters

Quantum efficiencies including quantum yields of photosystem II (F_v/F_m), the quantum yield of electron transport chain (ET₀/ABS), and efficiencies at which a trapped electron can move beyond Q_A^- in the electron transport chain (ET₀/TR₀) decreased during vase life for all cut flowers. F_v/F_m, ET₀/ABS, and ET₀/TR₀ were higher in leaves of cut flowers held under a light intensity of 50 μmol‧m^-2‧s^-1 (Fig. 3A-C). Quantum efficiencies were lowest in the leaves of the L10 flowers (Fig. 3A-C). In contrast, the specific energy fluxes such as energy absorption and trapping per reaction center (ABS/RC and TR₀/RC), the electron transport flux per reaction center (ET₀/RC), and heat dissipation were higher in cut flowers exposed to the light intensity of 10 μmol‧m^-2‧s^-1 (Fig. 3D-G). The performance index for the photochemical activity (PI_ABS) was higher in cut flowers exposed to the light intensity of 50 μmol‧m^-2‧s^-1 during the vase period (Fig. 3H). Treatment with sucrose did not affect PI_ABS of cut roses (Fig. 3H).

Photosynthesis rate and stomatal conductance

Photosynthesis rate and stomatal conductance in the leaves of cut roses were affected by light intensity. Higher photosynthesis rate and stomatal conductance were observed on the leaves of cut flowers exposed to the light intensity of 50 μmol‧m^-2‧s^-1. (Fig. 4A and B). However, the photosynthesis capacity of cut roses was decreased when the light intensity was increased (Fig. 4C). Although the photosynthesis capacity of cut flowers was not affected by external sucrose treatment, it was negatively correlated to the SSC in leaves of cut roses (Fig. 4A and D).

Relationships between CF parameters and photosynthesis rate, and between photosynthetic parameters and vase life

Correlation analysis was performed to determine the relationships between CF parameters and photosynthesis rate of cut roses (Fig. 5A). Most of the CF parameters (exception of ABS/RC and F_v/F_m) were significantly correlated with photosynthesis rate of cut roses (Fig. 5A). PI_ABS was highly positively correlated with photosynthesis rate (r = 0.832; p < 0.01). In contrast, ET₀/RC was highly negatively correlated with photosynthesis rate (r = 0.838; p < 0.01) (Fig. 5A).

To understand how CF parameters and photosynthesis rate influence the vase life of cut roses, correlations between these factors and vase life were analyzed (Fig. 5B). PI_ABS (p < 0.01), P_n (p < 0.01), ET₀/ABS (p < 0.05), and ET₀/TR₀ (p < 0.01) were positively correlated with vase life. In contrast, ABS/RC (p < 0.05), TR₀/RC (p < 0.05), ET₀/RC (p < 0.01), and DI₀/RC (p < 0.05) showed significantly high negative correlations with vase life (Fig. 5B). F_v/F_m value was not significantly correlated with the vase life of cut roses (Fig. 5B).

Discussion

An application of exogenous sucrose enhances flower opening and vase life of cut flowers in multiple species (Doi and Reid 1995;Liao et al. 2000;Singh et al. 2008;Ha et al. 2017) but the photosynthetic responses to light conditions during postharvest period and its influence on the longevity and quality of cut flowers have not been understood well. We found that most of photosynthetic parameters were significantly correlated with the vase life of cut rose flowers. In this study, light intensity at 50 μmol‧m^-2‧s^-1 increased photosynthesis capacity and stomatal conductance, thus delaying petal senescence and extending the vase life of cut rose flowers. The addition of exogenous sucrose significantly improved the water relations and vase life of cut roses regardless of light intensity. The maintenance of water balance is among the most important factors for the vase life of cut flowers (van Doorn 2012). Adverse water balance lead to premature petal wilting and bending of the pedicel, resulting in shortened vase life (Ha et al. 2019;Ha and In 2022;van Doorn 2012). It is well known that light intensity is involved in stomatal conductance and higher light intensity may stimulate the stomatal opening (Noichinda et al. 2007). Sucrose is well known it enhances water influx into petals of cut flowers due to the reduction of the osmotic potential in the vacuoles (Doi and Reid 1995;Ha and In 2022;Lü et al. 2010). Therefore, higher light intensity with exogenous sucrose contributed to increasing water transport efficiency by an increased stomatal conductance and consequently increased water absorption.

Carbohydrates are the major source of energy for the plant’s developmental and metabolic processes and their deficiency results in the petal senescence of cut flowers (Liao et al. 2000;Singh et al. 2008). Previous studies showed that a higher amount of SSC in petals is associated with a delay in the flower senescence of cut flowers (Aalifar et al. 2020;Ha et al. 2017;In and Lim 2018;Ha et al. 2019;Ha and In 2022. Change in the SSC in leaves because of exposure to different light intensities or spectra (Mastropasqua et al. 2016;Ohashi-Kaneko et al. 2007). Light is the driving force of photosynthesis activity for the production and accumulation of carbohydrates in plants. L50 flowers exhibited a higher photosynthesis rate but the SSC in leaves was lower than that of L10 flowers. Previous results have shown that the increased carbohydrate accumulation in leaves was associated with both down-regulation of photosynthesis and feedback inhibition on SSC synthesis (Jeannette et al. 2000;Wang et al. 2009). These findings are consistent with the inverse relationship between the photosynthesis rate and the SSC in leaves of cut rose flowers (Fig. 4D). The lower SSC in L50 flowers may be due to the carbohydrates in the leaves having been transported to the petals to provide energy for flower opening and other metabolic activities to prolong the vase life of cut flowers. These results indicate that the photosynthesis capacity of cut flowers held in vases plays an important role in extending their vase life of cut rose flowers. However, whether photosynthesis-derived SSC directly affects the petal senescence and vase life of cut roses is a matter for further study. Sucrose has two side effects on the photosynthesis activity of plants, depending on the plant species. The addition of sucrose can increase or decrease the photosynthesis rate of plants (Eckstein et al. 2012;Lobo et al. 2015). In some cases, such as strawberries, the addition of 3% and 5% sucrose did not affect or reduce the photosynthesis capacity of the plants (Hdider and Desjardins 1994). Previous study also showed that the presence of sucrose in the in vitro medium decreased the photosynthetic activity of the plant tissues (Eckstein et al. 2012). A previous study has indicated that external sucrose had a negative effect on the photosynthesis of two rose cultivars (Langford and Wainwright 1987). However, in this study, the addition of sucrose into the vase solution did not influence the photosynthesis rate of cut rose flowers. This is probably because the PSII activity including the apparent electron transport rate and effective quantum yield was not stimulated by external sucrose (Lobo et al. 2015). In addition, external sucrose may not influence the balancing process of sugar consumption and production in cut roses and the expression of the photosynthesis-related CAB genes in the leaves of cut roses (Eckstein et al. 2012;Hdider and Desjardins 1994).

The effect of the light spectrum and intensities on the photosynthetic performance of cut flowers or intact plants was explored recently (Aalifar et al. 2020;Bayat et al. 2018). Consistent with the previous study, our results showed that higher light intensity induced better photosynthesis performance in cut rose flowers during the vase period (Pan and Guo 2016). However, too high light intensity decreased photosynthesis capacity of cut flowers (Fig. 4C). In this study, CF parameters were strongly related to photosynthesis capacity of cut rose flowers. Cut roses exposed to the light intensity at 50 μmol‧m^-2‧s^-1 maintained a higher photosynthesis capacity by reducing heat dissipation (DI₀/RC) and elevating electron transport in the electron transport chain of the photosynthesis apparatus (ET₀/TR₀ and ET₀/ABS). The photosynthesis rates of cut rose flowers were also positively correlated with quantum yields of photosystem II (F_v/F_m). The high DI₀/RC and ET₀/RC rates are most probably related to the reduced F_v/F_m value and a higher photoinhibition occurrence rate. This result is because the PSII function and structure are damaged, causing most of the light energy absorption to be dissipated from the PSII reaction center (Moosavi-Nezhad et al. 2021). Light energy absorption per reaction center (ABS/RC) was highest in L10 flowers, this might be due to the inactivation of the PSII response center under unsuitable light intensity or stress conditions. In addition, the energy used to reduce Q_A by the reaction center unit of the PSII system (TR₀/RC) was higher in L10 flowers. The effect of light intensity on overall PSII photosynthesis capacity is also evaluated by measuring the photosynthesis performance index (PI_ABS), which is the most sensitive parameter of the chlorophyll fluorescence transient (OJIP) method (Gonzalez-Mendoza et al. 2007). The reduction in PI_ABS is associated with a low capacity for the development of the trans-thylakoid proton gradient (Moosavi-Nezhad et al. 2021). Thus, the decreased PI_ABS value in L10 flowers may indicate that the potential activity of the PSII system and damage/ repair ratio of protein D1 of the PSII system may be damaged under certain light conditions (Moosavi-Nezhad et al. 2021). These results indicate that the functionality of photosynthetic apparatus in cut roses can be assessed by the OJIP method. The OJIP method can point outed a negative effect of unsuitable light intensity or stress conditions on cut flowers’ photosynthetic performance.

In conclusion, the higher light intensity (50 μmol‧ m^-2‧s^-1) had a positive effect on the photosynthesis, postharvest quality, and vase life of cut roses. Our results on photosynthesis capacity and physiological and morphological changes during the vase period of cut roses under different light intensities provided valuable knowledge regarding the mechanism underlying the influences of light intensities on the vase life. Thus, for longer longevity of cut flowers, control of light conditions during the postharvest period or in retail facilities is important as well as the use of exogenous sucrose. Additionally, the strong correlation between photosynthesis and CF parameters showed that the OJIP transient method is an effective tool to evaluate the photosynthesis capacity of cut flowers, especially outside of laboratory.