Physiological Characterization of Flower Senescence in Long Life and Ephemeral Hibiscus (Hibiscus rosa-sinensis L.)

The most part of hibiscus plants produces short life flowers that last one day. Therefore they are called ephemeral and flower senescence is usually associated with petal in-rolling and abscission. Flowers are well shaped and different organs can be easily separated, these characteristics make the hibiscus a good model system for flower senescence studies. The aim of this work was to identify metabolic differences and hormonal profiles in petals of different flowers of hibiscus plants that show different flower longevity. Different hibiscus (Hibiscus rosa-sinensis L.) plants were screened under controlled environment for identifying different clusters. Among the plants screened the physiological studies were performed on the following cultivars: ‘Caribbean Tricolour’, ‘Caribbean Dark Pink’, ‘Caribbean Pink’, ‘Caribbean White’, ‘Caribbean White eye’, ‘Porto’ and ‘La France’. Flowers were detached from different clusters and placed in postharvest room for vase life determination. Flower harvesting was performed in the morning usually between 9:00 and 12:00 a.m. Detached flowers were placed in deionised water. Ethylene production, abscisic acid content, anthocyanins, total phenols and carotenoids were measured. All data were analysed and correlated with flower longevity. The ephemeral flowers lasted 12-18 h, while the longest flower life was 3-4 days in ‘Porto’. The ethylene production and ABA content were inversely proportional to flower longevity. In fact the longest flower life was observed in hibiscus plants that had the lowest ethylene production. INTRODUCTION Flowering potted plants are ornamental items that are characterised by a cultivation period in optimal growing conditions. Subsequently, when plants reach the desired commercial size, they can be transferred in hardening greenhouses or directly sent to the distribution chain (Nowak and Rudnicki, 1990). Usually flowering potted plants are packed with transparent films and stored or shipped to different selling destinations. The ornamental quality of flowering potted plants depends on the number and quality of flowers, mainly open flowers. Moreover, the nutrient management during cultivation is also very important. It has been demonstrated that water stress combined with phosphorus and nitrogen deficiency during cultivation improved the post-production quality (Hansen et al., 2005). The presence of flowers on plants depends on the flower longevity and turnover. Hibiscus plants usually produce flowers that last one day and they are classified as ephemeral. Flowering hibiscus potted plants are very sensitive to exogenous ethylene and its effect is visible at concentrations as low as 0.1 μl L. Ethylene induces flower senescence and in hibiscus plants causes buds abscission, compromising the ornamental quality (Høyer, 1996; Hansen et al., 2005). Physiological studies during hibiscus flower senescence showed that ethylene production and 1-aminocyclopropane-1-carboxylic acid (ACC) content increased in petals during flower development and senescence (Woodson et al., 1985). Exogenous applications of ACC or amino-oxyacetic acid (AOA) increased or reduced ethylene production respectively, with positive or negative effect on flower life. The ethylene production and accumulation during transportation or storage can have dramatic effect on bud abscission reducing the commercialization quality (Al-Saqri et al., 2003). Treatments with ethylene action ethylene inhibitors, such as 1-MCP or STS, Proc. IC on Qual. Manag. Supply Chains of Ornamentals Eds.: S. Kanlayanarat et al. Acta Hort. 755, ISHS 2007 458 extended flower life of one day compared to the controls. The 1-MCP treatment was effective only if applied continuously (Reid et al., 2002). Beside ethylene also abscisic acid (ABA), another plant hormone, increases during hibiscus flower senescence (Swanson et al., 1975). The endogenous levels of these two hormones may be linked with genetic background of cultivars and might be used as markers for classic breeding programs which have as target to extend flower life. The aim of this work was to study the endogenous variations of ethylene production, ABA content and flower pigments during flower senescence of different hibiscus cultivars characterised by different flower longevity. MATERIALS AND METHODS Plant Material Hibiscus (Hibiscus rosa-sinensis L.) plants were grown in greenhouse under natural environmental conditions at the Dip. Biologia delle Piante Agrarie, University of Pisa. The experiments were carried out on ephemeral (‘La France’) and long life (‘Porto’, ‘Caribbean White’, ‘Caribbean White eyes’, ‘Caribbean tricolour’) hibiscus plants. Flowers were harvested from May to July, period of the highest flower production, between 9:00 and 12:00 a.m. Detached flowers were immediately transported to the laboratory and placed in distilled water. All experiments were performed under controlled environment conditions (20°C T, 70% RH, 10 μmol m s PPFD, 12 h photoperiod). Flower life was determined in these conditions by daily observations. All measurements were carried out on one day fully open flowers. Ethylene Measurements Ethylene production was measured by enclosing flower organs in air-tight containers (250 ml). Two ml gas samples were taken from the headspace of the containers after 1 h incubation at room temperature. The ethylene concentration in the sample was measured by a gas chromatograph (HP5890, Hewlett-Packard, Menlo Park, CA) using a flame ionization detector (FID), a stainless steel column (150 x 0.4 cm ø packed with Hysep T), column and detector temperatures of 70 and 350°C, respectively, and nitrogen carrier gas at a flow rate of 30 ml min. Abscisic Acid Determination Petals (80-100 mg FW) were extracted with distilled water (water:tissue ratio 10:1 v/w) for 16 h at 4°C in the dark. Quantitative analysis was performed on crude aqueous extracts using a solid-phase radioimmunoassay based on a monoclonal antibody (DBPA1) raised against free (S)-ABA, as described previously (Vernieri et al., 1991). Determination of Carotenoids, Anthocyanins and Total Phenol Absorbance Total carotenoids for hibiscus petals were determined by extraction using methanol 99.9% as solvent. Petal samples were kept in a dark, cold room at 4°C for 24 h. Quantitative determination of carotenoids was carried out immediately after extraction. Absorbance readings were performed at 470 nm. Total carotenoids were calculated using Lichtenthaler’s formula (1987). Petals of the frozen tissue (50 mg) were extracted into methanolic HCl (1%). Samples were incubated overnight at 4°C in darkness. The concentration of anthocyanins was expressed in cyanidin-3-glucoside equivalents determined spectrophotometrically at 535 nm using an extinction coefficient of 29,600 (ε). Phenolic compounds were extracted from 30-50 mg of petals using 5 ml 1.2 M HCl in 99% methanol. Absorbance measurements were taken after overnight incubation at 4°C. Total phenolics were estimated by measuring absorbance at 320 nm using an UVVis spectrophotometer (Ke and Saltveit, 1989). The amount of total phenols was expressed as gallic acid equivalents.