Effect of Promoter and Inhibitors of Phenylalanine Ammonia-Lyase Enzyme on Stem Bending of Cut Gerbera Flowers

The main postharvest disorder of cut gerbera flowers is the stem bending and break. The cause of stem bending is not completely clear. Although the genetic background seems to play the major role, some plant hormones such as ethylene and cytokinins may enhance or reduce the incidence of stem break during postharvest life. The choice of withstand genotypes seems to be a good strategy. In fact, stem bending incidence strongly varies among cultivars. The aim of this work was to evaluate the role of PAL activity and tissue lignifications on stem bending. Cut gerbera flowers were obtained from local growers and immediately transferred to postharvest evaluation room. Cut flowers were screened and selected in order to have homogenous samples. Chemicals were applied as pulse treatments for 24 h using 2 mM amino-oxyacetic acid (AOA), 1 mM α-aminooxi-β-phenylpropionic acid (AOPP) or 100 μl L ethylene. During postharvest life the stem elongation, stem bending, PAL activity and vase life were measured. Results showed a strong variation of stem bending incidence among cultivars. Pulse treatments with PAL inhibitors such as AOA and AOPP strongly increased stem bending even after 1-2 days after treatments. Ethylene reduced the stem bending incidence increasing the PAL activity. INTRODUCTION The quality loss of cut flowers may depend on many factors. All the physiological disorders that affect the visual appearance compromise the quality and the commercial value of floriculture items. The main problems of ornamental perishables usually are flowers or leaf senescence. In some flowers such as gerberas the vase life may be defined by stem bending or break besides flower wilting. The stem bending is a postharvest disorder that usually occurs 10 cm below the capitulum (Wilberg, 1973). The causes of this postharvest disorder are not yet clearly identified. Some authors suggested that ethylene may be involved, because the 1-aminocyclopropane-1-carboxylic acid (ACC) content immediately increased in the bent stems (Mencarelli et al., 1995) associated with an increase of ethylene production. Gerberas belong to the Asteraceae family and are classified as not sensitive to ethylene (Reid, 1989), at least in the presence of common ethylene levels that can be found during the distribution chain. However, treatments with ethylene inhibitors did not prevent stem break. Therefore ethylene did not seem to be directly involved. It is known that postharvest treatments against bacteria growth may help to reduce stem bending (Kader and Rogers, 1986; Accati and Jona, 1989; Gerasopoulos and Chebli, 1999). In fact, bacteria in vase water may block vessels on the cut surface. Stem occlusion reduced the water uptake and increased the loss of turgidity (van Meeteren, 1978a,b; van Meeteren, 1980; van Doorn and Dewitte, 1994). Water losses reduce the ability of the stem to maintain the weight of flower head and the stem bends. The stem bending varies among cultivars and seems correlated to genetic background (De Jong and Garretsen, 1985; Werett et al., 1996). The flower head is pretty heavy, depending on capitulum diameter, usually is around 10-13 g at harvest time and the lack of lignification during stem elongation may reduce the resistance to flower weight (Cinotti et al., 2005). The youngest part of the stem is less lignified and might be responsible for stem Proc. IC on Qual. Manag. Supply Chains of Ornamentals Eds.: S. Kanlayanarat et al. Acta Hort. 755, ISHS 2007 472 bending. The aim of this work was to study the physiological and biochemical differences between cultivars that show and do not show stem bending. MATERIALS AND METHODS Plant Material Cut gerbera (Gerbera jamesonii H. Bolus) flowers were obtained directly from a cooperative of flower growers (Florexport, Viareggio Italy). Cut flowers were selected to avoid malformations or damage related to harvesting and transport handling. Flower stems were cut to 40 cm and placed in distilled water. All experiments were performed in a postharvest room equipped with a controlled environment maintained at 20°C, 70% relative humidity and 10 μmol m s light intensity for 12 h day by cool-white fluorescent lamps. A wide number of cultivars were screened for evaluating their postharvest behaviour. Treatments Cut gerbera flowers were placed in distilled water (control) or in a vase solution containing 2 mM α-aminooxyacetic acid (AOA, Sigma – Italy), 1 mM α-aminooxi-βphenylpropionic acid (AOPP, Wako – Germany) or 100 μl L ethylene (Sigma, Italy). Flowers were pulse treated for 24 h in the postharvest laboratory under the conditions described above. After the pulse treatment, all the flowers were placed in distilled water. The entire experiment was repeated twice. PAL Activity Stem samples were taken around 10 cm below the capitulum, about 1 g of stem was ground in pre-chilled mortar with 3 ml extraction buffer, containing 100 mM sodium borate and 14 mM β-mercaptoethanol. The pH was adjusted to 8.8. The homogenate was centrifuged at 14000 rpm at 4°C. The surnatant represented the crude extract and was used for enzyme assay. PAL activity was determined adding 100 μl of crude extract to 1.2 ml of assay buffer containing 200 μl 100 mM L-phenylalanine and 1 ml sodium borate. The reaction mixture was incubated at 40°C for 2 h. The reaction was stopped adding 250 μl HCl 5 N and absorbance readings were performed at 290 nm. Statistical Analysis The data are reported as means with standard errors. Each treatment was composed of 8 replicate stems. RESULTS The cultivars screening showed interesting results about vase life and stem bending incidence. The vase life varied among cultivars and ranged between 5 and 24 days (Table 1). The vase life was the minimum value between stem bending or flower wilting. Four cultivars did not show stem bending and vase life was considered ended when ray flowers wilted. On the contrary, five cultivars showed stem bending in all flowers used for the experiments. The stem break calculated on all the cultivars that showed stem bending was on average 9.7 cm below the flower head. Although in some gerberas the stem bending occurred just below the capitulum. The PAL activity was determined in four cultivars that showed 100% (‘Olina’ and ‘Dame Blanche’), 87% (‘Real’) and 0% (‘Dalma’) stem bending (Fig. 1). Results showed that the highest PAL activity was found in ‘Dalma’, while the lowest enzyme activity was found in those that showed 100% stem bending. Intermediate values of PAL activity were found in the ‘Real’ that showed 87% of stem bending incidence (Fig. 1). ‘Olina’ was used for further analysis using PAL inhibitors or promoter. Their effects on vase life and enzyme activity were analysed. The PAL inhibitors, AOA and AOPP, strongly induced stem bending, all flowers treated bent within 2 days. Moreover, these treatments strongly reduced PAL activity compared to controls (Fig. 2). Ethylene