Ethylene Response to Mechanical Stress Perturbation of Salvia Splendens L. Potted Plants

Mechanical treatment could be an efficient method to control growth of vegetable and ornamental species cultivated under greenhouse conditions. Mechanical growth control is able to induce a 20-50% reduction in the vegetative growth depending on the species, the type and intensity of stress applied. In any case, plant growth and morphogenesis are influenced by mechanical perturbation and common features among many different plant species are a decrease in elongation growth and an increase in radial expansion. Exogenous application of ethylene can result in morphological and physiological changes that resemble aspects of thigmomorphogenesis. Ethylene production occurs after mechanical stimulation of plants and many studies suggest that ethylene may have a role in radial expansion aspects of thigmomorphogenesis although it is unlikely to be the primary signalling molecule that controls all touch responses. The aim of the this work is to investigate the effects of mechanical perturbation caused by brushing potted plants of Salvia splendens L. cv. Flamex 2000. Attention was focused on the production of ethylene from different organs of the stressed plants and was monitored the time course of ethylene released as response to brushing treatments. The effects of the mechanical stress were compared to those induced by a common chemical agent usually applied to reduce transplant size. The results obtained showed that the dimension of Salvia splendens potted plants could be efficiently reduced following brushing treatments. Ethylene was released mainly in the apical shoots exposed to the mechanical perturbation but the production decreased after repeated treatments suggesting a mechanism of adaptation to the imposed stress conditions. INTRODUCTION Many greenhouse floricultural crops, bedding plants and vegetables transplants tend to grow taller than desired, do not respect the demands of market and require height control measures (Schnelle et al., 1999). Chemical and plant growth regulators are traditionally used to maintain plants at a certain size. However, using chemicals increase the risk of environment pollution, the efficacy of the treatments varies by chemical and species and the chemical compounds considerable increase the production costs. Mechanical conditioning is an excellent alternative for growth regulation (Latimer, 1998) and brushing has been shown to reduce stem elongation during plant development (Garner and Langton, 1997) but the use of this method is limited by a lack of information on how plants respond to application of the technique (Björkman, 1999). Brushing application has been previously studied in Salvia Splendens, a popular bedding plant analysing the involvement of abscisic acid in the plant response to mechanical stress (Vernieri et al., 2003). However, results suggest that the synthesis and the translocation of this hormone is not significantly affected by the imposed stress. Hormonal changes due to mechanical perturbation were detected in different plant species and, in particular, an increase in ethylene evolution was observed (Biro and Jaffe, 1984; Morgan and Drew, 1997). The aim of the present work was to investigate whether ethylene production from Salvia plantlets is affected by brushing treatments, and the possible relationship with the morphological modification induced by the stress. 421 Proc. I IC on Labiatae Eds.: B. Ruffoni et al. Acta Hort. 723, ISHS 2006 MATERIALS AND METHODS Plant Materials Seeds of Salvia splendens L. cv. Flamex 2000 were sown in plastic seed boxes filled with vermiculite. The seed boxes were placed in a growth chamber to germinate at a temperature of 25C and 300 μmol m sec PAR light. The emergence occurred after 1015 days. When the first pair of true leaves was visible, the seedlings were transplanted in plastic-packs with trunk-conic section (5.5 x 4.5 x 6.0 cm) filled with 1 soil: 1 vermiculite (v/v). Seedlings have been set in a growth chamber with 25C during the day and 16-18C during the night, 65-70% relative humidity and 16h (350 mol m sec of PAR). Seedlings were watered as required and fertilized twice a week with a nutrient solution. Ethylene Determination The quantitative analysis of the ethylene in gaseous samplings has been carried out through the use of the gas chromatograph (5890 HP) equipped with a FID detector and a metal column packed with Hayseed T (80-100 meshes). Two mL of gaseous samplings were withdrawn from each vial after 1h incubation with a gas tight syringe, and injected. The temperatures of the column and the detector were 70°C and 350°C, respectively. N2 was used as carrier at a flow rate of 30mL/min. Ethylene was quantified through the technique of the external standard. For ethylene analysis apical portion of the plants with two apical leaves were enclosed in 10-mL glass vials (Pyrex). For each treatment 3 repetitions were accomplished. Treatments Plants were brushed with an electric motor-equipped apparatus that moved brushes forward and backward above the plants. Each movement was counted as a single brush stroke. The brushing system is formed by sliding metallic bars (a brush consisted of a rod attached to a frame), where a strip of plastic PVC protects the rods. As the seedling grew, the heights of the plastic were adjusted daily. Mechanical stress was applied by brushing the tops (about 2 cm) of the plants. Every brushing action touched and bent the apical part of the plants that formed about 45 angle. No apparent damage was caused and the shoots returned to a vertical position immediately after brushing. Mechanical stress was provided by stimulation of petioles and leaves, and by back and forth movement of entire shoots (Garner and Langton, 1997). The time used for the treatment brushing was 30 minutes (in a total of 30 back and forth) four times a day. Chemical treatment was performed spraying a growth regulator, principle active Flurprimidol, and commercial name TopFlor® at the dosage of 0.5g/L. All the experiments had the same randomised blocks statistical design. The number of treatments and repetition varied with the experiment. RESULTS AND DISCUSSION Ethylene production was monitored in Salvia splendens seedlings before and during the application of a brushing treatment of 30 min. Ethylene production was also measured at the end of the treatment and 30 and 60 min during recovery (Fig. 1). Ethylene peak was observed after 15 minutes of brushing and after 90 minutes from the beginning of the mechanical stress. This pattern of ethylene biosynthesis suggests the presence of an initial reaction to the perturbation stress followed by a peak of ethylene that represents the recovery of normal growth conditions. Ethylene production was considered a signal of recovery also in other plant stress as chilling (Serrano et al., 1995) or ozone exposure (Tuomainen et al., 1997). The morphological changes in plants exposed to mechanical stimulation resemble the decrease in elongation and the radial expansion observed following ethylene treatments. In addition, ethylene production increases after touch stimulation in many plant species suggesting a possible role of mediation of this hormone in plant responses to touch (Mitchell, 1996).