Inhibitors of oxidative enzymes affect water uptake and vase life of cut Acacia holosericea and Chamelaucium uncinatum stems
Research highlights
▶ We studied effects of oxidative enzyme inhibitors on water relations of cut flowers. ▶ 12 inhibitors increased water uptake and vase life of Acacia foliage. ▶ 6 inhibitors increased water uptake and vase life of Chamelaucium flowers. ▶ There were significant correlations between water uptake, fresh weight and vase life. ▶ We conclude that oxidative enzymes contribute to xylem occlusion in woody cut stems.
Introduction
Acacia holosericea (Velvet Leaf Wattle) has cut foliage potential due to its silvery green silky phyllodes (leaves). However, it has a short vase life due to insufficient water uptake causing early wilting and/or desiccation of phyllodes (Damunupola et al., 2010). Water uptake by cut flower stems, including A. holosericea, decreases with time after being stood into water, possibly due to occlusions in the xylem (Van Doorn, 1996). Physical blockages of xylem due to air embolism (cavitation), small particles or bacteria and bacterial products are well known (Van Doorn, 1996). Physiological plugging as a result of defensive metabolic responses to wounding has also been reported (Van Doorn and Cruz, 2000, Van Doorn and Vaslier, 2002, Vaslier and van Doorn, 2003, Loubaud and van Doorn, 2004, He et al., 2006).
Wounding plant tissues activates mechanisms for defence and healing (Leon et al., 2001). Deposition of materials such as suberin, lignin and tannin in the xylem as a physiological reaction to wounding was suggested long ago (Dean and Kolattukudy, 1976, Halevy and Mayak, 1981, Van Doorn, 1996). Williamson et al. (2002) considered that hydrophobic suberin formation was an early (<12 h) response to wounding that led to cavitation and water deficit stress. Peroxidase is involved in suberin deposition in wound healing potato tuber tissue (Espelie et al., 1986). The wound signal is transmitted over a distance of at least 4 cm through pea stem tissue (Peck and Kende, 1998). With no delay in response intensity, a hydraulic or electric signal is possibly responsible for spread of the wound response.
Inhibition by antioxidants of wound-induced xylem occlusion has been reported to occur in Chrysanthemum both during dry and wet storage (Van Doorn and Cruz, 2000). Catechol oxidase and laccase are major phenoloxidases (Walker, 1995). Xylem blockage in Chrysanthemum was delayed by the catechol oxidase inhibitors, tropolone and 4-hexylresorcinol (Van Doorn and Vaslier, 2002). Both peroxidase and catechol oxidase were involved in physiological blockage in the lowermost 5 cm of dry stored Bouvardia flower stems (Vaslier and van Doorn, 2003). Similar physiological blockage also occurs in Astilbe flowers (Loubaud and van Doorn, 2004).
S-carvone is a monoterpene known to inhibit phenylalanine ammonia-lyase (PAL) activity and synthesis of wound-induced compounds, like suberin (Oosterhaven et al., 1995a). However, it also has antimicrobial activity (Oosterhaven et al., 1995b). In vase solution, S-carvone extended the vase life of flowers from members of the families Proteaceae [viz. Hakea francisiana (Williamson et al., 2002) and Grevillea ‘Crimson Yul-lo’ (He et al., 2006)] and Myrtaceae [viz. Baeckea frutescens and Chamelaucium uncinatum (Damunupola et al., 2010)]. However, there were no positive effects on vase life of A. holosericea and Chrysanthemum (Damunupola et al., 2010). Like S-carvone, treatment with a catechol oxidase inhibitor, 4-hexylresorcinol, delayed stem end blockage in Grevillea ‘Crimson Yul-lo’ inflorescences (He et al., 2006). Thus, blockage in cut Grevillea stems was suggested to be physiologically mediated.
Peroxidase, polyphenoloxidase and PAL activities after wounding are associated with lignin and phenol metabolism (Okey et al., 1997). As a rapid response to wounding in wheat roots, peroxidases from cell surface release into the apoplast where they can display both oxidative and peroxidative activities (Minibayeva et al., 2009).
In the present study, inhibitors of peroxidase and phenoloxidase (viz. catechol oxidase, laccase, etc.) enzymes were tested for woody A. holosericea and C. uncinatum stems. It was hypothesised that these inhibitors of oxidative enzymes would suppress enzyme activity in response to wounding and thereby prevent or reduce physiological occlusion of xylem vessels for these species.
Section snippets
Plant materials
Leafy stems of A. holosericea A. Cunn. ex G. Don (Velvet Leaf Wattle, Family Mimosaceae) around 42–55 cm long were harvested in mornings over winter from young (1-year-old; experiment 1; 30 June 2009) and established (3–4-year-old; experiment 2; 14 July 2009) plants growing at The University of Queensland Gatton campus nursery (152°20′ E, 27°33′S). Flowering stems of C. uncinatum Schau., Myrtaceae ‘Dancing Queen’ (Geraldton waxflower; Family Myrtaceae) around 30–35 cm long were harvested in the
Experiment 1 for Acacia
Effects of enzyme inhibitor pulse treatments on changes in relative fresh weight (RFW), water uptake (WU) and water loss (WL) for stems from young Acacia plants over 8 days in the vase were recorded (Fig. 1). Significant treatment effects were observed for WL, WU and RFW (Table 2).
AT had no effects on RFW (Fig. 1A), WU (Fig. 1AU) and WL (Fig. 1AL) (Table 2). Other peroxidase inhibitors were significantly positively effective: CH, from day 4 for RFW (Fig. 1A), on day 2 for WU (Fig. 1AU) and WL (
Discussion
Peroxidase inhibitors showed positive effects for WL, WU and RFW of Acacia stems in experiments 1 and 2. PD increased WU and RFW in 56% of measurements (Table 2, Table 3). Except for AT, the other three peroxidase inhibitors, CH, HQ and PD, prolonged vase life Acacia stems in both experiments (Table 5). PD and CS increased WU and RFW of Chamelaucium (Table 4). Three peroxidase inhibitors also increased Chamelaucium vase life (Table 5). That HQ was ineffective is possibly because the
Acknowledgements
Prof. Fisun Çelikel was the grateful recipient of an Endeavour Research Fellowship Award from the Australian Government. Mayati Che Husin (PhD student) and Dr. Iftikhar Ahmad Hashmi (visiting researcher) are thanked for their technical assistance in some experiments. Allan Lisle is gratefully recognised for advice on experimental design and assistance with statistical analyses. We thank the reviewers for valuable comments.
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