Harpins are proteins produced by plant pathogenic bacteria; they induce hypersensitive response in non-host plants. The possibility of harpin induced resistance was investigated in muskmelons, and the physiological mechanism was studied in this paper.
1. Harpin did not demonstrate any fungicide effect in vitro. The effect of postharvest dipping with different concentrations of harpin was studied in two Hami melons cultivars (cvs. New Queen and 8601). A concentration at 90 mg/L was the most effective treatment for reducing decay severity caused by Alternaria lternata,Fusarium semitectum and Trichothecium roseum. Higher concentration (120 and 200 mg/L) failed to promote better resistance against decay-causing agent and to cause phytotoxicity. Harpin induced local and systemic resistance against T. roseum. Efficacy of suppression against T. roseum lasted 5 and 8 days for ‘New Queen’ and ‘8601’ cultivars in harpin treated fruit. More mature fruit were less susceptibility to harpin.
2. Treatment of harpin at 90 mg/L increased activity of peroxidase (POD), chitinase (CHT), phenylalanine ammonia-lyase (PAL), chalcone isomerase (CHI) and polyphenoloxidase (PPO) in Hami melon(cv. 8601). Harpin resulted in an increase in production of total phenolic, flavanoid and lignin. The challenge with T. roseum enhanced furthermore activity of POD, CHT, PAL, CHI and PPO in the harpin treated, more accumulation was also found in total phenolic, flavanoid and lignin. The control had an increase resistance reaction after challenge with T. roseum; however, the intensity was weaker and the duration was shorter than the treated.
3. Treatment of harpin at 90 mg/L increased the production of H202 and O2- in Hami melon(cv. 8601). Harpin resulted in an increase in activity of superoxide dismutase (SOD), POD and glutathion reductase (GR), and a decrease in activity of catalase (CAT) and ascorbic peroxidase (APX). Harpin did not demonstrate any effect in content of ascorbic acid (AsA), glutathione (GSH) and malondialdehyde (MDA) and membrane integrity. The challenge with T. roseum enhanced furthermore accumulation of H202 and O2- in harpin treated, more activity was also found in SOD, POD and GR; however, the increase of CAT and APX was inhibited. The challenge slowered the reduction of AsA , GSH content and membrane integrity, and accumulated MDA.
4. Harpin at 50mg/L was sprayed 4 times to melons (cv. Yindi) in the field. The spray time was at 1 week before flowering, young fruit period, fruit enlarging period, and netting period. The latent infection rate of fruit was reduced significantly with harpin treatment. The latent infection rate decreased with increasing of spraying times. The four times of spraying showed the best effective control. Harpin increased the activity of POD,CHT,PAL,SOD and PPO, enhanced the content of total phenolic, flavanoid lignin and H202, reduced the activety of CAT.