Oxidative metabolism in ‘Valencia’ sweet orange [Citrus sinensis (L.) Osbeck] flavedo tissue treated with the abscission agent 5-chloro-3- methyl-4-nitro-1H-pyrazole (CMNP)

Abstract

Summary5-Chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) is a pyrazole derivative that promotes the abscission of mature citrus fruit and is being developed to facilitate mechanical commercial harvesting of mature fruit. Understanding the mode of action of CMNP in promoting abscission will assist in decisions on its commercial use and in the development of alternative, but superior compounds. Fully-mature ‘Valencia’ sweet orange trees were sprayed with 2.0 mM CMNP. Fruit detachment force (FDF) values, hydrogen peroxide (H2O2) concentrations, and malondialdehyde (MDA) concentrations, as well as superoxide dismutase (SOD), ascorbate peroxidase (APOD), glutathione reductase (GR), and peroxidase (POD) activities were measured 0, 1, 2, and 3 d after CMNP treatment (DACT). FDF values declined linearly from 65 N to 28 N after CMNP treatment. H2O2 and MDA concentrations were higher in CMNP-treated flavedo tissue (FT) compared to the untreated controls. SOD activity was higher in the FT of CMNP-treated fruit than in untreated controls and was largely responsible for the accumulation of H2O2. The activities of enzymes in the APOD/GR cycle (i.e., ascorbate peroxidase and glutathione reductase) declined significantly throughout the abscission process. CMNP-induced higher SOD and lower APOD and GR activities, which maintained elevated levels of H2O2. H2O2 may serve as a signal, in association with previously reported lipid signals, in the CMNP-mediated abscission of citrus fruit. POD activities increased up to 1 DACT, then declined to control values. POD may be involved in reducing free indole-acetic acid (IAA) concentrations in the FT, thus enhancing the sensitivity of the FT to ethylene, which is known to promote abscission in citrus fruit.