Decline in ascorbate peroxidase activity - A prerequisite factor for tepal senescence in gladiolus

Flower senescence was studied in Gladiolus cv. Snow Princess over five arbitrarily divided developmental stages (stage 1, half bloom; stage 2, full bloom; stage 3, beginning of witting; stage 4, 50% witting; stage 5, complete witting) in terms of changes in fresh weight, antioxidant enzymes (superoxide dismutase, SOD; ascorbate peroxidase, APX; glutathione reductase, GR) activities and membrane integrity. A significant decrease in tepal fresh weight was observed over the senescence period (after stage 2). Membrane integrity was studied by measuring lipid peroxidation [in terms of thiobarbituric acid reactive substances (TBARS) content] and membrane stability index (MSI) percentage. Maximum TBARS content was recorded in stage 4 (50% witting). This increase in lipid peroxidation over the senescence period was in close association with high degree of membrane deterioration expressed as decrease in membrane stability index percentage. A significant decrease (two and half-fold) in MSI% in stage 5 (as compared to stage 1) indicates complete membrane deterioration. Progressive increase in endogenous H(2)O(2) level was recorded over senescence period. Maximum H(2)O(2) content (19.7 +/- 1.4 mu mol g(-1) DW) was recorded at stage 5 (complete witting). Three different patterns were observed in antioxidant enzymes behavior over the senescence period. APX activity was declined significantly as, the flower entered stage 3 (beginning of witting) from full bloom condition (stage 2). Progressive and significant increase in SOD activity was measured as a function of time. Maximum SOD activity (24.2 +/- 0.8 U mg(-1) DW) was recorded in stage 5 (three-fold increase over stage 1). GR activity initially increased up to stage 4 (50% witting) and declined significantly thereafter (similar to seven-fotd). An increase in endogenous H(2)O(2) level during senescence may be the result of a programmed down-regulation of APX enzyme activity, which seems to be the prerequisite factor for initiating senescence process in gladiolus tepal. (C) 2005 Elsevier GmbH. All rights reserved.