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    Apoplastic anti-oxidants in pepper (Capsicum annuum L.) fruit and their relationship to blossom-end rot
    (Headley Brothers Ltd, 2006) Turhan, E.; Karni, L.; Aktas, H.; Deventurero, G.; Chang, D.C.; Bar-Tal, A.; Aloni, B.
    The aim of the present research was to study changes in the major anti-oxidative enzyme activities known to be associated with the apoplast, during the induction of blossom-end rot (BER) in bell pepper (Capsicum annuum L.) fruits grown under saline conditions. Pepper plants of the BER-sensitive cv. 'Mazurka' and less-sensitive cv. 'Selica' were grown under control, low- and high-salinity irrigation regimes. Fruits were harvested continuously and, after approx. 7 months under these conditions, BER symptoms started to appear (in the Spring season) and fruits were sampled for biochemical measurements. The intercellular washing fluid (IWF; apoplast fraction) of the fruit pericarp was prepared and enzyme activities in that fraction were determined. The results showed that production of reactive oxygen species (ROS) in the fruit apoplast was higher in 'Mazurka' than in 'Selica'. Apoplastic peroxidase and superoxide dismutase (SOD) activities, and the concentration of ascorbic acid, increased during 'Selica' fruit development, whereas the concentration of H 2O2 decreased. During the stage of rapid growth, when fruits are most susceptible to BER, apoplastic peroxidase activity was greatly enhanced in the healthy pericarp of BER-affected fruits compared to healthy fruits. Under saline conditions, apoplastic peroxidase and SOD activities, as well as ascorbic acid and H2O2 levels, increased in healthy fruits of the resistant cultivar 'Selica'; whereas, in healthy fruit of the BER-sensitive cultivar 'Mazurka', apoplastic peroxidase activity decreased while the level of ascorbic acid increased, but to a much lesser extent than in 'Selica'. SOD activity and H2O2 levels increased similarly in both cultivars with increasing levels of salinity. It is proposed that apoplast-associated peroxidase activity and ascorbic acid concentration are increased coordinately in response to salt stress and may be part of a protective anti-oxidation mechanism that determines cultivar sensitivity to BER.
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    Blossom-end rot is associated with impairment of sugar metabolism and growth of pepper (Capsicum annuum L.) fruits
    (Headley Brothers Ltd, 2006) Turhan, E.; Aktas, H.; Deventurero, G.; Karni, L.; Bar-Tal, A.; Aloni, B.
    Blossom-end rot (BER) in pepper fruits is known as a " physiological" disease which is evoked by climatic and osmotic stresses (e.g., high temperatures and salinity) and by deficit irrigation management. The physiological and biochemical mechanisms involved in the stimulation of BER are still a matter of dispute. It has been proposed that BER occurs under cultivation conditions that accelerate fruit growth beyond calcium supply to the growing tissue. In the present study, we investigated this hypothesis by recording fruit growth and sugar metabolism under normal and BER-inducing conditions. The results showed that pepper fruits grown in a ventilated-cooled greenhouse, which ameliorated BER, had a higher initial growth rate than those grown in a non-cooled greenhouse, but had lower calcium concentrations in the fruit pericarp. Fruits that grew without ventilation-cooling had higher concentrations of apoplastic and symplastic sucrose and reducing sugars, and higher levels of symplastic starch than those that developed in a cooled greenhouse. Concomitantly, fruits that developed in a non-cooled greenhouse had lower acid invertase, sucrose synthase and fructokinase activities than those that developed in a cooled greenhouse. Accordingly, the non-necrotic part of BER-affected fruits had higher concentrations of symplastic starch, reducing sugars and sucrose than healthy fruits. These findings indicate that the occurrence of BER in pepper fruits was associated with impaired sucrose metabolism in fruit tissues, which led to reductions in growth rate and final size. Thus, the present findings negate the notion that BER occurs when fruit growth rate is stimulated, and favour the idea that direct impairment of fruit sugar metabolism and of normal fruit growth are involved.
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    Öğe
    Changes in ascorbic acid concentration, ascorbate oxidase activity, and apoplastic pH in relation to fruit development in pepper (Capsicum annuum L.) and the occurrence of blossom-end rot
    (Headley Brothers Ltd, 2008) Aloni, B.; Karni, L.; Deventurero, G.; Turhan, E.; Aktas, H.
    We recently suggested that the occurrence of ascorbic acid (AsA) in the apoplast of bell pepper fruit may be part of the protection mechanism against blossom-end rot (BER). In the present study, we determined AsA concentrations, changes in pH, and the activities of ascorbate oxidase (AO) in the apoplast and symplast of pepper fruit, in relation to fruit development and the occurrence of BER. In healthy pepper fruit, the concentrations of AsA in the symplast and apoplast increased during fruit development, whereas the activity of AO decreased when measured at the pH typical of each developmental stage. The apoplastic pH decreased from 6.2 in young fruit to 4.8 in mature red fruit; while the symplastic pH did not change during fruit development. Following high-temperature treatment (35°C for 48 h in the dark), the concentration of apoplastic AsA, and the activity of galacturonic acid reductase (which participates in the AsA biosynthetic pathway), decreased, while there was an increase in apoplastic pH. The AsA content decreased in the apoplast of BER-affected fruit, and the pH was more alkaline (pH 6.9) than in healthy fruit (pH 6.0). In young fruit with BER, the apoplastic AO activity increased and the symplastic galacturonic acid reductase activity was inhibited. We suggest that a reduction in apoplastic AsA concentration, an increase in apoplastic AO activity, and changes in apoplastic pH in fruit prone to BER may be part of the mechanism which leads to the occurrence of BER symptoms. Whether or not there are causal relationships between these changes is not yet clear.