Search

SummaryWe 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 p...

SummarySuccess in grafting depends on the identification of a compatible rootstock (RS) that promotes rapid formation of vascular connections between the RS and the scion, and rapid resumption of root and shoot growth. In this study, we used compatible and incompatible Cucurbita rootstocks, with a melon (Cucumis melo L. ‘Arava’) scion, to identify physiological and biochemical factors in the scion-RS interface that could be associated with graft compatibility. Anatomical characterisation of the grafting interface showed that the regeneration and differentiation of vascular elements was similar, at first, in both compatible and incompatible grafted seedlings although, in the latter, a protective layer was formed, but did not interfere with communication between the RS and the scion. Water uptake and sugar distribution between the plant canopy and the roots, measured 14 d after grafting (DAG) were not correlated with compatibility or incompatibility. At 24 DAG, both water uptake and root sugar concentration...

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.

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.