To understand the factors affecting the incidence of blossom-end rot (BER), the effect of the Ca/K ratio (4/12-12/4, in me·L -1 ) in nutrient solutions and Ca concentration in fractions in the distal part of young tomato fruits immediately before BER symptoms appear were investigated for three seasons. The rate of BER incidence increased with a decrease in the Ca/K ratio in the supplied solutions in the summer and spring, but little difference was observed in the winter. Ca concentration was highest in winter and lowest in summer, and the concentration in fractions decreased with a decrease in the Ca/K ratio of the solutions. When the results of all three experiments were pooled, among the fractions, water-soluble Ca concentration was found to have the highest significance in the relationship to BER incidence. The risk of BER incidence in rapidly growing tomato increased to a critical level when water-soluble Ca in the distal part of the fresh fruit decreased to less than 0.20 μmol·g -1 FW. Multiple-regression analysis revealed that the concentration of water-soluble Ca, which is predominantly recovering apoplastic or cytoplasmic Ca 2+ , and total Ca, which has been translocated during fruit development, are significantly affected by solar radiation and Ca concentration in the supplied solution rather than air temperature.

Blossom-end rot (BER) in tomato has been generally reported as a calcium (Ca)-related physiological disorder influenced by cultivar and environmental factors. In our previous works, we found that different fruit-sized cultivars could share a similar threshold value of water-soluble Ca. In addition, seasonal susceptibility to BER was closely related to fruit growth rate. This study aimed to clarify the effect of fruit growth rate as a dominant factor determining the susceptibility in different fruit-sized tomato cultivars.

Seasonal changes in the water-soluble Ca concentration in the distal half of fruit tissue and the blossom-end rot (BER) incidence in relation to the fruit growth rate were examined in the medium-sized fruit tomato cultivar 'Cindy Sweet', which is less susceptible to BER than large-sized fruit tomato cultivars. The rate of BER incidence was higher in the plants fed low Ca solutions, 1/4Ca and 1/8Ca, compared to those fed Enshi standard solution in both spring and autumn cropping. It was significantly higher in plants grown in spring than in autumn. The water-soluble Ca concentration in the distal half of fruit tissue was negatively correlated with the BER incidence and was significantly lower in spring than in autumn. A marked increase in the incidence of BER was observed as the water-soluble Ca concentration in the distal half of fruit tissue decreased below 0.2 mmol・g -1 FW. This value was similar to our previous findings on the large-sized fruit tomato cultivar 'House Momotaro'. The fruit growth rate was 3-fold higher in the fruits from plants grown in spring, with its higher temperature and solar radiation, than in autumn. There was a significant negative correlation between the rate of fruit growth and water-soluble Ca in the tissue of the distal half of fruit. These results suggested that the vigorous fruit growth can lead to BER disorder by potentially causing the dilution of water-soluble Ca in fruit tissue and increased Ca demands for rapid fruit enlargement.

The incidence of blossom-end rot in relation to the Ca concentration in tomato fruits as affected by a long daily photoperiod was examined using supplemental lighting (60~168 μmol・m -2 ・s -1 of PPFD at plant canopies) in autumn cropping. Low Ca solutions (1/4Ca or 1/8Ca of Enshi standard solution) were supplied for the medium-sized fruit tomato cultivar 'Cindy Sweet'. Supplemental lighting significantly decreased both total and water-soluble Ca concentrations in distal fruit tissue and aggravated the incidence of BER in the tomato. No significant difference was observed in the fruit growth rate; however, a positive relationship was found between leaf stomatal conductance and the intensity of supplemental lighting. Therefore active leaf transpiration after sunset caused by supplemental lighting may have resulted in decreased Ca translocation into the fruit through xylem vessels. In addition to the vigorous growth and increased Ca demand of fruit resulting from a high temperature and strong solar radiation, reduced Ca translocation into the fruit due to a short dark period may aggravate tomato fruit susceptibility to BER in the late spring to mid-summer season. Key Words：Ca

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