Effects of Defoliation on the Occurrence of Internal Browning in Tomatoes Grown in Soilless Cultures

Internal browning is a physiological disorder that occurs in hydroponically grown tomatoes (Solanum lycopersicum L.) and is characterized by the browning of the inside of the fruit, while its outer surface appears normal (Fig. 1). Internal browning often occurs in the lower trusses of tomatoes cultivated via hydroponics that are planted in August; it presents a recurring problem to Japanese producers (Iwamoto et al., 1993; Terabayashi et al., 2010). Subsequently, internal browning is often not detected before shipping, and the affected fruits reach consumers via the distribution network. The cause of internal browning has not been determined. However, analysis of the inorganic components in affected fruits suggests calcium (Ca) deficiency as a factor because affected fruits contain lower Ca levels than those of normal fruits (Ishizuka et al., 2000; Suzuki et al., 2019). Additionally, it was found that internal browning occurs when the nutrient solution used contains a low concentration of Ca (Terabayashi et al., 2010). A related disorder of tomatoes is blossom-end rot, which may also be associated with Ca deficiency (Raleigh, 1939; Lyon et al., 1942). In this disorder, Ca concentrations at the site of the blossomend rot in tomato plants are low (Ward, 1973; Terabayashi et al., 1988). Defoliation is a method used to suppress blossom-end rot. It confirmed that when the leaf area is halved, the occurrence of blossom-end rot of ‘NDM0112’ (Nippon Del Monte Co., Ltd., Gunma, Japan) and ‘Summer Kiss’ (Nippon Del Monte Co., Ltd., Gunma, Japan) was reduced from 30% to 16% and from 18% to 6%, respectively (Sato et al., 2004). Since internal browning and blossom-end rot may develop due to similar factors, we hypothesize that defoliation may also suppress internal browning. To investigate the effects of defoliation on internal browning, we first investigated the effects of defoliation on internal browning of tomatoes planted in a rockwool cultivation system set-up in an actual glass greenhouse, and in a hydroponic device in a controlled environment chamber. In this experiment, we used a nutrient solution with a lower Ca concentration and higher NH4 concentration than that in the normal culture solution. This is because this type of nutrient solution has been associated with the occurrence of blossom-end rot (Ikeda and Osawa, 1988; Terabayashi et al., 1988). Second, we compared the effects of defoliation and lack of defoliation on the Ca concentration in tomato fruits.