Environmental factors influence the contents of taste components, such as capsaicinoid compounds, in the fruit of chili pepper (Capsicum annuum). The present research was conducted to evaluate the effect of water supply and harvesting date after flowering on sugar and capsaicinoid contents in fruit of the Japanese chili pepper cultivars 'Botankosho', 'Fushimiamanaga', 'Manganji', and 'Sapporo Oonaga Nanban'. The experiment was conducted in a greenhouse from April to October in 2016 and 2017. Three water supply treatments were applied: 260 mL (excess), 130 mL (standard), and 50 mL (drought) per application. Fruit were harvested at 20, 30, 40, and 50 days after flowering (DAF). Glucose, glutamic acid, and total sugar were measured using a portable spectrophotometer, and capsaicinoid content was measured by HPLC. Total sugar content and Brix tended to increase with later harvesting, whereas glucose content did not change significantly by DAF. Sugars in the fruit were dominated by fructose, and the ratio of fructose content to total sugar content increased as the fruit matured. Glutamic acid content in the fruit increased up to 40 DAF, and thereafter remained unchanged or decreased. The capsaicinoid content of the fruit increased with fruit maturation. Elevation in water supply induced an increase in the fruit glucose content and decrease in total sugar and glutamic acid contents. Previous studies of tomato (Solanum lycopersicum L.) found that total sugar and glucose contents decrease in response to increase in water supply. Therefore, it is suggested that sugar metabolism and accumulation differ in the fruits of tomato and chili pepper. The highest capsaicinoid content in chili pepper fruit was observed in response to the drought treatment.

The amounts of taste components, including those for pungency, in chili pepper fruit change depending on environmental factors. Our previous study revealed that the amount of capsaicinoid was significantly increased in chili pepper fruits that were cultivated under a drought stress condition. Therefore, the present experiment was conducted to determine the effect of drought stress on pungency and the expression levels of capsaicinoid biosynthesis genes in chili peppers. Japanese chili pepper cultivars 'Shishito' and 'Sapporo' were selected and cultivated in a greenhouse under a drought stress condition or an excess water supply condition. The fruits were used for morphological analysis, and the quantification of the capsaicinoid content in the placental septum was done using high performance liquid chromatography. Gene expression analysis was carried out using a quantitative reverse transcription polymerase chain reaction for 18 capsaicinoid biosynthesis genes. Based on the obtained gene expression patterns, we divided the 18 genes into three groups. The genes in group 1 (ACL, pAMT, Pun1, WRKY9, CaKR1, CaMYB31, FAT, and KAS I) showed higher gene expression levels in the drought stress condition than in the excess water supply condition in both cultivars at 20 DAF. The genes in group 2 (KAS III, BCKDH, ACS, BCAT, and 4CL) showed higher gene expression levels in the drought stress condition than in the excess water supply condition in only one of the cultivars at 20 DAF. The genes in group 3 (PAL, C3H, HCT, C4H, and COMT) did not show any significant differences in gene expression between the two treatments in either cultivar at all DAF. The genes in our experiment showed similar expression patterns in pungent parthenocarpic fruit and control fruit of 'Shishito' as in a previous study. Moreover, we found that the number of seeds tended to be lower in fruits cultivated under a drought stress condition, while capsaicinoid content was higher. It is possible that drought stress firstly affected the number of seeds in the fruits, and the decrease in the number of seeds subsequently caused changes in capsaicinoid biosynthesis.