The onset of apple [Malus sylvestris (L.) Mill. Var. domestica (Borkh.)Mansf.] fruit maturity is preceded by the production of ethylene, the ripening hormone, which induces fruit ripening. The amount of ethylene produced by the fruit correlates with the level of transcription of the ripening-specific 1-aminocyclopropane-1-carboxylate (ACC) synthase genes. We have found that an allele (MdACS1-2), which contains an inserted retroposon-like sequence at the 5'-flanking region, is transcribed at a lower level than the wild-type (MdACS1-1). MdACS1-2/2 homozygous fruit produce a lower level of ethylene at the climacteric stage than do the wild type fruit. We have also found that the preharvest drop rates of apple cultivars and strains of MdACS1-2/2 trees have less fruit drop than the MdACS1-1/1 or MdACS1-1/2 trees. Treatment of the MdACS1-1/2 trees with 1-MCP, an ethylene receptor blocker, further decreased fruit drop. Analysis of commercial apple cultivars for the presence of the MdACS1-2/2 allele may help in the early detection of apple cultivars with a low fruit drop rate.
We investigated sugar (solute) accumulation in watermelon [Citrullus lanatus (Thunb.) Matsum. et Nakai] fruits at the immature stage. Watermelon plants were grown hydroponically in a nutrient solution with an electric conductivity (EC) of 1.2 S⋅m−1 (EC 1.2 regime); then, fruits were harvested 21 days after anthesis. The flesh of each fruit was divided into seven different parts to measure the sugar concentration and water status. The results indicated that the sugar concentration was higher in the center of the fruit flesh than in the other parts, such as around the pericarp. Moreover, the lowest osmotic potential was observed in the center of the fruit flesh, indicating solute accumulation. Concurrently, when the transport of photosynthates in the fruit was investigated using the 13CO2 isotope, the active solute accumulation in the center of the fruit flesh was observed, supporting the observed sugar accumulation in this part. Consequently, this active solute accumulation and distribution occurred in the center of the watermelon fruit, as demonstrated by the data of osmotic pressure and sugar concentration and supported by the observed active photosynthate accumulation. Additionally, we investigated these measurements by increasing the nutrient solution concentration 14 days after anthesis. As a result, fruit growth was slightly inhibited using the EC 3.0 regime, and 13C translocation was also inhibited in the fruit, especially in its center. Even though the sugar concentration and osmotic pressure of the fruit flesh were not clearly affected by high nutrient solution concentrations, the cell turgor of the central flesh of the fruit grown using the EC 2.0 and 3.0 regimes was lower than that of the fruit grown using the EC 1.2 regime. Treatments with higher nutrient concentrations might have negative effects on immature watermelon fruits.