Changes in the harvesting window and quality of apple fruit cultivated under long-term high temperature and CO2

Atmospheric temperatures and CO₂ concentrations are expected to increase in the future. Apple trees that have been cultivated in the same location for decades are particularly vulnerable to these environmental changes. While most of the experiments on apple fruits under climate change have been conducted using potted trees in the short term or in small chambers, we designed field-based, long-term greenhouse experiments under controlled environments to assess potential future changes in the acceptable harvesting window and fruit quality. In 2015, apple saplings grafted on M.26 rootstock were planted to establish three mini-sized populations with two rows of eight trees, each including six ‘Tsugaru’ (early maturing) and six ‘Fuji’ (late maturing) cultivars. Subsequently, in 2018, they were enclosed in independent greenhouses to keep them under ambient temperature and CO₂ concentration (GH-A), 3 °C higher and ambient CO₂ (GH-T), or 3 °C higher and 200 ppm elevated CO₂ (GH-C) before sprouting to after harvesting from 2019 to 2022. Our 4-year investigation revealed that the optimum harvesting period for ‘Tsugaru’ in GH-T and GH-C is earlier in the calendar date as is the spring phenology, remained unchanged based on days after full bloom (DAFB) compared to GH-A. However, for ‘Fuji,’ the optimum harvesting period was judged to delay approximately 10 days in the calendar date owing to slow coloration and soluble solid accumulation, although it also experienced similar advancements in spring phenology. Additionally, it is anticipated that the acceptable harvesting window will become narrow in the future owing to the rapid softening in firmness and decrease in acid concentration. Furthermore, high temperatures in GH-T tended to result in a decrease in fresh weight, firmness, acid concentration, starch rating, and color rating for both varieties compared to GH-A. In addition, for ‘Fuji,’ very few instances of watercore were observed. However, elevated CO₂ in GH-C tended to promote fresh weight for both varieties and increase the watercore appearance for ‘Fuji,’ mitigating the effects of high temperature. No other significant trends in the fruit quality indices were observed between GH-C and GH-T. Our results suggest that the overall fruit quality will reduce under future climatic conditions.