The number of leaves in ‘Delaware’ grape is an excellent indicator to determine the optimal timing for gibberellic acid (GA₃) application to fruit clusters to induce seedless berries. To predict the optimal timing for GA₃ application to ‘Delaware’ grape, we aimed to develop a method to estimate the accumulated leaf number (LN) on the second shoot (which grows from the second dormant bud from the end of the fruiting mother shoot) from the air temperature. This was undertaken using leafing data from a vineyard in Osaka, Japan from 1987 to 2019. Base temperature (Tb, °C), ranging from −2.5 to 12.5°C with step increases of 2.5°C, was estimated using the coefficient of regression determination (R²) between LN and effective accumulative temperature (EAT, °C·day) and two types of root mean square error (RMSE) (RMSE of LN and RMSE of the day when LN reached 10.0 leaves). The LN showed a good fit in linear regression with EAT at every Tb (−2.5 to 12.5°C); in particular, a Tb of 7.5°C yielded the highest R² and the lowest RMSE. From these results, we determined that the Tb for leaf emergence of ‘Delaware’ grape was 7.5°C. We then compared the estimation accuracy in predicting leaf emergence using EAT (Tb of 7.5°C) (proposed method) and constant leaf emergence rate per day of 0.36 leaves·day⁻¹ (conventional method). As the RMSE when using the proposed method was lower than the RMSE when using the conventional method, the proposed method was more suitable to predict of optimal timing for GA₃ application than the conventional method. The findings of this study could be used to predict the optimal timing for GA₃ application with high accuracy using LN on the measurement date and forecast values for temperature. Furthermore, the coefficient of the developed equation suggests that the ‘Delaware’ grape develops 4.0 leaves per EAT of 100°C·day with a Tb of 7.5°C.

“特拉华州”葡萄中的叶子数量是确定赤霉素（GA ₃）应用于水果簇诱导无核浆果的最佳时机的极佳指标。为了预测将GA ₃应用于“特拉华州”葡萄的最佳时机，我们旨在开发一种方法来估计第二芽（其从子母芽结束时的第二个休眠芽中生长出来）的累积叶数（LN） ）从空气温度。这是使用1987年至2019年日本大阪葡萄园的叶子数据进行的。使用回归系数估算基准温度（Tb，°C）在-2.5至12.5°C之间，逐步增加2.5°C。测定（R ²LN与有效累积温度（EAT，°C·day）和两种均方根误差（RMSE）之间的值（LN达到10.0叶片的那一天的LN RMSE和RMSE）。LN在每Tb（-2.5至12.5°C）下与EAT的线性回归显示出良好的拟合；特别是，Tb为7.5°C时，R ²最高，RMSE最低。根据这些结果，我们确定“特拉华州”葡萄叶片出芽的Tb为7.5°C。然后，我们比较了使用EAT（Tb为7.5°C）（提议的方法）预测叶片出苗的估计准确性和每天0.36叶片·天^-1的恒定叶片出苗率（传统方法）。由于使用本发明方法时的RMSE低于使用传统方法时的RMSE，因此与传统方法相比，本发明方法更适合于预测GA ₃施用的最佳时机。这项研究的发现可用于在测量日期和温度预测值上使用LN高精度地预测GA ₃的最佳应用时间。此外，发展方程的系数表明，“ Delaware”葡萄每100℃·天的EAT可生长4.0片叶子，Tb为7.5℃。