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Background and Aims Irrigation is an important management practice in viticulture. Irrigation scheduling established by previous researchers was based mainly on the irrigation level for optimal berry composition and size at harvest. Because eventual berry size and composition depend on the accumulation of daily growth, a more precise study must be implemented to establish the soil water potential (ψsoil) threshold to trigger irrigation at different development stages of the berry. Methods and Results Photogrammetry, 13C isotope labelling and other techniques were applied to study the relationship between ψsoil and real‐time change of berry expansion, net photosynthetic rate, and the distribution of photosynthetic products in 3‐year‐old Kyoho grapevines. The diurnal variation of berry size presented a regular pattern, shrinking in daytime and expanding at night. Analysis of a time series model revealed a close correlation between berry growth and ψsoil. With the decline in ψsoil, berry growth presented four phases of change: high speed growth phase, rapid growth phase, slow growth phase and shrinking phase. Conclusions The optimal ψsoil threshold to trigger irrigation at different berry developmental periods was established: −10 kPa after berry setting (berry diameter < 10 mm), −15 kPa during berry pea size (berry diameter 10–20 mm) and −20 kPa after veraison (berry diameter > 20 mm). Significance of the Study The established ψsoil threshold in this study has significant value for the implementation of precision irrigation, which favours normal berry expansion and composition while saving water.

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