Determination of thresholds to trigger irrigation of ‘Kyoho’ grapevine during berry development periods based on variations of shoot diameter and berry projected area

Abstract:【Objective】Irrigation is an important management practice in viticulture. Currently available reports mainly focus on the selection of water status indicators and the impacts of different irrigation treatments on fruit trees. Studies about determining the suitable soil water potential ranges at different phenological periods based on the real-time response of plant organs to soil water potential have not been reported. In this study, we conducted the experiment to determine suitable soil water potential ranges for‘Kyoho'grape.【Methods】In the experiment, eight-year-old potted‘Kyoho'grapevines were used as the experimental materials. During the first rapid growth period and veraison, the relationships between shoot growth, berry expansion and leaf net photosynthetic rate and soil water potential were analyzed through the photogrammetry of plant organs, real-time monitoring of soil water potential and measurements of leaf photosynthetic traits. Besides, cubic spline interpolation based on MATLAB software and time series model were applied to selecting soil water potential thresholds for irrigation. They generated smooth function curves of berry expansion rate against soil water potential. Then changes in slope of the regressed curve were used to distinguish stages of berry development.【Results】During berry first rapid growth period and veraison, shoot diameter decreased in daytime and increased at night, forming an irregular serrated growth curve in this period. MDS increased significantly when ψsoil was-8.89 kPa during the first rapid growth period and-9.21 kPa at varaison. In addition, MXDG and MNDG of shoot showed a significant daily decrease when ψsoil was below-17.34 kPa during the first rapid growth period and-16.46 kPa at veraison. The daily changes in berry projected area under timely irrigation and under water withholding both presented a regular diurnal pattern of shrinking in the daytime and expanding at night. Berry projected area presented a spiral cyclical growth under timely water supply, while its increase slowed down and finally shrank under no water supply. With the decline of soil water potential, berry growth presented four phases, the high speed growth phase (HSGP) , rapid growth phase (RGP) , slow growth phase (SGP) and shrinking phase (SP) . During the first rapid berry growth period, the soil water potential ranges of the above four stages were >-9.37 kPa, -9.37 kPa to-21.14 kPa, -21.14 kPa to-27.86 kPa and <-27.86 kPa, respectively, and the berry expansion rates in the corresponding stages were 0.10 mm²·h^-1 to 0.31 mm²·h^-1, 0.03 mm²·h^-1 to 0.10 mm²·h^-1, 0.00 mm²·h^-1 to 0.03 mm²· h^-1 and < 0.00 mm²· h^-1, respectively. Meanwhile, the soil water potential ranges of the four stages at veraison were >-10.31 kPa, -10.31 kPa to-22.05 kPa, -22.05 kPa to-32.83 kPa and <-32.83 kPa, respectively, and the berry expansion rate was 0.11 mm²· h-1 to 0.26 mm²· h^-1, 0.02 mm²· h^-1 to0.11 mm²· h^-1, 0.00 mm²· h^-1 to 0.02 mm2· h-1 and < 0.00 mm2· h-1, respectively. Leaf Pnpresented diurnal variations. It increased rapidly after sunrise and generally peaked around 10:00 am, then decreased with a midday rest. In the afternoon, it recovered slightly and fell to zero after sundown. During the experiment, the diurnal variation pattern of Pnwas not significantly influenced by the decline of soil water potential, while the diurnal maximum of Pnshowed a trend of decline. During the first rapid growth period, the diurnal maximum of Pnwas not significantly influenced by soil water potential within a range of0 to-27.3 kPa. However, it decreased significantly when rooting-zone soil water potential declined to-36.8 kPa at veraison.【Conclusion】The reasonable soil water potential ranges for eight-year-old‘Kyoho'grapevine was determined based on the expansion of berries, the leaf net photosynthetic rate and the growth of shoots. The suitable irrigation thresholds for‘Kyoho'grape during berry first rapid growth period and at veraison are-12.83 kPa to-15.67 kPa and-16.46 kPa to-22.05 kPa, respectively.