Dynamic simulation of diurnal course of stem diameter of apple tree (Malus domestica Borkh. Cv. 'Fuji').

Using soil-plant-atmosphere continuum (SPAC) theory and stem volume equation, dynamic changes in stem diameters of apple trees (Malus domestica Borkh. Cv. 'Fuji') were simulated. Numerical simulation results indicate that stem water potential fluctuated sinusoidally and gently on sunny and cloudy days, respectively. Fluctuations in the amplitude and trend were caused by sap flow and storage resistance, respectively. The change in stem-stored water was proportional to the change rate in water potential. Stem diameter increased with fluctuations in clear day; moreover, it varied slightly on cloudy days. Changes in stem diameter were caused by changes in water potential, which was determined by meteorological factors, soil water potential, and characteristics of plants. Introduction Information on a crop’s water status, which is required when planning irrigation programs, is best provided by physiological indicators. In the diurnal course of tree, stem diameter is a direct indicator of water status of a tree (Yatapanage and So 2001, Köcher et al. 2013). Micromorphometric method is most widely used for automatic irrigation of crops (Link et al. 1998, Ortuño et al. 2010, Cocozza et al. 2015). By establishing a diurnal course model of stem diameter, an in-depth study of stem variation as well as precision irrigation were conducted. Based on stem structure, Génard et al. (2001) constructed a model and simulated diurnal courses of stem with respect to different temperatures, diameters, and water potentials. John (1999) simulated diurnal and annual courses of stem under different densities and temperatures according to the law of dry matter distribution. However, aforementioned models require dozens of equations and many parameters, bringing difficulties in the applications of actual production. Moreover, currently available models usually cannot simulate daily variations in stem diameter due to environmental conditions. In this study, soil-plant-atmosphere continuum (SPAC) theory was integrated with tree diameter growth model, and then it was simulated with diurnal course of stem diameter. Thus, a new strategy was developed to simulate dynamic growth of stem diameter. Materials and Methods Experiments were conducted at the experimental station of Beijing Vocational College of Agriculture in 2017. Test materials were 4-year-old potted apple trees (Malus domestica Borkh. cv. ‘Fuji’). The rootstock was M. micromalus Makino. Cultivation management measures were same for all test trees, which had approximately the same crown size and stem thickness. The pot size was 0.3 m × 0.3 m × 0.5 m; all pots were wrapped with reflective films to prevent evaporation of water from soil. Soil moisture and nutrient conditions were favorable for the growth of potted apple trees. Soil moisture was controlled by quantitative irrigation, which was based on soil water potential. *Author for correspondence: <gaozhaoquan@sina.com>. College of Soil and Water Conservation, Beijing Forestry University, Beijing, 100083, PR China. Comprehensive Business Bureau, Ministry of Water Resources, Beijing 100053, China. Beijing Vocational College of Agriculture, Beijing 102442, P. R. China.