The phyllochron of Prunus persica shoots is relatively constant under controlled growth conditions but seasonally increases in the field in ways unrelated to patterns of temperature or radiation

• The phylochron of proleptic shoots was observed for Prunus persica.
• A growing degree hour model was developed to measure the phyllochron.
• The phyllochron of field grown trees increased seasonally, likely an aging effect.

Growth and development of plants are characterized by the addition of repeated units called phytomers that consist of a node, internode, leaf and axillary bud. The time elapsed between the addition of phytomers can be represented by the appearance of leaves at each node, or the leaf appearance rate (LAR). The inverse of LAR is the phyllochron, the chronological time or thermal time elapsed between the appearance of successive leaves along a stem. Plant genotype interactions with environment are responsible for the timing of the phyllochron. Temperature is important for the regulation of many plant processes. To what extent temperature regulates the phyllochron of fruit trees is not clear. Furthermore, the correct thermal time scale to employ for determining the phyllochron has yet to be determined. The objective of this study was to develop a growing degree hour (GDH) thermal time scale model for measuring the phyllochron of Prunus persica and to determine if temperature is the primary factor that influences the phyllochron under field conditions. A temperature-controlled growth chamber study was conducted to determine the appropriate temperatures to use in a GDH-based phyllochron model. We developed a model that is linear on both sides of a plateau-shaped optimum. The base temperature was 4 °C, the critical or maximum temperature was 40 °C and the optimal temperature range spanned 18–32 °C. This GDH model was then used to model the phyllochron of proleptic peach shoots of mature grown trees in the field for two years. Even though the GDH model accounted for much of the daily fluctuations in the phyllochron, the phyllochron generally increased as the season progressed in a manner apparently unrelated to seasonal temperature or daily radiation patterns.