Pistachio nut phenology studies in California address crop development as a function of heat unit accumulation

Elizabeth Fichtner, Farm Advisor, Tulare and Kings Cos

Louise Ferguson, CE Specialist, UC Davis

Narges Moosavi Mahvelati, Jr. Specialist, UC Davis and UC ANR

California pistachio growers have likely noticed the influence of the cool 2017 spring on crop development. This observation illustrates the limitations of modeling anticipated crop development based on calendar date. Nut development trends will also vary based on geographic area or local microclimates in which pistachios are planted, and are subject to influence from large-scale trends such as global climate change.  As a consequence, scientists utilize alternate methods to define developmental growth stages as a function of heat unit accumulation.  This allows for comparisons of crop development between cultivars across sites, years, and even decades. Thermal unit modeling may also be used to facilitate pest control, irrigation scheduling, and prediction of harvest time.   

Lu Zhang, Post-doctoral researcher, UC Davis

Some variability may exist between various model types; however, the unifying concept is that they strive to correlate milestones of plant development with accumulated heat. The time of fruit set is a biofix at which thermal unit (TU) accumulation is equal to zero.  Heat unit measurements are determined by tracking hourly data using data loggers installed in the field. Thermal units are calculated by taking a daily average temperature and subtracting the base temperature threshold. In pistachio models, 7◦C is utilized to indicate the base temperature beneath which growth does not occur. The work of Allan et al. 2014, focused on relating 6 variables (nut length, width, height, volume, penetrability, embryo size) to accumulated heat in each of 5 cultivars of pistachio.  

Pistachio nut development is defined in three overlapping stages. Stage 1 is defined by the growth of the pericarp to its final size; stage 2 is defined by the hardening of the endocarp (ie. shell); stage 3 is characterized by growth of the embryo (Figure 1).  In pistachio, Stages 1 and Stage 2 and Stages 2 and 3 have overlapping curves, indicating that portions of these developmental phases occur simultaneously (Allan et al 2014). Stages 1 and 3, however, are mutually exclusive of each other such that the pericarp is fully formed before embryo development is initiated (Figure 1).  The final volume of the pericarp is achieved after accumulation of approximately 670 TU (Figure 2).  In 2016, this would have occurred around the beginning of June in the southern San Joaquin Valley (SSJV). Stage 2, the process of shell hardening, requires approximately 2423 TU. Embryo growth (Stage 3) is initiated after around 900-1000 TU accumulated (mid-June in 2016; SSJV) and maximum embryo length requires approximately 1880 TU (mid-August in 2016; SSJV).  Stage 2 requires more heat units for completion then Stage 3.  The variability of these mean values is largely contributed by differences in cultivars.   

This year, we are building the pistachio nut growth models and strengthening the prediction ability of the model by continuing weekly fruit sampling from early fruit set to harvest. In addition to current equation of TU accumulation, we are also improving the method of calculating heat units using Asymmetric Curvilinear Growth Degree Hours (GDH) Model, as well as to predict the base, optimum and critical temperatures for pistachio growth in California. Our final objective is development of a model that predicts the number of heat units to complete all three growth stages in the Kerman, Lost Hills, Golden Hills, Kaleghouchi and Pete1 cultivars and to develop the software to convert calendar dates to thermal units based on historical temperature records and two weeks weather forecast. Growers will be able to predict optimal harvest date conveniently by logging into our website and inputting the location, cultivar and blooming date.  

Select References 

Allan, C.A., Ferguson, L., Bourgeois, G., Cristosto, C., Thur, K., Ramacho, F.A., Jiminez, F.J., Saracoglu, T. Pistachio nut phenology and development in five cultivars as a function of heat units. Report to the California Pistachio Research Board.  www.calpistachioresearch.org.