Modelling Fruitlet Growth of Pistacia vera as a Function of Temperature

The production of Pistacia vera spp. would benefit greatly from the ability to predict growth stages as a function of thermal units. In this two-year study, we determined the thermal unit requirement (TU) for the three stages of pistachio (Pistacia vera) fruit growth. The first stage is exocarp growth.  The second stage is hardening of the endocarp through thickening. The third stage is embryo growth. Five  Pistacia  cultivars,  ‘Kerman’, ‘Golden Hills’, ‘Lost Hills’, ‘Kalehghouchi’ and ‘Pete 1’ were compared. HOBO data loggers logged air temperature at eight California Central Valley locations. Weekly samplings of the developing fruits combined with the hourly temperature data were analyzed in a non-linear model for each growth stage.  The data was also analyzed using the software DJPheno (Degree-days estimator to predict phenological stages) developed by Agriculture and Agri-Food Canada.  Results from the two methods were compared. Using volume as an indicator of stage one, the thermal units to 95% of the asymptote differed between the two models.  The DJPheno model required 563 TU, a value between the TU requirement for 95% of the volume (680 TU) and of 95% (440 TU) of the length of the developing fruit in the non-linear model. Stage two and three were similar between the two models. Stage two required approximately 1900 TU and stage three required approximately 1750 TU.  With both methods, the cultivar ‘Golden Hills’ reached full embryo length before the other four cultivars.  With the DJPheno method the model efficiency for cultivar ‘Kalehghouchi’ was weakened by the lack of sample size due to its small presence in the California industry.