Physiology of Fruit Growth as a Function of Heat in Commercial Pistacia vera Species

During the summer season the growth of many plants is a function of temperature, specifically heat.  Determining the degree-day requirement (DD) is done by simultaneously measuring both the accumulated temperatures above a certain plant species determined threshold temperature along with the plant's growth and correlating the two. Once the degree-day accumulation for a specific cultivar has been collected this data can be formatted to stage-determining equations which can then be used as a prediction tool. Pistachio has three major growth stages; stage I relates to the growth of the empty fruit; stage II refers to a period of time where the pistachio shell increases in hardness; and stage three begins when the embryo begins growing until the pistachio splits, indicating the harvest readiness. This information can be used to direct field production practices. For example we will know if there is a difference in the timing of shell split among cultivars to determine which blocks to harvest first. In this study we have sampled from the ‘Kerman’, ‘Golden Hills’, ‘Lost Hills’, ‘Kalehghouchi’ and ‘Pete 1’ cultivars. In this first year the growth of five cultivars as a function of accumulated degree days from a local temperature monitor was measured at eight different locations in California. All stage I growth terminates at 500 DD. Stage II differed among cultivars with 'Lost Hills' hardening earliest at 1,600 DD and 'Kaleghouchi' being the slowest at 2,100 DD. 'Golden Hills' initiated stage III at 900 DD followed by 'Pete 1' at 1,000, Kaleghouchi at 1,100 and 'Kerman' and 'Lost Hills' at 1,200 DD. The total heat units necessary for all cultivars to fully ripen and split was approximately 2400 degree days, except 'Golden Hills' which required approximately 100 DD less, averaging 2300 degree days. All the cultivars were similar in their in their weight accumulation as a response to degree day accumulation.  The data resulted in a typical double sigmoid growth curve for all cultivars.