Introduction: The Dormancy Challenge in Commercial Pistachio Cultivation
The pistachio tree (Pistacia vera L.), as a characteristic plant of semi-arid regions, possesses an evolved mechanism called Winter Dormancy. This physiological process is crucial for protecting buds against environmental stresses, particularly severe cold. However, in many major pistachio-producing areas (including Iran, California, and the Mediterranean), due to climate change and warmer winters, trees often fail to receive their full Chilling Requirement. Failure to complete the chilling requirement leads to several phenomena: Delayed and irregular bud break, a low percentage of flowering (especially for flower buds), and ultimately, the phenomenon of “Blind Wood” (failure of lateral buds to activate). This article deeply examines the mechanisms of dormancy and the strategic application of chemical agents to overcome these challenges.
Section I: Physiology and Biochemistry of Dormancy in Pistachio Buds

1. Physiological Dormancy Stages:
   Winter dormancy is categorized into three key phases. First is Paradormancy, where dormancy is externally regulated (e.g., by the presence of active leaves or improper pruning). The critical phase is Endodormancy (True Dormancy), where control is internal, and breaking it requires fulfilling the chilling requirement. Finally, after enough chilling, the bud enters Ecodormancy, awaiting favorable external conditions (warm temperatures) to sprout.

2. The Role of Hormonal Balance:
   Bud dormancy is strictly controlled by the balance between inhibitory and stimulatory hormones. Abscisic Acid (ABA) plays a vital role in dormancy induction, and its level is high during Endodormancy. Cold exposure causes the degradation or conjugation of ABA, lowering its concentration. Conversely, Gibberellins (GAs) and Cytokinins are the main stimulants. Cold triggers an increase in these hormones, activating growth pathways.
3. Metabolic Changes:
   During dormancy, significant metabolic shifts occur in the bud cells, including an increase in non-structural sugars (as cryoprotectants), activation of calcium signaling pathways, and altered expression of genes related to growth and stress response.
   Section II: Scientific Models for Calculating Chilling Requirement (Chilling Units)
   Accurate chilling calculation models are essential for predicting the optimal time for bud break and making decisions about chemical application.

• Utah Model: This model simply counts the hours between 0^{\circ} \text{C} and 7.2^{\circ} \text{C} as one Chilling Unit (CU). Its simplicity is an advantage, but it fails to account for the reversal (neutralization) of accumulated chilling by warm temperatures (above 16^{\circ} \text{C}).
• Positive Model (or Cummings Model): This model is more accurate in subtropical climates as it incorporates the effect of warm temperatures, deducting “Negative Units” and effectively neutralizing accumulated chilling.
• Parabolic Model (Phenological Model): This model determines the efficiency of each temperature degree in fulfilling the chilling requirement based on a parabolic curve (with the optimum range around 5^{\circ} \text{C} to 8^{\circ} \text{C}). Due to its high accuracy in pistachio-producing regions, it is considered the reference model. The chilling requirement for pistachio, depending on the cultivar, is typically estimated to be between 800 and 1200 Chilling Units (CU) using this model. Chemical agents are necessary if this threshold is not met.
  Section III: Advanced Chemical Strategies for Dormancy Breaking (Bud Break Promotion)
  The application of chemicals should occur when the tree has met approximately 60% to 80% of its chilling requirement and is on the verge of Ecodormancy.

2. Mineral Oils:
   The precise mechanism of action involves mineral oils creating changes in cellular membrane permeability, increasing cellular respiration, and inducing controlled oxidative stress. This results in a higher internal bud temperature and the activation of metabolic pathways, simulating the effect of full chilling. The use of mineral oils with high sulfonation percentages and appropriate viscosity ensures better safety and efficacy.

2. Thidiazuron (TDZ):
   TDZ is a substituted phenylurea with potent cytokinin effects. It directly contributes to the initiation of growth signals by activating cell division and increasing endogenous cytokinin levels in the buds. Typical concentrations of TDZ in pistachio range from 50 to 150 ppm and are usually combined with mineral oils to enhance penetration.
3. Lower-Toxicity Alternatives:
   Research is focusing on using nitrogen-based compounds (such as those based on Calcium Nitrate and Ammonium Thiosulfate) as milder stimulants, especially in combination with light mineral oils. Their mechanism of action is linked to providing critical nitrogen supply during the initial phase of rapid growth.
   Conclusion and Future Outlook
   Pistachio dormancy management has evolved into a precise science that requires integrating climatic data, physiological knowledge, and accurate chemical application. Success is measured not only by an increase in bud break percentage but also by achieving the optimal synchronization of male and female flower bud break, which is a critical factor in enhancing the final yield of the pistachio orchard. Future research should focus on developing biopesticides and biological agents to fully replace high-risk chemical alternatives.
   For purchasing specialized chemical products and receiving scientific and technical consultation regarding pistachio tree dormancy breaking, please contact Mr. Ravanshad on WhatsApp:
   Mr. Ravanshad: 00989214773705