Is cutting method a reliable tool for estimating pistachio climatic requirements in warm Mediterranean conditions?

Most of the woody perennial temperate trees require cool and cold temperatures as chilling requirement during winter, followed by warm spring temperatures as heat requirements to break their winter dormancy and to achieve homogeneous and simultaneous flowering and regular yields. Estimating chilling and heat requirements is therefore essential for choosing appropriate pistachio cultivars for a given site and for insuring viable orchards. To this end, we tried to estimate chilling and heat requirements of pistachio cultivars using cuttings method under controlled environment during 2 years. Phenological stages of flowering buds were followed until bloom and chilling and heat accumulation were also calculated. Normally, chilling requirements are delineated from the relationship between chilling accumulation and heat requirements (GDH) and this relationship is a hyperbolic curve. Forcing units decrease as accumulated chilling units increase and then stabilize, reaching a threshold value beyond which the GDHs no longer increase. Therefore, chilling requirements for bud dormancy release could be chill units that buds had accumulated and for which correspond the threshold GDH value. Meanwhile, in our experiment and during two years, this relationship had different shapes for pistachio and sometimes most of the buds remained dormant. Thus, it was complicated to determine chilling requirements or/and the estimations were biased. Heat requirements are usually calculated from the date of bud dormancy release until the date when 50% of flowers are open. However, in our experiment flowering percentages were sometimes null or very low, making it impossible to estimate heat requirements for flowering. These results seem to be related to lack of carbohydrate reserve in the shoots used in the experiment. As a conclusion, it is difficult to estimate climatic requirements for pistachio in warm semi-arid conditions using this cuttings method, without taking into account the starch accumulation during the previous year in the wood.