Agro-climatic requirements and reproductive potentials of pistachio pollinator specimens grown in warm area in North Africa

The success of pistachio production depends on the best selection of female and male trees combined with perfect flowering synchronization. In warm areas, flowering is highly affected by climate change and selecting an appropriate pollinator for a female cultivar is challenging. In this study, seventeen local pistachio pollinators were under study in the arid Central Tunisian region. Bloom timing and chilling and heat requirements were determined based on 17 years of phenology records and using the Partial Least Squares (PLS) regression. Pollen production and quality were assessed. Three pollinator groups as early-, intermediate- and late-flowering ones were delineated and their median bloom dates spanned from DOY84 to DOY110, with bloom dates spread over 27 and 37 days, allowing perfect coverage of the bloom period of the female cultivar Mateur. Within the three groups, the most interesting candidates were H23a and H26a as intermediate-flowering males, H3a, H3b, H18, H23b and H26b as late-flowering ones and H30 as an early-flowering male covering the first flowers blooming. The pollen viability was high in all the studied pollinators (92-98 %). However, great differences existed in inflorescence density (in the range of 2-4) and pollen production per inflorescence (in the range of 191-424 mg). Based on flowering synchronization, the suitable ones had less variable chilling requirements in the range of 33-37 Chill Portions (CP) in warm areas against a significant variability of the heat needs (8783-17,302 GDH). This supported the key role of increased heat accumulation to compensate for the lack of chill and when chilling requirements are not fully fulfilled in warm areas until the chill threshold value of 25 CP. A significant delayed flowering period occurred for all pistachio pollinators when chill accumulation dropped below 25 CP, which may cause asynchronous flowering, leading to poor pollination. In conclusion, bloom timing and pollen production are common variables to characterize pistachio pollinators. The agroclimatic needs could be a relevant tool for planning the best combination linked to local conditions, while estimation improvement is required for warm areas. However, pistachio cultivation in warmer areas will be viable until a certain chill level. Pistachio cultivars will be able to grow, while lack of chill will be overcome with heat accumulation key role for chill compensation until a threshold of 25CP. These findings are of high interest for the warmest orchards, which will cover many important fruit-growing regions with Mediterranean climates all over the world with the expected climate change.