Warm–cold stratification breaks morphophysiological dormancy in Juniperus polycarpos seeds; implications for plant conservation under a changing climate

Juniperus polycarpos forms the uppermost tree-line in the highland open forests of the Middle East. Despite its critical role in ecosystem functioning and services, low seed viability rate and seed dormancy, global warming and human disturbances, have raised concerns on the species extinction. Improving seedling production through targeted seed treatments offers a promising conservation strategy. While stratification is known to alleviate dormancy in J. polycarpos, the most effective temperature regimes for dormancy break and precise classification of its dormancy remain undefined. Using the time-to-event analysis, we evaluated germination of both untreated and GA3-pretreated seeds at constant temperatures ranging from 5°C to 25°C for 30 weeks. In the second experiment, seeds underwent a total of 30 weeks of stratification under either warm–cold (20°C followed by lower temperatures) or cold–warm (5°C followed by higher temperatures) sequences. At a constant 20°C, maximum germination reached only 30% of viable seeds. In contrast, warm–cold treatment (16 weeks at 20°C followed by 14 weeks at 10 °C) produced 95% germination of viable seeds without GA3 pretreatment. This warm–cold stratification also increased embryo length by approximately 100%, suggesting deep morphophysiological dormancy—an adaptive trait to the region’s temperature and moisture patterns. Thus, the incidence of drought, rising temperatures, and inherently low seed viability may restrict J. polycarpos distribution by undermining natural regeneration. Defining optimal stratification protocols will, therefore, be essential for restoration and conservation efforts of this species under future climate scenarios.