Dual regime of flowering time and pollination enhances pollen tube development in Ziziphus.

Chinese jujube (Ziziphus jujuba Mill., 2n = 2x = 24) is a drought-tolerant, nutrient-rich fruit crop. However, its genetic improvement is constrained by protandry, low fruit set, and severe embryo abortion. Interspecies hybridization between Chinese jujube and Indian jujube (Ziziphus mauritiana Lam., 2n = 4x = 48) is further hindered by asynchronous flowering. We developed a dual-regime protocol combining temperature control and strategic heavy pruning to advance the flowering time of Indian jujube (cultivar 'Niunaidaqingzao', N) by 2 months, thereby synchronizing its anthesis with that of Chinese jujube ('Dongzao', D) and wild Chinese jujube ('Suanzao', S). In vitro artificial self-pollination (AS) and in vitro artificial cross-pollination (AC) were conducted to assess pollen tube elongation and ovary expansion. Triple AS (TAS) boosted pollen tube emergence to 59%-87% across the three genotypes, more than doubling in vitro spontaneous self-pollination (SSP) rates and outperforming single AS 1.4 to 2.7 times (P < 0.05). Ovary-swelling frequencies of TAS reached 68.52% in wild Chinese jujube S and 27.78% in Indian jujube N, indicating 2.85 and 2.14 times increases over SSP and 1.88-4.11 times increases over single AS. In ♀S × ♂D, ♀D × ♂S, and ♀S × ♂N crosses, triple AC (TAC) raised pollen tube emergence to 54%-72% (1.3-2.2 times of single AC) and ovary expansion to 26%-39% (1.4-1.9 times of single AC) (P < 0.05). These findings provide a practical and efficient strategy for overcoming asynchronous flowering and reproductive barriers of interspecies hybridization in genus Ziziphus, enabling the establishment of interspecies hybrid populations for downstream breeding programs.