Dual Regime of Flowering Time and Pollination Enhances Pollen-Tube Development in Ziziphus

Abstract 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 (Z. 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 two 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) (P0.05). 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.