Seedlessness in lemon (Citrus lemon) is due to

Seedlessness in lemon (Citrus lemon) is due to Self-incompatibility.
Emasculated unpollinated flowers also developed into seedless fruits, indicating that seedlessness contributes to parthenocarpy. However, gametophytic self-incompatibility has a major role in seedlessness in lemons by blocking fertilization at the bottom of the stigma.
After self-pollination, however, ovules began to abort at 2 weeks post-pollination, with ovules disappearing at 5 weeks, ultimately producing seedless fruits.