The tea tree (Melaleuca alternifolia) is known as an essential oil producer. This species is commercially important as a source of essential oil, especially in Australia. To improve its oil quality, research on artificial pollination between plus trees was carried out. The information about self-incompatibility and inbreeding depression due to self-pollination should be ascertained to support the pollination success. This study was aimed at examining the self-incompatibility of tea trees by conducting controlled self-pollination and cross-pollination. Controlled cross and self-pollination were carried out at a seedling seed orchard of tea tree, using four mother trees as experimental samples. Natural self-pollination was undertaken by bagging unopened flowers, without emasculation and counting the number of capsule sets. Open pollination was used as a control treatment, of which the number of unopened flowers and the number of capsule sets were counted. Observations revealed that no capsule was found from controlled self-pollination (with the index self-incompatibility = 0), even though natural self-pollination produced a low number of capsules (with the index self-incompatibility = 0.24). Therefore, it can be assumed that a high level of self-incompatibility was taken place in the tea tree. Inbreeding depression also existed in this species, revealed by the decreased number of capsule set, the lower seed germination rate of self-pollinated seeds than cross-pollinated seeds. The slower growth of self-pollinated seedlings than cross-pollinated seedlings in the nursery also indicated that inbreeding depression occurs.
