I. Vervaeke

Prefertilization barriers between different Bromeliaceae

Pollen germination on stigma, pollen tube growth in the style and pollen tube penetration of ovules have been studied of excised pistils, after crossing and selfing of 7 bromeliad species. Prefertilization barriers were observed after intraspecific, interspecific and intergeneric crosses. Satisfactory pollen germination in both selfed and crossed flowers showed that the stigma did not act as a barrier. Self-incompatibility in Aechmea B and Tillandsia A was confined in the style, which is characteristic for the gametophytic SI system. Unilateral incongruity between Aechmea A andAechmea B was not related to self-incompatibility, but could be explained by differences in style length. Incompatibility, interspecific and intergeneric incongruity were different with respect to the number of pollen tubes in the style, ratio of pollen tube length and style length, and the number of pollen tube abnormalities. The prefertilization barriers of intergeneric crosses between different subfamilies were not greater than these observed within a subfamily.

Viability and storage of bromeliad pollen

Several bromeliad species from two different subfamilies, were used to develop a reliable method to evaluate pollen viability. Pollen germination on a medium containing 20% sucrose, 0.001%H3BO3 and 0.5% agar was comparable to germination on a compatible stigma. Maximum germination was reached within 2 to 10 hours depending on the species. Based on this test, six species were considered as being good pollen donors with germination percentages between 49%and 83%. Furthermore, pollen from these species and cultivars could be stored in liquid nitrogen (–196 °C) without a considerable loss of viability. For all species, a dehydration period of 4 hours prior to cryopreservation and a rehydration period of 1 hour after cryostorage were essential. Greenhouse humidity influenced anther moisture content and cryostorability.

Pollen tube growth and fertilization after different in vitro pollination techniques of Aechmea fasciata

The effect of cut style and placental pollination on fertilization efficiency was studied for compatible Aechmea fasciata plants. These alternative in vitro pollination techniques resulted in lower penetration rates of the ovules by pollen tubes in comparison to pollination on the stigma. An explanation was found in the intervention of the normal pollination process,through which less pollen-pistil interactions were built up. After cut style pollination the percentage of ovule penetration by pollen tubes increased when a longer style part was left at the ovary. Probably fewer factors that control pollen tube growth are present in the lower style part. Pollen germinated on the ovules after placental pollination but only rarely penetration of the micropyle by a pollen tube occurred. Activation of the ovary, induced by in vivo prepollination for 6 hours, and pollination two days after anthesis did not increase the fertilization percentage. Grafting a style with active growing pollen tubes to ovules on the placenta (placental grafted style pollination) resulted in a higher fertilization percentage. Pollen tube growth through the style was essential for pollen tube guidance to the ovules and penetration of the micropyle.

Semi in vivo pollen tube growth of Aechmea fasciata

With semi in vivo pollen tube growth assays, stigmas are pollinated in vivo and, after a fixed time interval, the styles are isolated from the ovary and placed on culture medium in vitro. Semi in vitro pollination includes isolation of the stigma and style complex, followed by pollination and placing the stylar end on nutrient medium. After semi in vivo pollination more and longer pollen tubes protruded from the cut end of the styles into medium, in comparison to semi in vitro pollination. Medium with 3 g l−1 agar was better than that with 6 g l−1 agar for pollen tube growth after the tubes emerged from the cut style. Semi in vitro pollination of the reversed style indicated that pollen tube growth was not influenced by the direction of the style. Fructose and glucose inhibited pollen tube growth compared to sucrose. Swollen tips characterized tube growth inhibition. After semi in vivo pollination all generative nuclei had divided to give two sperm nuclei. The average distance between the last sperm nucleus and the pollen tube tip as well as the distance between the two sperm nuclei diminished in growing pollen tubes between 24 and 48 h after pollination. The arrangements between the vegetative and the generative nuclei did not differ in semi in vivo and in vitro cultured pollen tubes of Aechmea fasciata. This information is important to explain why fertilization rate is low after placental pollination in comparison to placental grafted style pollination of Aechmea fasciata. The data may also contribute to the improvement of in vitro fertilization methods in Bromeliaceae and other higher plants.

Inheritance of spineless leaves in Aechmea (Bromeliaceae)

Bromeliaceae are tropical ornamentals with a variety of colors and inflorescence shapes. Leaves with spined margins are characteristic for the genus Aechmea. Crosses between spineless and spined Aechmea cultivars were attempted in order to obtain a spineless offspring. Crossing spined and spineless genotypes resulted in a progeny with on average 50% spineless plants. The hybrids of two spineless plants had on average 75% spineless leaves. This indicated that the spineless character was dominant. Homozygous plants were selected; these plants are interesting for further breeding and propagation by seed.