An SEM Study of Pollen Tube Growth in Intra- and Interspecific Crosses between Lilium Species

As a part of our study on interspecific cross barriers in Lilium spp., we compared pollen tube growth in intraspecific (L. longiflorum) and interspecific (L. longiflorum x Lilium x `Mont Blanc') crosses, using scanning electron microscopy. After penetration into the stigma, pollen tubes in the intraspecific pollination grow straight along the longitudinal axis of the style. In the ovary they follow the papillate area between two rows of ovules, after which they turn and grow between two bordering ovules to penetrate the micropyles. In contrast, pollen tubes in the interspecific pollination show a winding growth pattern, terminating in the upper half of the style, which means an interspecific cross barrier. After pollination by the cut-style method directly on top of the ovary, interspecific pollen tubes enter the ovary, form sperm cells, grow straight along the papillate tissue between the ovules, and turn toward the micropyles, as in the intraspecific pollination. Some of them penetrate a micropyle; most of them stop in front of the micropyles, indicating that different stimuli lead the pollen tubes toward and into the micropyle, respectively. The number of micropyles in which a pollen tube can be observed is higher than the number of seeds obtained in previous breeding programs with the cut-style method. Thus, after cut-style pollination a cross barrier is found inside or just in front of the ovules. However, the low number of seeds obtained after intraspecific cut-style pollination makes it uncertain whether this barrier is caused by interspecifity or by the pollination method itself.