Abstract: The ultrastructural changes of pollen-stigma interaction were studied under artificial pollination in intersectional crosses of Populus lasiocarpa Oliv. × P. nigra L., P. lasiocarpa Oliv. × P. simonii Cart., P. lasiocarpa Oliv. × P. adenopoda Maxim. and P. lasiocarpa Oliv. X P. lasiocarpa Oliv. The results are as follows. Before pollination, the stigmatic cells of P. lasiocarpa containing large amounts of endo- plasmic reticulum, abund ant dictyosomes and well-developed mitochondria appear physiologi- cally active. Its outer wails are covered by a layer of cuticle with numerous branching channels toward the outer space. When the pollens of P. lasiocarpa and P. nigra are dusted, respectively, to the stigma of P. lasiocarpa, the degeneration of the stigmatic cells are rather slow. The cuticle of the stigma surface just adjacent to the alighting pollen is dissolved, but the cell contents still have large amounts of endoplasmic reticulum and the structure of the membrane system remains distinctly. When the pollens of P. simonii and P. adenopoda are forced, respectively, upon the stigma of P. lasiocarpa, the stigmatic cells degenerate rapidly. In addition to the disruption of the cuticle, heavy callose is deposited on the walls of the stigma cells adjoining the dropping pollens. Numerous vesicles are formed by the rough endoplasmic reticulum, and the electron density is increasing. The structure of the membrane system disappears gradually and becomes indistinct. Osmiophilic globuli occurs in the central vacuole. Plastids accumulate starch grains. The behavior of pollen tubes of P. nigra is similar to that of P. lasiocarpa. The pollen grains germinate rapidly and the pollen tubes penetrate into the stigma after the short-tube patten. Abnormal behavior is rare. A lot of minute exudates appear on the pollen walls. As for P. lasiocarpa × P. simonii and P. lasiocarpa × P. adenopoda, only half of the tubes can penetrate into the stigma after short-tube patten and is associated with callose deposition on the walls. The other half arresting on the stigma surface shortly after germination shows abnormal behavior; such as swelling, creeping, bursting, twisting and distortion with callose deposition on the tip of the pollen tubes. As a result of our experiments, It is concluded that the degeneration speed of the stigmatic cells may be a sign of incompatibility and the crossability in progamic phase may arrange as follows: P. lasiocarpa × P. nigra, P. lasiocarpa × P. simonii, P. lasiocarpa × P. adenopoda. The first one is compatible while the second and the third are incompatible.

Key words: Intersectional cross, Incompatibility, Pollen-stigma interaction, Populus