108
and actual fertilizatio n showed that the poor crossability with rye of both cv. Hope
and the CS/Hope 5B substitution line resulted from the absence of fertilization.
Pollen germination did not depend on whether wheat was pollinated with wheat or
rye pollen, or whether the wheat was readily or poorly crossable with rye. Studies
of pollen grain germination and pollen tube growth showed that the dominant alleles
of the crossability genes manifested themselves through the retardation and even-
tual inhibition of pollen tube growth at the style base and in the ovary wall. In Hope
the growth of al l pollen tubes was inhibited, whereas in CS/Hope 5B fertilization
was achieved in rare cases. The recessive alleles of the crossability genes do not
seem to have any infl uence on the growth of rye pollen tubes in wheat pistils.
Fertilization and early seed developmen t were studied by Wojciechowska and
Lange ( 1977 ) in the common wheat variety Chinese Spring after pollination with
rye and selfi ng, and in the common wheat variety Hope after selfi ng. Fertilization
with rye gave rise to a tetraploid embryo (2 n = 28) and a septaploid endosperm
(2 n = 49) in hexaploid maternal tissue. In all three combinations the fi rst pollen tube
reached the mycropyle about 40 min after pollination. When pollinated with rye the
migration of the sperm nuclei to the egg cell and the polar nuclei was delayed by
about an hour. The fi rst mitosis in the zygote of selfed Chinese Spring and Chinese
Spring × rye occurred at about the same time, as did the fi rst mitosis of the polar
nuclei. The subsequent development of both embryo and endosperm was slightly
faster for Chinese Spring × rye than for selfed Chinese Spring. The development of
the embryo and endosperm was monitored for 72 h after pollination.
D’Souza ( 1978 ) studied differences i n the germination percentage of rye pollen
on the stigmas and the rate of pollen tube growth of within the styles of wheat vari-
eties known to have good and poor crossability with rye. For this purpose, wheats
known to be readily crossable with rye, e.g., Chinese Spring, T. durum , and T. carth-
licum , were compared with varieties which have little or no crossability with rye,
e.g., Carsten VIII and T. monococcum. In wheat × rye crosses rye pollen germinated
as well on wheat stigmas as on rye. There was no retardation in the growth of the
pollen tubes, which penetrated the styles and reached the base of the styles regard-
less of the crossability of the wheat varieties with rye. Irrespective of the ploidy
level and the crossability with rye, the pollen germinated on the stigma and the pol-
len tubes reached the base within an hour of pollination. The failure of seed set in
poorly crossable genotypes when crossed with rye is thus not due to the differential
behaviour of the style, but probably to physiological differences within the ovules.
Jalani and Moss ( 1980 ) investigated the ma nifestation of Kr genes on pollen germi-
nation and pollen tube growth in a wide range of hexaploid wheat varieties crossed
with rye, with the aim of elucidating and identifying the site (s) of action of the
crossability genes. The results indicated that the crossability genes have little effect
on pollen germination or on the time taken for the pollen tubes to reach the micro-
pyle. These results were in agreement with those of Zeven and van Heemert ( 1970 )
and Lange and Wojciechowska ( 1976 ). The number of pollen tubes reaching the
micropyle is, however, affected by the Kr genes, as this number was greater in
highly crossable genotypes than in poorly crossable ones. There was a strong
correlation between the mean number of pollen tubes at the micropyle and seed set,
M. Molnár-Láng