GENETICS 22: 130 (Jan. 1937)
CYTOLOGICAL PHENOMENA CONNECTED WITH SELF-STERILITY IN THE FLOWERING PLANTS1
E. R. SEARS
United States Department of Agriculture and Missouri Agricultural Experiment Station. Columbia. Missouri
1 A thesis submitted to the faculty of the division of biology of Harvard University
in partial fulfilment of the requirements for the degree of Doctor of Philosophy.

pp. 135-137

Bud fertility

The ability to set seed when self-pollinated before anthesis is possessed by numerous self-sterile plants. EAST (1923) observed it in Nicotiana, SIRKS (1926) in Verbascum phoeniceum, YASUDA (1930) in Petunia violacea, KAKIZAKI and KASAI (1933) in Brassica pekinensis and Raphanus sativus, and ALAM (1936) in Eruca sativa. PEARSON (1929) reported bud fertility for Brassica oleracea var. capitata, and KAKIZAKI (1930) confirmed this finding.

Experiments conducted on broccoli showed it to be highly fertile in the bud. Table 2 gives the results of bud pollinations. All buds of reasonable size on a flowering branch were pollinated at one time and the branch covered with a bag. On succeeding days each flower was tagged as it opened.

Good sets of seed resulted from some buds pollinated two or three days before opening, and from practically all flowers pollinated four to seven days before opening. Had younger buds been pollinated, it is believed that many of them would have set seed. Selfed buds of plant 2 which opened on the ninth, tenth, and eleventh days probably failed to produce more seed because of the weakened condition of the plant and the stunted and abnormal condition of the buds. Tests the following year on a plant of' another generation-I-( 1)-showed that three flowers which opened on the tenth day after being pollinated had 4, 10, and 11 seeds, respectively.

Behavior of compatible pollen

The rapidity of germination of pollen after compatible, incompatible, and bud pollinations was found to differ. Several buds were pollinated by sister flowers, and open flowers on the same plant were pollinated at the same time, some compatibly crossed and some selfed. Cytological observations of stigmas at intervals showed that activity of pollen on the stigma is more rapid after bud pollination than after pollination of open flowers. Germination occurs sooner (in less than two hours), and tubes are more rapid in their growth into the stigma. Compatible pollen on open flowers, while slower than pollen on buds, germinates sooner than incompatible pollen on open flowers. The latter pollen, as pointed out earlier, never reaches the normal percentage of germination.

No striking differences occur between growth rates of compatible tubes in mature styles and of tubes in immature styles. Tubes could be found in the ovary in about 24 hours after pollination in each case. PEARSON (1933) reported that tubes reached the ovary in approximately the same amount of time after bud pollination as after compatible pollination at anthesis.

The fact that immature stigmas were more receptive than mature ones indicates that pollen is inhibited to some extent on the mature stigma even though it is compatible. Since this inhibitory effect is confined to the stigma, as is the inhibition of incompatible pollen, it seems very likely that incompatibility results from what is only a stronger expression of an effect also exerted on compatible pollen. In other words, an inhibitory reaction, of the type which would cause incompatibility if it were stronger, occurs between compatible pollen and the stigma.

Further evidence that compatibility in broccoli may involve an inhibitory reaction similar to that of incompatibility was furnished by the behavior of certain plants when changed to an unfavorable environment. In some instances the effect of moving a plant from the greenhouse to the garden was to destroy its fertility to pollen from certain plants, but not to lower its fertility to pollen from certain other individuals. This may be explained satisfactorily as due to pollen from certain compatible plants being normally somewhat inhibited, so that raising the general inhibitory ability of the plant by unfavorable environment increases the inhibition of this pollen until the dividing line between compatibility and incompatibility is passed. Pollen from other compatible plants is less inhibited, or is not inhibited at all, and the general increase in inhibitory effect does not force this pollen across the line into the incompatible class. A sharp dividing line between fertility and sterility is indicated by the rarity of partially fertile combinations.