In a series of mixed pollination experiments in maize (K. Murakami et al., 1972; M. Yamada and K. Murakami, 1978) the existence of the superiority of pollen grains from F1 plants in selective fertilization could be confirmed. It is assumed that the superiority must be corroborated by in vivo or in vitro germination experiments on pollen grains from inbred lines compared with those from the F1 plants.

In vitro germination experiments were carried out using three inbred lines (A34, C164, and Koshu-564) and three F1 hybrids of these lines (A34 x Koshu-564, C164 x Koshu-564, and A34 x C164), among which three F1 hybrids had shown the superiority in selective fertilization from mixed pollination (M. Yamada, in press). The Cook and Walden (1965) artificial medium used for the germination experiment consisted of 0.6% bacto-agar containing CaCl₂*2H₂O (0.03%), H₃BO₃ (0.01%) and sucrose (0.35 M). Fresh pollen grains of each material were spread over the surface of agar medium in Petri dishes. The inoculated dishes were incubated at 24 C for one, two, and three hours three times to estimate germination. Pollen activity was interrupted by flooding the surface of the medium with a solution prepared according to P. L. Pfahler (1967). The Petri dishes were then stored at 3 C until the observations were made. Pollen germinability was determined on the screen of a Sonic Digitizer with a graph pen, which could record automatically points, length, etc. on a disk. For observation of pollen grains, projections onto a screen with a 20 cm x 20 cm frame were measured twice in relation to the total number of pollen grains in the frame, number of pollen grains which had germinated and length of the tube.

As shown in Fig. 1, pollen grains from F1 plants in each F1 hybrid showed higher germination percentages at an earlier time (A34 x Koshu-564 and C164 x Koshu-564) and a longer pollen tube (A34 x Koshu-564 and A34 x C164). It is well known (J. A. Goss, 1968) that germinability of maize pollen is controlled by pollen per se, the pollen tube reaching a maximum length of 500 within 3 hours. On the basis of the results shown in Fig. 1, therefore, an experiment was designed to determine the germinability of each pollen grain on the screen within the first hour after inoculation. As shown in Fig. 2, the increment of percentages of germinated pollen grains occurred within thirty minutes after inoculation. It also appeared that the elongation of pollen tubes in pollen grains from the F1 hybrid was promoted unlike that of the tubes of the inbred lines. This preliminary experiment suggests that the superiority of pollen grains from F1 plants is controlled in the first stage of germination depending upon pollen grains per se which are derived from the F1 heterozygous plant. It is necessary to carry out in vivo experiments on silks after pollination, so as to correlate the results with those obtained from in vitro germination experiments.

Figure 1.

Figure 2.

Minoru Yamada and Yasunobu Ohkawa

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