Employing a composite gene-flow index to numerically quantify a crop's potential for gene flow: an Irish perspective.
The literature dealing with seed set on male sterile wheat plants by open pollination is critically reviewed with special attention to the influence of the distance from the pollen source and the pollinator: male sterile ratio. It is suggested to indicate seed set only as a percentage of the fertile analogue. Own investigations in Wageningen, the Netherlands, indicated that seed set on male sterile wheat plants on various distances from a pollen source of 6×6 m of a mixture of three spring wheat varieties amounted to maximally about 36%, but decreased quickly at increasing distances. Apart from an evident effect of the wind direction, in 1968 average seed set became less than 10 % after 1 m distance from the pollen source; in 1969 and under more favourable experimental conditions for the first meter from the pollen source a decrease to 25% took place, becoming 10% only after 3 m. It is recognized that for practical conclusions the pollen field size applied is too small. Both in 1971 and 1972 the seed set increased with increasing pollinator: male sterile ratio. This increase always was relatively small and, consequently, the kg yield hybrid seed per ha to be expected always decreased when this ratio increased. Applying a female strip width of 2 m and pollinator (mixture of five spring wheat varieties): male sterile ratios of 1:2, 1:1 and 2:1, in 1971 the latter ratio yielded only 60% of the former. In 1972 the same ratios were tested, but besides three male sterile strip widths were applied: 2, 3 and 4 m. In this year the calculated percentage seed set was much higher than that of the previous years (maximum almost 80%), probably as a consequence of more and larger pollinator areas and a more adequate way of establishing the 100% value of seed set. Again the kg yield hybrid seed per ha strongly decreased with an increase of the pollinator: male sterile ratio. The efficiency of the different ratios depends on the width of the female strip. Unfortunately, but according to the expectation, the kg yield hybrid seed per ha decreases, apart from the male to female ratio, fixing the 2 m value at 100%, to about 88% at 3 m and 72% at 4 m female strip width.