Fertilization by low numbers of pollen grains

Zea mays is recognised as a prolific pollen producer. It has been estimated that about 25,000 pollen grains are produced by one plant for each kernel on an ordinary ear with 1000 kernels. Allowing for dispersal, it is possible that at least 170 grains are available per silk during the course of pollen shedding (T.A. Kiesselbach, The Structure and Reproduction of Corn, Univ. Nebraska Press, 1980). The availability of large numbers of pollen grains competing for a limited number of ovules provides a selective advantage for the more vigorous pollen grains (Mulcahy, Science 206:20-23, 1979). Goss (Bot. Rev. 34:333-358) recognised it is difficult to estimate pollen viability by the application of single grains to silks.

During an investigation into the possibility of transferring DNA to individual pollen grains, it was necessary to estimate the minimum number of pollen grains required on a silk to effect fertilization. Detasseled maize plants (lines 256 and g25) were kept in a glasshouse separate from the same lines of plants which produced the pollen. The tassels of the pollen plants were shaken each morning to remove old pollen. Two hours later (ca. 11 a.m.) the freshly shed pollen was collected. Detasseled plants were transferred to the laboratory. Individual pollen grains were picked up on a pointed scalpel blade and transferred to silks exposed under a dissecting microscope. Detasseled plants exposed to similar conditions in the laboratory without application of pollen grains produced no seed.

Repeated application of pollen grains to parts of the silk other than the hairs did not produce any fertilization. When individual pollen grains were applied to the hairs of silks, varying levels of fertilization occurred depending on the number of grains present (Table 1).

The application of one to three pollen grains on an individual hair of a single silk produced a low level of fertilization (8-24%). The addition of one, two or three grains to two or more hairs on a single silk produced increasing seed set (25-83%). These preliminary results suggest satisfactory levels of fertilization can be achieved with relatively low numbers of pollen grains applied to the bases of silks. It is not known if a minimum number of grains is required on each silk for efficient fertilization due to reduced pollen viability or if this number is necessary to stimulate synergistic pollen tube growth.

Table 1. Application of individual pollen grains to the hairs of silks of maize plants.

J.C. Waldron

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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