Maize pollen grows with difficulty on artificial media. A good germination percentage is obtained on agar media, while in liquid media, probably because the water uptake is unregulated, the grains generally burst. The total tube length reached in vitro is very limited, compared with the length in natural conditions, probably owing to the lack of the metabolic support furnished in vivo by the style.
Most of the tube growth is observed in vitro in the first two hours, and within about six hours the growth stops. This trend is observed also when the pollen is stored for 24 hours at 4 C immediately after collection. Fig. 1 shows the tube growth during 24 hours of two inbred lines on standard medium (P. L. Pfahler, Genetics 52:513, 1965; D. Cheng and M. Freeling, MGCNL 50:11, 1976). The growth was measured after 2, 6, 10 and 24 hours. Storage appeared to have a favorable effect on the C123 line, not on the H3025. A favorable response to storage has been reported by P. L. Pfahler and H. F. Linskens (Theor. Appl. Gen. 42:136, 1972) and by C. Frova and W. A. Feder (Ann. Bot. 43:75, 1979), but it is clearly dependent also on the pollen genotype involved.
In order to obtain pollen germination and growth in liquid medium for particular experimental requirements, such as pollen treatment with hydrosoluble compounds, we tested a more complex medium (J. P. Mascarenhas, Am. J. Bot. 53:563, 1966). In Fig. 2 pollen germination percentage and tube length of the W22 line after four hours of incubation on standard medium, agarized and liquid Mascarenhas medium, are reported.
The result is clearly better in the last condition; however, we observed a great variability in the number of burst grains from one experiment to another.
We investigated the stylar component effect on pollen tube growth by supplementing the medium with water and ether silk extracts, or directly with stylar fragments. Since the contribution of stylar tissues to gametophyte development may derive from the metabolites present in the mature silks, or from those mobilized only as a response to the tube growth itself, we used both pollinated and not pollinated silks. Moreover, in order to verify the effect of pollen grains, which were present in the first treatment, pollen extracts were also added. In Fig. 3, as indicative data, the results concerning one of the tested lines (H3025), are reported. Similar results were obtained for the other lines (WF9, C123, B14). In general the silk extracts had an unfavorable effect on pollen growth for all the genotypes. Pollen extracts acted differently according to the pollen source used in the extract: in some cases growth was inhibited, whereas the B14 pollen seemed to have a favorable effect on the development of all the lines except H3025.
The silk fragments directly soaked in the medium revealed a clearly favorable effect on germination and particularly on pollen tube growth, even if the pollen was not in direct contact with them. In fact the silks were deposed on the bottom of the petri dish and covered with a thin layer of medium. Thus, it appears that the silks diffuse some growth factor, which is inactivated during the extractive manipulations, or that these extractive manipulations induce the production of some inhibitor substance.
M. Sari Gorla and M. G. Valsecchi
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