Effects of light gravity and humidity on archegonial development

The reduced effect of surface heterogeneity in soil cultures versus agar cultures, the increase in percentage of males in flat soil versus flat agar cultures, and our observations of the anomalous position of archegonia on the dorsal surface of gametophytes in agar cultures suggest that there may be additional differences between natural and laboratory cultured gametophyte populations.

Humidity, light, and gravity orientation are environmental parameters that can also differ sharply in nature from the environment of standard laboratory cultures. Wild gametophytes grow in diverse orientations with respect to gravity, and to some extent with respect to light. An additional significant consideration may be the degree of difference in light received between the two gametophyte surfaces. Gametophytes growing on a dark, non-reflective surface, as is the usual case in nature, experience a much sharper light-intensity gradient from one surface to the other than do gametophytes growing on a translucent agar substrate.

To investigate the effects of light and gravity orientation, gametophytes of Pteridium aquilinum were grown on an agar substrate in Petri dishes positioned above and below a fixed light source and in both normal (right-side-up) and inverted (upside-down) orientations with respect to gravity. Gametophytes in all orientations continued to produce some archegonia on their morphologically dorsal surface (air side), but significantly fewer when that surface was oriented toward the light source (Table 9.3). Regardless of orientation, the gametophyte surface facing away from the light source produced the greater total number of archegonia, and, on dorsal surfaces facing away from the light source, more archegonia were produced on that surface whether in normal or inverted orientation with respect to gravity. Gametophytes cultured on soil in normal orientation with respect to light and gravity produced very few archegonia on the dorsal surface, mimicking those observed in wild soil-grown gametophytes. Production of archegonia on the dorsal surface in agar grown cultures likely results from a reduced differential in light incidence due to the translucence of the medium, allowing much greater light incidence on the ventral surface of the gametophyte than occurs in field grown gametophytes.

Gravitational orientation did not exert an overriding effect on archego-nial position although gametophytes receiving normal orientation of gravity and light produced slightly fewer archegonia on the dorsal surface and fewer archegonia overall than did cultures with reversed gravity and normal light orientation. Because gametophytes in nature grow on substrates of all aspects, the anomalous minimal effects of gravitational orientation are not surprising.

Table 9.3 Effect of light orientation and gravity on the position and number of archegonia per gametophyte of Pteridium aquilinum Air side, morphologically dorsal surface. ASU-B, air side up, below light; ASD-A, air side down, above light; ASU-A, air side up, above light; ASD-B, air side down, below light. Results are an average of three replicates; the number per replicate ranged from 11 to 40 individuals (20 individuals per plate were sampled where possible, but plates with an atypical gravity/light orientation produced fewer archegoniate gametophytes). Mean values for % of archegonia on the dorsal surface that do not share a letter in common are significantly different at P < 0.05. Differences in total number of archegonia were not significant among treatments.

Table 9.3 Effect of light orientation and gravity on the position and number of archegonia per gametophyte of Pteridium aquilinum Air side, morphologically dorsal surface. ASU-B, air side up, below light; ASD-A, air side down, above light; ASU-A, air side up, above light; ASD-B, air side down, below light. Results are an average of three replicates; the number per replicate ranged from 11 to 40 individuals (20 individuals per plate were sampled where possible, but plates with an atypical gravity/light orientation produced fewer archegoniate gametophytes). Mean values for % of archegonia on the dorsal surface that do not share a letter in common are significantly different at P < 0.05. Differences in total number of archegonia were not significant among treatments.

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