The Role of Light in the Interrelated Processes of Morphogenesis and Photosynthesis in the Fern Gametophyte

The interrelationship of photosynthesis and photomorphogenesis in the gametophyte of the fern Pteridium aquilinum was examined in detail. High energy red light produces filamentous gametophytes composed of greatly elongated cells. Blue light of equal energy produces normal cordate gametophytes composed of isodiametric cells. The assimilative rates of the two forms are identical during the initial stages of development. However, the filamentous gametophytes eventually exhibit a decline in assimilation not matched by their normal counterparts. Although this decrease in assimilation of gametophytes exposed to red light may be traced to a decrease in the photo-synthetic rate due to a decrease in chlorophyll synthesis, these events occur subsequent to the establishment of the filamentous mode of growth and are therefore a result, not a cause, of filamentous development. Gametophyte assimilation is also impeded under low energy blue light, due to a reduction in the photosynthetic rate at this low energy; and the initial filamentous stage, in which the cells remain normally isodiametric, is not exceeded. The addition of extra exogenous sucrose restores the capacity for growth and normal cordate prothalli result. It may therefore be concluded that the ultimate form of the gametophyte is under the control of a photoreceptor that is sensitive to blue light, and activated at a relatively low energy level. Red light at any level is ineffective in promoting normal cordate development, but is effective in supporting growth through vigorous photosynthesis, yielding a filament composed of greatly elongated cells. The eventual photosynthetic decline of the filamentous form is due directly to this particular growth mode. Low energy blue light is sufficient to effect form (the cells remain isodiametric), but insufficient to provide the necessary energy for growth through vigorous photosynthesis. Therefore, in order for the gametophyte to reach maturity it must be supplied with light energy that not only satisfies the requirement for form but the requirement for growth as well. The implications of these requirements are discussed in the text.