Early development in fern gametophytes: interpreting the transition to prothallial architecture in terms of coordinated photosynthate production and osmotic ion uptake.

Gametophytes of Onoclea sensiblis L. were grown under various light and media-ion conditions to gain a better understanding of the source/sink relationships between photosynthetic and ion-absorbing cells. There was a clear interdependency between green cell and rhizoid functions, such that the growth and development of the rhizoids was completely dependent on the internal delivery of photosynthates from green cells, and conversion of the one-dimensional filament into the two-dimensional prothallus required monovalent cations that could only be provided by rhizoid uptake. The need for monovalent cations was related to osmotic demands of dividing and expanding cells; prothallial development was blocked by monovalent cation deficiency, and the system resorted to Na+ uptake to support cell expansion when K+ was absent. Surgical excisions of filament cells further demonstrated the high degree of coordinated growth between the light-absorbing and ion-absorbing regions. It was also learned that excised sub-apical cells of the protonemata, like the intensively studied apical cell, were capable of remodelling remnants of the filament into a normal prothallus.