An experimental system for studying interrelationships between the embryo and megagametophyte of Pinus strobus during seed germination

An experimental culture system was devised that provided a reliable means for conducting quantitative studies on interrelationships between embryos and megagametophytes of Pinus strobus L. during germination and early seedling development. By controlling imbibition rates, it was possible to synchronize germination of intact megagametophytes and obtain cultures with uniform morphological characteristics for use in biochemical studies. Early seedling development was affected by time of removal of the megamametophytes from the embryos; organs were larger if magagametophytes were left intact for longer periods of time. By using a nitrate reductase semimicro assay, enzyme activity was detected in embryos and shoots of intact megagametophytes, but activity declined as the seedlings aged. Activity within roots increased as seedlings matured and may be localized in the root tip. Nitrate was produced within the megagametophyte; however enzyme activity was dependent upon sustained, direct access to the inducing substrate, nitrate, in the culture medium. When megagametophytes remained intact for the initial 8 days of germination, seedlings developed epicotyls, and nitrate reductase activity within the roots was significantly greater than in 6-day cultures; in essence, by 8 days these seedlings appeared to have passed a threshold and were autotrophic.