On the time of embryogenic pollen grain induction during sexual development of Nicotiana tabacum L. plants

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Abstract
The previously-found correlation of P-grain frequency in situ (pollen grains which are able to form embryos in culture) with floral induction and sex balance of tobacco plants (Nicotiana tabacum L. var. Badischer Burley) was studied in more detail in order to find out in which stage of sexual development P-grain induction takes place. To know the time of P-grain induction is important for attempts to intervene more specifically in plant development, particularly with chemicals, with the aim of inducing P-grain formation. It was found that the floral stimulus was not involved in the control of both sex balance and P-grain formation. Rather, sex balance and P-grain formation were controlled by factors operating during flower development, that is, after floral induction period. Furthermore, both phenomena seemed to be controlled by the same factors, since changes in P-grain frequency and sex balance followed the same time course when flowering plants were transferred from short-day conditions at 24° C to 18° C and vice versa. These transfer experiments also revealed that the process of P-grain induction starts early in flower development, that is, well before meiosis (about five weeks before anthesis), but that the potential P-grains can return to normal gametophytic development until after meiosis. Pollen embryogenesis is regarded as a form of induced apogamy and is discussed in relation to an alternation of generations.