Physiological genetics of pisum.

The flowering behaviour of plants which respond both to vernalisation and photoperiod has been described by many writers, although long-day plants have been used more extensively than short-day plants. From his studies on winter and summer rye, Gregory (1948) postulates a linear sequence of processes leading to flower initiation. In winter rye, which has a quantitative low-temperature requirement, the operation of these processes is largely dependent on a precursor synthesised as a consequence of low-temperature treatment. In spring rye the precursor is already present in the germinating seed, Lang and,Melchers (1947), using a biennial variety of Hyoscyamus niger, postulate that a qualitative low-temperature requirement must be satisfied before photoperiodic reactions can occur. However, vernalisation may replace exposure to long-day photoperiod in accelerating flowering in a limited number of plants, Owen et: al, ( 1940) found that some individuals of one biennial beet variety were so strongly influenced by low temperatures that they developed seed stalks in non-inductive short days. Aitken (1955) found that an early variety of subterranean clover can initiate flowers at a low node as a result of either sufficient vernalisation without long days, or exposure to a sufficiently high-photoperiod without prior low-temperature treatment. In the present work a similar equivalence of vernalisation and long days in causing acceleration of flower initiation in several late pea varieties is reported. This equivalence is not expected on the theories of flowering postulated by Gregory (1948) and Melchers ( 1952), and possible reasons for this apparent disagreement are discussed in Part II of this thesis.