The tetrad phase of microsporogenesis in trees with reference to the annual cycle

Abstract

LUOMAJOKI, A. 1984. The tetrad phase of microsporogenesis in trees with reference to the annual cycle. —Hereditas 101:179–197. Lund, Sweden. ISSN 0018–0661. The timing of the tetrad phase of microsporogenesis in eleven tree species growing in natural conditions in Finland was studied in dates and in period unit (p.u.) and degree-day (d.d.) heat sums. The different methods were compared. Latitudinal variation and variation between successive years in generative development were also studied. The total lengths of tree meioses were evaluated and the duration of the tetrad phase wasmeasured. The zero point of the active period of trees was sought. The variation in northern marginal populations was evaluated. The synchrony of the PMCs and the surrounding somatic cells and various effects of temperature and climate are discussed. The earliest species (of genera Larix, Populus, Abies and Picea) move into the active period soon after p.u. heat sums start to accumulate. Pinus sylvestris has a larger safety margin against late frosts. The magnitudes of the heat sums at a given phase have no consistent correlation to the relative earliness or lateness of an individual year. Photoperiodism has no apparent effect on the inititation of meioses in the spring. Latitudinal differences in the lengths of early intervals of the generative cycle are small or negligible. The winter dormancy is the most important element of adaptation of trees to severe climates, provided that the active period is not too long. The adaptation of Scots pine to northern conditions primarily functions by shrinking the safety margin of the winter dormancy. The transition between the major elements of the annual cycle is probably gradual.