Embryogenesis and germination in rye (Secale cereale L.) : II. Biochemical and fine structural changes during germination.

When rye embryos imbibe water they rapidly return to a condition of biochemical and structural complexity. Three stages of imbibition can be recognised: Phase I a short period (10 min) of physical wetting; Phase II a longer period (1 h) when little further imbibition occurs, followed by Phase III a continuous phase of active water uptake. The latter coincides with an increase in respiration rate and an increase both in the number of mitochondria and of cristae within them. Changes in fine structure become evident in all organelles in Phase III, after 2 h of imbibition. In the unimbibed embryo endoplasmic reticulum is present only as short crescents associated with electron lucent bodies, but in Phase III the endoplasmic reticulum proliferates to form many surrounding cirlets. After 6 h these circlets become fewer and instead the endoplasmic reticulum is seen in close association with the nuclear membrane. Concurrently incorporation of radioactive uridine and thymidine is first detectible. This suggests that the large increase in protein synthesis occurs on new ribosomes present on the reticulum associated with the nuclear membrane. For the first 6 h protein synthesis must occur either on polysomes within the dense packing of ribosomes or on these circlets of endoplasmic reticulum associated with electron lucent bodies.