A geometric analysis of somatic embryo formation in carrot cell cultures

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Abstract
Cell cultures of the domesticated carrot, Daucus carota L., are a convenient model system for studying somatic embryogenesis. Various geometric measurements were used to characterize the different stages of carrot embryo formation in an attempt to define the major morphological events associated with the transitions to successive stages. Contrary to the qualitative impressions obtained from previous research, a developmental stage with a distinctive growth pattern is observed between the globular and heart stages of carrot embryogenesis. After the globular stage, which is characterized by isodiametric expansion, the present analyses demonstrate that the future hypocotyl axis undergoes considerable elongation before the emergence of the cotyledons that marks the initiation of the heart embryo. Because of the elongate shape of this intermediate stage, it is termed the oblong embryo. Incipient procambium can also be recognized in the central axis of the oblong embryo. The formation of a torpedo embryo from a heart embryo does not involve any apparent changes in the relative growth of various parts of the heart embryo but rather results from the continuous increase in overall size. These results are discussed in terms of their implications for future investigations into the physiological mechanisms of somatic embryogenesis.