PHASES OF GROWTH

The sequence of stages through which a higher plant has passed in its development from a ertilized egg to an adult organism, is known as its ontogeny or ontogenesis.

In the juvenile phase, a young seedling plant displays one or more distinctive characteristics of both a morphological and physiological nature. These distinguish a juvenile plant from an adult.  The juvenile phenotype gradually disappears during subsequent growth, and under natural conditions it is replaced by the adult (mature) phase in those parts of capable of changing from vegetative to flowering meristems under “normal” inductive conditions.  In some plants, a transitional phase of development can be distinguished between the juvenile and adult phases during which the potential to flower gradually increases. the plant in which maturation has occurred. Maturation occurs when the apical meristems become capable of changing from vegetative to flowering meristems under “normal” inductive conditions.  In some plants, a transitional phase of development can be distinguished between the juvenile and adult phases during which the potential to flower gradually increases.

When a plant is induced to flower, the shoot apical meristem changes from forming vegetative structures to forming reproductive organs (Hackett and Murray, 1997).  Typically, it is an external environmental stimulus, such as day length or chilling that will trigger this change.  Under such a normal environmental stimulus the apical meristem of a juvenile plant is not capable of perceiving or responding to the signal and remains vegetative. Under certain circumstances,  juvenile plants can be induced to initiate flowers but will revert to the non-flowering juvenile state for several years (Hackett, 1987).  However, it is often difficult to devise methods to promote flowering that will not also advance maturation (Hackett, 1985).  In many conifers, flowering can be induced on juvenile shoots by gibberellins or auxin/gibberellin combinations (Pharis and King, 1985; Pharis and Kuo, 1977; Pharis et al., 1976, 1980; Ross et al., 1981).

In general, the duration of the juvenile phase is proportional to the potential ultimate size of the plant, being shortest in annual herbaceous species and progressively longer and more noticeable in perennial and woody plants.  In trees it may sometimes persist for many years.

Phase change from juvenile to adult is an epigenetic change, in that there are phenotypical changes that are the result of changing gene expression, not mutation.  When an adult plant is regenerated from an embryo (either zygotic or somatic), juvenile traits are again expressed.  Gene expression for dihydroflavonol reductase (DFR) was studied in adult and juvenile leaf lamina tissue of English Ivy (Hackett and Murray, 1997).   DFR activity was detected in juvenile leaves treated with sucrose and light, but not in mature phase discs.  It was determined that the lack of DFR activity was because there was no accumulation of DFR mRNA because of a lack of transcription of the DFR gene in mature phase discs.  Lack of expression of this gene was also reported for adult phase stem tissue.  This gene was expressed in juvenile leaves and stems. The specific reason why the  DFR locus was not transcribed was unknown.  In Arabidopsis, both light and temperature independently regulate the floral promoter (FT) gene (Poethig, 2003).  However, there are other genes that also regulate temperature sensitivity of flowering time.  These genes (FCA and FVE) act upstream from the FT gene and seem to be involved in phase change.