Auxin: Cytokinine Ratio May Determine Plant Stem Elongation
Endogenous levels of phytohormones seldom correlates to growth and development. However, in Abies nordmanniana, a IAA: cytokinin ratio below one corresponds to meristematic activity in the apical meristem, whereas a high ratio about 50 was observed in the elongating stem tissue.
Although auxin has been known to influence cell elongation since the 1930’es,we are still lacking an understanding of how auxin participates in maintaining controlled stem elongation during a growing season. In a changing environment auxin initiates developmental adjustments of processes such as tropisms, apical dominance and cell proliferation, and auxin synthesis as well as transport are influenced by irradiance and temperature, all factors that may explain the weak correlations between hormone levels and plant development as well as unexplainable peaks. Where auxin is able to stimulate cell elongation by itself, it is the interaction between auxin and cytokinin that is required to maintain cell division. It is well documented that auxin and cytokinin regulates each other, and thereby developmental processes such as embryogenesis, meristem development and shoot branching. However, auxin and cytokinin are today still being evaluated separately regarding stem elongation. A model system where the development of the apical meristematic is separated from the process of stem elongation is idle to probe into the possible regulatory function of the auxin-cytokinin ratio in determining stem elongation. Such a model does exist in gymnosperms. In conifers such as spruce, pine and abies, a none elongating shoot initial is formed within the apical meristem each year, but the elongation thereof occurs first the flowering year. A cellular barrier exists between the two types of tissue. These plants are therefore unique by having separated the process of forming a shoot initial from the process of stem elongation. In these species of gymnosperms, top leader elongation is concealed to at short well-defined time period as bud burst occur in late May, and elongation of the preformed shoot initial occurs in June/July, with some variation between years.
We have made year round hormone determinations in several parts of Abies nordmanniana. Although we observed a dramatic changes in the level of hormones during the growing season, as well as between different types of tissue, is was not possible to make firm conclusions to how the observed hormonal changes might regulate plant development. Furthermore, although weekly samples were taken in the period of growth, unexplainable variation in hormone levels was always determined.
During a reevaluating of the data obtained, it became apparent, that if the auxin-cytokinin ratio was used as a determinator of stem elongation, the unexplainable hormone fluctuations disappeared. The period of fast stem elongation corresponded to the period where the auxin-cytokinin ratio was very high, around 60, and as soon as the elongation process began to ceases, this ratio dropped to below 2. If the same ratio was determined within the apical bud, where the none-elongating shoot initial were formed, the ratio stayed below 1 throughout the entire developmental period, except at the very early phase of initiation, where a ratio of 2 was observed.
The literature contains only very few data comparing the level of auxin to cytokinin. However, studies of hormonal levels in elongating buds in Lupinus are in agreement with the hypothesis that it is the auxin: cytokinin ratio that determine elongation. A cytokinin: auxin ratio above 1 was observed in elongating bud tissue independent of the actual hormone levels, whereas a low ratio corresponded to a low elongation rate. These authors concluded that although the data from each hormone may be suggestive they do not relate completely to observed growth pattern. The ratio of auxin and cytokinin may thus control stem elongation independent of the actual level of each hormone. The threshold value seems to be around 1. Cell elongation occurs when the level of auxin are above the level of cytokinin making the auxin: cytokinin ratio exceeding 1 whereas a lower ratio probably favors meristematic activity.
