How Well Understood Are the Processes that Create Dendroclimatic Records? A Mechanistic Model of the Climatic Control on Conifer Tree-Ring Growth Dynamics - Dendroclimatology - page 51

radial cell dimension of the tracheid, the experimental data on the kinetics of cell
wall deposition indicate that the average rate of deposition of cell wall does not dif-
fer much during the growing season and supports the statement that the leading role
in determining final cell wall thickness is played by the duration of this process and
by the radial size of the tracheid in which the cell wall was deposited. Mechanisms
for the control of tracheid radial expansion through the predetermination of final
tracheid radial dimension and cell wall thickness may be mediated through con-
trol of the rates of synthesis of different components of the tangential and radial
walls.
The process of tracheid differentiation is, as a matter of fact, a process of imple-
mentation of the genetic program of differentiation, starting at the level of cambial
cells and finishing with secondary cell wall formation (Fukuda 1996 ; Graham 1996 ;
Hertzberg et al. 2001 ; Chaffey et al. 2002 ; Ito and Fukuda 2002 ; Goujon et al. 2003 ;
Kirst et al. 2003 ; Nieminen et al. 2004 ) . Schrader et al. ( 2003 ) , for example, showed
that the expression of specific members of the auxin transport genes are associated
with different stages of vascular cambium development and demonstrated that trees
have developed mechanisms to modulate auxin transport in meristem in response
to developmental and environmental cues. A variety of anatomical parameters of
tracheids among trees and years indicate that the eventual result is not absolutely
determined, and that it depends on the local conditions where the differentiation
occurs. The process of differentiation can be presented as a series of events, in
which the duration and intensity of each stage depend on the previous one. Then,
in changed conditions, if two events are carried on further in space and in time,
their deterministic relationship may change. If we identify processes most sensitive
to the influence of external factors, we see that the external signal should be most
clearly perceived by the cambial zone. These external effects will leave their mark
on further processes of differentiation and, ultimately, the anatomical characteris-
tics of tracheids. As a result, the direct influence of environmental conditions on the
process of tracheid enlargement, as recorded in their final anatomical characteris-
tics, will be significantly smaller. There are a number of indirect data that indicate
that the events occurring during cell division in the cambial zone can have a strong
influence on the ultimate sizes of tracheids. Already, at this early stage of differenti-
ation, biochemical changes of the primary cell wall are necessary for radial growth
of cells and determination of its rate (Taiz 1984 ; Catesson 1990 , 1994 ; Pritchard
1994 ) .
In a recent review, Somerville et al. ( 2004 ) stated that progress integrating bio-
physical, developmental, and genetic information into useful models of plant cell
wall will require a system-based approach. They presented a cyclical diagram which
emphasizes that the expansion of the cell wall and integration of a new cell plate dur-
ing cytokinesis are components of the cell cycle. 'Thus, we infer that many of the
genes involved in primary cell wall synthesis and modification will be found to be
controlled by factors that control other aspects of the cell cycle' (Somerville et al.
2004 , p 2210). This process determines the bridge between the first and last stages
of tracheid differentiation through the activity of genes involved in the cell cycle
and cell wall thickening.