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 48

tracheid. Recent work on root development may, however, indicate the kinds of
mechanisms that may be involved. Baluska et al. ( 1994 , 1996 , 2001 ) hypothesized
and substantiated the existence in growing roots of a so-called 'transition zone'
between the root meristem and the elongation zone. The main significance of this
'transition zone' is as a sensory zone of the root that monitors diverse environmen-
tal parameters and effects appropriate responses (Baluska et al. 1996 , 2001 ) . For
example, gravistimulation significantly changes the distribution of the relative ele-
mental growth rate pattern along the growing root (Mullen et al. 1998 ) . The real
mechanisms are still unclear. Perhaps they operate through interactions of the gene
expression responsible for the control of coordinated growth processes with hor-
mones and growth regulators (Savidge 2000 a ; Baluska et al. 2001 ) . For our purposes
the definition of such a special zone is very likely. This is because the results we
have presented here lead us to expect the existence of some specific mechanisms
of external control of cell production and enlargement at the edge of the cambial
zone. We can only hypothesize that such mechanisms are related to growth rate and
'movement' of cells through the cell cycle, especially through the G 1 -phase. These
mechanisms would provide the link between growth rate near the edge of the cam-
bial zone and further radial enlargement. Using a simplified scheme (i.e., that the
division rate at the edge of the cambial zone controls the ultimate radial diameter
of the tracheid) clarifies the positional control of cell growth within the cambial and
enlargement zones. Both zones (dividing cells and enlargement) are characterized
by a high rate of primary wall expansion (elongation), although the second stage has
a higher rate of linear growth.
The importance of auxin in this process is clear (Rayle and Cleland 1992 ;
Casgrove 1993 ; Brett and Waldron 1996 ) . Precise determinations of gradients of
IAA distribution within growing xylem and phloem show that IAA can be the hor-
monal signal for positional control of cell growth (Uggla et al. 1996 , 1998 , 2001 ) .
The results clearly indicate that IAA concentration is higher within the cambial
zone with dividing cells and decreases to zero at the end of the zone of enlargement
(Uggla et al. 1996 , 1998 , 2001 ) . We have not here entered the discussion of the
origin and maintenance of a constant level or the total value of IAA (is its control
external, from the shoot—or internal, from the cambial zone itself?). Such a pic-
ture of the distribution of one of the main hormones that is closely related to cell
wall growth and cell growth regulation supports some of the assumptions we have
made in a way we cannot get from direct evidence. Uggla et al. ( 2001 ) describe
graphically the generalized distribution pattern of IAA, carbohydrates, and sucrose-
metabolizing enzyme activities across the cambial zone, zones of enlargement and
maturation. If we compare the measured concentration of IAA from experiments
by Uggla et al. ( 1996 , 1998 , 2001 ) and specific growth rate from our evaluations,
we see that the maximal linear extension growth rate of cells in the enlargement
zone coincides with decreasing IAA concentration (Uggla et al. 2001 , Fig.1).The
sucrose concentration decreases in the same direction, and SuSy (sucrose synthase,
an enzyme) activity increases in the enlargement zone and falls during cell wall
thickening.
All the main processes involved in the radial growth of developing tracheids, as
well as in the formation of the secondary wall, are coordinated by the position of the