Agriculture Reference
In-Depth Information
scion
('Queen Cox') and scion hydraulic resistances derived from measurements of Lp and the
hydraulic resistances of the graft union estimated by difference
Table
.
The calculated rootstock shank, rootstock shank
+
graft union
+
Rootstock
Rootstock shank
+
shank
graft union
+
scion
Scion
Graft union
Rootstock
Rr
Rrus
Rs
Ru
'M.
'
.
.
.
.
'M.
'
.
.
.
.
−
'MM.
'
.
.
.
.
All differences between rootstocks highly significant.
Reproduced from Atkinson
et al.
(
) with permission.
rate increases (Landsberg
et al.
,
). Apparent changes in con-
ductance with increases in fluxat low fluxrates may be a consequence of the
importance of osmotic forces at these rates (Boyer,
; Lakso,
).
Rootstocks differ in their resistance to flow. This was deduced by Olien and
Lakso (
' were under
greaterwaterstressatmid-day,asmeasuredbystemwaterpotential,thanthose
on 'M.
), who found that 'Empire' apples on 'M.
' and 'M.
' despite lack of effect on transpiration,
stomatal conductance or the ability of the scion stem to conduct water. Part
of this effect could be attributable to the smaller root systems of dwarfing
rootstocks and lower root/shoot ratios of trees budded on them (McKenzie
', 'MM.
' and 'MM.
). Part is due to differences in specific conductivity
between rootstocks. Atkinson
et al.
(
; see also pp.
-
) showed the resistance of rootstock
shanks to be greater the more dwarfing the rootstock (Table
.
), and the
graft unions between 'M.
' and 'Queen Cox' to provide a large
resistance to flow whereas that for the union between 'MM.
' and 'M.
' and 'Queen
Cox' did not do so.
The more dwarfing rootstocks also increase the resistance to flow of scions
worked on them, though the effect is less pronounced than the changes in
rootstock resistance
per se.
Variations in the size of trees on the same rootstock are accompanied by
variations in root resistance, larger trees having higher root conductivity. The
slope of this relationship is, however, less on 'M.
' and 'M.
' than on more
vigorous rootstocks (Olien and Lakso,
). The rootstock effect on conduc-
tivity cannot, therefore, be fully accounted for by a simple linear relationship
between tree size (and consequently root size) and root system conductivity.
Xylem vessel number and radius markedly affect resistance; flow through
capillaries of the dimensions of xylem vessels is proportional to the fourth
power of the capillary radius (Hagen-Poiseuille flow equation). In general, the
