Agriculture Reference
In-Depth Information
100
g
l
50
LA
expansion
0
5
Days since watering
10
15
Figure 12.7
Responses of apple leaf stomatal conductance (
g
l
)
and
leaf area expansion to soil water stress on outside-grown potted
trees. Reproduced from Lakso (1985),with permission.
The different responses of root and leaf growth to drought arise from their
differing levels of osmotic adjustmentand inthe different consequences of their
poor hydraulic linkages, due to immature xylem, in their different environ-
ments. For root tips, when water potential is suddenly reduced there is rapid
osmotic adjustment. This allows for partial recovery of turgor. The cell walls
also loosen and growth resumes. Just-unfolding leaves have very low petiole
conductance and do not show rapid osmotic adjustment or cell wall loosening.
Consequently, when subject to drought their growth is inhibited (Hsiao and
Xu,
) and they also wilt readily.
The impact of these effects on shoot and leaf growth on orchard productivity
depends on tree age, planting density and vigour of growth in the absence of
serious water stress. The effects of checking the growth of young trees, before
the orchard canopy has attained its final dimensions, will normally be very
adverse. However, once the canopy has attained its optimal dimensions and
leaf area on an orchard basis, some reduction in annual shoot growth is not
necessarily disadvantageous.
Water stress effects on stomatal conductance are much less severe than those
on leaf expansion (Figure
). Stomatal conductance is a major factor con-
trolling both transpiration and photosynthesis per unit leaf area but whereas
water vapour conductances of approximately
.
cm s
−
permit maxi-
.
-
.
mum rates of photosynthesis (Lakso,
), transpiration continues to increase
with stomatal conductance above these levels. Stomatal conductances in apple
