Agriculture Reference
In-Depth Information
1.3.1 IMPACT ON FRUIT CROPS
The recurrent developmental events of phenology and seasonality in veg-
etative flushing or bud differentiation distinguish trees from annual or ag-
ricultural crops. The fact that trees live over multiple growing seasons
implies that every year there is a considerable renewal cost of some organs
(leaves and fine roots), and that trees are more responsive or susceptible
to climatic changes. When horticultural productivity is the goal, the al-
location of resources toward reproductive processes must be maximized.
However, the tree must also preserve its growth potential for future years;
thus, a delicate C balance must be maintained between vegetative and re-
productive needs. In spring, stored sugars and nutrients support actively
growing shoots and inflorescences. Competition occurs between vegeta-
tive and reproductive meristems, and the fruit is growing essentially on
the currently produced photosynthates. Fruits represent a major C sink in
tree crops. The relationship between fruit load and photosynthetic activ-
ity (Palmer et al., 1997), as well as the effects of several climatic vari-
ables, has been intensively studied (Buwalda and Lenz, 1995; Wibbe et
al., 1993). Among all tree crops, cultivated fruit trees are the ones most
adequately supplied with water and nutrients; thus, there should be few, if
any, constraints to a positive CO
2
response (Janssenet al., 2000).
Climatic change effects are not caused by a single factor (e.g., elevated
CO
2
), but originate from complex interactions among various factors such
as atmospheric CO
2
, air temperature, nutrient supply, tropospheric ozone
level, UV-B radiation, drought frequency, etc. A reduction of stomatal
conductance under elevated CO
2
might have a significant effect on water
transport in trees, since the latter is roughly proportional to stomatal con-
ductance. Hydraulic conductivity was reported to decrease with elevated
CO
2
(Tognetti et al., 1996) but this effect is very species-specific. A de-
crease in stomatal conductance in response to CO
2
enrichment is common-
ly observed in many crops. However, under elevated CO
2
, the increase
in WUE is usually greater than the reduction of stomatal conductance,
especially under drought conditions. Among all plant organs, fine tree
roots generally show the greatest response to elevated CO
2
. In addition
to increases in fine-root density, trees may enhance their nutrient uptake
capacity through alterations in root morphology and architecture. Trees
grown under elevated CO
2
initiate more lateral root primordial, leading to
increased root branching and a more thorough exploration of the soil. In
