Agriculture Reference
In-Depth Information
Minchin et al. (
) showed experimentally that apple fruitlets import photo-
synthate at a rate matching their utilization capacity, which in turn can adapt
to supply. Increases in availability of photosynthate cause only a small increase
in import at first but over several hours further and much larger increases
occur, suggesting an increase in enzyme activity induced by increased avail-
ability. Short-term reduction in photosynthate supply resulted in an incapacity
of the fruitlet to return to its initial import rate, indicative of the reduction in
activity of enzymes. Where two fruits were supplied by a single leaf fed with
CO the partitioning between them responded to changing their relative
temperatures.
Net carbon exchange and
orchard productivity
Dry matter increment in an orchard over the season is dependent on:
The percentage of available sunlight intercepted (Chapter
).
The irradiance per unit surface,
The efficiency with which light energy is used in dry matter production.
Carbon losses by respiration.
Theyieldoffruitdependsonthetotaldrymatterincrementandtheproportion
of this directed into fruit.
Percentage light interception is the major factor determining differences
in yield between different orchard systems, especially in the years before at-
tainment of final canopy dimensions (Chapter
). It also contributes to the
higher yields at lower latitudes where growing seasons are longer and leaf area
durations greater (Heim et al. ,
).
Irradiance varies with latitude, cloud cover and altitude (Wagenmakers,
; Jackson,
,
). It generally increases with decreasing latitude down
, further increases as the equator is approached being confined
to autumn and winter months. Growing-season irradiance is, however, higher
over most of the summer months in fruit-growing areas of New Zealand (
to around
S)
S) as well as being about
than those of South Africa (
% higher than
N). It decreases with cloud cover, so is higher in
arid regions, and increases with altitude. Light energy is the driving force of
photosynthesis although not the sole controlling factor. The photosynthesis
of a single, fully exposed, leaf is light-saturated at a relatively low irradiance
level, but this saturation level is higher the greater the irradiance under which
the leaf is grown. The leaf area receiving light energy above any specific
level is, moreover, a function of above-canopy irradiance as well as the light
transmitting characteristics of the canopy (Chapter
those in Kent, England (
). Assuming a closed
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