Agriculture Reference
In-Depth Information
proportion of their assimilates into the harvested product, i.e. have a higher
'harvest index' rather than a higher total biological yield (Gifford and Evans,
).Similarlyforapples,theuseofdwarfingrootstockshasresultedinamuch
increased ratio of crop yield to total dry matter production. Their perennial
growth habit, however, introduces further complexities. In the early years after
planting it is usually desirable that the trees grow rapidly, as well as bear crop,
in order to attain high levels of light interception, hence potential yield, on an
orchard basis. Moreover, excessive cropping can inhibit fruit bud formation,
although not necessarily as a result of effects on carbohydrate partitioning, and
reduce cropping in the following year. It can also reduce shoot growth in both
the year of cropping and the subsequent year, so influencing future cropping
potential. Knowledge of the relative 'sink strength', i.e. power to attract assim-
ilates, of different tissues and organs throughout the year is therefore basic to
effective tree management.
In general the underlying basis of sink strength (assuming a pressure-flow
mechanism of phloem translocation) is thought to be the ability to lower the
concentration of photosynthate in the sieve elements servicing the sinks and
thus establish a favourable concentration gradient between source and sink
(Wardlaw,
). This is achieved either by rapid metabolic use of the carbo-
hydrate being transported or its conversion into a storage compound.
The supply of assimilate to any particular sink, either directly or after stor-
age, depends on source limitations and on the competitive strength of the sink.
There are many examples of source-limitation, e.g. photosynthesis is low in
the shaded parts of trees and the supply of carbohydrates in spring, from stor-
age, can be reduced by premature autumn defoliation (Abusrewil and Larsen,
).
An approximate measure of net carbon increment and its partitioning over
the season is given by changes in dry matter, more than
% of the dry matter
produced by the plant originating from photosynthesis by the leaves (Hansen,
shows the cumulative dry matter production of fruiting and
non-fruiting apple trees. Up to approximately the end of August (Northern
hemisphere) the total dry matter increment of the fruiting trees exceeded that
of the non-fruiting trees, i.e. the latter were sink-strength limited. Throughout
the season the dry matter increment of root and stem was much greater in
defruited than fruiting trees, showing the effects of the fruits, as the dominant
sink, on vegetative growth.
Relative sink strength is not, however, an intrinsic and unvarying property:
it is a consequence, not a primary cause, and varies with the time of season
and the physiological state of the tissues or organs. Shoots form terminal buds
and cease growth early in the season. Fruits, especially those of early maturing
cultivars, approach their final size well before cessation of leaf activity. Stem
thickening and root growth may occur much later and different patterns of
). Figure
.
