riod [78,79]. In addition, the source must be able to accumulate enough reserve carbon to allow for envi-
ronmental conditions (clouds, water stress, temperature fluctuations) that may interfere with photosyn-
thetic processes even in the light [80]. How the plant decides which carbon is destined for export and
which is to be stored is not fully understood.
Many tightly regulated metabolic steps control the accessibility of photosynthetically fixed carbon
to the phloem transport system. Control at the source end is governed largely by rates of photosynthetic
incorporation of CO 2 , but for photosynthetic rate to have any direct effect on the rate of phloem transport,
the carbon must be fixed into phloem-mobile intermediates (predominantly sucrose in most agronomi-
cally important crops). The flow of carbon into soluble sugars, which are synthesized in the cytoplasm of
the photosynthetic cell, is regulated by complex biochemical interactions, which direct the export of fixed
carbon out of the chloroplast [81]. Carbon not released from the chloroplast is retained as insoluble starch
and will not be immediately available for phloem transport [82]. In addition, once synthesized, soluble
sugars can be siphoned off into the vacuole for storage, and this carbon also would not be available for
phloem transport [78].
The phloem loading process, which establishes the high solute level in the phloem, must therefore
compete with storage processes also occurring in the chloroplast and vacuole, which can divert substan-
tial amounts of photosynthetically fixed carbon from the phloem loading site. Control of phloem trans-
port by source tissues is therefore exerted largely by control of the availability of phloem-mobile solutes
and not directly by the rate of photosynthesis per se. Indeed, environmental factors that reduce rates of
photosynthesis do not necessarily result directly in lowered rates of phloem transport. This is because
stored carbon, either in the source tissues or in storage tissues along the pathway, can be mobilized to
maintain the high solute levels in the phloem [78].
One key contribution of source leaf metabolism, arising from the combination of photosynthetic ac-
tivity and membrane transport between cellular compartments, is therefore the control of amounts and
probably the types (sugars or amino acids) of assimilates that have access to the loading sites. Source leaf
metabolism, therefore, directly regulates the overall composition of the phloem sap [34], including both
organic and inorganic constituents. In general, though, the set point for rates of phloem transport is es-
tablished in the sink tissues, where these nutrients are utilized.
B. Regulation by Sinks
A typical higher plant has a myriad of sink tissues that depend on the source leaves for photoassimilates.
Reproductive sinks (flowers, seeds, fruits) are of prime agronomic importance, and as a result most stud-
ies of sink regulation of phloem transport have tended to focus on carbon partitioning to these sinks. How-
ever, reproductive sinks represent only a small proportion of potential sinks on a plant, and we are now
beginning to realize that during the growth period, carbon partitioning to other sinks, particularly tempo-
rary vegetative sinks, can be important in determining final crop yield.
- Vegetative (“Buffering”) Sinks
During the translocation process, carbon is continuously diverted from the phloem to surrounding
parenchyma cells for temporary storage. Parenchyma tissues of leaves, petioles, stems, and roots can all
act as sinks for assimilates, which are usually stored in the form of starch. These stored reserves can be
drawn on and reloaded into the phloem under conditions of reduced photosynthesis [78–80,82] (e.g., dur-
ing adverse environmental conditions) or when sink demand increases (e.g., during the reproductive
phase of plant growth) [83]. An amplified version of this type of sink activity is seen in perenniating or-
gans such as tubers and taproots and also in ray cells of woody species, in which large amounts of carbon
are diverted to storage to allow for regrowth of vegetative tissues in the next growing season. The phe-
nomenon of alternate bearing in perennial tree crops may also reflect this type of sink activity: that is, car-
bon diverted to vegetative storage sinks in nonbearing years may be utilized for crop production in the
subsequent bearing year.
The vegetative “buffering” sinks, therefore, have the unique property of being able to act both as
sinks for assimilates and as sources of assimilates for phloem transport, depending on the carbon needs
of the plant at a particular growth phase or under the prevailing environmental conditions. Sinks of these
types can, therefore, regulate phloem transport by coarse control of the assimilates available to the sieve
elements along the phloem transport path.
PHLOEM TRANSPORT OF SOLUTES IN CROP PLANTS 461