otic. If permeability is too low, very small amounts of the material will enter the sieve tubes. If perme-
ability is too high, much xenobiotic would enter the sieve tubes but would leak out rapidly once it moved
away from the supply region. If the supply region consists of leaves, the material would be carried back
into the leaves in xylem; therefore, the xenobiotic would never reach the sink (target). Figure 2 provides
models of movement of two materials of different membrane permeabilities. Kleier [307] combined ion
trapping and permeability models to predict the mobility of xenoblotics through the phloem.
For all of these models, the required low membrane permeability of xenobiotics (for movement
through phloem) results in an accumulation of the xenobiotic in the sink region within sieve tubes (see
Figure 2). This buildup results from continuous flow of solution into the sink that is driven by unloading
of osmotically active materials and water following the movement of osmoticum. Because some leakage
of xenobiotic does occur, its accumulation in sink phloem results in significant delivery of xenobiotic into
sink cells.
It seems reasonable to develop pesticides that attach to sites used by materials normally loaded into
and unloaded from phloem. These xenobiotics would then use the loading and unloading mechanisms of
metabolites. Other than auxin herbicides, to date, only glyphosate has been reported to use a phosphate
transporter [308], and this use occurred not by design but by serendipity.
VIII. EXTRATERRESTRIAL AGRICULTURE
If humans are to inhabit a lunar base or travel to Mars, their needs must be met, at least in part, as they are
on Earth, by plants. Interest in this topic is illustrated by support that the National Aeronautics and Space
Administration (NASA) has provided for research (see, e.g., Refs. 309–313 and references therein).
If food is to be provided most effectively for space travelers, a high HI is needed. A NASA-funded
project on wheat is illustrative. In an attempt to achieve the highest possible food and O 2 production by
wheat, temperature at or near the optimum for photosynthesis was used, together with high irradiance. An
HI of 0.25 was obtained, but when the temperature was lowered, an HI of nearly 0.50 was obtained (FB
Salisbury, personal communication). It is also important that both volume and energy be used efficiently.
To get maximum production per unit volume, a high irradiance level must be provided. However, as ir-
radiance level was increased, efficiency of light utilization in photosynthesis fell [309]. Therefore, a sat-
isfactory irradiance-volume compromise must be developed. This need for efficiency of volume utiliza-
tion drove the development of a “super dwarf” (25 cm) variety of wheat [313]. Under controlled
conditions an HI of 0.52, greater than obtained with semidwarf varieties, was obtained.
If NASA’s goals of human exploration of deep space are to be attained, methods of providing a crew
with plant-produced food, O 2 , and pure water from transpiration, as well as recycling nutrients through
plants and removal of CO 2 , must be developed in a way that most effectively uses both volume and
energy.
IX. SUMMARY
Understanding the mechanisms of assimilate transport is useful or even essential if crop inputs, such as
pesticides, fertilizer, and water, are to be used economically. Understanding these processes is also help-
ful in developing the most effective use of both traditional and molecular genetics in enhancing crop
production.
Assimilate partitioning is the most critical component of translocation processes that determine eco-
nomic productivity. When the economic product is the result of sexual reproduction, establishment of
sinks is the critical step in productivity, for it is during floral development that maximum yield potential
is established. Once established, that potential might be lowered but it cannot be increased. Genetic and
environmental factors interact to limit yield. It appears that sink formation is the limiting component of
crop production in many situations, for experimental evidence supports the proposition that during
grain/seed filling, the systems are sink limited, even to the extent that there is a feedback of assimilate on
the rate of photosynthesis. For crops in which the economic product is vegetative, it appears that genetic
and environmental determinants affect the establishment of sinks; once sinks are established, however,
their growth is more likely to be source limited.
Over the last several decades, the mechanism of long-distance translocation of assimilates through
sieve tubes has been established as an osmotically driven pressure flow system. Several different nonre-
PRODUCTION-RELATED ASSIMILATE TRANSPORT 441