The Plant - Agroecology: The Ecology of Sustainable Food Systems - page 38

paramount importance in agriculture. Humans select plants
that shunt more photosynthate to the part of the crop that
is to be harvested, at the expense of other plant parts.
The harvestable or harvested portion of most crop
plants usually has limited photosynthetic capacity itself,
hence yields depend a great deal on carbohydrate that is
transported through phloem cells from photosynthetically
active parts of the plants to the harvestable parts
(Figure 3.2).
In ecological terms, we often refer to carbon partition-
ing as a “source, path, and sink” phenomena. The source
is usually the leaf, the chloroplasts in particular. Much
detailed research has been done on the physiology and
biochemistry of the actual transfer of carbon out of the
chloroplast and into transport paths. A complex set of
chemical locators and enzymes are active in this process.
Once in the phloem, carbon then moves through the stem
to grain, flowers, fruits, tubers, or other parts, which are the
sinks. At this point there is phloem “unloading” and sink
uptake. The actual transfer from vascular strands to sink
tissue is often based on a sugar concentration gradient.
The products of photosynthesis are compounds of car-
bon, oxygen, and hydrogen that make up an average of 90%
of plant dry matter. Therefore, there is a close relationship
between whole plant photosynthesis and whole plant pro-
ductivity. Overall photosynthetic rates are related to rates
per unit leaf area, as well as to the production of new leaf
area, but they are also dependent on the rate of transfer from
source to sink. Carbon is kept in the area of leaf develop-
ment while new leaves are forming; only after all leaves are
formed can the transfer to other sinks take place. After the
canopy closes, crop photosynthesis and growth depend
mainly on net CO 2 fixation per unit leaf area.
Over the growing season, the various sinks of the plant
compete with each other for the supply of fixed carbon
produced by the leaves, with the result that some parts of
the plant accumulate more biomass than others. The mech-
anisms regulating this partitioning of photosynthate within
the plant are not well understood, though it is clear that the
process is dynamic and related to both environmental con-
ditions and the genetically determined developmental pat-
terns of the plant. Ways of modifying carbon partitioning
in crop plants are being explored by researchers; one exam-
ple involves the development of perennial grain crops,
where the challenge is to balance the partitioning of carbon
between the vegetative body of the perennial plant (espe-
cially the roots and stems) and the grain.

Source

Path

Sink

FIGURE 3.2 Carbon partitioning.

and oxygen make up approximately 95% of the average
plant's fresh weight.
The elements that make up the other 5% of living plant
matter must come from somewhere else — namely the
soil. These elements are plants' essential nutrients. They
are needed to form the structures of the plant, the nucleic
acids directing various plant processes, and the enzymes
and catalysts regulating plant metabolism. They also help
maintain internal osmotic balance and have a role in the
absorption of ions from the soil solution. If an essential
nutrient is not available in adequate supply, the plant suf-
fers and does not develop properly. In agriculture we have
learned how to adjust the supply of these nutrients in the
soil to meet the needs of our crops.
The three nutrients that are required in relatively large
amounts, and have played such important roles as inorganic
fertilizers in agriculture, are nitrogen, phosphorus, and
potassium. These are classified as macronutrients. Plants
vary in the actual amounts of these nutrients they require.
Since each plant variety has become adapted to different
habitats with different environmental conditions, it makes
sense for there to be such variation in nutrient requirements.
A review of some of this nutritional variation can tell us a
lot about proper crop selection and fertility management.

Nitrogen

Nitrogen is needed in large amounts by plants, but at the
same time is the most universally deficient nutrient. It
occurs in every amino acid, and as a result is a major
component of proteins. Nitrogen is therefore involved in
some way with up to 50% of dry plant biomass. It is
required in enzyme synthesis, with a deficiency affecting
almost every enzymatic reaction. Since nitrogen forms
part of chlorophyll and is required in its synthesis, it is no
wonder that nitrogen-deficient plants show the yellowing
that is indicative of limiting amounts of this nutrient in
the soil. Adequate supplies of nitrogen are also needed for
normal flowering and fruit set in all plant species. Plants
commonly have 1 to 2% nitrogen as a proportion of dry
weight, but contents above 5% are not uncommon.

N UTRITIONAL N EEDS

Photosynthesis provides a plant with a large portion of its
nutritional needs — energy, and carbon and oxygen for
building important structural and functional compounds.
Together with hydrogen — derived from the water that
enters plant roots as a result of transpiration — carbon