Geoscience Reference
In-Depth Information
There are about 70 biogenic elements in plants bodies. With at least nine of them
being present in substantial, easily detected amounts (C, H, O, P, K, N, S, Ca, Mg),
they are known as macroelements. Among these important elements, carbon enjoys
a special position since it is a part of our Earth's countless natural cycles. Except for
C, H, and O, the other six macroelements belong to the products of two basically
different processes: weathering of minerals and decomposition of plant residues.
They participate in the completion of photosynthetic products with their content
within 1 kg of dry plant matter usually being more than 1 g. The remaining biogenic
elements occur in concentrations less than 100 mg in 1 kg of dry matter. Because of
their tiny amounts found in plants, they are denoted sometimes as trace elements or
more precisely as microelements. Among these relatively scarce elements, those
manifesting somewhat larger concentrations between 100 and less than 0.1 mg/kg
of dry plant matter are Cl, Fe, B, Mn, Zn, Cu, Mo, and Ni, respectively. Such tiny
concentrations enable and provide rates of biochemical reactions essential for the
production of special types of compounds. The plants accept macro- and microele-
ments dominantly in the form of water solutions through their fi ne hair-size roots.
The acceptance of these elements in the gas phase is not frequent. Fine hair-size
roots are short, have lengths of no more than a few millimeters, and live only for
several days. They accept the elements as water solutions under the driving force of
the gradient of total water potential.
The root system is important for withdrawing nutrients from the soil into the
plant. However, an extensive root system does not mean that the plant is more effec-
tively supplied by nutrients - it is merely a prerequisite assuring that nutrients will
be absorbed provided that all factors decisive for soil fertility are optimal. On the
other hand, under restricted nutrient supplies, a larger root system may allow a more
effective “pumping” of nutrients into the plant. Generally, essential nutrient sup-
plies are continually adjusted by specifi c nutrient demands from the crop.
11.1
Photosynthesis
Photosynthesis is the conversion of energy from the sun into sugars and generally
into a plant's body. It is chemical synthesis that is initially conditioned by sunlight.
Energy gained by absorption of the sunlight in chlorophyll is used for the synthesis
of organic compounds from such simple substances as carbon dioxide and water.
Chlorophyll is the green pigment dominant in leaves that is uniquely specialized to
absorb light. The gained energy is fi rst utilized in photolysis when water molecules
are split into gaseous oxygen and hydrogen ions. The process is accompanied by the
release of electrons that move in the electron transport chain that is also well known
as the respiratory chain in the fi nal stage of aerobic respiration (Figs.
11.1
and
11.2
).
Within the chloroplast there is an enzyme that enables the synthesis of adenosine
triphosphate - an adenosine molecule with three inorganic phosphates. Briefl y, it is
called ATP enzyme. The synthesized ATP preserves and protects energy up to the
times when energy is needed. At those times, releases of the needed energy are
