Lecture 12: Plant Nutrition
1. The chemical composition of plants provides clues to
their nutritional requirements
Every organism is an open system connected to
its environment by a continuous exchange of energy and materials.
In the energy flow and chemical cycling that
keep an ecosystem alive, plants and other photosynthetic autotrophs
perform the key step of transforming inorganic compounds into organic ones.
At the same time, a plant needs sunlight as its
energy source for photosynthesis and raw materials, such as CO2 and inorganic
ions, to synthesize organic molecules.
The root and shoot systems extensively network a
plant with its environment.
2. Plants require
nine macronutrients and at least eight micronutrients
Mineral
nutrients are essential chemical elements absorbed from soil in the form of
inorganic ions.
For
example, plants acquire nitrogen mainly in the form of nitrate ions (NO3-).
Yet, as
indicated by van Helmonts data, mineral nutrients
from the soil make only a small contribution to the overall mass of a plant.
About 80
- 85% of a herbaceous plant is water.
Because
water contributes most of the hydrogen ions and some of the oxygen atoms
incorporated into organic atoms, one can consider water a nutrient too.
However,
only a small fraction of the water entering a plant contributes to organic
molecules.
Over 90%
is lost by transpiration.
Most of
the water retained by a plant functions as a solvent, provides most of the mass
for cell elongation, and helps maintain the form of soft tissues by keeping
cells turgid.
By
weight, the bulk of the organic material of a plant is derived not from water
or soil minerals, but from the CO2 assimilated from the atmosphere.
Of the
15-20% of a herbaceous plant that is not water, about
95% of the dry weight is organic substances and the remaining 5% is inorganic
substances.
Most of
the organic material is carbohydrate, including cellulose in cell walls.
Thus,
carbon, hydrogen, and oxygen are the most abundant elements in the dry weight
of a plant.
Because
some organic molecules contain nitrogen, sulfur, and phosphorus, these elements
are also relatively abundant in plants.
3. The symptoms of
a mineral deficiency depend on the function and mobility of the element
4. Soil characteristics are key environmental factors in
terrestrial ecosystems
The
texture and chemical composition of soil are major factors determining what
kinds of plants can grow well in a particular location.
Plants
that grow naturally in a certain type of soil are adapted to its mineral
content and texture and are able to absorb water and extract essential
nutrients from that soil.
Plants,
in turn, affect the soil.
The
soil-plant interface is a critical component of the chemical cycles that
sustain terrestrial ecosystems.
5. Soil
conservation is one step toward sustainable agriculture
It takes centuries for a soil to become fertile
through the breakdown of soil and the accumulation of organic material.
However, human mismanagement can destroy soil
fertility within just a few years.
Soil mismanagement has been a recurring problem
in human history.
To understand soil conservation, we must begin
with the premise that agriculture is unnatural.
In natural ecosystems, mineral nutrients are
usually recycled by the decomposition of dead organic material.
In contrast, when we harvest a crop, essential
elements are diverted from the chemical cycles in that location.
In general, agriculture depletes minerals in the
soil.
To grow a ton of wheat, the soil gives up 18.2
kg of nitrogen, 3.6 kg of phosphorus, and 4.1 kg of potassium.
The fertility of the soil diminishes unless replaced by fertilizers, and most crops require far more water than the natural vegetation for that area.
6. The metabolism of soil bacteria makes nitrogen
available to plants
It is ironic that plants sometimes suffer
nitrogen deficiencies, for the atmospheres is nearly 80% nitrogen.
Plants cannot use nitrogen in the form of N2.
It must first be converted to ammonium (NH4+) or
nitrate (NO3-).
In the short term, the main source of nitrogen
is the decomposition of humus by microbes, including ammonifying bacteria.
All life on Earth depends on nitrogen fixation,
a process performed only by certain prokaryotes.
In the soil, these include several species of
free-living bacteria and several others that live in symbiotic relationships
with plants.
The reduction of N2 to NH3 is a complicated,
multi-step process, catalyzed by one enzyme complex, nitrogenase:
N2 + 8e- + 8H+ + 16ATP -> 2NH3 + H2 + 16ADP +
16Pi
Nitrogen-fixing bacteria are most abundant in
soils rich in organic materials, which provide fuels for cellular respiration
that supports this expensive metabolic process.
7. Symbiotic nitrogen fixation results from intricate
interactions between roots and bacteria
Many
plant families include species that form symbiotic relationships with
nitrogen-fixing bacteria.
This
provides their roots with a built-in source of fixed nitrogen for assimilation
into organic compounds.
Much of
the research on this symbiosis has focused the agriculturally important members
of the legume family, including peas, beans, soybeans, peanuts, alfalfa, and
clover.
8. Mycorrhizae are
symbiotic associations of roots and fungi that enhance plant nutrition
Mycorrhizae
(fungus roots) are modified roots, consisting of symbiotic associations of
fungi and roots.
The
symbiosis is mutualistic.
The
fungus benefits from a hospitable environment and a steady supply of sugar
donated by the host plant.
The
fungus increases the surface area for water uptake and selectively absorbs
phosphate and other minerals in the soil and supplies them to the plant.
The
fungi also secrete growth factors that stimulate roots to grow and branch.
The
fungi produce antibiotics that may help protect the plant from pathogenic
bacteria and pathogenic fungi in the soil.
Almost
all plant species produce mycorrhizae.
This
plant-fungus symbiosis may have been one of the evolutionary adaptations that
made it possible for plants to colonize land in the first place.
Fossilized
roots from some of the earliest land plants include mycorrhizae.
Mycorrhizal
fungi are more efficient at absorbing minerals than roots, which may have
helped nourish pioneering plants, especially in the nutrient poor soils present
when terrestrial ecosystems were young.
Today,
the first plants to become established on nutrient-poor soils are usually
heavily colonized with mycorrhizae
9. Mycorrhizae and
root nodules may have an evolutionary relationship
Mycorrhizae evolved very early, probably over
400 million years ago in the earliest vascular plants.
In contrast, the root nodules in legumes
originated only 65-150 million years ago, during the early evolution of
angiosperms.
The common molecular mechanism in the roots two
major symbiotic relationships suggests that root nodule development was at
least partly adapted from a signaling pathway that was already in place in
mycorrhizae.