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.