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CHAPTER 37

PLANT NUTRITION

OUTLINE

I. NUTRITIONAL REQUIREMENTS OF PLANTS

A. THE CHEMICAL COMPOSITION OF PLANTS PROVIDES CLUES TO NUTRITIONAL REQUIREMENTS

  • PLANTS DERIVE MOST OF ORGANIC MASS FROM THE C02 OF AIR, BUT THEY ALSO DEPEND ON SOIL NUTRIENTS IN THE FORM OF WATER AND MINERALS.

B. PLANTS REQUIRE NINE MACRONUTRIENTS AND AT LEAST EIGHT MICRONUTRIENTS

  • MACRONUTRIENTS INCLUDE C, H, 0, N, AND OTHER MAJOR INGREDIENTS OF ORGANIC COMPOUNDS.

  • MANY MICRONUTRIENTS HAVE CATALYTIC FUNCTIONS AS COFACTORS OF ENZVMES.

C. THE SYMPTOMS OF MINERAL DEFICIENCY DEPEND ON THE FUNCTION AND MOBILITY OF THE

ELEMENT

  • DEFICIENCY OF A MOBILE NUTRIENT USUALLY AFFECTS OLDER ORGANS MORE THAN YOUNGER ONES.
  • THE REVERSE IS TRUE FOR NUTRIENTS THAT ARE LESS MOBILE WITHIN A PLANT.

II. SOIL

A. SOIL CHARACTERISTICS ARE KEY ENVIRONMENTAL FACTORS OF TERRESTRIAL ECOSYSTEMS

  • VARIOUS SIZES OF PARTICLES DERIVED FROM THE BREAKDOWN OF ROCK ARE FOUND IN SOIL, ALONG ORGANIC MATERIAL (HUMUS) IN VARIOUS STAGES OF DECOMPOSITION.
  • ACIDS DERIVED FROM ROOTS CONTRIBUTE TO PLANTS' UPTAKE OF MINERALS WHEN H+ DISPLACES MINERAL CATIONS FROM CLAY PARTICLES.

B. SOIL CONSERVATION IS ONE STEP TOWARD SUSTAINABLE AGRICULTURE

  • IN CONTRAST TO NATURAL ECOSYSTEMS, AGRICULTURE DEPLETES THE MINERAL CONTENT OF SOIL, TAXES WATER RESERVES, AND ENCOURAGES EROSION.
  • THE GOAL OF SOIL CONSERVATION STRATEGIES IS TO MINIMIZE THIS DAMAGE.

III. THE SPECIAL CASE OF NITROGEN AS A PLANT NUTRIENT

A. THE METABOLISM OF SOIL BACTERIA MAKES NITROGEN AVAILABLE TO PLANTS

  • NITROGEN-FIXING BACTERIA CONVERT ATMOSPHERIC N2 TO NITROGENOUS MINERALS THAT PLANTS CAN ABSORB AS A NITROGEN SOURCE FOR ORGANIC SYNTHESIS.

B. IMPROVING THE PROTEIN YIELD OF CROPS IS A MAJOR GOAL OF AGRICULTURAL RESEARCH

  • SUCH RESEARCH ADDRESSES THE MOST WIDESPREAD FORM OF HUMAN MALNUTRITION: PROTEIN DEFICIENCY.

IV. NUTRITIONAL ADAPTATIONS: SYMBIOSIS OF PLANTS AND SOIL MICROBES

  1. SYMBIOTIC NITROGEN FIXATION RESULTS FROM INTRICATE INTERACTIONS BETWEEN ROOTS

AND BACTERIA

  • THE DEVELOPMENT OF NITROGEN-FIXING ROOT NODULES DEPENDS ON CHEMICAL CROSS-TALK BETWEEN RHIZOBIUM BACTERIA AND ROOT CELLS OF THEIR SPECIFIC PLANT HOSTS.
  • THE BACTERIA OF A NODULE OBTAIN SUGAR FROM THE PLANT AND SUPPLY THE PLANT WITH FIXED NITROGEN.
  1. MYCORRHIZAE ARE SYMBIOTIC ASSOCIATIONS OF ROOTS AND FUNGI THAT ENHANCE PLANT

NUTRITION

  • THE FUNGAL HYPHAE OF BOTH ECTOMYCORRHIZAE AND ENDONRYCORRHIZAE ABSORB WATER AND MINERALS, WHICH THEY SUPPLY TO THEIR PLANT HOSTS.
  1. MYCORRHIZAE AND ROOT NODULES MAY HAVE AN EVOLUTIONARY RELATIONSHIP
  2. THERE IS EVIDENCE THAT ROOT NODULE DEVELOPMENT DEPENDS ON MOLECULAR MECHANISMS OF SIGNALING AND ROOT-CELL RESPONSES THAT EVOLVED FIRST IN MYCORRHIZAE.

V. NUTRITIONAL ADAPTATIONS: PARASITISM AND PREDATION BY PLANTS

A. PARASITIC PLANTS EXTRACT NUTRIENTS FROM OTHER PLANTS

  • THEY DO SO EITHER DIRECTLY BY TAPPING INTO THE HOST'S VASCULAR TISSUE OR INDIRECTLY VIA MYCORRHIZAE.

B. CARNIVOROUS PLANTS SUPPLEMENT THEIR MINERAL NUTRITION BY PREYING ON ANIMALS

  • THIS PREDATION IS MOST COMMON IN ECOSYSTEMS WITH NUTRIENT-POOR SOIL.