soil particles
- Sand: 0.02 - 2 mm diameter
- Silt: 0.002 - 0.02 mm
- Clay: < 0.002 mm
humus
- Remains of dead organisms and other organic matter.
- Prevents clay particles from packing together and forms a crumbly soil that retains water but is still porous enough to aerate roots adequately.
- Increases the soil’s capacity to exchange cations and serves as a reservoir of mineral nutrients that return gradually to the soil as microorganisms decompose.
topsoil
Mineral particles released by weathering
+
humus
pores
- In topsoil, plants are nourished by the soil solution, the water and dissolved minerals in the pores btwn soil particles.
- Pores also contain air pockets. After heavy rainfall, water drains away from the larger spaces in the soil, but smaller spaces retain water b/c water molecules are attractive to the negatively charged surfaces of clay and other soil particles.
loam
- Enough small silt and clay particles to provide ample SA for adhesion and retention of minerals and water.
- Large spaces btwn sand particles enable efficient diffusion of oxygen to the roots.
- The most fertile topsoils have pores that are about 1/2 water and 1/2 air.
cation exchange
- Roots acidify the soil sol’n by releasing CO2 from respiration and pumping H+ into soil.
- CO2 + H2O –> H2CO3, which releases H+ upon dissociation.
- H+ ions in soil sol’n neutralize the negative charge of soil particles, causing release of mineral cations into the soil sol’n.
- Roots absorb cations.
- A soil’s capacity to exchange cations is determined by the number of cation adhesion sites and by the soil’s pH.
hydroponic culture
When plants are grown in mineral solutions instead of soil.
cytochromes
The proteins in the e- transport chains of chloroplasts and mitochondria.
most common deficiencies in plants
- Macronutrients, not micronutrients.
- Phosphorus, potassium, and ESP nitrogen.
chlorosis
- Yellowing of the leaves. Caused by:
- Magnesium deficiency.
- Iron deficiency; even though chlorophyll contains no iron, iron ions are required as a cofactor in one of the enzymatic steps of chlorophyll synthesis.
mineral mobility
- If a nutrient moves about freely, symptoms of deficiency appear first in older organs b/c young, growing tissues are a greater sink for nutrients that are in short supply. The mechanism for preferential routing is the source-to-sink translocation in phloem, as minerals move along w/ sugars to the growing tissues.
- Vice versa b/c older tissues may have adequate amounts that they retain during periods of short supply.
resistance to aluminum toxicity
Secretion of organic acids (ex: malic, citric acid) by roots. These acids bind to free Al ions and lower the levels of toxic Al in the soil.
why’s flooding bad for plants
- Deprives roots of oxygen
- Can injure plants as ethanol and other toxic products of alcoholic fermentation by the plant accumulate.
gene for flood-resistant rice
Submergence 1A-1
- The protein regulates the expression of genes that are normally activated under anaerobic conditions, such as those that code for alcohol dehydrogenase, an enzyme that breaks down ethanol.
- The heightened expression of Sub1A-1 in flooding-intolerant varieties of rice increases the alcohol dehydrogenase levels of the plants and confers tolerance to submergence.
one type of ‘smart’ plant
Takes advantage of a promoter (a DNA sequence indicating where transcription of a gene starts) that more readily binds RNA polymerase (the transcription enzyme) when the phosphorus content of the plant’s tissues begins to decline. This promoter is linked to a “reporter” gene that leads to production of a light blue pigment in the leaf cells.
rhizosphere
- rhizobacteria are soil bacteria with especially large populations in the rhizosphere, the soil layer that surrounds the plant’s roots.
- microbial activity within a plant’s rhizosphere is 10 to 100x higher than in nearby soil bc the roots secrete nutrients such as sugars, amino acids, organic acids.
