Mineral nutrition

Question 1

Mineral nutrition of plants

Answer 1

1. Essential elements
2. Function in plants
3. Principles of mineral uptake: accumulation, selectivity and transport
4. Mycorrhiza
5. Nitrogen assimilation
   Living things are composed of the same types of chemical elements as non-living things
   Mechanistic view: life is chemically based and obeys universal physical and chemical laws
   Number of essential minerals required by plants:
   - Macronutrients = required in large amounts
   - E.g (obtained from water/carbon dioxide) H, C, O, (obtained from soil) N, K, Ca, Mg, P, S, Si
   - Micronutrients = required in trace amounts
   E.g (obtained from soil) Cl, Fe, B, Mn, Na, Zn, Cu, Ni, Mo

Question 2

Essential elements

Answer 2

Nitrogen = proteins, nucleic acids, chlorophyll, deficiency = chlorosis
Potassium = enzyme activator, concentrates in meristems, stomatal regulation, deficiency = chlorosis and brown tips
Calcium = middle lamella, movement of substances through cell membranes, deficiency = terminal bud dead, young leaves hooked and withered, roots die
Phosphorus = respiration, cell division, high energy compounds, deficiency = stunted growth, leaves darker green, lower leaves purple
Magnesium = chlorophyll, enzyme activator, deficiency = yellowing of leaves between veins
Sulphur = part of some amino acids (e.g cysteine and methionine), Fe-S proteins important in electron transfer, deficiency = chlorosis, dead spots, veins lighter in colour
Iron = respiration, chlorophyll formation, deficiency = large veins remain green but chlorosis in remainder
Manganese = enzyme activation, photolysis, deficiency = dead spots, veins remain green, chlorosis effects on young leaves
Many mineral nutrients (metals) are constituents of specific enzymes
Often one limiting nutrient
Uptake influenced by environmental conditions – soil pH, saturation
Can also be harmful in high amounts

Question 3

Uptake of inorganic nutrients (ion uptake)

Answer 3

Uptake takes place through epidermis of root and is usually an active process requiring expenditure of metabolic energy
Aided by mycorrhiza, particularly phosphorus uptake
Transport across root mainly along symplast pathway
Evidence that ions are loaded into xylem vessels
Mineral composition of roots different to soil (e.g higher conc of K+ in roots), so ion uptake is selective
Some ions are taken up against a chemical potential gradient which requires an input of energy – e.g active uptake
Nitrate and phosphate must be transport against an electrochemical gradient – both anions

Question 4

Transport of organic nutrients into shoot

Answer 4

Mineral nutrients transported via transpiration stream upwards in xylem, some move laterally from xylem into surrounding tissue of roots and shoots, others transported to leaves
Active transport systems supply specific cells with nutrients
Mg, Fe and N needed to build chlorophyll in leaves
Nutrients imported into leaves via the xylem may be exchanged with the phloem and translocated with sucrose (in assimilate stream)

Question 5

Mycorrhiza

Answer 5

Intimate, mutualistic and symbiotic association between root and fungus
Helps roots obtain mineral nutrients, fungus gets sugars in return
2 types – ectomycorrhiza, endomycorrhiza
Ecto = very important to forest trees (conifer and some broad-leaf), fugus forms a sheath (mantle) around the root and grows between cells to from Hartig net, form mushroom fruiting bodies
Endo = present on roots of most plants (80% species), fungus penetrates outer cortex ad makes intimate contact with the cell membrane but doesn’t enter protoplasm, phosphorus supplied to plants cells via the arbuscules
Orchid mycorrhizae
Orchid family has more than 20,000 species
All mycoheterotrophic at one stage, usually seeds
Seeds very small with no nutrient reserves
Fungal symbiont provides carbon during germination

Question 6

Nitrogen assimilation

Answer 6

Forms of nitrogen available to plants:
- Nitrate NO3^- = important in many ecosystems and for crops
- Ammonium NH4^+ = in acidic soils
- Nitrogen N2 = only plants that have symbiotic relationship with N fixing prokaryotes
- Organic nitrogen = parasites, carnivorous plants

Question 7

Nitrate assimialtion

Answer 7

Requires energy and reducing power
Negative ion taken up against electrochemical gradient

Question 8

Nitrogen assimilation

Answer 8

Root symbiosis – legumes, alder, bog myrtle, mountain avens = rhizobium bacteira and actinomycete
Leaf symbiosis – gunnera, some ferns, some liverworts = cyanobacteria
Energy requiring process catalysed by nitrogenase
Nitrogenase is large protein complex associated with the prokaryote partner, contains Mo and Fe and catalyses reaction N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16 ADP + 16 Pi
Requires lots of energy supplied by plant, reduced ferredoxin, anaerobic conditions
In legumes, nitrogenase activity inhibited by o2, nodule cells make leghaemoglobin which binds o2, rhizobium forms an infection thread through root hair, root cells undergo mitosis to form nodule

Question 9

Nitrogen pollution

Answer 9

Limits plant growth
Human activity increases n emission and deposition
Short term acts as fertiliser, long term negative effects
Disrupts natural cycle C:N rations
Bryophytes most sensitive
Eutrophication and acid rain
Favours growth of invasive species
Phase shifts of plant communities
Global problem