Mineral Nutrition (1-3)

Question 1

By what is the ion-uptake of higher plants characterized?

Answer 1

1. Selectivity: Certain mineral elements are taken up preferentially, while others are discriminated against or almost excluded
2. Accumulation: The concentration of elements can be much higher in cellsap than in the external solution
3. Genotype: There are distinct differences between plant species in their ion uptake characteristics

Question 2

What is Suberin?

Answer 2

1. Suberin is present on underground parts, woody stems, and healed wounds
2. Suberin is a polymer whose structure is very poorly understood.
3. Like cutin, suberin is formed from hydroxy or epoxy fatty acids joined by ester linkages
4. It is presence in the Casparian strip of the root endodermis, which forms a barrier between the apoplast of the cortex and the stele

Question 3

What type of membrane transport proteins are there?

Answer 3

1. Channel proteins act as membrane pores, and their specificity is determined primarily by the biophysical properties of the channel.
2. Carrier proteins bind the transported molecule on one side of the membrane and release it on the other side.
3. Primary active transport is carried out by pumps and uses energy directly, usually from ATP hydrolysis, to pump solutes against their gradient of electrochemical potential.

→Channels and Carriers can mediate the passive transport of solutes across membranes (by simple diffusion or facilitated diffusion), down the solute’s gradient of electrochemical potential.

Question 4

Please explain the nomenclature of transport proteins?

Answer 4

Question 5

What are the driving forces behnid the transport?

Answer 5

• the driving force for the facilitated diffusion of an uncharged solute across a membrane is its concentration gradient
• whereas for an ion it is its electrochemical gradient

Question 6

What is the Nerst equation?

Answer 6

• The Nernst equation allows the direction of the net diffusive flux of an ion at a given membrane potential and temperature to be determined

Question 7

How high is the energy demand for solute transport in plants?

Answer 7

Question 8

Why is the concept of one protein carrier per ion to simple?

Answer 8

1. The original concept of a single protein-mediated mechanism of ion transport (one carrier system for each ion) did not sufficiently describe the kinetics of when wide concentration ranges were tested
2. At concentrations above 1 mM, for example, the kinetics of K uptake differ considerably from those at lower concentrations

Question 9

What is C min?

Answer 9

1. Cmin defines the concentration at which net uptake of ions ceases before the ions are completely depleted
2. Cmin is the lowest concentration at which roots can extract an ion from the soil solution
3. Cmin for nitrate can vary from between more than 50 AM to less than 1 JuM depending not only on the plant species but also on the environmental conditions

Question 10

How does the pH of the external solution influence the uptake of nutrients by plant roots?

Answer 10

1. effects of solution pH on the chemical species present in solution itself
2. effects the apoplasmic pH and so the concentrations of ions present in the apoplasm
3. influences the rhizosphere pH for the proton electrochemical gradient and the driving force for proton-coupled solute transport
4. solution pH can affect ion transport by protonation/deprotonation of amino-acid residues of transport proteins

Question 11

How does the pH of the external solution ifluence for example the borum (B) uptake?

Answer 11

1. The rate of B uptake decreases strongly when the pH of the external solution is increased
2. This closely related to the decrease in the ratio of boric acid, which is the substrate of the transporter catalysing boron uptake by root cells, to the borate anion

Question 12

What other factors besides the pH affect the ion uptake of the roots?

Answer 12

Main source of energy in non-photosynthesizing cells and tissues (including roots) is respiration. Thus, all factors that affect respiration can influence ion accumulation

1. Temperature
2. Oxygen: As oxygen lowered, the uptake of ions such as K and P is decreased
3. Carbohydrates: carbohydrate concentration, respiration and N uptake decrease within a few hours after excising roots which cuts off the photosynthate supply from the shoot

Question 13

Please explain the concept of competition in the context of ion uptake!

Answer 13

1. the transporters catalysing ion uptake are rarely specific and ions can compete directly for transport
2. competition is influenced by the properties of the transporter itself and by the concentrations of differentions in solution
3. competition occurs particularly between ions with similar physicochemical properties (valency and ion diameter),

Question 14

Describe the competition between potassium (K) and ammonium (NH₄)?

Answer 14

1. NH₄ is quite effective in inhibiting K influx, the reverse (inhibition of NH₄ uptake by K) is rarely observed
2. Ammonium competes with K for transport through both AtAKT1 and AtHAK5 and also reduces the expression of AtHAK5,
3. whereas K. does not appear to affect the expression or activity of the major ammonium transporter
4. A substantial proportion of ammonium may not be taken up as NH₄ but as NH₃

Question 15

What potential effect does the the application of K and Ca fertilizers have?

Answer 15

1. Might induce Mg deficiency in crop plants
2. This is because the Kations of K⁺ and Ca²⁺ inhibit the uptake of Mg²⁺ by the plant roots

Question 16

Please describe the competition of sulphate and molybdate!

Answer 16

1. Sulphate and molybdate are thought to enter root cells through the same proton-coupled symporters
2. Increasing the sulphate concentration in the rooting medium reduces molybdate uptake strongly
3. In soils containing high concentrations of Mo, S fertilization may be an effective tool to reduce excessive Mo uptake
4. the competition may become a critical factor in soils containing little Mo

Question 17

Please describe the competiton between Sulphate and selenate?

Answer 17

1. Sulphate and selenate are taken up into root cells through the same proton-coupled symporters
2. increasing sulphate concentration in the substrate strongly decreases Selenate uptake by roots and selenium accumulation by plants
3. increasing selenate concentration in the substrate often increases sulphate uptake and accumulation by plants

Question 18

Please describe the competition between chloride and nitrate?

Answer 18

1. Chloride concentrations in plant tissues, particularly in roots, can be reduced strongly by increasing nitrate availability
2. This isused to decrease the nitrate content of plant species such as spinach which tend to accumulate large amounts of nitrate in saline soils
3. the competing effect of chloride on nitrate uptake may impair N-uptake by the plants

Question 19

Please explain the competition between Nitrate and Ammonium?

Answer 19

1. Increasing the availability of ammonium strongly suppresses nitrate uptake
2. Increasing nitrate supply generally has little or no effect on ammonium uptake
3. When nitrogen is supplied as NH₄NO₃, Ammonium is taken up in prefernce to Nitrate

Question 20

What are the effects of extracellular Calcium (Ca²⁺)?

Answer 20

1. stimulation of cation and anion uptake by extracellular Ca²⁺ at low rhizosphere pH
2. Ca²⁺ counteracts the negative effects of high H⁺ concentrations
3. the presence of Ca²⁺ in the rhizosphere solution shifts the uptake ratio in favor of K⁺ at the expense of Na⁺(particularly important in saline environments)

Question 21

What are Cation-Anion Relationships and how are they influenced by the overall nutrient uptake?

Answer 21

• The uptake of one nutrient can influence the uptake of another indirectly through effects on

1. the membrane potential,
2. the proton electrochemical gradient
3. via feedback regulation through plant growth or metabolism

Question 22

What is a fundamental difference in the assimilation of NH⁴⁺ and NO^3- and how does this influence the Cation-Anion relationship?

Answer 22

1. assimilation of NH⁴⁺ produces protons
2. assimilation of NO^3- is correlated with an equirmolar consumption of H⁺

• Nitrogen nutrition has a strong effect on c-a relationship →about 70% of the cations and anions taken up by plants are either NH⁴⁺ or NO^3-
• Plants supplied NH⁴⁺ are generally characterized by a high cation/anion uptake ratio ⇔ plants supplied NO^3- by a low cation/anion uptake ratio

Question 23

How do solutes enter the xylem?

Answer 23

1. Through ion channels or uniporters, if their electrochemical gradients allows it
2. Their transport is coupled to the proton electrochemical gradient generated by the plasma membrane H⁺-ATPase or directly to ATP hydrolysis
3. The key role of the H⁺-ATPases in the plasma membrane of parenchyma cells in xylem loading is now well established

→H⁺ are pumped into the xylem both to generate a negative membrane potential and to acidify the xylem sap

Question 24

How does the phloem flow differ from the xylem flow?

Answer 24

1. flow rate and direction of flow are closely related to phloem unloading at the sink
2. organic compounds are the dominant solutes in the phloem sap
3. transport takes place in living cells (sieve tubes)
4. the unloading of solutes at the sink plays an important role
5. phloem pH 7-8⇔xylem pH 5-7

Question 25

How is the phloem mobility for different nurtrients?

Answer 25

1. for macronutrients it is generally cosidered high except for Ca
2. for micro nutrients it is intermediate with the exception of Mn
3. Most of the Ca demand of growth sinks is covered by the Xylem

Question 26

What are the main problems associated with foliar fertilization?

Answer 26

1. Limited leaf wetting and spreading of the applied nutrient solution when treating hydrophobic leaves and when applying unformulated sprays (i.e., in absence of adjuvants)
2. Spray run-off due to low solution retention rates
3. Washing off by rain following the application of nutrient sprays
4. Low penetration rates of the applied nutrient solution due to, for example, the nature of the leaf surface treated or to the effect of environmental conditions on solution physico-chemistry and plant performance (e.g., stomata opening or closure)
5. Rapid drying of spray solutions, particularly at low relative humidity and high temperature
6. Limited rates of distribution of certain nutrients such as Ca or Fe, from the sites of foliar uptake to other plant parts due to low phloem mobility
7. Limited amounts of foliar-applied macronutrients to meet the plant demand that can be supplied by one foliar spray without inducing phytotoxicity (exception being urea)
8. Leaf damage (necrosis, burning)

Question 27

How has the perception and usage of foliar fertilization changed over the years?

Answer 27

1. Traditionally, foliar fertilization was used to correct nutrient deficiencies however, there is an increasing trend to using nutrient sprays in the absence of deficiency symptoms,
2. particularly for elements with limited phloem mobility such as Ca, B, Zn, Fe or Mn

Question 28

Under which coditions is Foliar fertilization particulary advisable?

Answer 28

1. Low Nutrient Availability in Soils
2. Decrease in Root Activity during the Reproductive Stage
3. Increasing the Protein Content of Cereal Grains
4. Avoiding the Occurrence of Physiological Disorders in Horticulture
5. Bio-fortification