Nutrition and transport in plants

Plant macronutrients Nitrogen - nucleic acids, proteins, coenzymes Sulphur - proteins, coenzymes Phosphorus - nucleic acids, phospholipids, coenzymes, ATP Potassium - water balance, stomatal opening, protein synthesis Calcium - stability of walls and membranes, regulates many plant responses Magnesium - component of chlorophyll, activates many enzymes

Micronutrients - mainly cofactors Chlorine (ionic balancing) iron - cytochromes boron Manganese Zinc copper molybdenum Nickel The most usual symptom for deficiency of nutrients is chlorosis (leaves go yellow)

Water Makes up 95% of the weight of a cell. Plants lose a lot of water - around 300 litres for every Kg of carbon fixed. Plants must take up this water without taking up high concentrations of soil minerals - exclusion and selectivity Plants must have mechanisms to transport water efficiently up to 100m.

Water potential Water potential (  )is the force that drives water movement through the plant and into the atmosphere. Pressure increases   p = pressure potential Solutes decrease   s  = solute potential water moves down the gradient of its potential.

A demonstration of water potential Manometer split into 2 with a semi-permeable membrane 0.1molar solution Pure water H20H20  p = 0  s = -0.23Mpa  = -0.23Mpa 1Mpa = 10 atm = 10kg/cm Car tyre typically pumped up to 0.2Mpa

Water relations of cells Flaccid cell 0.4m sucrose Pure water Turgid cell  s 0.4m sucrose = -0.9 Mpa  =  soln (-0.9 Mpa) -  cell (-0.7Mpa) -  p (0Mpa) = -0.2Mpa Water flows out of the cell into the solution until  cell =  soln Cell becomes plasmolysed  =  soln (-0 Mpa) -  cell (-0.7Mpa) -  p (0Mpa) = 0.7Mpa Water flows into the cell from the solution until  p =  cell  cell = -0.7Mpa  p= -0Mpa

Ion selectivity Plants must take up ions selectively They do this by having transport proteins The energy for transport comes from ATP powered hydrogen extrusion to produce a gradient of H+ ions and an electrochemical potential difference between the inside and outside of the cell. Ions can either diffuse in or be pulled in by the negative charge.

Transport systems Outside Inside -70mV Cell membrane ATP ADP H+ H+ pump K+ H+ K+ Cation uptake H+ A- Co-transporter H+ S Transport of neutral solutes

Short distance transport (cell to cell) Water and ions have 2 routes to the endodermis –through cell walls (apoplast) –through the cells (symplast) The outside of the cells in the Endodermis in the root is impermeable to water due to a suberised casparian strip. Water and ions must enter cells before they can cross the endodermis. ION SELECTIVITY

Routes to the endodermis Apoplastic Vacuole Plasmodesma Cytoplasm Cell wall tonoplast Plasma membrane Symplastic Trans- membrane

3 Mechanisms of water movement Capillarity - Water will rise up capillaries, (like xylem vessels) but the distance is only a few centimetres at best. Root pressure - plants selectively take up ions and water will follow by osmosis. Not capable of providing the volume of flow and would result in toxic concentrations of ions Transpiration pull.

Transpiration pull The atmosphere has a very low water potential (-700Mpa) and cells in the leaf lose water to it. Water moves by osmosis from neighbouring cells until it reaches the bundle sheath cells. Bundle sheath cells take water from the xylem The whole column of water in the xylem moves up. The vacuum in the roots pulls water in.

Transport of solutes in the phloem Sugars are actively loaded into the phloem in the leaves, and actively removed from the phloem in parts of the plant like the roots that need sugar. Any sugar that leaks out is pumped back in by the companion cells Water follows by osmosis, and the difference in water potential between the leaves (high sucrose, high negative water potential) and the roots (low sucrose, low negative water potential) drives a bulk flow of the sugar solution.

Mass flow of solutes in the phloem Shoot Root Sucrose loaded in the leaves Bulk flow of solution Water follows by osmosis Sucrose removed Water follows by osmosis High  Low 

Symbiotic nitrogen fixation and mycorrhizae Certain microorganisms like Rhizobium can form symbiotic associations with plants whereby the microbes receive organic acids in exchange for nitrogen fixed by the microbe. Fungi are very efficient at taking up minerals from the soil. An association between plants and fungi may dramatically increase mineral uptake.