Mineral nutrition of plants

1. Mineral nutrition of plants -P -K Plant Physiol Biotech 3470 Chapter 12 Lecture 11 March 2, 2006 -N -S -Ca From Rost et al. “Plant biology”, 2nd edn -Fe -Mg

2. Plants need elements other than C to grow and develop • Must integrate carbon from PCR cycle with other inorganic minerals taken up by roots from environment • Together, these elements are the building blocks of complex molecules (proteins, nucleic acids, etc.) • Mineral (inorganic) nutrition dependent on C metabolism and vice-versa • History • Need for understanding how plants gain nutrition from environment started during Industrial Revolution • People no longer grew their own food! • Need to optimize growth conditions to feed more people • Key finding: central role of NPK fertilizer to boost yield

3. Hydroponic growth facilitated the discovery of essential mineral nutrients • Further definition of “essential”: Sachs (mid-19th century) used hydroponic culture • now used for vegetable production • roots are cultured in solution, not in soil • More modern growth media • Hoagland’s solution → now slightly MODIFIED • Murashige and Skoog (M + S) • Solutions have high nutrient levels relative to soil • Required because the supply is often not replenished frequently • Hydroponic culture can be as simple as a plant supported in an aerated pot • If roots waterlogged, what happens to yield? Ca N K S Mg P Fe B Mn Zn Cu Mo

4. Hydroponic culture techniques come in different flavors Fig. 12.1 • Main disadvantage of simple solution culture → as plant grows, it selectively depletes certain minerals • When one becomes limiting, growth will slow significantly • Can grow in vermiculite/perlite (inert, non-nutritive) and refertilize daily • Commercially, it is often cheaper and easier to continuously bathe roots in a nutrient solution (nutrient film technique) • Aerates • Standard nutrient level maintained • Continuous process monitoring • To define “essential”, researchers need inert materials contributing low levels of nutrients (NO METAL PARTS!) Fig. 12.2

5. There are 17 essential elements required for plant growth What defines an “essential” element? • In its absence the plant cannot complete a normal life cycle • The element is part of an essential molecule (macromolecule, metabolite) inside the plant • Most elements fall into both categories above (e.g., structural vs. enzyme cofactor) • These 17 elements are classified as • 9 macronutrients (present at > 10 mmol / kg dry wt.) • 8 micronutrients (< 10 mmol / kg dry wt.) • Environmental (silicon [dust]) and/or cultural (from equipment, water, impure salts) contamination make assigning “essentiality” difficult • Essentiality of micronutrients (0.1→1 ug/L!) especially difficult to establish • Difficult to detect low concentrations → push detection limit of common analytical techniques (e.g., flame spectrometry)

6. The availability of some minerals to the plant for growth is dependent on environmental conditions • There may be high levels of nutrient present in soil but it is not in a metabolically useful form • e.g., Fe • Need to supply a lot • Dependent on pH (precipitates out of solution) • Fe2+ more bioavailable (soluble) • Many plant “diseases” are actually mineral deficiencies (common: Mn, B, Cl) • Some inessential elements are still beneficial to plant health → required at sub-micronutrient concentrations • Na (in C4 plants involved in transporting C between bundle sheath and mesophyll cells) • Si (in cell walls; prevents lodging) • Co (by N-fixing bacteria)

7. The absence of essential elements causes deficiency symptoms • Essential because of their metabolic functions • Characteristic deficiency symptoms shown because of these roles • Typical deficiency responses are • Chlorosis: yellowing; precursor to • Necrosis: tissue death • Expressed when a supply of an essential metabolite becomes limiting in the environment • Element concentrations are limiting for growth when they are below the critical concentraion • This is the concentration of nutrient in the tissue just below the level giving maximum growth www.ridgetownc.com

8. Concept of critical concentration illustrated Fig. 12.3 • Above critical concentration, there is no net benefit (e.g., yield increase) if more nutrient is supplied • Below critical concentration, nutrient level limits growth! • Not shown on diagram: all elements eventually become toxic at very high concentrations • This is more common for micronutrients in polluted environments “HEAVY METAL” contamination

9. Limiting nutrient levels negatively affect growth • Plant responses to limiting nutrients usually very visible: affects yield/growth! • Again, chlorosis and necrosis of leaves is typical • Sometimes straightforward relationship • e.g., in chlorosis (lack of green color), • N: chlorophyll component • Mg: cofactor in chlorophyll synthesis • Sometimes NOT • Symptoms dependent on species and nutrient mobility Ctrl - P - N - Fe - Ca Fig. 12.4

10. Let’s briefly discuss cellular roles and deficiency symptoms for the big 3 essential elements N: • Abundant in atmosphere but metabolically unavailable to non-legumes • Usually absorbed as nitrate (NO3-) and reduced to ammonia (NH4+) in the plant • Agronomically, N is always limiting • There is a direct relationship between N supplied and yield! • Component of proteins, NAs (bases), PGRs, chlorophyll • Symptoms of deficiency: slow growth, leaf chlorosis • Mobilized from older leaves to sinks as soluble amines –NH3 and amides • Therefore, older leaves show first signs • Also accumulate anthocyanin pigments→ because C skeletons can’t make chlorophyll, amino acids, etc…. (no N!) • This is a typical nutrient stress response! Rost et al. “Plant biology”, 2nd edn O C CH3 N CH3

11. Phosphorus is the most limiting element in natural environments P: • Present in soil as phosphoric acid ( H3PO4 ) • pH < 6.8: H2PO4-→ orthophosphate→ most bioavailable form • Deprotonated at higher pH → less available • PO4 tends to precipitate and form unavailable complexes with • Metals • Organic molecules • Present at <1 µM in most soils → it is the most limiting element for plant growth! • Component of • Hexose-P • Nucleotides (P-backbone) • ATP! • Symptoms of P deficiency include • Reduced yield, short stems • Intense green colour • Anthocyanin synthesis • Mobilized from sources to sinks (young leaves) as for N Rost et al. “Plant biology”, 2nd edn

12. Potassium is essential for controlling plant cell size K: • Most abundant cation • Supplied as potash: K2CO3 • High solubility, leaches from porous soils • Biochemical functions: • Enzyme cofactor - activates enzymes of photosynthesis and respiration (pyruvate kinase in glycolysis) • An osmoregulator in vivo → controls cell size • e.g., guard cell H2O uptake → controls stomata size • Balances charge of anions like Cl- • Mobile - often leaves show deficiency first • Symptoms- chlorosis, necrosis, lodging of stems Rost et al. “Plant biology”, 2nd edn We will focus on the central role of nitrogen in metabolism in the next lecture…