Plant Nutrition

How plants acquire and use mineral nutrients Mineral Nutrition How plants acquire and use mineral nutrients 1. Why is mineral nutrition important? 2. What are the essential mineral nutrients? classification systems 3. Mineral nutrients in the soil nutrient availability adsorption to soil particles effects of pH 4. Roots and mineral nutrient acquisition root structure depletion zones 4. Mycorrhizae 5. Nitrogen - the most limiting soil nutrient

Why is mineral nutrition important? 2. In most natural soils, the availability of mineral nutrients limits plant growth and primary productivity. Nutrient limitation is an important selective pressure and plants exhibit many special traits related to the need to acquire and use mineral nutrients efficiently.

2. What are the essential mineral nutrients? Macronutrients - present in relatively high concentrations in plant tissues. N, K, P, Ca, Mg,S, Si Nitrogen is most commonly limiting to productivity of natural and managed soils. Phosphorus is next most limiting, and is most limiting in some tropical soils. Micronutrients - present in very low concentrations in plant tissues.

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.)

All mineral nutrients together make up less than 4% of plant mass, yet plant growth is very sensitive to nutrient deficiency. Not considered mineral nutrients PP05T011.jpg

Micronutrients are present in very low concentrations ppm Very low concentrations, but still essential because of specialized roles in metabolism PP05T012.jpg

PP05T040.jpg

I. Plant Nutrients C. Macro/Micronutrients Hydroponics allowed us to see what was needed The necessary nutrients are those the plant can not grow with out Come in two categories 1. Macronutrients (C, O, H, N, S, P, K, Ca, Mg) Majority of the time used for the main organic compounds 2. Micronutrients (Cl, Fe, B, Mn, Zn, Cu, Mo, Ni) Mostly cofactors for particular enzymes (Fe -> Cytochromes

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

Soils particles are generally negatively charged and so bind positively charged nutrient ions (cations). Cation Exchange Capacity refers to a soil’s ability to bind cations. PP05050.jpg NH4+, NO3-, Cl-, PO4-2, SO4-2

Soil pH influences availability of soil nutrients. PP05040.jpg

Roots Provide large surface area for nutrient uptake - Root hairs

Root hairs

Depletion zones - regions of lower nutrient concentration -develop around roots PP05070.jpg Fig. 5.7

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Root hairs Root hairs

4. Roots and mineral nutrient acquisition Fine roots and root hairs “mine” the soil for nutrients. Mycorrhizal hyphae do this even better.

K+ K+ K+ Clay particle H+ K+ K+ K+ K+ K+ Root hair

Vesicular Arbuscular Mycorrhiza Inside root Intercellular mycelium Intracellular arbuscule tree-like haustorium Vesicle with reserves Outside root Spores (multinucleate) Hyphae thick runners filamentous hyphae Form extensive network of hyphae even connecting different plants

Mycorrhizae Ectotrophic mycorhizae

Why mycorrhiza? Roots and root hairs cannot enter the smallest pores

Nitrogen fixing bacteria Genus: Rhizobium N2 NH4 Supply of electrons

Ion uptake

Active uptake Proton pumps establish an electrochemical gradient. Net Outside cell (positive) Net positive charge Net negative charge Inside cell (negative)

Cations enter root hairs via channels or carriers

Anions enter root hairs via cotransporters.

Concept of critical concentration illustrated 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

Analysis of plant tissues reveals mineral deficiencies

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

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 Ctrl - P - N - Fe - Ca

Chlorosis

Necrosis

Stunted growth