Plant Nutrition

Mineral NutritionHow 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. Mycorrhizae5. 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?1. In its absence the plant cannot complete a normal life

cycle2. 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

Micronutrients are present in very low concentrations

ppm

Very low concentrations, but still essentialbecause of specialized roles in metabolism

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

• 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. 12.1

Fig. 12.2

Hydroponic culture techniques come in different flavors

Soils particles are generally negatively charged and so bindpositively charged nutrient ions (cations).

Cation Exchange Capacity refers to a soil’s ability to bind cations.

NH4+, NO3

-, Cl-, PO4-2, SO4

-2

Soil pH influences availability of soil nutrients.

Roots

Provide large surface area for nutrient uptake

- Root hairs

Root hairs

Fig. 5.7

Depletion zones - regions of lowernutrient concentration -developaround roots

17

Root hairs

Root hairs

Fine roots and root hairs “mine” the soil for

nutrients. Mycorrhizal hyphae do

this even better.

4. Roots and mineral nutrient acquisition

Clayparticle

Root hair

H

K

K

KK

K

K

K

K

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 hyphaeeven connecting different plants

Mycorrhizae

Ectotrophic mycorhizae

Why mycorrhiza?

• Roots and root hairs cannot enter the smallest pores

Nitrogen fixing bacteriaGenus: Rhizobium

N2 NH4

Supply of electrons

Ion uptake

Outside cell (positive)

Net positive charge

Net negativeNet negativechargechargeInside cell

(negative)

Proton pumps establish an electrochemical gradient.

Active uptake

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

CtrlCtrl - P- P- Ca- Ca

- N- N - Fe- Fe

Chlorosis

Necrosis

Stunted growth