Mineral nutrition/nutritional deficiencies/physiological disorders of rice

 

            The lecture and this handout focus on mineral nutrition of rice and some physiological disorders of rice.   A note at the end of the handout is meant to be a guide to diagnosing sick rice plants.

 

Required elements for plant growth and development:

 

            C H O P K N ‘ S   Ca Fe   Mg   B   Mn   CuZn   Mo(Na)+Cl   (Si,Ni)

 

Obtained from water and CO2              +(Na only required by C4 plants)

            C         carbon

            H         Hydrogen

            O         Oxygen

 

Obtained from the soil:                                      Macro nutrient or micro nutrient for rice

            N         Nitrogen                                                                       Macro

            P          Phosphorus                                                                  Macro

            K         Potassium                                                                     Macro

            S          Sulfur                                                                           Macro

            Ca        Calcium                                                                        Macro

            Fe        Iron                                                                              Micro

            Mg       Magnesium                                                                   Micro

            B          Boron                                                                           Micro

            Mn       Manganese                                                                   Micro

            Cu        Copper                                                                                    Micro

            Zn        Zinc                                                                              Micro

            Mo       Molybdenum                                                                Micro

            Na       Sodium                                                             Micro

            Cl         Chlorine                                                                       Micro

            Si         Silicon                                                                          Macro

            Ni        Nickel                                                                          Micro

 

More on iron exclusion

Flood —> Reducing environment as opposed to oxidizing environment

            Fe +++ —> Fe++

The Fe ++ is toxic in large quantities but due to its widespread presence, rice has evidently developed mechanisms where the soluble Fe++ is converted to Fe+++ and deposited in the rhizosphere, on the outside of rice roots and in the roots themselves sequestered to prevent damage to leaves and stems.

 

Mn also becomes reduced and soluble in flooded soils.  Tissue levels of Mn are fairly high in rice and would be toxic in many plant species growing in non-flooded environments such as wheat or soybeans.  Almost all nutrient elements have ranges of deficiency, sufficiency and toxicity (see figure below).

 

 

Consequently, the roots absorb more Mn and the tops receive less Mn with the addition of silica to the nutrient media (this may be the result of reduced transpiration with the addition of silica)

 

            Also, as mentioned earlier rice roots accumulate Fe as a protective mechanism to prevent damage to leaves and stems.

 

 

 

So silica decreased Fe++ and increased Fe+++ (ferric)

 

Also “poisoning rice roots with KCN - (inhibitor of enzyme activity for terminal election transport) Table 1-11 p236 Japan Rice Phys.

                                                                                               

                                                                           ‘59Fe absorption’ b

Treatment

Root  respiration a

Stems & leaves

Roots

Control

32.6

0.28

1.01

KCNc

0.3

1.65

3.30

NaN3c

0.2

0.80

2.33

DNPc

2.5

1.29

2.70

 

              a microliters 02/100mg fresh weight/hour

              b mg/plant

              c KCN = Pottasium cyanide; NaN3 = Sodium azide; DNP = Dinitrophenol

 

So, when respiratory poisons are in place, more Fe is taken up and deposited in roots & tops.  When the plant is allowed to respire normally (control) then Fe levels stay low.

 

A note on mobility - nutrients fall into 2 general categories - mobile & immobile - mobile nutrients can be freely translocated from 1 plant part to another in deficiencies.

 

Mobile - N, K, Mg, P, Na, Cl, Mo

            Immobile - Calcium, sulfur, iron, boron, copper, Zn

These mobilities are relative but some elements are readily moved from one part of the plant to another while other elements cannot be so moved.

 

            Also nutrients are classified by the relative amounts needed.

 

            See Tables 5.1 p 104 in Taiz & Zeiger

 

 

 

Element

Relative Number of atoms relatives to Molydenum

 

H

60,000,000

 

C

40,000,000

 

O

30,000,000

Macro

N

1,000,000

 

K

250,000

 

Ca

125,000

 

Mg

80,000

 

P

60,000

 

S

30,000

 

Si

30,000

Micro

Cl

3,000

 

Fe

2,000

 

B

2,000

 

Mn

1,000

 

Zn

300

 

Cu

100

 

Ni

2

 

Mo

1

 

Uptake & exclusion

            Active - Si                               

            Passive - Arsenic (As)             

            Partial Exclusion - Fe+++ Active

 

Roles of nutrients in plant by Function

 

Plant mineral nutrients according to biochemical function

            N -       Constituent of amino acids, nuclear acids, protein

            S -       Constituent of some nucleic acids, protein & lipoic acid, coenzyme A, thiamine PPi, glutathione, necessary for energy storage or structural integrity

            P -       sugar phosphates, nucleic acids, coenzymes, phospholipids, phytic acid

            B -       Complexes with mannitol and other constituents of cell walls

            Si -       Deposited as amorphous silica in cell walls

 

Nutrients remaining in ions

            K -       Cofactor  for over 40 enzymes including soluble starch synthase

            Na -     C-4 & CAM plants not rice

            Mg -    Cofactor many enzymes but constituent of chlorophyll

            Ca -     Constituent middle lamella of cell walls; second messenger in metabolic regulation

            Mn -    Cofactor for some enzymes required for PS 02 evolution.

            Cl -      Required to PS 02 evolution.

 

Nutrients uninvolved in election transfers

            Fe -      Cytochrome & non-heme protein in PS, respiration

            Cu -     Ascorbic acid oxidase

            Zn -      Component alcohol dehydrogenese, CuZn superoxide dismutase

            Mo -    Nitrate reduction

            Ni -      Urease cofactor

 

Nutrient uptake

 

Different parts of the root take up different nutrients differently and different species

 

            K+, NO3-, NH4+, PO4 can be absorbed by all parts of the root.

 

            In corn and rice, the root tip takes up NH4+  faster than other parts of the root.

 

            Lateral roots and not root hairs are primarily responsible for silica uptake by rice.

 

Nitrogen uptake and metabolism

 

            Nitrogen (N) is present in the soil as NO3- , NH4+, amino acids, protein and as urea. Ureases in the soil quickly convert urea to NH4+  and CO2.  Sources of the urea are both plants and microorganisms.

            The reaction for nitrate assimilation occurs in both the roots and the shoots and both use nitrate reductase:

 

NO3-  + NAD(P) H + H+  + 2 e-  →     NO2 -   +NAD(P)  + H2O

 

Nitrate reductase containers molybdenum, is a dimer (2 subunits) and without nitrate reductase,   NO3-  accumulates in plants.  The form of the enzyme in the shoots is activated by light, deactivated by darkness.  The activation of the enzyme in the light is accomplished by dephosphorylation (phosphate is removed) of the enzyme and the deactivation is accomplished in the dark by phophorylation (addition of a phosphate to the enzyme).  Most of the nitrate is accomplished in the leaves.

 

 

Nitrate reductase:

 

NO2-  +  6 Ferredoxinreduced  + 8 H+  + 6 e-  →     NH4+ + 6 Ferredoxinoxidized  + 2 H2O

 

Nitrite (NO2- ) is potentially toxic in plants

 

The ammonium (NH4+)  in plants is incorporated into amino acids by a series of enzymes but the central hub for incorporation of NH4+ into plants starts with glutamine synthetase (GS) and glutamate synthase (GOGAT).   Asparagine and other amino acids are synthesized with amine (-NH2) added via the GS/GOGAT cycle.

 

 

 

Besides this, as leaves senesce (grow old), the proteins in the leaves are broken down to amino acids which are transported to new leaves to synthesize new proteins.   Plant proteinases break down the proteins in senescing plant parts.  When the different proteinases have done their work, the component amino acids remain for making new proteins.

 

Physiological disorders of rice plants associated with mineral nutrition

 

Akiochi

            Associated with a series of problems including decline of soil fertility, rice straw taken for fuel and manufacturing materials.  The plant growth deteriorates with age.  Often the roots rot.  The problem is associated with warmer regions in Japan also with iron sulfide formation, hydrogen sulfide, and Helminthosporium leaf sport.  Nutrtionally, K+ deficient plants lack Fe++ oxidizing power.  Also deficiencies in K, Ca, Mg, Mo , P or Si are susceptible to iron toxicity.

 

Akagare

            Similar to Akiochi except occurs in earlier plant development than Akiochi.  Three main types of akagare include Type I - iron toxicity;  Type II - zinc deficiency (associated with high soil carbonates and bicarbonates) and Type III - Iodine toxicity.

 

Type I - iron toxicity in rice

            In the reduced environment of a flooded rice field, Ferrous iron is abundant in the soil and leads to toxicity if the plant does not detoxify it.   Rice plants do this three ways:

(1)   Oxidation of ferrous iron to ferric iron in the rhizosphere;

(2)    Chelation as ferric iron on root surfaces;

(3)   Retention of ferrous iron sequestered inside the root .

 

Symptoms - bronzing of leaves including leaves which are purplish orange, yellowish brown, reddish brown, yellowish brown and brown leaves.

 

Note: Plants deficient or unhealthy roots cannot exclude iron effectively .  See poisoned root table above and P or Si deficient roots cannot exclude Fe++ .

 

Power of visual cues vs quantitative plant and soil analysis: Often a trained and experience eye and some relevant questions can reveal the nature of a "problem" rice field because often the changes found in a "sick" rice plant are subsequent to the initial stress or stresses causing the conditions and are secondary rather than causative.

 

Akagare Type II - zinc deficiency

            High pH

            Bicarbonate buildup

            Sulfides

 

Zinc is critical to several oxidation/reduction enzymes.  Notable among these is CuZn superoxide dismutase and alcohol dehydrogenase.

 

In our normal environment, oxygen is required for sustaining life and also a source of considerable stress.  Electrical charges in plants and animals lead to the production of superoxide:

 

O2  + e-  →     O2- (superoxide)

 

Superoxides combine with membranes and quickly lead to degeneration of cells and cell function if not detoxified.  One source of reducing oxygen comes from light which cannot be used in photosynthesis.  During cool conditions <65◦ F, the ability of the plant to photosynthesize is reduced.  If light conditions are fairly high (high irradiance), the stress resulting is called cool temperature/high light stress.  Particularly, the chloroplasts are affected in these cases (although the rest of the cell and plant are also potentially affected).  Chloroplasts have a CuZn superoxide dismutase which converts O2-  to a less toxic molecule:

 

O2- + 2 H2O →      2 H2O2

 

A deficiency of zinc is potentially devastating for chloroplasts as one of the key photoprotective mechanisms is blocked.

 

Alcohol dehydrogenase:

 

In a flooded soil, the rice root is initially deficient in oxygen and to survive the plant carries out anaerobic (no oxygen) respiration.  One of the byproducts of anaerobic respiration is ethanol (CH3CH2OH) which is potentially toxic to plants.  The enzyme which detoxifies alcohol (alcohol dehydrogenase) has a zinc cofactor.

 

G−CH2OH + NAD(P) +   →          G−HC=O  + H + NAD(P)H

 

Where G is a carbon-group (CH3 ─   , H3CCH2  ─  , H3C H2CH2  ─  , etc.)

Consequently, after flooding alcohol dehydrogenase is necessary for healthy function of the plant and without the zinc cofactor, alcohol dehydrogenase cannot function effectively.

 

Akagare Type III  Iodine toxicity

 

I levels high in these plants.   Some brown algae contain 1% iodine.  Iodine is not an essential plant nutrient although iodine concentrations in plants can be important to humans and animals eating those plants.  I content in higher plants can range between 0.2 and 0.5 ppm (parts per million).  In some areas in Japan, iodine (I) contents are too high for healthy functioning of plant and lead to akagare.  Roots are compromised in this condition as well. 

 

Summary of physiological disorders- when you are asked questions such as “what is wrong with this rice?”

 

            We would all like to have neat categories but the questions are not always so neatly presented to us.  Often, when you are asked what is wrong with a plant or field, you need to do an experiment or two to determine the answer.  The experiment can be simple like adding some nutrient or water or to transplant a healthy plant to the area or to transplant a plant from that area to another area.   One note, experiments in rice fields have a short life span.  Often, before an answer can be obtained, the whole field will be “treated” with the “best” or most promising treatment.  At any rate, above are some well-studied nutrient deficiency/physiological disorders.   Remember that temperature, herbicides, insecticides, poor quality water, insects, disease organisms and salt can exacerbate a problem so look for areas of the field and clues as well as looking at the “sick” plants.  Look at the roots of a plant.  A healthy, flooded V7 stage rice plant that can be easily uprooted by tugging on the plant has a serious problem because the root system should be extensive and quite difficult to uproot by this stage.  Often several stresses combine.  Also, young plants are quite susceptible to stress as they have few reserves.  Prolonged periods of cool temperatures (<65 degrees F) exacerbate problems.   Also, combinations of high light and cool temperatures are especially destructive.   It’s nice to study these notes but it’s also good to study plants.  Use your experiments in the class as an opportunity to observe and impose stresses on rice.