Law of Minimum

An important concept to remember is that one has to "feed" plants before the plants can provide us with food.   As you have learned in previous exercises, plant "food" consists of carbon dioxide and water (sources of C, H, O), and 16 elements (N, P, K, S, Mg, Ca, Fe, Mn, B, Cu, Zn, Mo, Na, Ni, Si and Cl). The 16 elements should be present in a water-soluble form so that a plant can take them up.  The16 nutrients are divided into primary nutrients (N, P, K), secondary nutrients (S, Ca, and Mg) and micronutrients (Fe, Mn, B, Zn, Cu, Mo, Na, Ni, Si and Cl). Even if only one nutrient is missing from the soil (or hydroponic) solution the plant will not develop and produce normally. This notion was postulated by Justus von Liebig in his Law of the Minimum.  The Law of Minimum maintains that yield is proportional to the amount of the most limiting growth resource.   As you recall, such growth resources are nutrients, light, temperature, water and space.

Justus von Liebig

Deficiency Symptoms

The corn plant on the left is nitrogen-deficient. It developed deficiency symptoms which include stunted growth, chlorosis (yellowing), and necrosis (death). Nitrogen is part of a chlorophyll molecule (right, below).  As you recall, chlorophyll is the green pigment that plays an important part in photosynthesis.  If nitrogen is limiting, chlorophyll molecules cannot be synthesized. The plant loses its green color, and can't photosynthesize  As a result, the N-deficient plant does not produce required carbohydrates.  Older leaves  develop  deficiency symptoms earlier, because N is translocated inside the plant from the older leaves to the younger ones.

Below are several examples of nutrient deficiencies. Some of these minerals are involved in the formation of biologically active molecules, such as pigments (chlorophyll, carotenoids, etc.), nucleic acids (DNA and RNA), energy molecules (ATP, NADPH) and enzymes. All of these molecules have different important functions within a plant cell. Nucleic acids, for example, carry an organism's genetic information, ATP provides energy for the reactions within a cell, while enzymes catalyze the reactions.

chrolophyll

Let's briefly talk about enzymes. An enzyme is a protein (sometimes RNA) that functions as a biological catalyst. 

• Enzymes are encoded by genes. Sequence of DNA in the genes codes for a sequence of aminoacids. Aminoacids are assembled together by ribosomes.  When this amino acid chain is released from a ribosome, interactions between aminoacids cause unique folding of the protein.  This uniquely folded protein, sometimes associated with co-enzymes and metals, functions as a biological catalyst.
• Enzymes are very selective in the substrates they act upon and in the kinds of the reactions they catalyze.  Rubisco, an enzyme involved in photosynthesis, catalyzes the conversion of ribulose1,5-biphosphate to two molecules of 3-phosphoglycerate.  

Considering how many biological reactions take place inside an organism (bacteria, plant or human), you can only imagine how many different enzymes there are!  The product of one enzymatic reaction is usually a substrate for another enzyme.  This sequence of enzymic reactions in an organism is known as metabolism. If an enzyme (or any other important biological molecule) is not produced inside the cell due to a mineral deficiency, then the biological reactions catalyzed by this enzyme do not take place, the organism's metabolism is severely impaired, and deficiency symptoms develop.

Nitrogen (N) deficiency N-deficiency is the most common nutrient deficiency. N is a part of a chlorophyll molecule, aminoacids, proteins, and many other important bioldogical molecules. Older leaves of nitrogen- deficient plants are yellow from the tip outside, the plant is light green. Stalks of the N-deficient plants short and slender. Leaves drop. Excess N may cause K deficiencies. Potato, carrot, beets grown with excessive N, show prolific shoot growth with small underground organs. Excess N can lead to splitting of tomato fruits as they ripen.

Leaf of N-deficient corn (top); N-deficient barley leaves, healthy leaf on the bottom

P-deficient and healthy lettuce

Phosporus (P) deficiency Second to N, P is often the limiting element in soils. Older leaves of P-deficient plants are purple or dark green. Stalks short and thin. New growth is weak and stunted. P-deficient plants show poor  flowering and fruiting. Phosphorus is important for structure and function of nucleic acids and energy molecules (ATP, NADP).

Potassium (K) deficiency Potassium is imporntant in  many enzymes that are essential for photosynthesis. Like N and P, potassium is freely translocated inside the plant, so the deficiency symptoms first occur on the older leaves. Lack of potassium causes leaf margin chlorosis, followed by necrosis from the outside to the midvein. K-deficient grasses are more prone to root infections, and are easily bent to the ground (lodged) by rain or wind. Researchers from U. of Georgia suggest that K-deficient cotton plants are more susceptible to fungal infections. They suggest split K applications (half at planting, half as side-dressing), and use foliar fertilization if the deficiency occurs.

K-deficient corn (above), K-deficient cucumber (right)

Ca-deficient tomato

Calcium (Ca) deficiency Calcium is often limited in acidic soils that recieve abundant rainfall. When calcium is deficient, terminal bud dies, young leaves are hooked, because Ca++ is not easily translocated inside the plant. Dying back occurs at tips and margins, foliage may become distorted. Stalk dies off at the terminal bud. Root systems may be damaged by the root tip death. Calcium is bound to enzymes, it also participates in cell wall formation. Calcium is required for cell division and is required for normal membrane functions. Excess Ca may cause boron or magnesium deficiencies.

Sulfur (S) deficiency Because enough sulfate is present in most soils, sulfur deficiency is fairly uncommon. S is not easily translolcated inside the plant, so sulfur-deficient plants develop interveinal chlorosis on younger leaves first. Necrotic spots are usually not present. Sulfur is essential for protein structure, it also occurs in vitamins.

S-deficient corn (right)   S-deficient cotton plant and healthy plants (left)

Magnesium (Mg) deficiency Mg is a part of the chlorophyll molecule, it is also important for activating some enzymes. Plants lacking magnesium have leaves with interveinal chlorosis. Leaves may redden, develop dead (necrotic) spots; tips and margins sometimes cup upward. Stalks are usually slender. Magnesium deficiency is rarely a problem in most soils. Excessive magnesium, on the other hand, can induce potassium deficiency due to interference with K uptake and utilization.

Mg-deficient cucumber plants (right)

Iron (Fe) deficiency Iron often becomes poorly soluble and therefore limited in soils with neutral or basic pH. Fe-deficient plants develop interveinal chlorosis occuring first on younger leaves. In severe cases, younger leaves become white with necrotic lesions. Iron is important because it is a part of some enzymes. Its ability to undergo oxidations and reductions (Fe2+ <->Fe3+) is essential for electron transport in many biochemical reactions inside the plant.

A leaf of Fe-deficient peanut plant

Deficiencies making front pages... Here is how a recent journal PLANT PHYSIOLOGY desecribed its cover(right): the interveinal chlorotic sunflower leaves shown in the photograph suffer from Fe chlorosis. Fe chlorosis occurs mainly on calcareous soils with nitrate as the exclusive N form, and leaves are frequently chlorotic in spite of abundant Fe concentrations. Kosegarten et al. (pp. 1069-1079) have shown that pH of the intercellular space ("apoplast") regulates Fe3+ reduction and thus Fe2+ transport across the cell membrane. Microscope imaging combined with the fluorescence ratio technique revealed high apoplastic pH at cellular sites in the interveinal area of young leaves due to nitrate nutrition (see inset of the interveinal area). In the interveinal area, Fe3+ reduction was depressed at sites of high apoplastic pH, thus inducing leaf yellowing. In contrast, apoplastic pH in the xylem vessels (see related inset) was low even with nitrate nutrition, and, due to high rates of Fe3+ reduction at low apoplastic pH, the tissue around the leaf xylem remained green.

Deficiency symptoms could be sometimes confused with herbicide injuries. Refer to the following web pages for an illustrated list of some herbicide injuries on common crops:
http://www.btny.purdue.edu/Extension/Weeds/HerbInj/InjuryHerb1.html

For more information on plant mineral nutrition and role of  various nutrients, visit:
http://maine.maine.edu/~thomascb/nutri.html

Why do deficiency symptoms differ?
Deficiency symptoms for any nutrient depend on two factors:

• the role of the element in the plant;
• whether or not the element is translocated from older leaves to the younger ones.

Ability of a nutrient to be translocated depends on its mobility in the phloem. The mobility is determined by solubility of the chemical form of the element. Symptoms vary somewhat between species, and according to the severity of the problem, the growth stage, and complexities resulting from deficiencies of two or more elements.

The first modern hydroponic systems were developed in France and England during the 17th century.  Although long before the Europeans experimented with hydroponics, Aztecs grew many of their staples using "chinampas" or floating garden plots.

Hydroponics is the technology of growing plants in a nutrient  solution with or without the use of an artificial medium (vermiculite, sand, gravel, etc.) to provide mechanical support.  Hydroponic systems are classified as liquid or aggregate, respectively.  The vast majority of hydroponic systems are enclosed in greenhouses to provide temperature monitoring, reduce evaporation, and to protect the systems from unfavorable weather conditions.
Several hydroponic techniques have been developed in the recent years:

• Nutrient Film Technique. A thin film of nutrient solution is driven by gravity through plastic-lined channels. The roots grow inside the channels and form a tangled mat.
• Floating hydroponics. Usually used to germinate seeds in beds floating on top of a nutrient solution. Lettuce is grown in this manner in 2.5 cm-thick plastic floats for 4-6 weeks. This technique is very similar to chinampas.
• Aeroponics. Plants are grown in holes of expanded polystyrene panels. Plant roots are suspended in midair beneath the panel and enclosed in a spraying box. Aeroponics is valuable for the rooting of stem cuttings and in the production of leafy vegetables. Space is used more efficiently in this system.
• Aggregate hydroponics systems.  A solid, inert medium provides support for the plants. As in liquid systems, the nutrient solution is delivered directly to the plant roots.

Click here to obtain some practical advice on hydroponic production from Cornell scientists.

About the experimental setup
In this exercise we will use an aggregate/wick hydroponics system.  Solid medium (vermiculite) will provide support for the growing plants, nutrient solution will be driven into the medium by the capillary action. You will replace the mineral solution every week to compensate for the removal of the nutrients by the  plants and pH changes resulting from this removal.
Mineral solutions were prepared based on the Hoagland Mineral Solution No2 for Hydroponic Culture.  The medium contains phosphates, which act as a buffer to prevent rapid pH changes in the solution. Chelating agents are added to the solution to prevent ions (mostly divalent metals) from precipitating.

Click here to learn more about chelating agents and buffers.

You may choose from -N,  -P, -K, -Ca, -S and control  solutions.  You may also decide to work with tall fescue, lettuce, cucumber or a corn plant.

1. Decide which crop and which deficiency your group would like to work with in this exercise. Formulate the hypothesis you want to test. Identify the dependent and independent variables, as well a control treatments.

2. Dilute the stock solution 5 times (i.e. 1 part of the stock per 4 parts of distilled water).

3. Add prepared mineral solution to the white bucket so that there is approximately 9 cm (3.5 inches) of liquid in it.

4. Place 2 sheets of cheesecloth inside the green pot. Pull the cheesecloth through the orifices in the green pot, so that when the green pot is inserted into the white bucket, cheesecloth is immersed into the mineral solution.

5. Fill the green pot with vermiculite.  Wet vermiculite with the appropriate mineral solution.

6. Plant the seedling into vermiculite.

7. Place the green pot inside the white bucket with the mineral solution.

8. Clearly label the pot with your group number, date and treatment. Move the hydroponic assembly into the designated part of the greenhouse.

For more information, email us at maxtep@ufl.edu, mcmahon.43@osu.edu.