Topic 3

Factors Affecting Plant Growth

We will review these factors because of the limiting effects they have on use of plant nutrients.

Definition of growth - The progressive development of an organism.

Usually expressed as dry weight (total of the part we're interested in such as grain), height, length, diameter

Growth of an annual plant related to time is an S shaped curve for an or one growing season for a perennial plant.

 

 

growth

time

 

 

Growth related to the factors affecting it.

G = f (X₁, X₂, X₃ .....Xn)

G = measure of growth

Xi = growth factors

The factors that affect plant growth can be classified as genetic or environmental.

A. Genetic Factor

1. Field crops - Yield potential is determined by genes of the plant. A large part of the increase in yield over the years has been due to hybrids and improved varieties. Other characteristics such as quality, disease resistance, drought hardiness are determined by the genetic makeup. Corn hybrids are an example of a dramatic yiel incease resulting from genetics. Genetic engineering is now becoming an important tool in changing a plants potential.

2. Nursery crops and turf - not interested in total growth as much as appearance. Ex. is Bermudagrass

1. Coastal bermudagass- As a forage the grower is interested in yield and feed quality.

2. Tifdwarf - Golf greens - interested in appearance, cover, wear resistance not how much total growth occurs.

3. Variety and Plant Nutrient needs-Hybrid corn producing 200 bu/ac requires more plant nutrients than a hybrid producing 100 bu/ac. As potential crop yields are increased, the plant nutrients required are increased. Current research in the Soil Science and Genetics department is concerned with developing corn hybrids that use nitrogen more efficiently - Produce more grain per pound of N - fertilizer.

4. A producer has control over the genetic factor by his choice of variety.

Field crops - highest yielding, disease resistant, etc.

Nursery - Best appearance - dwarf vs larger shrubs

B. Environmental Factors

definition - All external conditions and influences affecting the life and development of an organism.

The following are regarded as the most important environmental factors

• Temperature

• Moisture supply

• Radiant energy

  Composition of the atmosphere

• Soil aeration and soil structure

• Soil reaction

• Biotic factors

• Supply of mineral nutrients

• Absence of growth-restricting substances

Each can be a limiting factor in plant growth. These environmental factors do not act independently example - inverse relationship between soil moisture and air

a. Temperature - A measure of the intensity of heat. Plant growth occurs in a fairly narrow range - 60 - 100 degrees F

1. Temperature directly affects

photosynthesis

respiration

transpiration - loss of water

absorption of water and nutrients

2. The rate of these processes increases with an increase in temperature responses are different with different crops

cotton vs collards or potatoes

fescue vs bermuda grass

These generalizations hold within a crops range of adaptation

3. Temperature also affects soil organisms nitrifying bacteria inhibited by low temperature. pH may decrease in summer due to activities of microorganisms

4. Soil temperature affects water and nutrient uptake

b. Moisture supply - Plant growth restricted by low and high levels of soil moisture

1. can be regulated with drainage and irrigation

2. good soil moisture improves nutrient uptake

If moisture is a limiting factor fertilizer is not used efficiently.

c. Radiant energy

Quality, intensity and duration of light are important

1. Quality can't be controlled on a field scale - Feasible on specialty crops

2. Intensity of light (brightness) is an important factor.

photosynthesis light intensity

Corn with upright leaves being bred to intercept more light

3. Duration - Photoperiodism - Plant behavior in relation to day length

- long day plants - flower only if days are longer than same critical period - 12 hours Grains and clovers

- short day plants - flower only if days are shorter than a critical period soybeans.

- indeterminate - flower over a wide range of day lengths. Tomato, cotton, buckwheat

Some crops fail to flower in certain geographical areas

Chrysanthemums can be made to bloom by controlling photoperiod.

d. Composition of the atmosphere

CO₂ makes up 0.03 per cent of air by volume. Photosynthesis converts CO₂ to organic material in the plant. CO₂ is returned to atmosphere by respiration and decomposition

In a corn field or closed greenhouse CO₂ level may drop and become a limiting factor in growth.

Increasing CO₂ can increase crop yields respiration of plants and animals - decomposition of manure or plant residue may release CO₂

greenhouse crops

Plant growth and quality can be enhanced by supplemental CO₂. Growth responses have been shown with tomatoes, lettuce, cucumbers, flower crops, greens, peas, beans, potatoes

Air Quality

Air pollutants in sufficient quantities are toxic to plants sulfur dioxide - provides sulfur at low levels

carbon monoxide

hydrofluoric acid

e. Soil aeration

Compact soils of high bulk density and poor structure are aerated poorly.

Pore space is occupied by air and water so the amount of air and water are inversely proportional to the amount of oxygen in the soil. On well drained soils, oxygen content is not likely to be limiting to plant growth.

Plants vary widely in their sensitivity to soil oxygen. Paddy rice vs tobacco

f. Soil reaction

- pH influences availability of certain nutrients ex phosphate availability low on acid soils. Al is toxic to plants

diseases affected by pH

Potato scab controlled by keeping pH below 5.5

g. Biotic factors

disease - heavier fertilization may increase vegetative growth and susceptibility to disease

Root knot nematodes reduce absorption so more fertilizer is necessary.

insects

weeds - compete for moisture nutrients light

allelopathy - harmful substances released by roots.

h. Mineral nutrients essential nutrients - any element that functions in plant metabolism

Non-mineral nutrients (from water and air)

carbon, hydrogen, oxygen

Primary nutrients

nitrogen, phosphorus, potassium

Secondary nutrients

calcium, magnesium, sulfur

Micronutrients

copper, manganese, zinc, boron, molybdenum, chlorine, iron

Beneficial to some plants

cobalt, vanadium, sodium, silicon,

i. Absence of growth - restricting substances

High concentrations of plant nutrients

aluminum, nickel, lead - associated with sewage disposal, wastes from industry, mines, etc.

organic compounds - phenols, oil