FACTORS AFFECTING PLANT GROWTH
Growth: Progressive development of an organism (plant in this case).
Can be expressed as dry weight (of the plant part of interest); height, length, diameter etc.
growth
time
Figure 1: Typical growth pattern of an annual crop
G = f (X1, X2, X3..............Xn)
G =measure of growth
Xi = growth factors
There are two main classes of factors: Genetic and Environmental
A. Genetic Factor
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Extent to which a plant may develop is determined or limited by the genetic make-up of
the plant
Over the years use of hybrids (maize and other crops) has brought about increases in
yield
Genetic constitution also determines other characteristics e.g. quality, pest/disease
resistance, drought hardness etc.
Farmer has control over the genetic factor through choice of variety
High yield potential is often associated with high input requirements e.g. nutrients
Thrust →development of hybrids that use nitrogen more efficiently. i.e. produce more
grain per
per kg of nitrogen fertilizer.
B. Environmental Factors
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Important environmental factors include:
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Temperature
Moisture supply
Radiant energy
Composition of the atmosphere
Soil aeration and soil structure
Soil reaction
Biotic factors
Supply of mineral nutrients
Absence or presence of growth-restricting substances
Each of the above factors can be a limiting factor in plant growth and these
environmental factors do not act independently e.g. inverse relationship between soil
moisture and air.
1. Temperature: (a measure of intensity of heat).
- Plant growth occurs in a fairly narrow range of 15 – 40 oC for most crops
- Temperature directly affects
Photosysthesis:
o Effect differs from spp to spp and with CO2 content of the atmosphere, the
intensity of light, duration of light of a given intensity.
o If light is limiting, temperature has little effect on photosynthesis.
o If CO2 is limiting, but light is not, photosynthesis is increased by an increase in
temperature
Respiration:
o In general respiration takes place more slowly at low temperatures and increases
as temperature rises
Transpiration:
o Rates of transpiration are low at low temperatures and increase with rising
temperature
o Water losses may exceed water intake by plant under conditions of excessive
transpiration → wilting
Absorption of water and nutrients
o Influence of temperature is modified by spp. But with a number of plts,
absorption increases with a rise in temperature of the rooting medium from 0oF to
approximately 60oF to 70oF
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o Absorption by the roots is retarded at lower soil temperature which may be caused
by:
i)
Reduced respiratory activity
ii)
Reduced cell membrane permeability
Temperature indirectly influences plant growth through its effects on microbial activity for
example, low temperature inhibits nitrifying bacteria.
Extreme max temperatures
o Combined with low humidity cause wilting. ( dis-equilibrium between the
absorption of water by the roots & loss of water through transpiration)
o Temporary or permanent wilting : high temperature alone rarely causes death
unless water is limiting.
Extreme min temperature
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Can result in frost formation
Most annual crops cannot withstand frosts .
Besides the extreme case of frost crops, annuals in particular, have thresholds below
which their physiological activity is greatly reduced. These thresholds vary from 3 oC for
wheat to 12 oC for rice. Threshold for maize is 10 oC.
Concept of heat units:
For a plant to complete its growth cycle, it has to acquire a certain quantity of heat (provided
other conditions are ideal). This heat is measured in heat units. Long duration varieties have a
higher heat unit rating than short duration varieties
Table 1: Mean corn yield comparisons among 4 maturity zones according to heat units
(Ontario)
Maturity group according to corn Heat Unit
rating
2 400 – 2 500
2 500 – 2 700
2 700 – 3 100
3 100 – 3 400
Mean yield (1977-1979) – 3 year average:
t/ha
5. 67
7.44
7.85
9.36
Heat units received by a plant for a given period is given by:
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[Mean daily temperature for the growth period– base temperature ] x period (duration in days)
Suppose maize plants are subjected to four consecutive days with mean daily temperatures of
20oC; 22 oC; 19 oC and 24 oC respectively, then the units received during the 4 days are given by:
[(20 + 22 +19 + 24)/4 -10]*4 =45
Plants often have a requirement of fluctuating/alternating day and night temperature: related to
balance between photosynthesis and respiration:
Suppose we have 14 hours of daylight and 10 hours night, if average day and night temperatures
= 24 oC and 16 oC respectively , the mean daily temperature is given by:
[(24*14) + (16*10)]/24 = 20.7 oC
Causes of frost/type of frost
i)
ii)
Advection frost: Results from movement of a cold air mass at or below 0 oC into an
area.
Radiation frost: Results from considerable heat loss from the earth at night. The rapid
cooling of the land at night causes an inversion of temperature. Air over the cool soil
becomes heavy and does not rise. Thus the temperature of this area reaches freezing
point →Frost
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At night - air at ground level may be cooler than air high above the ground (
say 100m ), as a result of radiative cooling. This condition is refered to as a
temperature inversion.
Ground relief (topography) will affect temperature. E.g. At night cold air
collects in low-lying areas such as river valleys {cold air is denser than warm
air. The former seeks the lower level available}.
The lower the latitude, the higher the temperature
Movement of the earth around the sun causes a cyclic variation in the amount
of solar energy reaching the top of the earth’s atmosphere (troposphere), in
any latitude.
At the equator, amount of energy received does not fluctuate much during the
course of the year.
Differences between summer and winter temperatures increase with
increasing latitude.
Occurrence radiation frost is favoured by:
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Absence of a cloud cover (clouds prevent loss of the long wave radiation from the
earth and re-radiate it back to the earth thereby maintaining high temperature above
the ground.
Calm nights (no wind). Wind tends to mix cold and warm air leading to reduced
likelihood of frost occurrence
Dry atmosphere (Explain why this is the case)
Occurrence in Zimbabwe
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Mostly radiation frost
Expected end of May – mid August
Occurs most often at high altitudes
Expected in the valleys. Cold air is heavy and tends to collect at the bottom of valleys
Frost Protection
Long term methods( these tend to be passive
i)
ii)
iii)
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Choice of growing season
Choice of growing area
Breeding (for frost tolerant varieties )
Soil management/ husbandry
- Heavy clay with high water content remain warmer at night , reducing the
chances of frost occurring while light sandy soils tend to warm rapidly during
the day and lose heat rapidly at night thereby increasing chances of frost
occurrence.
Frost barriers (e.g. live hedges), to prevent cold air draining to lower slopes where
susceptible crops may be growing
Short-term (these are rather active)
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Covering whole plant/s (plastic; straw).
- e.g. Covering with sheet of polythene (greenhouse effect).
- Relatively cheap method
- Fastens ripening
- Necessity to replace/ remove sheet every year
- Plastic tunnels
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Use of fans or wind machines Mixing to continue until likelihood of frost ends
- Can offer 2-3 oC against frost
- Circulate air. Cooling is faster when air is still. Each wind machine can
protect a maximum of 8 – 10 acres.
Sprinkling with water / irrigation water
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- Sprinkle water on foliage
Explain how frost control is achieved in this method. What precautions should be taken
when using this method?
What are the advantages and disadvantages of this method?
Heating e.g. heaters; heating lamps and fires. ( can combine heaters with fans)
Smudge pots- smoke effects the control. Blanket effect – re-radiation of long. wave
radiation
Mulch prevents rapid loss of heat from the soil
Wet compact soil conducts heat better than a one.
2. Moisture supply
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plant growth restricted by low and high levels of soil moisture (specific effects are
discussed in later chapter in this module)
can be regulated with drainage and irrigation
good soil moisture improves nutrient uptake
if moisture is a limiting factor fertilizer is not used efficiently.
[graph on “effect of N and moisture on the yield of wheat straw”: Adapted from
Fernandez and lard, 1989. – Higher yields were obtained when when irrigation was
carried out when AW(moisture) in the 5-30cm soil horizon was at higher % than at lower
once. AW % range fro 1% to 61%. Higher yields yields were obtained at higher N levels
for each AW level. (assertate in box file)].
3. Radiant energy
Quality, intensity and duration of light are important (photosynthesis; vernalization etc)
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Quality can't be controlled on a field scale - Feasible on specialty crops
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Intensity of light (brightness) is an important factor. (graph on assertate). Different plants
respond differently to varying light intensity e.g. Corn/maize attains its maximum DM
production (or photosynthesis) at higher insolation than sunflower which in turn requires
higher insolation than tobacco.
Generally, most plants are able to make good growth at light intensities less than full day
light.
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Duration - Photoperiodism - Plant behavior in relation to day length / plant response to
daylength
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.
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Day neutral plants - flower over a wide range of day lengths. Tomato, cotton, buckwheat
What are Short –Long day plants
Some crops fail to flower in certain geographical areas
Chrysanthemums can be made to bloom by controlling photoperiod.
4. Composition of the atmosphere
CO2 makes up 0.03 per cent of air by volume. Photosynthesis converts CO2 to organic material
in the plant.
Sources of CO2 in the atmosphere:
i)
ii)
respiration of plants and animals
decomposition of organic residues by micro-organisms
In a field or closed greenhouse CO2 level may drop and become a limiting factor in growth.
For greenhouse crops, plant growth and quality can be enhanced by supplemental CO2. (CO2
enrichment). Positive growth responses have been shown with tomatoes, lettuce, cucumbers,
flower crops, greens, peas, beans, potatoes
Air pollutants/gases such as SO2, CO and hydrofluoric acid in sufficient quantities are toxic to
plants. (At low concentrations, SO2 provides sulphur).
5. Soil structure and composition of the air.
Soil structure influencfes/determines the bulky density of a soil.
The higher the BD, the more compact the soil and the smaller the amount of pore space.
Compacted soils are poorly aerated. High BD leads to restricted plant growth.
Under field conditions, oxygen diffusion into the soil is determined largely by moisture level of
the soil.
In well drained soils, with good structure oxygen content is not likely to be limiting to plant
growth..
Plant sensitivity to soil oxygen varies widely from crop to crop viz rice vs tobacco
6. Soil reaction (pH)
- influences availability of certain nutrients e.g. phosphate availability is low on acid soils.
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- Different plants thrive under different pH ranges
- effects on microbial activity
- By keeping pH at certain levels some diseases can be controlled e.g. potato scab ccan be
controlled by keeping pH below 5.5
7. Biotic factors
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diseases
Root knot nematodes reduce absorption so more fertilizer is necessary.
Insects pests
weeds
8. 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
Nutrients beneficial to some plants: cobalt, vanadium, sodium, silicon
9. 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
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