Effect of Light on Plant
growth:
Light energy influences almost all the aspects of plant life directly or
indirectly. Thus, it controls plants structure, form, shape, physiology and
growth, reproduction, development and local distribution.
Phenology
Refers to the timing of seasonal activates of plant in relation to changes in
environmental condition. On the basis of light factor certain ecologists have
classified plants into sciophytes or shade loving or photophobic plants, which
have best growth under low intensities of light. For example beech, spruce, firs
and heliophytes or photophilous plants, which have best growth in full sunlight.
Examples: Pine, willows and birch.
Photoperiodism
Actual duration or length of the day (photoperiod) is a significant factor in
the growth and flowering of a wide variety of plants. The controlling effect of
photoperiod is called photoperiodicity.
In this basis, the angiosperms are divided into 3 categories. They are:
Long day plants
Long day plants, which bloom when light duration is more than 12 hrs per day.
Examples: wheat, Beet roots, carrot, oats and rye.
Short day plants
Short day plants, which bloom when light duration is less than 12 hrs per day.
Examples: Tobacco, Dahlia, hemp and cosmos.
Day neutral plants
Day neutral plants, which show little response to length of day light.
Examples: Cucumber, cotton and potato.
Light also affects movement in some plants. This is called heliotropism or
phototropism.
Light promotes the growth in most of the plants by promoting the development
of chlorophyll, photosynthesis, synthesis of growth hormone and stomatal
opening.
Effect of temperature on plant growth:
Sometimes temperatures are used in connection with day length to manipulate the
flowering of plants. Low Temperature causes chilling in plant which blocks the
movement of nutrients; minerals transport system and photosynthesis activity
i.e. Tomato. Low temperature in some crop effect the pollination and seed
setting i.e. wheat. Which decrease the yield. High temperature retards the plant
growth and effect the fruiting. Thermoperiod refers to daily temperature change.
Plants produce maximum growth when exposed to a day temperature that is about 15
to 35°C higher than the night temperature. This allows the plant to
photosynthesize (build up) and respire (break down) during an optimum daytime
temperature, and to curtail the rate of respiration during a cooler night. High
temperatures cause increased respiration, sometimes above the rate of
photosynthesis. This means that the products of photosynthesis are being used
more rapidly than they are being produced. For growth to occur, photosynthesis
must be greater than respiration.
Buds of many plants require exposure to a certain number of days below a
critical temperature (chilling hours) before they will resume growth in the
spring. Peaches are a prime example; most cultivars require 700 to 1,000 hours
below 45°F and above 32°F before they break their rest period and begin growth.
This time period varies for different plants. The flower buds of forsythia
require a relatively short rest period and will grow at the first sign of warm
weather.
Effect of rainfall on plant growth:
Plants use a optimum amount of water. Example: Water requirement for rice is
300 to 950 mm, for sorghum it is 300 to 650 mm.
Rains, to a major extent, provide this water, through the soil, for the
development and growth of plants.
Rainfall is the first source of water. Irrigation can supplement rainfall to
supply crop water needs.
Rain fall decrease the soil PH that is most useful to available plant growth
nutrients in salty soil.
Importance of water for plants:
Need for the physiological processes to complete plant life cycle i.e. seed to
seed;
A primary component in photosynthesis and respiration;
Responsible for turgor pressure in cells (Like air in an inflated balloon,
water is responsible for the fullness and firmness of plant tissue).
Maintains temperature of plant body;
Helps in the uptake of nutrients from soil;
A solvent for minerals and carbohydrates moving through the plant ;
Serves as a medium through which nutrients and other solutes move in the
plants.
Thus, water from rainfall is vital for plant growth.
Less rain or breaks in monsoon rainfall results in moderate to sever moisture
stress.
Such moisture stress affects
Root and shoot development
Flowering
Pollination and fertilization
Grain filling
Crop yields.
Thus, water from rainfall is vital for plant growth
Equally damaging is excess rainfall
Causes flooding of fields
Water stagnation affects the growth processes
Continuous rains during flowering time affect
Fertilization Grain formation and filling
Favors plant diseases and pests
Interferes with many farming operations like
Seedbed preparation,
Sowing,
Harvesting,
Threshing, and
Processing of crops resulting in low crop yields.
Effect of Humidity on plant growth:
Moisture is a very important factor in growth of plants and is defined as the
ratio of water vapor in the air to the amount of water in the air. The relative
humidity in the air is used by the plants and is crucial for the transpiration
of the plants. Transpiration is at its peak cutting hot, windy and dry days
while transpiration slows down during cool and humid days.
Humidity plays a major role in plant growth, and its effects are often
underestimated or overlooked. To the right you will see the "tools of the trade"
for controlling humidity...the dehumidifier and the humidifier.
Plants breathe through tiny openings on the undersides of their leaves called
stomata. Plants can (and do) open and close their stomata under certain
conditions, for example if heat becomes excessive and causes a plant to start
loosing more water than it can take up, the plant will close it's stomata to
slow down the water loss.
The ideal humidity range for healthy plant growth is 50% humidity, plus or minus
10%. Unfortunately, by closing the stomata and slowing evaporation the plant
also has slowed down its cooling mechanism. This causes heat to build up in the
plant tissue, and in temperature too hot the plant actually cooks itself. It is
important to understand the opening and closing of the stomata and how it, in
turn, controls plant transpiration.
Plant transpiration is how plants breathe. Plants do not have lungs, however, so
when molecules of gas and water vapor are released from the stomata they tend to
just hang there in the absence of any breeze. That is why it is so important to
have box fans or oscillating fans in a garden to circulate the air (in addition
to exhaust fans). These fans are actually like the plant's lungs, and without
them the plants would have no way of moving fresh CO2 molecules into contact
with their plant tissue. The plants would slowly choke on their own transpired
gasses and water vapor.
As water evaporates from the surfaces of leaves, the surface tension of the
water molecules tend to pull the next water molecule along behind it, up through
the plant's veins. Water is pulled up through the plant stem, which is pulled
from the plant's roots. This creates a negative water pressure in the root zone
and allows the roots to suck moisture up out of the root zone like a straw. The
process of water absorbing into the plant through the roots is known as osmosis.
Which brings me back to humidity. Water vapor is humidity. As a plant
transpires, the humidity immediately surrounding the leaves will become
saturated with water vapor. Now, the entire plant transpiration cycle is
controlled by evaporation. When gasses surrounding a leaf become saturated with
water vapor (100% humidity), there is no place for the next molecule of water
vapor to evaporate to.
The end result is that water vapor is not evaporating, so water is not being
drawn up from the root zone...and neither are any nutrients. If nutrients are
not being taken up, than developing fruits are not getting the food they need to
be healthy. This is exactly why high humidity will cause blossom end rot in
fruiting tomatoes just like a Calcium deficiency. It is another reason why it is
so important to keep box fans and oscillating fans in the garden area to keep
the air circulating.
So, evaporation controls plant transpiration. High temperatures and low humidity
therefore both cause fast transpiration. Fast transpiration means your plants
will be taking up and using lots of water (and nutrients). This is fine, unless
you were feeding your plants strong to begin with. Your plants can only handle
so much fertilizer within a specific period of time.
So if you now have warm temperatures, low humidity, and fast transpiration rates
you may find your plants are using a little too much fertilizer a little too
quickly. Leaf tip burn is usually a sign of this. Under these circumstances you
can feed your plants with a weaker nutrient solution.
Or, if all other plant growth influencing factors are in their ideal ranges, you
can try to maximize plant growth by adding CO2 (in which case you should
experience heavy plant growth without showing any signs of stress or damage.
Effect of Carbon dioxide and Oxygen on plant growth:
The manufacturing of sugar by plants requires the presence of carbon dioxide and
hence it is one of the vital elements for plant growth. It is a known fact that
plants can use as much as 1500 parts per million of carbon dioxide. In case the
natural carbon dioxide available in the air is not enough, there exist Carbon
dioxide injectors that promote enhanced plant growth. Oxygen is essential for
plant respiration and utilization of photosynthesis byproducts.