Effect of Abiotic Stress on Plant growth stages.

(Shehzad Ahmad Kang, Gojra)

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.

SHEHZAD AHMAD KANG
About the Author: SHEHZAD AHMAD KANG Read More Articles by SHEHZAD AHMAD KANG: 9 Articles with 12819 views I am chairman of SFAD Seed & Services.
I do M.Sc(hons) Plant Breeding & Genetics from University of Agriculture,Faisalabad,Pakistan.
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