What Affects Plant Growth

It should come as no surprise that environmental conditions can affect how well a plant grows and how widely they are distributed.  And when those conditions are not ideal, they become limiting factors in the growth of plants.  Most problems in plants result from environmental stresses, so it is important to understand the four major environmental factors affecting plant growth:  light, temperature, water, humidity, and nutrition.

How Light Affects Plant Growth

The quantity, quality, and duration of light affect plant growth.  Light quantity refers to the intensity or concentration of sunlight.  It is most intense in summer months, and least intense in winter months.  The more sunlight that is present, the more food a plant can produce through photosynthesis (up to a point).  When sunlight decreases, so does food production. 

It is possible to alter the light quantity occurring in nature.  To reduce the amount of sunlight, you can plant in areas near large shade trees.  You can also use shade cloth above outside plants or on the roof of greenhouses to reduce the amount of sun reaching the plants below.  You can also increase the amount of light available to plants by placing reflective material around the plants or augmenting the natural light with grow lights.

The color or wavelength of light that reaches a plant determines the light quality.  Light is created from a combination of colors with differing wave lengths.  Black is due to the complete absence of light.  Sunlight that is visible to humans is white light.  When passed through a prism, sunlight or white light separates into its various visible light colors or wavelengths, specifically red, orange, yellow, green, blue, indigo, and violet from longest visible wavelength to shortest visible wavelength.   The sun emits other colors and wavelengths not visible to humans that are even longer (e.g. far-red, infrared, and heat), or shorter (e.g. ultraviolet), than the visible wavelengths.  Colors with shorter wavelengths (i.e. blue/violet) have the most energy.

While humans see best in yellow/green regions of the light color spectrum, the same color region is inefficient for photosynthesis.  Plants can synthesize food more efficiently by absorbing red and blue light.

It is impossible for artificial light sources to duplicate the quality of sunlight.  Incandescent lights offer a broad range of light color, but are low in the amount of blue light emitted. They also produce a lot of heat.  The light produced has more far-red light than red light, which leads to changes in the plant’s form.  This is called photomorphogenesis.  Far-red light causes plants to stretch, or develop elongated stems and/or larger leaves.  For incandescent lights to drive photosynthesis, the intensity of the light would need to be increased to levels where the resulting heat would become problematic.

Fluorescent light emits the blue and red wavelengths (with no far-red) which is desirable for photosynthesis.  But the quantity of light emitted is very low and inefficient for photosynthesis.  They only become beneficial when the light can be placed very close to the plant as with African violets or when germinating.  The red light promotes the germination of many varieties of plant seeds.  Fluorescent grow lights most closely emulate the production of sunlight. Cool white fluorescent bulbs are likely more cost effective, but are less effective from a photosynthesis perspective.

Light duration, or the photoperiod, is how long a plant is exposed to or has been deprived of sunlight.  In nature, this is the proportion of light and dark in a 24-hour day.  The effects on plant development by light duration is photoperiodism.  It was originally thought that flowering of plants was triggered by the length of light periods in a day, but it was later discovered that it was instead the length of dark period that promotes flower development.

There are three categories of plants based on how flowering responds to the duration of light or dark.  They are short-day, long-day, and day-neutral plants.

Short-day plants require longer periods of darkness at night uninterrupted by light to flower.  Many spring and fall flowering plants are short-day/long-night plants.  Long-day plants require longer days and short nights to flower.  These include all summer flowering plants and many vegetables.  Day-neutral plants will flower regardless of the length of day or night.  Many of these are tropical plants with origins in the equatorial regions of the earth. 

How Temperature Affects Plant Growth

There are several ways that temperature can affect a plant’s growth and productivity.  For example, there are warm-season crops and cool-season crops.  If the days are too warm and long, cool-season crops will bolt or develop flowers/seeds.  If the days are too cool, warm season crops like tomatoes will not develop fruit at all. 

The change in temperature from night to day is known as the thermoperiod.  When the day temperature is about 10-15 degrees higher than the night temperature, plants produce maximum growth.   This allows plants to produce food via photosynthesis and respire during the day and reduce the rate of respiration at night. 

Photosynthesis must occur at a faster rate than respiration for plant growth to occur.  If the temperature is too high, it can lead to respiration using up stored carbohydrates faster than photosynthesis can produce them.  If temperatures are too low, photosynthesis and plant growth can be slowed producing lower yields.  But different plants have different optimum temperature ranges.

Some plants can withstand cold temperatures, and are referred to as [cold] hardy plants.  Those that cannot are called non-hardy.  Some plants require a certain number of days at low temperatures for proper growth to occur.  Winter injury can occur if non-hardy plants are exposed to temperatures that are too low or if low temperatures that they might normally withstand occur too early in the fall or late in the spring.  Winter injury can also occur if the plants become dried out or desiccated.   When the soil is frozen, it drastically restricts the flow of water into plants. 

How Water Affects Plant Growth

Water has many important functions in a plant.  In addition to being a primary material used in photosynthesis, water is the main medium for transporting minerals from the roots to stems, flowers, and leaves as well for transporting food for storage or use.  Water makes up most of the plant’s cell protoplasm and helps to maintain turgor pressure throughout the plant.  Water is responsible for regulating transpiration by regulating the opening and closing of the plant’s stomata.  The release of evaporated water through stomata during transpiration helps to cool the plant.

Relative humidity is a ratio of the amount of water vapor present in air at a given pressure and temperature to the maximum amount that the air could hold at that same pressure and temperature.  It is always expressed as a percentage.  For example, if a pound of air at a given temperature and pressure can hold at most 5 grams of water vapor and it is currently holding 3 grams of water vapor at that same temperature and pressure, then the relative humidity is 3/5 or 0.6, which yields a relative humidity of 60%.

Water vapor diffuses when air with one relative humidity encounters air with a different relative humidity.  The vapor moves from the area of higher relative humidity into the area of lower relative humidity until both air masses reach an equilibrium and have the same relative humidity.  The greater the difference in relative humidity, the faster the diffusion will occur.  Warm air can hold more water vapor than cold air.  So, if the temperature increases while the amount of vapor in the air remains the same, then the relative humidity will drop.  This concept is what drives respiration in plants.

The air between cells within a leaf has a very high relative humidity, almost 100%.  This is almost always higher than the relative humidity outside the plant.  When stomata on a plant open, this difference in relative humidity cause the water vapor inside the plant to diffuse into the air outside the plant.  This creates a tiny high humidity dome of air around each stoma which then slows the transpiration rate and cools the surface of the plant around the stomata.

How Nutrition Affects Plant Growth

Plants need both water and nutrients to survive.  The nutrients are used throughout the plant for various processes like photosynthesis and respiration.  They react with complex carbohydrates produced and stored during photosynthesis to create simple carbohydrates and to release energy.

Plants get their water and minerals primarily through the fibrous portions of their root system.  Water from the soil is pulled from the soil into the roots up the stems to the leaves and out the stomata during transpiration as previously discussed.  Nutrients are often dissolved in the soil water and absorbed as the plant absorbs the water from the soil.  Most of these nutrients are absorbed as charged particles called ions.  Those that are positively charged are called cations, while those negatively charged are called anions.

Nutrients move to and collect at the root surface, either by diffusion along a concentration gradient or by encountering the tips of the roots.  Once located near the surface of the root, nutrients may require chemical energy for the roots to absorb them.

Water can be absorbed by both passive and active processes.  When absorbed passively, no chemical energy is required.  As leaves transpire and moisture is loss, the water and nutrients get pulled through the plant’s vascular system from the roots to the stems to the leaves to replace the transpired water vapor.  However, active absorption does require chemical energy, which means respiration needs to occur to convert stored carbohydrates to convert them to chemical energy.  If no oxygen is present, then respiration cannot occur, and the stored carbohydrates cannot be converted to energy, preventing the active absorption of water and minerals.  If photosynthesis is hindered or prevented for any reason (such as lack of sunlight, water, or carbon dioxide), then sugars cannot be produced to use as energy, which can lower the active absorption of water and minerals.  If it is a dormant period in the plants life cycle, which is often the case in winter, then all processes slow, which mean few nutrients will be absorbed.

Once nutrients are transported from the roots through the plant’s vascular system to the living cells that need them, they must pass through the cell membrane to enter the cell. This absorption can occur three ways.

In the first method, an entire ion pair moves through the cell membrane into the cell and no ions are displaced or exchanged from the cell.  If the cell is actively transporting the ion using energy, then each of the two particles that makes up the ion pair are pulled through the cell wall separately.  Most negatively charged anions are actively absorbed in this manner using energy.

In the second method of absorption, the cell exchanges one charged ion for another with the same charge.  For example, a hydrogen cation (H+) is often released so that another cation like potassium (K+) can be absorbed.  Cations are often passively absorbed in this manner without energy.

While both absorption methods above can be either passive or active, the third method is always active requiring energy to occur.  It is called the carrier system, and it occurs when specialized chemicals in the cell membrane called carriers attract an ion outside the cell membrane through a chemical change or reaction.  The carrier then releases the ion inside the cell where it combines with other ions through subsequent reactions to prevent it from exiting the cell.

For a plant to grow and remain healthy, active absorption of nutrients must be present in addition to passive absorption.  Healthy plants cannot rely on passive absorption of minerals alone.