The natural conditions in which plants and trees grow are neither uniform nor controlled. Many changes or fluctuations, even if they are temporary, can have a negative impact on and stress plants. The factors which can lead to stress can be one of two types: abiotic or biotic. Stress can have serious repercussions on various phases of a plant’s growth and, ultimately, crop productivity.

While a few stress factors can be measured in advance and prevented, most have to be dealt with after their occurrence. Thus, it is important to be able to recognize their effects.

Abiotic and Biotic Stress

Abiotic Stress

Abiotic stress factors stem from the environment in which the plant or tree grows and include light, temperature, moisture, nutrients, and soil conditions.

Light

Stress due to light usually occurs due to a lack of it. Light is necessary for germination, growth, flowering, fruiting, and proper ripening, so it is important that plants receive enough of it. In the case of photoperiodic plants, even the duration of light can be important. The amount of light plants receive can be measured in lux by a lux meter.

Temperature

Plants are affected if the temperature rises or falls below the optimal range. Trees are more resilient than annuals or biennials to severe temperatures, and seedlings are the most sensitive group. Both proper ambient and soil temperatures are crucial for growing plants and are measured in degrees Celsius or Fahrenheit.

• High temperatures can cause direct irreversible damage to plant tissue, hasten reproduction at the cost of biomass accumulation, and reduce yield. Heat can indirectly affect plants by increasing transpiration and causing plant water deficits.
• Cold temperatures are important, as there is a minimum temperature which all plants need to function. Plants and trees can suffer from chill stress, which results in reduced plant growth, delayed flowering, seed setting and maturity, and lowered yield. The temperature below which chill stress occurs differs based on the type of plant, and is 0 to 4oC for temperate fruits, 8oC for subtropical fruits, and 12oC for tropical fruits.

Moisture

One of the most common stresses that plants will likely experience, especially temporarily, is due to water.

• Drought: A lack of water can occur due to acute or chronic drought, as well as indirectly due to water deficits created through high temperatures. Its effect on plants can be measured through Leaf Water Potential (LWP) of leaves or other tissues in field pressure chambers. 0.3 – 2.5 MPa is the normal LWP range for plants. Drought can affect plant growth, phenology, photosynthesis, and reproduction; ultimately, this decreases yield. In severe cases, the whole crop can be lost.
• Waterlogging: Waterlogging occurs when water accumulation exceeds drainage. This can happen due to flooding (natural causes) or irrigation (manmade causes). When the soil is waterlogged, the amount of gases, including oxygen, diffused in soil decreases. The limited oxygen is not enough to support the proper growth of roots and hampers the development of the whole plant. The anaerobic conditions in the soil encourage the growth of many pathogens, leading to disease. Self-poisoning in roots can also result due to the change in metabolism in anaerobic conditions.

Nutrients

Stress due to nutrients can be one of two types: deficiency, when the essential elements are not present in optimal concentrations, and toxicity, when the elements are in excess.

• Nutrient Deficiency: Deficiency can lead to chlorosis, discoloration, stunted growth, and necrosis. Lack of nutrients, especially of the three major nutrients – nitrogen, phosphorus, and potassium, or NPK – can severely limit crop plant growth and productivity. The other macronutrients – such as calcium, magnesium, and sulphur – as well as the eight important micronutrients – boron, chlorine, copper, iron, manganese, molybdenum, nickel, and zinc – which are important for metabolism, must be present in quantities necessary for plants to function properly.
• Toxicity: Conversely, too much of an element, usually due to the over-application of fertilizer, can stress plants, leading to fertilizer burn, chlorosis, leaf discoloration, stunted or excessive growth, and necrosis. This happens due to:
  • Change in the soil pH,
  • Inhibition of water absorption by roots,
  • Nutrient imbalance, leading to improper proper absorption of other nutrients (an excess of nitrogen, for example, hinders uptake of phosphorus), and
  • Leaching of unused fertilizers into the soil, which affects soil fertility.

Both nutrient deficiencies and toxicities can be measured through:

• Visual observation of specific symptoms in the field,
• Testing the soil for the elements in the laboratory, and
• Plant analysis of plant parts, usually leaves, in the laboratory.

Soil Conditions

Salinity is one of the main soil factors causing stress in plants. Salinity results when soils have a concentration of soluble salts high enough to affect plant growth. Electrical conductivity (EC) of soil and water samples is used to measure salinity and is expressed as dS/m. Soils are considered saline if the EC is 4 dS/m or more. Nearly 20% of irrigated agricultural lands in the world are saline.

Salinity can occur due to natural causes, such as through the weathering of soil, and due to manmade causes, such as when intensive agriculture replaces perennial vegetation, and irrigation brings up salts stored underground. Salinity inhibits plant growth, because

• It prevents absorption of water by plants through osmosis,
• The accumulated salts in the plant injure transpiring leaves, and
• Chronic salinity can cause salts to seep into the subsoil, causing sodicity, which destroys soil texture.

Biotic Stress

Biotic stress arises from the other living organisms that the plants coexist and interact with. Biotic stress factors such as pathogens and herbivores can threaten food security and result in heavy monetary loses.

These stressors can be detected by visual field observations of symptoms, and the presence of causal factors can also be detected through field observations or through laboratory examinations.

Pathogens

Pathogens are organisms that cause disease in plants and can be viruses, bacteria, protists, fungi, or nematodes. They can attack all the parts of the plant, affecting plant growth in myriad ways and reducing the productivity of crops. They can be transmitted by insects, wind, water, and even people.

Herbivores

Herbivores can be small pests or mammals which feed on the leaves and other parts of the plant and can even damage entire crops in severe cases.

• Small pests include insects, mites, snails, slugs, and all kinds of bugs. In the case of insects, caterpillars and mature insects can feed on plants. Large-scale outbreaks of pests such as locusts, or fruit flies can damage whole regions in extreme cases. In addition, many pests act as vectors of disease.
• Rodents do not cause just stress – they can eat an entire plant, or even entire crops, and have previously resulted in famines.
• Ungulates – such as cows, goats, and sheep – when allowed to graze uncontrolled, can cause varying levels of loss in biomass and threaten crop yield.

Detecting Stress Factors

The effect of stress can influence the physiology of plants, change regulatory networks, and alter gene expression. It is often difficult to pinpoint the exact cause of stress in plants, as more than one factor is usually involved, and they will interact synergistically. For example, saline soils are also likely to be suffering from waterlogging, and this makes it difficult to separate the effects of the various stress factors. Therefore, a fair level of expertise is required to identify stress factors. The availability of field instruments for the detection of some factors makes non-invasive measurement easier, with the purpose of narrowing down stress factors without further stressing the plants.

A number of measurements instruments can be used to detect stress in plants, crops and forests. For example:

• Canopy analysis and measuring leaf area index and photosythentically active radiation using a plant canopy analyzer.
• Root analysis measuring root growth and changes using a root imager.
• Measurement of photosynthesis, respiration, transpiration, stomatal conductance, PAR and internal CO­2 using a photosynthesis analyzer.
• Measurement of chemical concentrations, color, photochemical reactions such as photosynthesis, and physical or optical properties such as film thickness, index of refraction, and extinction coefficient using a spectrometer.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

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