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Stress factors due to the presence of pathogens

There are several groups of organisms that cause damage to plants. Without doubt, one of the most harmful is the insects group, as it can cause great damage to crops. Insects can be classified depending on the way in which they cause harm to the plant: sap-sucking insects, insects that gnaw shoots, tunnelling insects, vectors of pathogenic agents, etc.

Infestation by nematodes is increasingly important, due to the practical disappearance of conventional chemical disinfectants. These insects which live in the ground usually attack the roots, leading to deformities and gallnuts, but the symptoms are extremely visible in the top part of the plant, in the form of withering, yellowing, loss of vitality and rot.

Infection due to fungi, bacteria or viruses is also important. The virulence of these infectious agents depends to a great extent on the resistance of the plant, and the conditions in which it grows. The symptoms triggered in a plant due to infection by a pathogenic organism are quite varied: necrosis and deformities, rot and exudation, or alterations in colour, such as the foliar mosaic pathology caused by some viruses. Depending on the state or severity of the infection, losses in production can be quite considerable.

Stress factors due to causes associated with phenological periods

From the germination of the seed to the senescence phase, the plant undergoes vegetative development and is reproduced, based on a series of phases that make up its biological cycle.

This development is marked by a series of physiological and metabolic changes that permit the growth of the plant and the differentiation of its structures, depending on the phenological period it is undergoing, with regard to its sensitivity to any variation on the optimum development conditions. Stress factors coincide with states such as embryogenic development, budding, flowering, fruit set, and ripening of the fruit, during which period the plant is more sensitive due to the complex interior changes it undergoes. The entry of the plant into the different phases is determined genetically and depends on the environmental conditions.

The common sign that each process is starting to take place is a change in the endogenous hormone contents, which are followed by changes in the internal distribution of the plant metabolytes. These phases of sensitivity also coincide with the critical states of growth for obtaining optimum production.

Stress factors due to climatic causes

Among these, the most important are those caused by low temperatures (stress due to cold or frost) or high temperatures (stress caused by excessive heat). Plants suffer severe damage that limit their optimum development, above or below a certain temperature, depending on the plant. Exposure to the cold paralyses enzymatic activity and induces a fall in the fluidity of the cellular membranes, which means that the conveyance of water and nutrients through them may be affected, and the plant may stop producing. If the temperature falls sharply, the plant runs a risk of freezing, with the ensuing formation of ice crystals inside the cells, leading to considerable cellular dehydration.

On the contrary, prolonged exposure to extremely high temperatures causes damage to the cells due to the inactivation of enzymes and de-naturalisation of proteins, leading to an increase in the fluidity of the cellular membranes, i.e., the permeability of solutes through them becomes altered. The heat also intensifies plant respiration in relation to photosynthesis which may lead to paralysation of growth.

Mediterranean climates have hot summers with not only high temperatures, but also intensive drought.

What is known as water stress may therefore include a shortage of water (drought), and also an excess of this element (root asphyxia). By way of example, the first of these occurs when plant transpiration is greater than its water absorption. One of the first signs is a loss of cellular turgidity, which leads to the dehydration of the cell, closing of the stomata and a reduction in the photosynthetic capacity.

Lastly, physical damage caused by climatic agents such as wind or hail, which lead to the rupture of parts or all of the plant, with the ensuing delay or loss in production that this brings.

Stress factors associated with crop management

Crop-growing practices carried out on crops to optimise production may cause damage to the plants and the soil in which they grow, if they are not carried out properly. For example, the use of pesticides may lead to phytotoxicity in the plant, as a result of phytosanitary treatments.

On the other hand, due to poor water management, the salinity of the soil is fast becoming a great problem in international agriculture. Stress due to excess salinity causes toxicity problems in plants due to the high saline concentration, whilst at the same time, producing an osmotic effect that prevents the plant from absorbing the water and nutrients it needs.

The lack of essential nutrients is evident through the development of different physiological and/or morphological symptoms, depending on each case. What is known as deficiency stress, is often not due to the insufficient presence of a certain element in the soil, but to the fact that this element in present in a form that cannot be absorbed by the plant.

On the other hand, plants may suffer physical damage due to improper use of farming tools and machinery. Pruning, damage to roots as a result of farming activities, etc.: when injuries are caused to plants, the severity of these injuries does not so much depend on the damage in itself, but the risk of the plant suffering an infection, since the injury may act as a port of entry for pathogenic organisms.

Correct transplanting is another important farming practice. The crucial factor determining the development of the seedling during the first phase of growth is a good root activity.