Info

Short term Long term

Activation of defence system

Damage

Acclimation

Short term Long term

Activation of defence system

Acclimation

Damage

Fig. 3. General plant response against stress conditions caused by a deviation of a given nutrient solution factor from optimum conditions, due to a high or a low level of the factor. In the short term, a movement away from optimum conditions may activate the defense mechanisms that plants have to overcome the stress. In the long term, the response may depend on the severity of the stress. Under mild stress, plants may acclimate to the new conditions. However, under severe stress, plants may be seriously damaged and may even die. The question marks point to the limits of the different ranges of a given factor. The specific values underlying these question marks should be uncovered by researchers in order to plan precise strategies for growers of soilless systems to achieve the best yields and product qualities. Different values might result when different cultivars, developmental stages or levels of other factors are considered.

widely measured in this respect are: the photosynthetic activity, the antiox-idant capacity and oxidative stress, the content and partitioning of several compounds in the plant (carbohydrates, hormones, amino acids, and nutrient elements), the activity of specific enzymes, plant water relationships and the expression of specific genes. It is important to point out, though, that the effect on these processes may depend on the tolerance of the species or cultivar, the level of the factor and the duration of the stress. For example, in the short term, plants may activate their defense mechanisms against stress. However, in the long term, plants may acclimate to a mild stress or may be seriously damaged if the stress was severe (Fig. 3). The resulting severity of the stress in the plant may be evaluated through measurements of chlorophyll fluorescence and/or oxidative stress. In addition, similar symptoms might be a result of different stresses. In that case, it is important to carry our additional measurements to find out the cause of the stress. For example, measurements of nutrient content in plant tissues to assess possible nutrient deficiencies, water potential and osmotic adjustment for a possible osmotic (water and salinity) stress, Na+ and CF in the leaves for a possible salinity stress, root respiration and ADH activity for a possible hypoxia/ anoxia, root growth and water uptake for a possible stress due to low solution temperatures, and root growth and root respiration for a possible stress due to high solution temperatures. In conclusion, in order to give a correct diagnosis, it is important to take into account the following aspects: (i) keeping in mind the growing conditions; (ii) analyzing the severity of the stress and (iii) finding out the cause of the stress.

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Growing Soilless

Growing Soilless

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