**4. Interaction between stress: water deficiency and aluminum toxicity**

In tropical countries, the occurrence of periods of water scarcity is common in regions where the soil is weathered and with high levels of toxic Al. However, with global climate changes, the occurrence of water deficiency has become more frequent. *Toxic Aluminum and Water Deficit Interaction in Plants: Physiological Aspects and Chemical… DOI: http://dx.doi.org/10.5772/intechopen.111418*

Thus, the interaction between toxic Al and water deficiency potentiates the reduction of plant growth and production.

Under non-stressful conditions (absence of water deficiency and Al toxicity), cell growth can be explained by cell expansion resulting from the action of the enzyme xyloglucan endotransglycosylase—XET (EC 2.4.1.207). This enzyme promotes the cleavage and reformation of bonds between the xyloglucan chains (**Figure 5**) allowing cell expansion to occur due to the entry of water into the cell and an increase in cell pressure potential [41].

However, the interaction between water deficiency and toxic aluminum results in lower water influx into the cell and inhibition of XET activity. This set of events implies less cellular expansion of the root system with negative repercussions on plant growth and production (**Figure 5**). Therefore, toxic aluminum has as its primary target the root system whose elongation rate is considerably reduced when there is an interaction between toxic aluminum and water deficiency [8, 42].

The interaction between toxic Al levels and reduced water availability induces an increase in leaf contents of important compatible osmolytes such as glycine betaine, proline, and trehalose. However, a greater accumulation of compatible osmolytes does not prevent the decrease in plant growth [7, 43] represented by the dry mass of roots and shoots, and leaf area [43, 44].

The photosynthetic machinery is greatly affected by the interaction between aluminum toxicity and water deficiency because the concentration of photosynthetic pigments is considerably reduced [8] due to the production of free radicals that lead to lipid peroxidation, degradation of chlorophyll, and carotenoids [9, 45]. The degradation of chloroplast pigments may originate from lower root absorption and reduced

#### **Figure 5.**

*Cell expansion in the presence (left) and absence (right) of water and aluminium. The symplastic and apoplastic influx of H2O into cells increases the activity of the cell wall enzyme xyloglucan endotransglucosylase (XET) and the pressure potential on the cell wall. XET improves cell wall extensibility which is favored by intracellular water influx. These biochemical and physical phenomena imply cell expansion (left). Toxic aluminum reduces water influx into the cell and decreases XET activity. This implies less cell expansion (right). Source: Figure adapted from Yang et al. [42].*

accumulation of magnesium in plant leaves due to toxic Al since magnesium is an integral part of the chlorophyll molecule [46].

Despite the large production of ROS such as O2 − , H2O2, and OH− due to the interaction between toxic Al and water deficiency, plants activate detoxification enzymatic mechanisms. In this context, the enzymes superoxide dismutase and guaiacol peroxidase have their activities increased to reduce the production of ROS and lipid peroxidation [9].

The mineral metabolism of plants is affected by the interaction between Al toxicity and water deficiency because the joint action of these two limiting factors reduces the calcium, magnesium, and phosphorus content in the leaves and roots of plants [47]. Essential macronutrients absorbed mainly by mass flow such as calcium and magnesium can be found in lower levels in plants due to the lower flow of water in the soil-plant-atmosphere system under conditions of water deficit.

This negative effect of water deficiency can be potentiated by Al toxicity. For example, in acid soil with an Al content of 12 mmol L−1 (soil depth of 0–20 cm) and with 50% of its pores filled with water, the contents of nitrogen, phosphorus, potassium, calcium, magnesium, zinc, and manganese were reduced in maize plants under the interaction of stresses [48]. Similarly, in soybean plants, the contents of nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, copper, and manganese are reduced under the interaction of stresses. Furthermore, root density and biomass production of soybean and corn is strongly reduced by the interaction between toxic aluminum and water stress [48].
