About this Research Topic
Ionic stress, induced by excessive minerals in their ionic state, is a major problem for agriculture and forestry. To cope with the problem of ion excess caused by, in particular, salt ions and heavy metals, interest in perennial plants that can be employed for afforestation and for sustainable land use is increasing. Afforestation may contribute to solving the problem of soil reclamation, as it is a sustainable land use system that serves as an alternative to agriculture. Frequently, trees are more tolerant to ion stress than agricultural crops. Moreover, perennial plants have a great potential for carbon sequestration and forest products can be used for fodder, energy, and timber production.
Ionic stress affects tree physiology in a variety of ways. Tree growth is restrained because water uptake is inhibited by the osmotic effect of salt ions or heavy metal-impaired water channel. In addition, soil-exchangeable cations and anions taken up by roots and transported to shoots via the transpiration stream may accumulate to toxic ion concentrations within the tree. Apart from osmotic stress and ion-specific effects, high toxic ions also induce oxidative stress. Reactive oxygen species (ROS) are a potential threat because they can injure membranes by lipid peroxidation, can disturb the redox regulation of proteins and may even cause DNA breaks.
In general, higher plants have evolved various adaptation mechanisms such as osmotic adjustments, anatomical modifications in the roots to exclude toxic ions, intracellular compartmentalization of these elements, and the accumulation of compatible solutes, all of which increase tissue tolerance to osmotic stress and toxic ion concentrations. There are genotypic differences among tree species that underlie variations in the capacity to cope with toxic ions at the cellular and whole-tree level, for example via ion uptake, transport, and compartmentation in roots, wood, and leaves. However, excessive toxic ions (e.g. salt ions and heavy metals) in the soil result in competitive interactions with essential mineral nutrients, decreasing nutrient availability, impairing nutrient uptake, and leading to nutritional imbalances in trees. It is worthwhile to examine the uptake of essential nutrients as well as the physiological adaptations that perennial trees undergo to withstand long-term ionic stress.
Maintaining ROS homeostasis is critical for plant cells to cope with excessive ionic environments. Antioxidant systems keep ROS under control for extended periods of time. Interestingly, ROS also act as second messengers that induce antioxidant defenses in woody species.
The genetic and transcriptomic bases for physiological salt acclimation are salt sensing and signalling networks that activate target genes. Genetic engineering has been used for improving salt and heavy metal tolerance of economically important trees. The target genes keep ROS under control, maintain the ion balance and restore water status. Moreover, biotechnological approaches such as mycorrhization and polymer amendment hold great promise for ionic homeostasis and improving the mineral nutrition of stressed trees, thus expanding their future utilization in soil reclamation and timber production under ionic stress. The stress signaling networks in water, ionic and ROS homeostasis is of particular interest in woody plants.
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