Arbuscular Mycorrhizal Fungi Provide Complementary Characteristics that Improve Plant Tolerance to Drought and Salinity: Date Palm as Model

Abstract

In arid areas, date palm (Phoenix dactylifera L.) is considered crucial to the ecosystem establishment and maintenance as it protects the surrounding vegetation against desert influences and provides adequate microclimate to the understory crops. Palm grove, the keystone of the oasian ecosystem balance, is often subjected to severe environmental constraints such as nutrient-poor soil, long-term drought, high temperature, salin soil, and desertification. These constraints cause not only reduction in the production of dates, the principal food of humans and animals in the desert, but also accentuate the fragility of this ecosystem that is no longer able to buffer the effects of climate fluctuations. One of the main challenges for agricultural success in arid land is the efficient exploitation of soil, not only as an agricultural resource base but also as a living and fragile system, to guarantee its long-term stability and productivity. Among the soil fertility factors, the biological component is the most important since the agronomic potentialities of a soil depend on it. The management of soil microorganisms as providers of key ecological services is at the forefront of governing sustainable soil fertility by controlling cycles of major plant nutrients. These organisms, often referred to as “ecosystem engineers,” “biocontrol agents,” “biofertilizers,” or “bioenhancers,” can participate in improving plant growth and nutrition, strengthening plant performance, restoring ecosystems, and combating pests and pollution. The most important providers of these ecological services are arbuscular mycorrhizal (AM) fungi (AMF) which can form symbiotic association (mycorrhiza) with roots of most land plants. Mycorrhiza refers to a mutual association or symbiosis between plants and soilborne fungi that colonize the cortical tissue of roots during periods of active plant growth. The ability of the root systems to establish beneficial symbiotic relationships with soil microorganisms represents one of the most successful strategies that land plants have developed to cope with abiotic and biotic stresses imposed during the colonization of terrestrial ecosystems. Mycorrhizas are multifunctional: the multiple benefits gained from complementary characteristics that AM symbiosis provide can be characterized agronomically by increased growth and yield, physiologically by improved nutrients statut and water relations, and ecologically by improved ecosystem stability and preservation. The fungal mycelium that extends from the mycorrhizal roots forms a three-dimensional network linking the root and the soil environment beyond the nutrient depletion zone. It constitutes an efficient system for water and nutrient uptake, scavenging nutrient-poor conditions. The mycelium also contributes to the formation of water-stable aggregates necessary for good soil quality. Mycorrhizal performances are more pronounced under harsh conditions including poor soil, water scarcity and soil salinity. As a result, mycorrhizal plants are often more competitive and better able to tolerate environmental constraints than are non-mycorrhizal plants. In this chapter, we will compile and discuss the current knowledge concerning mycorrhizas occurrence and effectiveness in increasing plant performance in terms of growth, nutrition and protection against detrimental effect of abiotic stresses. An overview of most persuasive and effective uses of AMF to improve date palm growth and productivity in the context of the harsh conditions of arid land is highlighted.