Salinity stress in date palm (Phoenix dactylifera L.): tolerance, mechanisms and mitigation

The salinization of soil and irrigation water is becoming an increasingly serious constraint for the cultivation of fruit crops, especially in arid and semiarid areas. The effect of salinity of different ionic compositions on date palm growth and tolerance at different stages of growth and development has been described in this review; the tolerance is based on the type of salt stress: osmotic effect, specific ion toxicity, and/or nutritional imbalance. Up to recently, the mechanisms underlying salt tolerance of date palms are far from being completely clarified. In response to salinity stress, date palm adapts via different mechanisms, including ion homeostasis (Na exclusion by the root system and the ability to sustain a proper K/Na ratio), reactive oxygen species scavenging, and osmotic adjustment to limit water loss and protect the photosynthesis process. Much work is needed at the molecular and biochemical levels toward a full understanding of the mechanisms of salinity tolerance in date palms. The expression analysis of some genes (PdGLX1, PdPIP1;2, PdDJ-1 and PdVIK) promote date palm growth by enhancement of methylglyoxal (as toxic oxygenated short aldehyde) detoxification activity and decreasing the accumulation of reactive oxygen species under salt stress environment. The extent of tolerance mechanisms is strongly cultivar-specific; so, supplementary research is required to recognize the genotypic variation in salt adaptation among date palm cultivars in the long term and thus start new breeding programs. The cyclic use of saline water and good quality water and avoidance of irrigating trees with high saline water at sensitive stages of growth are, also, good and safer strategies to reduce the negative impact of salt stress.