Physiological, biochemical, and molecular responses of fruit trees to root zone hypoxia

Plant hypoxia (low-oxygen stress or oxygen deficiency stress) is due to soil waterlogging, flooding, or soil compaction. In agricultural areas that already experience hypoxia in the root zone, and in agricultural production areas where hypoxia has never been observed, the potential for soil waterlogging and/or flooding is expected to increase as result of extreme weather events, such as intense storms and hurricanes, associated with global climate change. Hypoxia is considered a major abiotic stress to fruit trees with adverse effects on plant growth and development, cell division, cellular metabolism, energy consumption, transcriptional regulation, mineral uptake, plant metabolism, and physiological variables including leaf water potential, stomatal and hydraulic conductances, photosynthesis, hormone biosynthesis, carbohydrate mobilization, and reactive oxygen species (ROS) generation/scavenging. Phytohormones including auxin (IAA), ethylene (ET), gibberellin (GA), and abscisic acid (ABA) have a key function in responses to root zone hypoxia. Fruit trees exhibit a series of physiological, biochemical, and molecular responses by changing their metabolisms to survive hypoxic conditions in the root zone. Cutting-edge technologies such as engineering of rootstocks, identification of QTL (quantitative trait loci) through gene mapping and phenotyping, and marker-assisted breeding may help in developing new rootstocks for low soil oxygen conditions. This review provides an overview of the current state of knowledge of biochemical, physiological, and molecular mechanisms in fruit trees in response to root zone hypoxia that have not yet been reported, and provides insights into fruit trees' adaptions to hypoxic stress conditions.