Multiple abiotic stresses occurring with salinity stress in citrus

Citrus, one of the most important fruit crops in the world, is sensitive to many environmental stresses including salt stress. The negative effects of stresses often lead to poor tree growth and reductions in fruit yield and quality. Under natural conditions, citrus trees often experience multiple stresses at the same time so there are direct and indirect interactions between salinity and almost all physical abiotic stresses that include flooding, drought, nutrient deficiency, high irradiance, high temperature, and high atmospheric evaporative demand. In addition, salinity stress also has direct effects on roots predisposing trees to biotic environmental stresses including attack by root rot, nematodes and bacterial disease. The agronomical and physiological responses of citrus exposed to two or more stress factors, can differ depended on stress intensity or duration. Since citrus leaf Cl− accumulation has been linked to water use, for example, other environmental factors including high CO2 concentration, lowered temperature and high relativity humidity which decrease leaf transpiration, can improve the salt tolerance. Citrus rootstocks known to be tolerant to root rot and nematode pests, can become more susceptible to these biotic stresses when irrigated with high salinity water. Root pests can, in turn, affect the salt tolerance of citrus roots and may increase salt uptake. Moderate salinity stress, however, can reduce physiological activity and growth allowing citrus seedlings to survive cold stress and can even enhance flowering after the salinity stress is relieved. In this review, we discuss the currently available information about the effects of salinity in citrus trees from an agronomic and physiological point of view, and how these responses interact with other abiotic/physical and biotic environmental factors. Short-term potential benefits of moderate stresses including salinity, will also be discussed.