Abiotic Stress Resistance

Abstract

Citrus, one of the most important fruit crops in the world, is sensitive to many environmental stresses often leading to poor tree growth and reductions in fruit yield and quality. Citrus is most often grown in warm climates with well-drained soils, therefore acceptable growth conditions depend upon the quality and quantity of irrigation water and the risk of cold temperatures. Citrus species do not develop a powerful root system and in well-drained soils of subtropical semiarid zones might be subjected to water deficit especially during hot dry summers. Then, these areas often require supplemental irrigation that may prompt the use of low quality water thereby increasing soil’s salt concentration. Even when irrigation water is of good quality, the use of fertilizers and other agro-chemicals raises the likelihood of salts to rise in the soil causing salinity stress, especially high chloride, which in turn is rather detrimental to citrus growth and fruit quality and yield. Negative soil characteristics such as excess calcium, high pH and mineral imbalances also affect citrus fruiting. In calcareous soils, for example, the high pH causes Fe-immobilization in unavailable forms for plant absorption thus causing iron deficiency. In addition, in areas characterized by scarcely drained soil, flooding can affect the soil structure depleting O2 for the radical tissues and provoking a reduction in iron solubility. Moreover, these damaging stresses do not come alone but in combination, in some cases acting according to an additive effect thus leading to a more restricted plant development. In this chapter, citrus plant behavior under the main abiotic stress conditions, such as drought, salinity and low temperatures will be deeply described, taking into account that important differences among genotypes have been described in their response. The effect of heat, flooding and heavy metal stresses will be also considered although the reader can refer to the existing literature to examine in depth these abiotic stresses. In conclusion, the consequences of the combined effect of more than one stress type at once, occurrence that normally mimics the natural environmental conditions, will be also reviewed.