ABSTRACT
The growth of plant roots through the soil is a complex and misunderstood process. Roots acquire water and mineral nutrients from the soil while providing a mechanically sound base that anchors and supports the shoot system as it grows up and outward toward light. The soil system is complex and challenges the root system with numerous biotic and abiotic stresses. Abiotic challenges include salt stress and mineral nutrient deficiency. Water can be either limiting (water stress) or excessively available to the level where it actually reduces or depletes oxygen (hypoxia and anoxia respectively) in the soil. Soil compaction can limit the flux of water and oxygen through the bulk soil to the rhizosphere as well as mechanically impede the growth of the roots. A typical soil also contains numerous impenetrable obstacles such as stones and rocks. In response to these challenges, biochemical and physiological response mechanisms have evolved to both avoid and tolerate these different types of abiotic stress. Many of the important aspects of molecular and biochemical processes that impart stress tolerance have been described, but stress avoidance by differential tropic curvature is not well understood. Despite a body of classical work that was begun as far back as the 18th century, very little attention has been paid to these important mechanisms.
