ABSTRACT
Surviving periods without water is one of the greatest challenges faced by many plants. To cope with this challenge, plants have developed several strategies: either adaptation mechanisms which allow them to survive the adverse drought conditions, or the possession of particular growth habits to circumvent or avoid drought. Both mechanisms must have arisen under the same evolutionary constraints to enable plants to cope with low water availability. Plants avoid drought by completing their life cycle during the wet season when sufficient water is available. This strategy has been adopted by many flowering annuals. Another drought-avoidance mechanism is the formation of deep roots which allows better access to groundwater resources. The development of deep roots is an example where it is difficult to distinguish between avoidance and adaptation mechanisms. Most adaptation mechanisms are constitutive and are also present during non-stressful conditions. The objective of drought-adaptation mechanisms is to decrease transpiration and to improve water up-take. The development of succulence in leaves and roots, sunken stomata, reduction of transpiring surfaces even by the shedding of leaves or the presence of specialized photosynthetic pathways (C4 and CAM plants) are examples of drought-avoidance mechanisms. In summary, adaptation includes modifications of a plant on the morphological, anatomical and/or biochemical level to cope better with water-deficit.
