Drought

First, we begin by defining drought. The NOAA defines drought as "a deficiency in precipitation over an extended period, usually a season or more, resulting in a water shortage causing adverse impacts on vegetation, animals, and/or people. It is a normal, recurrent feature of climate that occurs in virtually all climate zones, from very wet to very dry.  Drought is a temporary aberration from normal climatic conditions, thus it can vary significantly from one region to another." http://www.nws.noaa.gov/om/brochures/climate/DroughtPublic2.pdf 

"VegDRI combines satellite-based observations with climate and biosphysical information to map drought’s effect on vegetation at a one-kilometer resolution"

Drought can be measured many different ways today including, ways to measure vegetation response to drought. The monitoring and analysis of drought have long suffered from the lack of an adequate definition. Drought indices have "slowly evolved during the last two centuries from simplistic approaches based on some measure of rainfall deficiency, to more complex problem-specific models."  http://dx.doi.org/10.1175/1520-0477(2002)083<1149:AROTDI>2.3.CO;2   "The twentieth century concluded with the development of the Drought Monitor tool, which incorporates Palmer's index and several other (post Palmer) indices to provide a universal assessment of drought conditions across the entire United States."  http://dx.doi.org/10.1175/1520-0477(2002)083<1149:AROTDI>2.3.CO;2
Generally, droughts are categorized in four groups: meteorological, hydrologic, agricultural, or socioeconomic. Both meteorological and hydrologic refer to water availability, while agricultural and socioeconomic relate more to drought impact. https://www.fs.fed.us/sites/default/files/DROUGHT_book-web-1-11-16.pdf

drought.unl.edu/DroughtBasics/TypesofDrought.aspx

Now that we have defined drought and the resources used to measure it, we can begin discussing what adaptations and responses plants have to drought. "Global plant productivity that once flourished under warming temperatures and a lengthened growing season is now on the decline, struck by the stress of drought." https://www.nasa.gov/topics/earth/features/plant-decline.html
A vascular plant's anatomy, morphology and physiology is designed to obtain and retain water. Most vascular plants can undergo short time periods of drought but long term generally results in plant death. "Experts consider a plant to be drought tolerant if it can withstand a moderate period of limited moisture. This does not imply that a drought-tolerant plant prefers hot, dry conditions or that the drought will not adversely affect the plant. Woody plants are typically more tolerant of water stress than herbaceous plant varieties because they can store more energy in their roots and woody tissues." https://content.ces.ncsu.edu/coping-with-drought-a-guide-to-understanding-plant-response-to-drought Limited soil moisture content can cause a chemical release of abscissic acid in plants that causes closing of the stomata. Closing the stomata causes reduced transpiration which allows plants to cool off and reduced photosynthetic production which will negatively affect the plant by causing nutrient deficiency. Plants that are adapted to hot, dry environments, such as cacti, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. Some other short-term, physical, survival mechanisms include: wilting, marginal leaf scorch and loss of some foliage in an effort to preserve energy. "In the long term, after severe drought, twigs and branches may die back. You may notice a reduction in flower and fruit production and a decrease in leaf size." https://content.ces.ncsu.edu/coping-with-drought-a-guide-to-understanding-plant-response-to-drought Some plants can under go dormancy and often appear dead during a period of drought stress and return to life when conditions have improved; these plants are referred to as resurrection plants.
 Plants have also exhibited expression of different gene controls under drought stress. "A range of tools, from gene expression patterns to the use of transgenic plants, is being used to study the specific function of these genes and their role in plant acclimation or adaptation to water deficit." http://www.publish.csiro.au/fp/FP02076

Because of climate change and the increasing temperature of the earth, continued research on plants and their ability to survive arid environments will be crucial in areas like agricultural and land management practices. https://f1000research.com/articles/5-1554/v1

http://drought.unl.edu/