ABSTRACT
Agricultural drought is defined as ‘‘a climatic excursion involving a shortage of precipitation sufficient to adversely affect crop production or range productivity.’’[1,p.2] For centuries, plants have been classified based on their response to drought. Theophrastus, the Greek philosopher and botanist (371/370-288/ 287 B.C.), divided plants into groups according to their need for water. He said, ‘‘For there are some plants which cannot live except in wet; and again these are distinguished from one another by their fondness for different kinds of wetness . . .Others . . . seek out dry places.’’[2,p.31-33] In the early 1900s, researchers tried to define ‘‘drought resistance’’ exactly. Maximov[3] followed the definition of Kearney and Shantz[4] and Shantz,[5] and defined drought-resisting plants as those that ‘‘resist drought by storing up a supply of water in their fleshy bodies, to be used when none can be obtained from the soil . . .To this type belong succulents, such as cacti and Agave, and many epiphytes. Plants of a non-succulent type, but with large water reservoirs in their stems or in their underground organs, e.g., many trees of the African grasslands, which spring into bloom before the rains, are also included by Shantz in this group.’’[3,p.309] Levitt’s[6]
definition of drought-resisting plants is widely taught today. He divided them into two groups: droughtavoiding and drought-tolerating plants.[6,p.355]
Drought avoidance can be achieved through restriction of water loss or by expansion of the root system to reach a greater supply of water.[7,p.3] Tolerance is the ability of an organism to perform well, or survive, despite the existence of a stressed condition within its tissues.[7,p.35] The distinction between droughtavoiding and drought-tolerant plants is not always clear, and Levitt, [6,p.418] added groups such as ‘‘tolerant avoiders’’ to cover ‘‘more complicated’’ situations.
NEED FOR QUANTITATIVE DEFINITION
