Are The Terms “drought-tolerant” And “irrigation Requirement” Related?
Physiologically, tolerance means that the organism tolerates the stress without suffering major damage or is able to continue to function in a stress-induced state. Plants may do this through drought avoidance, physiological adaptations that afford tolerance, or through efficiency mechanisms. Drought avoidance may be escape, such as a plant that reproduces quickly, thereby avoiding drought, or through conservation mechanisms such as stomatal closure, change in leaf canopy or orientation, or cuticular resistance. They may also avoid drought by developing an extensive root system. Physiological adaptations may include osmotic adjustment, changes in cellular elasticity, and dessication tolerant enzymes, all of which help the plant maintain turgor. Efficiency mechanisms include high water use efficiency (WUE). WUE is often defined as growth per units water provided, photosynthesis per units water, etc. Mechanisms that enhance drought tolerance reduce WUE because growth and carbon assimilation are reduced by leaf firing, rolling, etc. It is important to note that any plant will require frequent irrigation after planting to ensure survival and establishment. For established plants, the irrigation requirement is defined as the water needed, exclusive of rainfall, for a crop to grow without water stress. It is the difference between ET (evapotranspiration or plant water use) and rainfall. ET may be calculated using one of a number of models, all of which have inherent uncertainty. Using climatological data, one can estimate ET and thus calculate the irrigation requirement on a monthly or annual basis as the difference between ET and effective rainfall. Effective rainfall is rainfall that is estimated to remain in the root zone of the plant. Effective rainfall is the result of total rainfall minus losses to drainage and percolation below the root zone.
