Different root types have different temporal and morphological patterns of development, functionality and adaptation. Root based adaptation to stresses can not be assessed on a root system basis, but must be employed on an individual root type basis. Three different types of root are developed by most seedlings: tap root, basal roots, and lateral roots. These have been demonstrated to have temporally and spatially different developmental and functional patterns. If a stress occurs prior to the onset of an especially sensitive type of root or after that type has shut down functionally, the seedling will demonstrate resistance, when it is not correctly resistant. Timing of screening treatments and scoring of the results is, therefore, extremely critical. Different genotypes within a species demonstrate strong differences in numbers and timing of root initiation and functional maturity for each of the root typos. In addition, different types of root demonstrate sensitivity/tolerance to different chemicals, suggesting that functionality and, therefore, resistance/tolerance mechanisms may differ. All root types present on a seedling must be scored for resistance/sensitivity to the stress, even if morphological/physiological symptoms are not readily apparent. The technology (knowledge, software and equipment) necessary to detect tolerance/resistance and to establish genetic selection schemes is available. Root type differences and the potential for genetic selection will be discussed.
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