The role of root system architecture and root hairs in promoting anchorage against uprooting forces in Allium cepa and root mutants of Arabidopsis thaliana

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Abstract
The role played by lateral roots and root hairs in promoting plant anchorage, and specifically resistance to vertical uprooting forces has been determined experimentally. Two species were studied, Allium cepa (onion) which has a particularly simple root system and two mutants of Arabidopsis thaliana, one without root hairs (rhd 2‐1) and another with reduced lateral root branching (axr 4‐2). Maximum strength of individual onion roots within a plant increased with plant age. In uprooting tests on onion seedlings, resistance to uprooting could be resolved into a series of events associated with the breakage of individual roots. Peak pulling resistance was explained in a regression model by a combination of a measure of plant size and the extent to which the uprooting resistance of individual roots was additive. This additive effect is termed root co‐operation. A simple model is presented to demonstrate the role played by root co‐operation in uprooting resistance. In similar uprooting tests on Arabidopsis thaliana, the mutant axr 4‐2, with very restricted lateral development, showed a 14% reduction in peak pulling resistance when compared with the wild‐type plants of similar shoot dry weight. The uprooting force trace of axr 4‐2 was different to that of the wild type, and the main axis was a more significant contributor to anchorage than in the wild type. By contrast, the root hair‐deficient mutant rhd 2‐1 showed no difference in peak pulling resistance compared with the wild type, suggesting that root hairs do not normally play a role in uprooting resistance. The results show that lateral roots play an important role in anchorage, and that co‐operation between roots may be the most significant factor.