THEORY OF MINIRHIZOTRON ROOT DIRECTIONALITY OBSERVATION AND APPLICATION TO WHEAT AND CORN

Root growth orientation, termed root directionality (RD), is a property of root systems conveying information on response to soil environment and about branching character in root development. Observation of the orientation and configuration of the fine, branched roots in soil is very important for understanding response to soil environment, but is currently difficult or effectively impossible. Root directionality may be determined with root observation tubes (minirhizotron, (MR) utilizing the statistical probability that, for a given soil depth increment, the ratio of root numbers impinging on the upper sides to numbers on the lower sides (the directionality ratio, DR) of MRs installed at an angle to the vertical is a function of the vector mean of RD. The theory of RD observation by MRs is presented with calculations indicating that DR increases with greater downward RD and with increased installation angle of MRs. Measurements of DRs were conducted in MRs installed in spring and winter wheat (Triticum aestivum L.) and corn (Zea mays L.) crops established on a Typic Argiboroll soil. The majority of the measurements yielded DR values >1 in the upper 0.5 m of the root zone, indicating downward RD, shifting to lower DR values (many <1.0) at soil depths >0.5 m, indicating sideward to upward RD, or less downward RD. The greater probability of observing axile (seed- or stem-attached) roots in the upper root zone and lateral (branch) roots at greater soil depth is an explanation. The majority of RD values of wheat observations were considerably higher than for corn. The greater downward RD observed for wheat is believed to be linked to a greater degree of root branching in corn and possibly to more favorable water availability for corn.