Temperature and the Development of the Taproot and Lateral Roots of Four Indeterminate Soybean Cultivars1

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Abstract
Soil temperatures vary with depth and time within and among years. An experiment was conducted to quantitatively evaluate the effects of temperature on the development of the taproot and lateral roots of four soybean (Glycine max (L.) Merr.) cultivars at four temperatures. ‘Beeson’, ‘Hawkeye 63’, ‘Wayne’, and ‘Harosoy 63’ soybeans were grown at 17, 21, 25, and 29°C, using a specially designed, constant‐temperature, water bath system. The water bath was located in a temperature‐modified greenhouse where air temperature ranged from 20 to 35°C during the 12‐month experimental program. Serial measurements of taproot and lateral root tip location and number of primary nodes were recorded on alternate days until 19 days after emergence. A primary node was defined as the position at which one or more 1st order laterals emerge from the taproot. The rate of taproot extension increased with temperature, decreased with time at temperatures greater than 17°C, and was greater for the Beeson and Hawkeye 63 cultivars than for the Wayne and Harosoy 63 cultivars when averaged over time. The rate of lateral root extension increased with temperature, increased with time at temperatures less than 29°C, and was greater for the Beeson and Hawkeye 63 cultivars than for the Wayne and Harosoy 63 cultivars at temperature treatments greater than 17°C when averaged over time. The number of primary nodes varied with depth, time, and cultivar. The greatest number of nodes occurred 2.5 to 7.5 cm below the vermiculite surface. The Beeson and Hawkeye 63 cultivars had fewer primary nodes than the Wayne and Harosoy 63 cultivars. The results indicate that the depth of soybean root penetration can be altered to increase available water during periods of stress by cultivar selection and changes in soil temperature.

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