Studies of soil water drawdowns by single radish roots at decreasing soil water content using computer-assisted tomography

Application of computer-assisted tomography to the attenuation of X-rays has been used to compare the drawdowns in soil water content associated with radish roots at starting soil water contents (theta (v)) of 0.3 cm(3)/cm(3) and 0.1 cm(3)/cm(3), respectively. Decreasing soil water content results in an increase in the appearance of 'beam hardening'. Decreasing soil water content from 0.3 to 0.1 cm(3)/cm(3) caused the transpiration rate to decrease by 6-10 times. This was presumably due to a reduction in the water potential gradient across the root membrane. The transpiration rate decreased less rapidly than did the water content at the soil-root interface, suggesting some osmotic adjustment by the leaves. This osmotic adjustment would allow the plant to maintain transpiration rate even at relatively low soil water content. The drawdown distances associated with roots growing at the lower soil water content were 8 times smaller than those at the high soil water content and the value of theta (v) at the soil-root interface at the end of the transpiration period was 2.5 times lower. The radish roots exhibited a temporary slight decrease in diameter after the transpiration commenced followed by a significant temporary increase. However, root diameter stabilised around its original value when the plant attained an almost steady water uptake rate. Despite the complexity arising from 'beam hardening', CAT scanning can provide valuable information on processes at the root-soil interface.