Combined soil physical stress of soil drying, anaerobiosis and mechanical impedance to seedling root growth of four crop species

Soil compaction often creates combined physical stresses of drought, anaerobiosis, and mechanical impedance in field soil. This paper aims to analyze the effect of combined and independent soil physical stresses on crop root growth to find out the species-specific response to the physical stresses, which has not been reported before. Drying stress without the increase of mechanical impedance was evaluated in a very loose pot soil environment. This drying stress did not modify the root elongation rates of rice and pea by the 48 h exposure to the stress environment. For maize and cotton, however, mild drying stress (-80 kPa Psi w) enhanced root elongation by 17-18%, but severe drying stress (-900 kPa Psi w) reduced it by 17-21% as compared with the control environment (-10 kPa Psi w). The combined stress of drying and mechanical impedance nearly stopped the root elongation in all the species, while that of anaerobiosis and mechanical impedance did not stop the elongation of rice and. cotton; cotton elongated about 32% of control environment. In maize, root diameter was reduced by the severe drying stress due to the reduction in the number of cortical cell layer and diameters of both central cylinder and xylem vessel. In contrast, cotton showed a significant increment of cortex diameter, although overall diameter was not statistically increased by the severe drying stress. The ability of cotton to continue elongation under anaerobiosis and mechanical stress implied the higher penetration ability to the hard pan layer under the anaerobic condition just after the heavy rainfall.