Mechanical Impedance Effects on Oxygen Uptake and Porosity of Drybean Roots 1

Autor: A. J. M. Smucker, T. E. Schumacher
Rok vydání: 1981
Předmět:
Zdroj: Agronomy Journal. 73:51-55
ISSN: 1435-0645
0002-1962
DOI: 10.2134/agronj1981.00021962007300010012x
Popis: Low oxygen concentrations and mechanical impedance are two components of soil compaction often implicated in the reduction of crop yields. The relative importance of these two stresses to the root system has not been thoroughly assessed. A system was developed to study the effect of mechanical impedance on root respiration and morphology. Oxygen uptake, by intact plant root systems subjected to three levels of mechanical impedance, was determined polarographically by measuring the oxygen concentration of nutrient solutions before and after flowing through a root media of 1 mm, 3 mm, or no glass beads. Oxygen uptake rates by drybean (Phaseolus vulgaris L.) roots were independent of solution flow rates > 6.0 ml/min' when inlet pO2 was 0.21 atm. Mechanical impedance reduced dry weight, fresh weight, volume, and length of roots after 8 days of treatment. Dry matter percentage of mechanically impeded roots was greater than the control. Roots subjected to mechanical impedance were deformed, branched more frequently, less porous, and consumed more oxygen per unit fresh weight. It is suggested that a greater oxygen supply may be required at the root surface to prevent anoxia in mechanically impeded roots. Additional index words: Anoxia, Root respiration, Phaseolus vulgaris L. S compaction substantially alters the environment surrounding plant roots. Anoxia, oxygen deficiency, has been implicated as a primary factor contributing to reduced yields on compacted soils (22). Mechanical impedance also reduces growth and modifies the morphology of roots growing in compacted soils (16). Several investigators (2, 8, 21) have demonstrated a greater than additive reduction of root growth when mechanical impedance and low oxygen concentrations occur together. One explanation for these 1 Contribution from Michigan State Univ. Financed in part by the Michigan Agric. Exp. Stn. and Michigan Dry Edible Bean Production Research Advisory Board. Michigan Agric. Exp. Stn. J. Article No. 9208. Received 29 Oct. 1979. 2 Graduate research assistant and associate professor of Soil Biophysics, Dep. of Crop and Soil Sci., Michigan State Univ., E. Lansing, M1 48824. observations could be that mechanical impedance induces changes in the root system, predisposing them to an oxygen deficiency. The maintenance of an optimum oxygen concentration in root tissue is a function of both soil and plant properties. Soil physical properties (i.e., porosity, water content, etc.) determine the rate at which oxygen diffuses to the root surface. The lower rate of oxygen diffusion to the surface of roots growing in compacted soils is well documented (15). Several plant properties determine the rate of diffusion and consumption of oxygen within root tissues. Properties affecting the concentration of oxygen in roots include radial and longitudinal diffusivity of oxygen, membrane permeability, uptake rate of oxygen, root diameter, and porosity (11, 12). Mechanical impedance appears to increase the diameter of roots (3, 17). The effects of mechanical impedance on other root properties affecting oxygen concentrations in roots are unclear. Although root porosity has not been measured for mechanically impeded roots, there are indications that the porosity of roots may decrease when subjected to soil compaction. Goss and Walter (6) observed distorted cortical cells in barley (Hordeum vulgare L.) roots growing in a rigid 1 mm glass bead system. Distorted cell growth could cause compression of the intercellular spaces and reduce the diffusivity of oxygen and perhaps other gases through root tissue (4). Tackett and Pearson (21) demonstrated that roots of cotton (Gossypium hirsutum L.), mechanically impeded by a high subsoil bulk density, produced more carbon dioxide than non-impeded roots. A greater production of carbon dioxide by mechanically impeded roots implies a greater oxygen requirement. The objective of this study was to determine the influence of mechanical impedance on the porosity, oxygen uptake and other morphological and physiological properties of drybean (Phaseolus vulgaris L.) roots which in turn influence the internal oxygen concentration of the roots. Published January, 1981
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