, Volume 145, Issue 2, pp 157-166

Prediction and measurement of gap water vapor conductance for roots located concentrically and eccentrically in air gaps

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Water movement between a root and the soil depends on the hydraulic conductances of the soil, the root, and the intervening root-soil air gap (Lgap) created as roots shrink during soil drying. To measure Lgap, segments of young cylindrical roots of Agave deserti about 3 mm in diameter were placed concentrically or eccentrically within tubes of moistened filter paper at a known water potential. As the width of the air gap between the filter paper and a concentrically located root was made smaller, measured Lgap increased less than did predicted Lgap based on isothermal conditions. For gaps of the size expected in the soil during water loss from roots (e.g., 10% of the root radius), the underprediction was about 70% and was primarily caused by a lowering of the root surface temperature accompanying water evaporation. As a root segment was eccentrically moved toward the filter paper, the measured Lgap increased. For the most eccentric case of touching the filter paper, the measured Lgap was 2.4-fold greater than for the concentric case, compared with an infinite Lgap predicted if the water potential were constant around the root surface. When a root touched soil with a water potential of -1.0MPa, Lgap estimated using a graphical method increased about 2.3-fold and the overall conductance of the root-soil system increased by 31% compared with the concentric case. For markedly eccentric locations of roots in air gaps, Lgap, which can be the principal conductance initially limiting water loss from roots to a drying soil, can be about 60% of the value predicted for the concentric isothermal case.