Shrinkage of Attached Roots of Opuntia ficus-indica in Response to Lowered Water Potentials—Predicted Consequences for Water Uptake or Loss to Soil

Attached 2-month-old roots of the succulent plant, Opuntia ficus-indica, shrank 0.4% radially during periods of maximal transpiration under wet conditions. In contrast, reversible decreases in diameter of nearly 20% occurred for these roots as their ambient water potential (ψ) in the vapour phase decreased from −0.01 to −10 MPa over 8 d, the changes being slightly more rapid at 40 °C than at 10 °C. Such substantial diameter changes became progressively less with root age, from a 43% decrease in diameter at 3 weeks to a 6% decrease at 12 months Root shrinkage was slight when ψ was decreased from −0.01 to −0.3 MPa, the latter ψ being similar to the root water potential. As ψ was further decreased from −0.3 to −10 MPa, water movement out of cortical cells caused considerable root shrinkage. The root hydraulic conductivity (Lp) decreased only 30 to 60% for a change in ψ from −0.01 to −10 MPa compared with a decrease of over 106-fold for the soil hydraulic conductivity over this range. The overall conductivity of the soil, the root-soil air gap, and the root was predicted to be dominated by Lp for ψsoil above −0.3 MPa. As simulated ψsoil decreased below −0.3 MPa, the root-soil air gap initially became the primary limiter of water loss from the roots. Below −5 MPa for 1-month-old roots and below −2 MPa for 12-month-old roots, the soil became the main limiter of water loss. Thus, water uptake from wet soils apparently was mainly controlled by root properties Water loss to drying soils was controlled by the development of a root-soil air gap around shrinking roots during the initial phase of soil drying and by the reduction of the soil hydraulic conductivity at even lower ψsoil.