Long-term hydraulic acclimation to soil texture and radiation load in cotton

The concept of root contact hypothesizes that the absorbing roots grown in sandy soil are only partially effective in water uptake. Co-ordination of water supply and demand in the plant requires that the capacity for water uptake from the soil should correspond to an operational rate of water loss from the leaves. To examine how the plant hydraulic system responds to variations in soil texture or evaporative demand through long-term acclimation, an experiment was carried on cotton plants ( Gossypium herbaceum L.), where three grades of soil texture and three grades of evaporative demand were applied for the whole life cycle of the plants. Plants were harvested 50 and 90 d (fully grown) after sowing and root length and leaf area measured. At 90 d hydraulic conductance was measured as the ratio of sap flow (measured with sap flow sensors or gravimetrically) and water potential. Results showed that for plants grown at the same evaporative demand, those in sandy soil, where rootspecific hydraulic conductance was low, developed more absorbing roots than those grown in heavy-textured soil, where root specific conductance was high. This resulted in the same leaf specific hydraulic conductance (1.8 ¥ 10 4 kg s 1 Mpa 1 m 2 ) for all three soils. For plants grown in the same sandy soil, those subjected to strong evaporative demand developed more absorbing roots and higher leafspecific hydraulic conductance than those grown under mild evaporative demand. It is concluded that when soil texture or atmospheric evaporative demand varies, plants co-ordinate their capacities for liquid phase and vapour phase water transport through long-term acclimation of the hydraulic system, or plastic morphological adaptation of the root/leaf ratio. Key-words : evaporative demand; hydraulic conductance; leaf area; root length; root-to-shoot ratio; soil texture.