Root growth in different soil depths and sap flow of barley (Hordeum vulgare L.) and pea (Pisum sativum L.) under drought

Root systems that proliferate in deep soil domains, when the upper most layers are progressively drying, have been proposed to confer crop drought resistance through the utilisation of deep water. A greenhouse study was conducted to identify root growth dynamics in three soil layers (0-26, 36-62 and 71-97 cm) and to relate them to sap flow rates and soil water content under mild drought. To that end, two crops with different root systems, peas (dicot) and barley (monocot), were grown in a loamy sand soil in 14x100 cm transparent pots and subjected to intermittent and terminal drought. In peas, no increased root proliferation (measured as root length) was observed in the lowest soil layers when the topsoil was dry and sap flow rates, during terminal drought, were similar in the control and the water stressed treatment. However, barley showed an enhanced root growth in the 36-62 cm layer due to the intermittent drought and in the 71-97 cm layer due to terminal drought. Moreover, sap flow rates, as measured during terminal drought, demonstrated increased water use in barley during the early stages of the drought. Our findings show the importance of the drought characteristics (timing, duration and severity) in plant response to drought and highlight the need for development of genotypes with increased root length in deeper soil domains under drought conditions