Evaluation of TDR Use to Monitor Water Content in Stem of Lemon Trees and Soil and Their Response to Water Stress

Abstract

The purpose of the study was to compare the response of TDR‐determined stem (θstem) and soil (θsoil) water content to different irrigation managements. θstem (L L−1) was measured with three‐rods TDR probes (70 mm) installed vertically or horizontally into predrilled holes in the trunk of 5‐yr‐old lemon [Citrus limon (L.) Burman f.] trees in a semiarid region (Israel). Four irrigation treatments were established to deliver 100% (“full”), half of this amount on one side of the tree (“50%”), same amount as in 50% but applied to alternate sides (3‐wk intervals) of the tree (“split”), and 0% (“dry”) of the orchard's normal irrigation volume (typically 150–200 m3 wk−1 ha−1). Treated sewage water (σw = 0.9 dS m−1) was used to irrigate the trees for 75 d (end of June and mid September). Changes in θsoil, and θstem were monitored at weekly intervals. Leaf water potential and temperature measurements were used to verify the achieved water stress levels. θstem of the full treatment fluctuated by about 0.02 to 0.03 L L−1 above and below the “prestress” reference level (beginning of the season). In contrast, θstem of the 50% treatment declined (by about 0.07 L L−1) steadily over the season. For the split and dry treatments, θstem decreased by about 0.12 L L−1 relative to the reference level. θsoil and θstem values suggest that some surplus irrigation was applied to the full and 50% treatments. It was found that water stress was reflected in TDR‐measured θstem changes but that these changes were too small for routine irrigation control.