Feasibility of using LVDT and watermark sensor for irrigation scheduling in plum
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The usefulness of continuous measurements of soil and plant water status for automated irrigation scheduling was studied in a drip-irrigation experiment on plum (Prunus salicina `Black Gold'). Soil matric water potential (psi(soil)) was measured with granular matrix sensors (Watermark) and short period variations of trunk diameter (TDV) with linear variable displacement transducers (LVDT). From TDV measurements, maximum daily shrinkage (MDS), evolution of maximum trunk diameter (MXTD) and trunk growth rate (TGR) were calculated. Their performance was compared to discrete measurements of midday stem water potential (psi(stem)). The Watermark sensor data were correlated with plant water status during all the season (r(2) = 0.56**), but were highly variable (CV approximate to 35-50%). TDV parameters were better related to psi(stem) and less variable in our conditions. During most of the fruit growth period (Stages II and III), MDS was higher in the less irrigated treatments than in the control and well correlated (r(2) = 0.89**) to psi(stem). However, after harvest, when TGR was higher, this correlation decreased (r(2) = 0.73-0.52**) and so did the slope (between MDS and psi(stem)) as the season progressed, suggesting tissue elasticity changes, but then, TGR differences between control and stressed treatments were better related to plant water status.