Testing a novel microtensiometer sensor in a citrus orchard for feedback control irrigation scheduling

Monitoring the plant water status is necessary to identify appropriate irrigation scheduling parameters. Stem water potential (Ψ_stem) is considered the standard measure of crop water status and its measurements have been conducted by using the Scholander pressure chamber (PC) which do not allow continuous monitoring of crop water status. More recently, microtensiometers have been developed to monitor the water potential of the trunk (Ψ_trunk) continuously, potentially overcoming the drawbacks of PC-based measurement.

This study was conducted to test the reliability of the new water status indicator, Ψ_trunk, measured by microtensiometer, comparing it with Ψ_stem values measured with a PC in a 30-year-old mandarin trees.

The research was carried out during the 2022 and 2023 irrigation seasons, on three plots, each with a specific irrigation method. In one of the plots, a sprinkler irrigation system is installed and the irrigation is managed by the farmer (Traditional Irrigation, TI). In the other two plots, a subsurface drip irrigation system is implemented and two irrigation strategies are applied: i) Full Irrigation (FI), in which the entire evapotranspiration is returned, and ii) Deficit Irrigation (DI), consisting in the application of a water saving strategy (1 July - 15 August). In each plot, a representative tree was selected and, starting from July, Ψ_trunk was monitored using two microtensiometers (FloraPulse, CA, USA) embedded directly in the trunk. Measurements cycles of Ψ_stem were taken by the PC on two covered stems, from 6:00 am to 6:00 pm every three hours, on TI tree the day after and three days after the irrigation event in both the 2022 and 2023 irrigation seasons. For DI and FI trees, the same measurements cycles days usually precede and follow the irrigation days. In addition, only in 2022 Ψ_stem were measured weekly at noon.

The Ψ_trunk monitored by the microtensiometer was influenced by the irrigation strategies applied. The greatest variations were observed in the TI thesis, where more negative Ψ_trunk values were recorded the day before irrigation. In both the FI and DI thesis, the seasonal variation of Ψ_trunk was more limited compared to TI. The water potential values on the stem were generally more negative than on the trunk, as would otherwise be expected, but the cycles of daily measurements, carried out with the PC, showed that the most negative values were usually recorded on the stem at 3:00 pm, whereas on the trunk they were recorded from 1 to 4 hours later. The correlations of the averaged values of Ψ_stem and Ψ_trunk showed value of the coefficient of determination R²= 0.43 when all the dataset was considered. However, when the dataset was split according to irrigation strategy, R² increased for FI and TI trees, R² =0.64 and R² =0.60 respectively, while it decreased for DI trees (R² =0.28).

In conclusion, the FloraPulse microtensiometer demonstrated the possibility of providing a better understanding of crop water potential variations in the SPA system, but it is necessary to identify Ψ_trunk thresholds for feedback control irrigation scheduling different from those already well defined in literature for the Ψ_stem.