Approach for using trunk growth rate (TGR) in the irrigation scheduling of table olive orchards

Abstract

The management of regulated deficit irrigation with a continuous measurement of the water status would allow obtaining an accurate estimation of the water needs. However, although different types of sensors are available, the threshold and daily management of these data are not clearly defined. Trunk diameter fluctuations are a good example of these type of data. The trunk growth rate (TGR) is considered an early indicator in olive trees. However, the daily TGR values are very changeable, and only cumulative values of TGR show a clear trend. The number of irrigation works using this indicator is scarce. The TGR thresholds considered in these papers are the average of values over a period, and this makes it difficult to use when preparing a daily schedule. The aim of this work is to present an approach that allows using daily TGR data. During the 2015 season, an irrigation experiment was carried out in the Doña Ana farm, a table olive orchard near Seville (Spain). The trees were 30 years old with a space of 7 × 4 m and they were irrigated using two drip lines with a flow rate of 2.5 l h−1. The experiment began in spring and involved three treatments. Control trees were irrigated to maintain the midday stem water potential values at around −1.2 MPa before pit hardening started and at −1.4 MPa after this point. The trees under a mild water stress treatment (MI) were irrigated in the same was as the Control trees, except from DOY (day of the year) 161, the beginning of pit hardening, to DOY 237, when threshold value decreased to −2.0 MPa. The Moderate water stress trees (MO) were irrigated in the same way as the Control trees, except in the same period that MI, but with a threshold value of −4.0 MPa. The midday water potential pattern and leaf conductance pattern suggested that the level of water stress in both treatments was low, slightly higher in MO than in MI at the end of the water stress period. The shoot elongation suggested a period of water stress before DOY 161 in MO trees that the midday stem water potential and leaf conductance did not detect. The maximum daily shrinkage (MDS) signal indicated water stress conditions during the drought period, but both treatments presented similar values. The trunk growth rate (TGR) indicator was useful when maximum perimeter curves, average TGR and daily TGR were used together. All the periods of water stress identified by other indicators were detected when the three TGR data were used. An approach to use these three sets of data is discussed herein.