Assessment of maximum daily trunk shrinkage signal intensity threshold values for deficit irrigation in lemon trees

The response of adult Fino lemon trees (Citrus limon L. Burm. fil.) on sour orange (Citrus aurantium L.) to an irrigation schedule based exclusively on maximum daily trunk shrinkage (MDS) measurements was studied during the 2005–2006 and 2006–2007 seasons. Plants irrigated above their crop water requirements (T0 treatment) were compared with plants under deficit irrigation, whereby the MDS signal intensity (actual MDS/reference MDS) threshold values were maintained at around 1.15 (T1 treatment), 1.25 (T2 treatment) and 1.35 (T3 treatment). Cumulative crop evapotranspiration (ETc) values reached 536.9 and 719.4 mm during the first and the second season, respectively, and the cumulative amounts of applied water in the deficit irrigation treatments were 662.4 mm (T1, 2006–2007 season), 396.3 mm (T2, 2005–2006 season), 554.0 mm (T2, 2006–2007 season) and 220.3 mm (T3, 2005–2006 season), which generated mild, moderate and severe water stress in T1, T2 and T3 plants, respectively. Results indicated that measurements of MDS are suitable for scheduling irrigation, except for rainy periods of low evaporative demand. Therefore, to improve the precision of irrigation management, some changes in the irrigation protocol should be introduced, for instance, using higher MDS signal intensity threshold values and/or a lower irrigation frequency. According to market demand, lemon fruits were harvested on two occasions, showing no effect of irrigation treatment on total yield and total number of fruits per tree. T2 and T3 treatments resulted in a lower yield and number of fruits per tree at the first harvest and modified fruit characteristics. In contrast, the yield at first harvest and number of fruits per tree was not affected in T1 (92% ETc) plants and fruit characteristics were hardly impaired.