The impact of tree phenology on the response of irrigated avocado: The hysteretic nature of the maximum trunk daily shrinkage

The goal of this study was to check the feasibility of the use of dendmmeters to monitor water stress levels in avocado trees (Persea americana Mill, cv. Hass) during the period of 2015-2017. This water stress was generated naturally during events of high vapor pressure deficit (VPD) that normally take place during the Israeli spring (May) and late autumn (September). It was also generated artificially by considering two irrigation regimes, one providing the crop water needs estimated using lysimeter data, and one applying 75% of that amount. The different water regimes and climatic conditions allowed us to investigate the impact of seasonal atmospheric changes on transpiration (T) and on the maximum trunk daily shrinkage (MDS) of the trees. The MDS captured well the differences between the irrigation treatments, showing over 100 mu m increase at peak summer in response to the treatment providing 75% of the irrigation dose applied to the well-watered treatment. We found that T increased linearly with the daily average value of VPD up to around 1.3 kPa, above which the slope of the linear relationship decreased with the increase in VPD. This reduction of T induced moderate changes in MDS under high VPD. The relationships between MDS and VPD and between MDS and T depended on the season, exhibiting substantially lower MDS during the spring and early summer. The hysteretic nature of these relationships indicates that MDS values are related to tree phenology: a VPD of 1.3 kPa corresponds to an MDS of 50 mu m in the first half of the year (flowering and fruit set) and to an MDS of 150 mu m during the second half of the year (fruit growth and maturation). Finally, we found that MDS was highly correlated with the stem water potential, regardless of the irrigation treatment or time of year, and is, therefore, a reliable stress indicator in avocado trees as long as VPD and phenological stage of the tree are considered.