Abstract
Little information is available on the diurnal behaviour of water potential and leaf conductance on pistachio trees despite their relevance to fine tune irrigation strategies. Mature pistachio trees were subject to simultaneous measurements of stem water potential (Ψx) and leaf conductance (gl) during the day, at three important periods of the irrigation season. Trees were grown on three different rootstocks and water regimes. An initial baseline relating Ψx to air vapor pressure deficit (VPD) is presented for irrigation scheduling in pistachio. Ψx was closely correlated with VPD but with a different fit according to the degree of water stress. No evidence of the variation of Ψx in relation to the phenology of the tree was observed. Furthermore, midday Ψx showed more accuracy to indicate a situation of water stress than predawn water potential. Under well irrigated conditions, gl was positively correlated with VPD during stage II of growth reaching its peak when VPD reached its maximum value (around 4 kPa). This behaviour changed during stage III of fruit growth suggesting a reliance of stomatal behaviour to the phenological stage independently to the tree water status. The levels of water stress reached were translated in a slow recovery of tree water status and leaf conductance (more than 40 days). Regarding rootstocks, P. integerrima showed little adaptation to water shortage compared to the two other rootstocks under the studied conditions.
Highlights
Cultivated for a long time in Mediterranean semiarid areas, pistachio (Pistacia vera L.) has been considered as a drought-tolerant species (Spiegel-Roy et al, 1977; Behboudian et al, 1986; Rieger, 1995), mainly for its ability to survive under extreme conditions of water stress (Kanber et al, 1990; Goldhamer, 1995)
Ψx was closely correlated with vapor pressure deficit (VPD) but with a different fit according to the degree of water stress
Measurement of Ѱx before dawn had showed its limitation even when statistical water stress differences were detected between treatments from the fact that it was not representative of a steady state; the maximum of Ѱx did not take place only at predawn but a few hours later, when trees were in a rehydration process from stage II to stage III (Figs. 3B and 3C)
Summary
Cultivated for a long time in Mediterranean semiarid areas, pistachio (Pistacia vera L.) has been considered as a drought-tolerant species (Spiegel-Roy et al, 1977; Behboudian et al, 1986; Rieger, 1995), mainly for its ability to survive under extreme conditions of water stress (Kanber et al, 1990; Goldhamer, 1995). Precision in irrigation has taken another level and the recourse to the study of the different mechanisms connecting the tree water relations to water deficit strate-. The water status of a tree or a tree organ embodies a dynamic property controlled by the relative rate of water loss and water uptake. Plant water potential is certainly the most commonly used parameter as indicator of water status and irrigation timing in fruit trees (Klepper, 1968), mainly for its physiological relevance in the control of the water pathway in the tree (Ritchie & Hinkley, 1975). The time of taking this measure has made the subject of several studies and disagreement between researchers, where the effectiveness of detecting water stress was compared between measures at predawn and midday in many species
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