BIOLOGIA PLANTARUM 51 (4): 735-740, 2007
735
Daily variations in water relations of apricot trees
under different irrigation regimes
M.C. RUIZ-SÁNCHEZ1,3, R. DOMINGO2,3 and A. PÉREZ-PASTOR2,3*
Dpto. Riego, Centro de Edafología y Biología Aplicada del Segura, CSIC, P.O. Box 164, E-30100 Murcia, Spain1
Dpto. Producción Vegetal, ETSIA-UPCT2, and Unidad Asociada al CSIC de Horticultura Sostenible
en Zonas Áridas, UPCT-CEBAS3, Paseo Alfonso XIII, 52,E-30203 Cartagena, Spain
Abstract
Mature apricot (Prunus armeniaca L. cv. Búlida) trees, growing under field conditions, were submitted to two drip
irrigation treatments: a control (T1), irrigated to 100 % of seasonal crop evapotranspiration (ETc), and a continuous
deficit (T2), irrigated to 50 % of the control throughout the year. The behaviour of leaf water potential and its
components, leaf conductance and net photosynthesis were studied at three different times during the growing season,
when they revealed a diurnal and seasonal pattern in response to water stress, evaporative demand of the atmosphere
and leaf age. The deficit-irrigated trees showed, among other effects, a pronounced decrease in leaf water potential
(\w), decreased in leaf conductance (gs) and no osmotic adjustment. For this reason, gl and \w can be considered good
indicators of mature apricot tree water status and can therefore be used for irrigation scheduling.
Additional key words: leaf stomatal conductance, leaf water potential, net photosynthesis, Prunus armeniaca L., water deficit.
Introduction
In Mediterranean agro-systems, as in many other semi-
arid zones in the world, water has become a precious
resource due to its scarcity. There is, therefore, a constant
need to improve water use efficiency by means of
accurate irrigation scheduling based on physiological
indicators which show information on crop water status
(Naor and Cohen 2003). However, there is no general
agreement on the most suitable indicator (Katerji et al.
1988).
The most widely used approach for evaluating plant
water status has been to determine leaf water potential,
either as predawn readings (Domingo et al. 1996, Ferreira
et al. 1997), or midday readings (Girona et al. 2006), as
well as stem water potential (McCutchan and Shackel
1992). Gas exchange parameters, such as variations in
stomatal conductance, have also been used as water
status indicators (Harrison et al. 1989). More recently,
some sensors used to measure sap flow and trunk
diameter fluctuation have provided useful information
about water status because of the continuous nature of the
measurements; they have also proved to be very robust
(Goldhamer et al. 1999, Remorini and Massai 2003,
Ohashi et al. 2006, Ortuño et al. 2006).
Our knowledge of the water transport through the
soil-plant-atmosphere continuum for apricot plants is
limited, since studies on drought resistance mechanisms
have focus on young plants during short-time water stress
periods (Torrecillas et al. 1999, Ruiz-Sánchez et al.
2000). Consequently, more comprehensive information
of water relations in mature trees growing in a field is

Received 17 March 2006, accepted 27 August 2006.
Abbreviations: ci - internal CO2 concentration; ETc - crop evapotranspiration; ETo - reference evapotranspiration; gs - leaf stomatal
conductance; LM/LA - leaf mass/leaf area ratio; PAR - photosynthetically active radiation; PN - net photosynthetic rate;
SM/DM - water saturated mass/dry mass ratio; Tl - leaf temperature; VPD - vapour pressure deficit; \o - leaf osmotic potential;
\os - leaf osmotic potential at full saturation; \p - leaf pressure potential; \w - leaf water potential.
Acknowledgements: To Dr. A. Torrecillas for critically reading the manuscript. The study was supported by CICYT (HID1999-0951
and AGL2000-0387-C05-04 and -05) grants to the authors. A. Pérez-Pastor was a recipient of a research fellowship from the
Ministerio de Educación y Ciencia of Spain.
* Corresponding autor; fax : (+34) 968325433, e-mail: alex.perez-pastor@upct.es

M.C. RUIZ- SÁNCHEZ et al.
736
needed if we wish to optimise both water use and orchard
management.
For these reasons, the aim of this paper was to study
plant-water relations (leaf water potentials and leaf gas-
exchange) throughout a growing season in mature apricot
trees growing in a orchard. The physiological basis of
diurnal and seasonal variations and the sensitivity of
these parameters to water deficits are discussed.
Materials and methods
The experiment was performed during 1997, in a 20 000
m2 plot of a commercial orchard, located in Mula valley,
Murcia, Spain (37º57’ N, 1º25’ E, 350 m above sea
level). The soil is loam-textured (sand: 29.85 %; silt:
42.92 %; clay: 27.21 %) and classified as a Xeric
Torriorthent. It is highly calcareous, has a pH of 7.8, and
a low organic matter content and cationic exchange
capacity. The available water capacity is about 0.15 cm3
cm-3. The climate is semiarid Mediterranean with hot and
dry summers; annual evaporation and rainfall for the
experimental period was 1470 and 436 mm, respectively.
The plant material consisted of mature apricot trees
(Prunus armeniaca L. cv. Búlida, on Real Fino apricot
rootstock), spaced 8 u 8 m, with an average ground cover
of 52 %. Trees were drip irrigated using one drip
irrigation line per row, with seven emitters per tree, each
with a flow rate of 4 dm3 h-1. The irrigation water had
low electrical conductivity (0.6 dS m-1).
Two irrigation treatments were considered: a control,
irrigated at 100 % of seasonal ETc (685.4 mm year-1) and
a continuous water deficit treatment, irrigated at 50 % of
the control treatment all year. Irrigation amounts were
scheduled on weekly based on crop coefficients
(Abrisqueta et al. 2001), reference crop evapo-
transpiration (ETo), as determined from data collected
the previous week in a U.S. Weather Bureau class A pan
(on bare soil and located on a weather station in the
orchard), and the estimated application efficiency (95 %).
Treatments were distributed in a completely randomised
block design, with four blocks. Each block consisted of
two rows of seven trees. Other details concerning the
tree, irrigation and climate characteristics have been
described in Pérez-Pastor et al. (2004).
Leaf water potentials, gas-exchange parameters and
environmental conditions (air temperature, relative air
humidity) were measured from predawn to sunset at 2-h
intervals and on three clear days representatives of the
growing season: 18th March (spring), 24th July (summer),
28th October (autumn). Climatic conditions were measured
by an automatic weather station located in the orchard.
Leaf water potential (\w) was measured in mature
leaves located on the south facing side, from the middle
third of the tree (two leaves per tree and four trees per
treatment), with a pressure chamber (model 3000, Soil
Moisture Equipment Corporation, Santa Barbara, CA,
USA) following the recommendations of Turner (1988).
After measuring \w, the leaves were frozen in liquid
nitrogen and osmotic potential (\o) was measured after
thawing the samples and extracting the sap, using a
Wescor 5500 (Logan, USA) vapour pressure osmometer.
Leaf pressure potential (\p) was derived as the difference
between leaf osmotic and water potentials. Osmotic
potential at full saturation (\os) was measured on leaves
adjacent to those used to measure predawn leaf water
potential. Eight leaves per treatment were allowed to
reach full \p by dipping their petioles in distilled water
for 24 h in darkness at 8 ºC (Yoon and Richter 1990).
The rehydrated leaves were weighed (water saturated
mass) and frozen in liquid nitrogen, and then \os was
measured following the same methodology as for \o.
Osmotic adjustment was estimated as the difference
between the \os of deficit and control plants.
Leaf conductance (gs) and net photosynthetic rate (PN)
were measured in a similar number of leaves, using a
field-portable, closed gas-exchange photosynthesis
system (LI-6200, LI-COR, Lincoln, USA) incorporating
IRGA (LI-6250). Each leaf was enclosed within a fan-
stirred one-litre chamber. The mean return flow rates of
air circulating within the closed system and the leaf to air
vapour pressure deficit for all measurements were around
450 mmol s-1 and 2 kPa, respectively. The CO2 analyser
was calibrated daily with two standard CO2/air mixtures.
Leaf temperature was measured using a portable infrared
thermometer (Everest Interscience, Fullerton, USA) with
0.5 ºC accuracy and 0.1 ºC resolution. Five
measurements were taken on four trees per treatment,
ensuring that the entire area detected by the sensor was
totally occupied by a single leaf in full sunlight.
Volumetric soil water content (Tv) was determined
using a neutron probe (Troxler mod 4300) that had
previously been calibrated for the site. Four 1.4 m access
tubes were located in each treatment (one per block) in
the wetted area close to the second emitter from the tree
trunk. Soil moisture was determined at 10 cm intervals.
Soil moisture at 10 cm was determined gravimetrically.