Description of the problem
The
avocado crop in Israel has expanded rapidly in the last 30 years, now accounting
for approximately 12,000 ha, with average national yields ranging between 9 and
13 tonne ha-1 (Lomas, 1992). Most of the commercial orchards are concentrated
in the central coastal plain, where the climatic conditions satisfactorily
meet the crop requirements.
The
avocado's importance in the Israeli fruit industry has increased, due to the
increase in exports: almost twofold, from 1988 to 1992. In 1992 the avocado
fruit export, mainly to the European market, reached 52.5 thousand tons,
comprising 4.5% of the total fruit exports of the country, third in importance
after citrus and apples (Affleck, 1992; FAO, 1992). However, the commercial production
of avocado in Israel is restricted by large variations of yield among years
that may reach 40-60% depending on the cultivar, climatic and site-specific
factors (Lomas, 1992). This significant yield variability produces large
fluctuations of export volumes between years, that cause serious marketing
problems and economic losses to avocado growers in Israel. Therefore,
information relating to proper crop management becomes increasingly important.
Several
studies have been carried out in order to determine the factors affecting
avocado yield in Israel. Among the main factors that have been described as
directly affecting the avocado yield, considerable emphasis has been placed to
the climate, the irrigation regime and the factors affecting the process of
water uptake from the soil by the roots (Kalmar and Lahav, 1977; Lahav and
Kalmar, 1977; Lahav and Trochoulias, 1981; Lahav and Kalmar, 1982; Levinson and
Adato, 1991; Lomas, 1988; Lomas, 1992; Lomas and Zamet, 1994; Michelakis et
al., 1993; Natan et al., 1991; Scholefield et al., 1980; Schroeder, 1976;
Sedgley, 1977; Sedgley and Annells, 1981; Shilo, 1986; Steinhardt and Tomer,
1988; Steinhardt et al., 1989; Sterne et al., 1977; Whiley et al., 1988b).
Climatic
factors influencing the evaporative atmospheric demand are responsible for
variations in water use during the growing season. The influence of adverse
temperatures occurring during the critical stages of flowering and fruit set,
has been pointed out as one of the main factors responsible for the low
fertility and yield variations observed in avocado orchards in Israel.
Conditions of high temperatures accompanied by low relative humidity during the
spring season cause a massive abscission of fruits and leaves in avocado,
reducing the canopy evaporative area and the potential fruit yield (Argaman,
1983; Gafni, 1984; Levin, 1981; Lomas, 1988; Lomas, 1992; Tomer, 1977). It is
well known that at times of high evaporative demand, water stress develops in
the canopy of avocado trees, as a consequence of an excessive transpiration
rate over the rate of uptake and
conduction of water from the soil. If soil water stress develops at the first
stages of fruit set, a strong competition for water will develop between the
fruits and the leaves. As a result of such competition, the leaves extract
water from the young avocado fruits, which shrink very severely and drop (Lahav
and Kalmar, 1982). Tree physiological responses to such environmental
conditions determine the adaptation and performance of irrigated avocado trees
growing in semi-arid environments (Scholefield et al., 1980).
Among
the principal management factors influencing avocado yield, the irrigation
regime has been extensively studied under various soil and weather conditions
in the main cultivated areas in Israel (Kalmar and Lahav, 1977; Lahav and
Kalmar, 1977; Lahav and Trochoulias, 1981; Lahav and Kalmar, 1982; Natan et
al., 1991; Steinhardt and Tomer, 1988; Steinhardt et al., 1989). The effect of
adequate irrigation is determinant during the periods of heat load occurring in
the flowering and fruit set stages in order to avoid canopy water stress.
Irrigation strengthen and accelerates the natural ability of the trees to adapt
to harsh conditions (Levinson and Adato, 1991).
The
maintenance of a tree water status adequate for achieving maximal yield depends
on the ability of the avocado root system to take up the required available
water from the soil. Avocado roots may supply enough water to satisfy tree
requirements, as long as their activity is not limited by external or internal
factors (Borys, 1986; Gefen, 1981; Lahav and Trochoulias, 1981; Lomas and
Zamet, 1994). However, under high evaporative demand conditions like those occurring in the spring season in Israel, even the water supplied by
irrigation is not enough to prevent canopy water stress and fruit drop (Honing
and Lavee, 1989). This fact has been attributed to a limited size of the root
system in the early spring season (Gefen, 1981).
The
spatial distribution of avocado roots is affected by the pattern of soil
moisture and soil hydraulic conductivity (Atkinson, 1980). One method for
increasing the supply of water available to the tree is through increasing the
total soil volume occupied by the roots (Taylor et al., 1983). Previous studies
have demonstrated that avocado root distribution closely follows the wetted
soil volume (Levinson and Adato, 1991; Michelakis et al., 1993), but it is not
clear whether enlarged wetted soil volume increases root-mass production and
water uptake capability of the total root system, as compared with a limited
wetted soil volume. Therefore, we tested the effect of enlargement of the
wetted soil volume on root growth, water uptake and canopy water status of an
adult avocado orchard, during periods of high atmospheric evaporative demands
in the flowering and fruit set stages. Measurements of leaf water potential,
canopy temperature and tree transpiration were utilized to monitor the effect
of enlargement of the wetted soil volume on avocado canopy water status.
Hypothesis and Objectives
In
our study we considered two factors influencing the avocado water status: (i)
the effect of wetted soil volume on the capacity of the tree to take up water
from the soil in order to maintain an adequate canopy water status, and (ii)
the effect of severe climatic conditions on tree water status during critical
growth stages.
The
main hypothesis associated with the experiment was that the development of a
more extensive root system by enlargement of wetted soil volume via irrigation
will improve the water uptake from the soil, and will therefore prevent
deterioration of the tree water status during periods of environmental stress.
Determination of a threshold value of available water required in the active root
zone to maintain an adequate water status of the avocado tree will facilitate
the control of irrigation for a minimum water stress risk.
Flowering
is a major event in the growth cycle of avocado. In Israel, conditions of
environmental stress during the flowering season are associated with
considerable yield loss in commercial orchards. Consequently, the secondary
objective of this study was to determine the effect of high evaporative demands
occurring during the flowering and fruit set period on avocado tree water
status and yield potential.