Proceedings
of The World Avocado Congress III, 1995 196 - 204
IMPROVING AVOCADO TREE WATER STATUS UNDER SEVERE
CLIMATIC CONDITIONS BY INCREASING WETTED SOIL VOLUME
T. Cantuarias E. Tomer
Y. Cohen Institute of Horticulture ARO,
Institute of Soils and Water Bet-Dagan POB 6
ARO, Bet-Dagan POB 6 50250,
Israel.
50250,
Israel.
Abstract
An irrigation
experiment was conducted in an adult avocado orchard in order to study the
effect of enlargement of the wetted soil volume on tree water status under
conditions of high evaporative demand, like those registered in Israel in the
early spring during the avocado flowering and fruit set stages. Enlargement of
the wetted soil volume from 25% to 75% increased root growth rate and improved
tree water status and transpiration response to high evaporative demands.
1. Introduction
Among the main
factors that have been described as detrimentally affecting avocado yield in Israel,
the influence of adverse climatic conditions on canopy water status during the
flowering and fruit set stages is of great importance (Levinson and Adato,
1991; Lomas, 1992; Lomas and Zamet, 1994). Periods of high air temperature
accompanied by low relative humidity cause a massive abscission of flowers
fruit and leaves, thus reducing the potential yield of avocado trees (Tomer,
1977; Argaman, 1983). Under these extreme climatic conditions even the water
supplied by irrigation is not enough to avoid canopy water stress and fruit
drop (Honing and Lavee, 1989). This fact has been attributed to a limited size
of the tree's root system at the early spring season, which reduces the
absorptive surface able to uptake water from the soil (Gefen, 1981).
This limited water uptake
capacity may be improved by enlargement of the soil volume occupied by the
roots that will moderate canopy water stress developing under severe climatic
conditions. This hypothesis was tested in the present study by applying two
different wetted soil volumes to modify root system distribution.
2. Material and methods
The experiment was conducted
during two years (1993-1994) in a 14-year-old avocado orchard cv. Ettinger
planted at 6x6 m spacing, in the Gilat Experimental Station in the northern
Negev (31º20’ N; 34º40’ E), at 150 m above sea level. The soil of the
experimental field is a loessial sandy loam, with a uniform soil profile to a
depth of 180 cm (Steinhardt and Tomer, 1988). Two irrigation treatments with
different wetted soil volumes were applied, each one on four experimental
plots: (i) irrigation by one drip lateral line along the row line, wetting 25%
of the soil surface, and (ii) irrigation by five drip lateral lines,
distributed in parallel to the row line, wetting 75% of the soil surface. Both
treatments received the same amount of water on a weekly basis, but with a
different frequency: irrigation was given twice a week for the 1-drip line
treatment, and once a week for the 5-drip lines treatment.
Tree transpiration rate (T)
was determined from sap flow measurements by means of a calibrated heat-pulse
technique, as described by Cohen et al. (1981) and Cohen (1991).
. Root distribution was
examined at trenches (100 cm depth, 300 cm length, 50 cm width) dug
perpendicular to the row line on two trees per treatment. Water pots were
placed in the bottom of the trenches to maintain high relative humidity, and
the trenches were covered with a thick double black- plastic sheet. Roots were
monthly counted on both walls of the trench using a 18 cm grid. After counting,
root tips were removed from the wall surface.
Soil water content was
measured with the neutron scattering method on two trees per treatment. Eight
aluminum access tubes were placed perpendicular to the trunk, and five soil
depths were monitored (25, 55, 85, 115 and 145 cm).
Leaf xylem water potential
(LWP) was measured during the morning on shaded and sunlit leaves on two trees
per treatment, using a pressure chamber (Arimad 2, Kibbutz Kfar Charuv,
Israel). Sunlit canopy temperature (Tc) was measured during the
morning with a hand-held infrared thermometer on four trees per treatment
(Model 43S, focal plane 8 cm., Telatemp Corp., Fullerton CA, USA).
Weather measurements were
recorded at 2 m height from an automated meteorological station located 300 m
outside the orchard. These data were used to compute the potential
transpiration (TP) according to a modified Penman-Monteith equation
(Fuchs et al., 1987).
3. Results
Soil water distribution was
more extensive in the treatment irrigated by 5-drip lines. In this treatment
the end of the wetting front was located at 225 cm from both sides of the row
line, as compared to 110 cm for the treatment irrigated by 1-drip line. In both
treatments water was mainly extracted from the first 55 cm of the soil profile.
Figure 1 shows the average rate of root tips appearance measured at different distances
from the row line on both treatments during May-June 1994, when root activity
was at its maximum. In both treatments, a gradual reduction of root appearance
with increasing distance from the row line was observed. The 1-drip line
treatment developed more roots in the first 50 cm from the row line. However,
at larger distances from the row line, the treatment irrigated by 5- drip lines
showed higher rates of root appearance. Average rate of root appearance during
the period was higher for the 5-drip lines treatment, 8.1 root tips m2 day-1, as compared
to only 5.5 root tips m2 day-1 in the
1-drip line treatment. Analysis of root distribution with depth form soil
surface indicated that 69% and 80% of the total roots are concentrated in the
first 36 cm of the soil for the 1-drip line and the 5-drip lines irrigation
treatments respectively, confirming the characteristic shallow distribution of
avocado roots (Levinson and Adato, 1991; Michelakis et al., 1993).
Figure 2 shows
the course of T and TP for both treatments measured during a 16-day
period on May 1994. Transpiration values correspond to the average data of 4
trees per treatment. Arrows indicate the irrigation dates for the 5- drip lines
treatment. This treatment showed higher average T rates during the whole
period, as compared to the 1-drip line treatment. However, the considerable
variability in transpiration found within trees of the same treatment does not
allow to consider the average T values in absolute terms. As seen in the
figure, T of the 5-drip lines treatment increased immediately after the
irrigations, and decreased approximately four days afterwards. Such variations
of T with time after the irrigation may be explained by a larger water uptake
from the soil that allows the increment of T after the irrigation, and by a
reduced soil water availability towards the end of the irrigation interval.
However, these variations of Tp are also a consequence of an
increase in Tp observed after the irrigations, and a coinciding
decrease of Tp occurring approximately four days after the
irrigation. These results indicate that avocado transpiration is influenced
both by soil water availability and by climatic conditions.
Therefore, in
order to isolate the effect of soil water availability on the transpiration
response to climate, we analyzed data collected during a period when both
treatments were daily irrigated, hence soil water availability was not limited.
Figure 3 presents the variation of the avocado T/Tp ratio with
increasing Tp values registered during an 11-day period in May 1994,
when irrigation was daily applied to both treatments because of the occurrence
of severe hot spells. The T/Tp ratio of both treatments
increased with higher Tp, however a larger increment was observed in
the treatment irrigated by 5-drip lines. Analysis of these data indicated that
T increased by nearly 15% and 20% in the 1-drip line and 5-drip lines
treatments respectively, following a change in Tp from 8.5 to 9.4 mm day-1
(11%). These results indicate that under non-limiting soil water availability,
avocado T increases at a higher relative rate as compared to the increment in T.
Figure 4 presents
the simulated variation of the avocado T/Tp ratio with increasing air
temperature and relative humidity. The T/Tp ratio was computed from
the modified Penman-Monteith equation utilized in this study, considering
climatic parameters and canopy structure data measured in the field as inputs
of the model. Three levels of canopy water stress were considered in the
simulation by introducing different canopy conductances (0.10, 0.08 and 0.05 cm
s-1 for none, moderate and severe stress). The figure shows an
increase of T/Tp ratio with increasing air temperature irrespective
of ale water stress level, while no significant effect of increasing relative
humidity on T/Tp, was observed.
Figure 5
presents the variation of LWP measured at noon on both treatments during
April-May 1994. Under a given T rate, LWP of the trees irrigated by
5-drip lines was higher &an in the trees irrigated by 1-drip line, both on
sunlit and shaded leaves. Although data were taken only from days when the soil
was wet - to eliminate the effect of soil water availability on xylem
potential- changes in LWP were poorly related to Tp - This
poor relationship may be associated with the response shown in Figure 3, of a
higher relative increment of T compared to the increment of Tp
that maintains a favorable tissue water status.
Figure 6 shows the canopy
temperature difference between the treatment irrigated by 1-drip line and the
treatment irrigated by 5 lines (Tc1-Tc5), as
measured at three time periods during the morning, on April-May 1994. Tc1-Tc5 reached its maximum value of 1.5-2.0 ºC
between 8:30-9:30 h, indicating the development of a more severe canopy water
stress in the 1-drip line treatment. Towards the noon, Tc1-Tc5
decreases, and even positive differences were observed, probably as a
consequence of an increased stomatal resistance affecting both treatments
during these hours.
4. Discussion
Enlargement of wetted soil
volume and root growth of an avocado orchard improves water uptake and allows an
increased T rate under conditions of high evaporative demand, thus preventing
canopy water stress. Under non-limiting soil water availability, avocado tree
transpiration increases in a higher relative rate as compared to the increment
of Tp. This situation can only occur if stomata remain open and no
restrictions to available soil water are imposed. Under high atmospheric
demands, the increment in air temperature will raise leaf temperature over that
of the air. Higher leaf temperature will then increase the saturated water
vapor pressure in the substomatal chambers of the leaf, thus promoting the
evaporation rate and water loss from the canopy, and explaining the increment
of the T/Tp ratio observed under these conditions.
The improved
water uptake and favorable canopy water status achieved by enlarging the wetted
soil volume were expressed as higher LWP and lower Tc during periods
of high evaporative demands.
Acknowledges
Special thanks are given to
Mr. Yefet Cohen, Mr. Shimshon Shooker and Mr. Mario Rippa who conducted the
field work, and to Mr. Vladimir Kutzii for technical support.
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