ABSTRACT- Supplemental irrigation under prolonged drought conditions has a key
role in providing water for transpiration of rainfed fig trees. The effect of different times and amounts of supplemental irrigation at different distances from the tree trunk on quantity and quality of Estahban rain-fed fig production was evaluated during two years. A randomized complete block design with four replications on fig cultivar of Sabz was used to conduct the experiment. Treatments of supplemental irrigation included three different application positions including close to tree trunks (NT); 1-1.1 m from tree trunk (UT) and outside of tree canopy (OT). Three different quantities of irrigation water including no supplemental irrigation (control), 1000 and 2000 liters irrigation water per tree, and with two different supplemental irrigation times in early spring and midsummer were also used. Results showed higher soil water content for irrigation during early spring, near tree trunk with 2000 liters irrigation water per tree. Despite the reduction in total soluble solids (TSS), supplemental irrigation improved the yield, size and skin color of fruits compared to the control. In both years, fig yield was higher in NT and OT treatments compared to UT. Irrigation out of canopy produced more fruits with higher quality. A non-significant difference between yields in irrigation water amount treatments during the second year indicated the adequacy of 1000 liters per tree. Application of 1000 liters, out of canopy in mid-summer would be recommended to fulfill marketing goals and sustainable use of regional water resource under drought conditions in rain-fed fig orchards.
Rainfed agriculture is a major source of food production worldwide, such that nearly 80% of the global cropland is rainfed. This provides 60-70% of the world’s food supply
(Falkenmark and Rockström, 2004). Although this system results in lower productivity and dependability compared with irrigated farmlands, it is still considered the principal method of food production for the increasing world population (Oweis and Hachum, 2003).
Iran has been the fourth producer and exporter of figs with an average of 75,833 tons production in the last two decades (1993-2013) (FAO, 2016). Most of the fig trees in Iran are cultivated in Estahban region, where 90% of dried fig in Iran is produced (Jafari et al., 2012). Fig production in the Estahban area is located mostly on foothill slopes of the Zagros Mountains. In these dryland orchards, rainwater harvesting is a traditional practice for supplying water by using micro-catchments built perpendicular to the slopes for collecting rain
water. Fig trees can be grown in a variety of soils ranging from coarse sand to heavy clay soils (Morton, 1987). Deep, gravelly, and alluvial soils in the Estahban plains together with flood waters from upland streams have provided favorable conditions for infiltration of water and storage in the soil profile.
Fig production under rain-fed conditions is highly dependent on precipitation, and fluctuation in annual precipitation is a major challenge for rainfed fig producers.
Under prolonged drought conditions, severe damage occurs in rainfig plants that are normally tolerant to water shortage (Gholami et al., 2012; Hallaç Türk and Aksoy, 2011; Karimi et al., 2012; Stover et al., 2007). Drought incidence results in massive leaf abscission and reduction in fruit quantity and quality (Hallaç Türk and Aksoy, 2011; Tehrani et al., 2016). Extensive drought events in Iran have seriously affected rainfed fig trees and in 2010, it resulted in the loss of more than 10% of the trees as a result of which fruit production was reduced by more than 80% (Jafari et al., 2012).
Under drought conditions, soil water content is severely reduced, thus reducing absorption of water and mineral nutrients by plants (Rostami and Rahmei, 2013). Water stress is induced by climatic, edaphic, and agronomic factors, and the vulnerability of plants to
drought conditions depends on the degree of water stress, together with accompanying stress factors, plant species, and the stage of plant growth (Demirevska et al., 2009).
According to previous studies, the use of techniques such as mulching (Aragüés et al., 2014; Jafari et al., 2012), potassium nutrition (Honar and Sepaskhah, 2015), micro-catchment construction (Sepaskhah and Moosavi-Fard, 2010; Sepaskhah and Fooladmand, 2004), and pruning (Kamgar-Haghighi and Sepaskhah, 2015; Leonel and Tecchio, 2010) can reduce the negative effects of drought on fig trees. Although fig trees show efficient water uptake and water use capacities, supplemental irrigation in years of belowaverage rainfall would have a significant role in providing water for transpiration and high annual water productivity (Abdel Razik and El Darier, 1991).
Supplemental irrigation can be defined as “the addition of a limited amount of water to otherwise rainfed crops, when rainfall fails to provide essential moisture for normal plant growth, in order to improve and stabilize productivity”
(Oweis et al., 1999). Similar to other arid and semi-arid regions, the tendency to use supplemental irrigation in Estahban fig orchards has increased in recent years (Kamyab, 2015; Sharifzadeh et al., 2012). Previous research showed the positive role of supplemental irrigation in improving the morphological characteristics and yield of rain-fed fig trees in the area under drought conditions (Honar and Sepaskhah, 2015; Kamgar-
Haghighi and Sepaskhah, 2015). Supplemental irrigation at the inappropriate time and quantity of water may have negative effects on fig trees. Nevertheless, there is a lack of information about the water needs of fig trees (Dominguez, 1990). Since the high use of water for fig irrigation could lead to a local shortage of water resources, especially in areas characterized by limited agricultural water (Abdolahipour and Kamgar-Haghighi, 2015), knowledge of accurate fig orchard needs will help clarify the discussion of supplemental water usage.
As we practice supplemental irrigation at the end of precipitation season, timing and amount of supplemental irrigation should be predicted. Nevertheless, there is a lack of information about the amount, timing, and application position of supplemental irrigation to achieve higher efficient use of water in the area.
The main objective of the present study was to evaluate the effects of supplemental irrigation on rainfed fig yield, yield quality and soil water variation in relation to irrigation timing, the quantity of water used, and its application position from tree trunks.
MATERIALS AND METHODS
The experiment was conducted in a farmer orchard in Estahban County, Fars Province, Iran (altitude, 1749 m; latitude, 29°07′ N; longitude, 54°04′ E) in 2013-2015.
Extreme temperatures in the region are in the range of 7 to 41°C. Annual average rainfall is about 354 mm with minimum and maximum values of 92 and 739 mm, respectively (Bagheri and Sepaskhah, 2014). The average relative humidity is 45%; however, it is reduced during the fruit maturing and harvest period in summer. Most of the precipitation occurs during late fall and winter. Meteorological information during the experimental period was obtained from a meteorological station in the region (Fig. 1).
The soil is gravelly loam texture with the top 150 cm composed of 30% sand, 48% silt, and 22% clay on fine soil particle basis (less than 2 mm) and also 30% gravel in volumetric sampling method. The sample contained a pH of 7.54, electric conductivity (EC) of 1.34 dS/m, field capacity (FC) of 31% and permanent wilting point (PWP) of 14% (volumetric method).
The different growth stages of fig tree must be taken into account for efficient water management of an orchard, especially when supplemental irrigation strategies are to be used. A diagram describing the annual life cycle of the fig tree is shown in Table 1.
For the conditions of Estahban area, shoot growth takes place from mid-April to mid-May. Leaves usually become fully expanded in May, depending on environmental conditions. The flowering and fruiting occurs from April to July. Fruit maturation starts in August and may last until temperatures drop
in October. At the end of the growth period, the leaves fall and the tree enters its rest period. Environmental factors such as temperature, photoperiod, and humidity affect the development and yield of the fig trees (Flaishman et al., 2007).
A number of rainfed fig cultivars are grown in the Estahban region (Fars Province, I. R. of Iran), and among them, Sabz cultivar (Smyrna type) is the dominant one (Bagheri and Sepaskhah, 2014). The Sabz fig tree is a cultivar with suitable vegetative and reproductive characteristics, round canopy, vertical growth, dense foliage, and usually 3-4 trunks (Faghih and Sabet-Sarvestani, 2001).
The experiment was performed on 72 uniform, 45year-old, edible fig cultivars of Sabz fig trees. In the study area, as in other rainfed orchards of the region, trees had been planted 10 m apart and the canopy diameter was about 3.2 m. Different treatments of supplementary irrigation were applied. The cultural practices and pollen source (Pouz Donbali cultivar) were similar for all trees.
The experiment was conducted in a split-split plot design over a randomized complete block design (RCBD) with four replications and 18 fig trees in each block. Treatments of supplemental irrigation included three different application positions from the trunk, using three different quantities of irrigation water, and with two different supplemental irrigation times. The volume of irrigation water for each tree was measured by using a flow meter installed at the inlet of the irrigation pipe.
Irrigation treatments based on the position of application from trees were: (1) irrigation in a microcatchment close to tree trunks (NT); (2) irrigation water applied in three holes placed 1-1.1 m from tree trunks under tree canopies for trees with almost 3.2 m canopy diameter (UT); and (3) irrigation applied in four holes outside of tree canopies placed 2.1-2.2 m from tree
trunks (OT) for trees with almost 3.2 m canopy diameter (Fig. 2).
Soil water content was measured at 30, 60, 90, 120 and 150 cm soil depths using the neutron scattering method (CPN® 503 ELITE HydroprobeTM) with onemonth interval. Access tubes were installed for trees in the first block at three different distances from the trunk in the closest possible place to the irrigation area. It was difficult to install the tubes for all trees and below the 150 cm depth, due to the gravelly texture of soils in the area. Previous studies on fig orchards in the area (Honar and Sepaskhah, 2015; Kamgar-Haghighi and Sepaskhah, 2015) indicated that it would be necessary to measure soil water content below the 90 cm depth. The time intervals between irrigation events and soil water content measurements are shown in Fig. 3.
To evaluate fruit production, the fruits of each tree were collected through the harvest period and became dry in the sun. Fig harvesting takes place from August to October in the region (Table 1). Fruit weight was measured using a digital balance with a sensitivity of 0.001 kg.
To study the pomological characteristics, the collected figs were graded to three different commercial grades (AA, A, and B) by using local commercial methods of grading. In these methods, fig fruits with larger diameters and lighter skin color are considered as higher quality fruits. The best quality of fig has got light yellow color, with 3 to 4 cracks on it (Faghih and Sabet-Sarvestani, 2001).
Fig. 1. Mean daily agrometeorological data for Estahban
Table 1. Different growth stages of fig tree
|Rest||Vegetative||Flowering||Pollination Cell enlargement and development||Harvest||Fall|
|Jan Feb||Mar||Apr||May Jun Jul||Aug Sep||Oct||Nov||Dec|
Fig. 2. Different irrigation application positions from tree trunk treatments in the experiment for tree with a canopy cover diameter about 3.2m (gray area: irrigation positions, hatch area: tree trunk, black points: access tube for measuring soil moisture, NT: near the tree trunk, UT: under the tree canopy and OT: out of tree canopy). Treatments based on time of irrigation were (a) in early spring and (b) in mid-summer and treatments based on the quantity of applied irrigation water were: no supplemental irrigation (control), and either 1000 or 2000 liters irrigation water per tree.
variance analysis performed independently for each year, major effects were considered to be statistically non-significant if the threefactor interaction (irrigation positions, amount and time) or two-factor interaction were significant. Differences between means were compared by Duncan’s multiple
range test at 5% level of probability.