Efficient Irrigation for Optimum Fruit Quality and Yield in Apples (2024)

Experiment description and irrigation regimes.

The experiment was established at the University of Idaho Pomology Orchards at the Parma Research and Extension Center during spring and summer of 2002. ‘Autumn Rose Fuji’ trees on RN 29 (Nic29) rootstock were planted at 1.52 × 4.27-m spacing (1540 trees/ha). The experimental design was a randomized complete block with five irrigation treatments and five replicates. Each block contained 10 trees per plot (i.e., per irrigation treatment). The experimental site had a semiarid climate with an annual precipitation of ≈297 mm and a sandy loam soil of pH ≈7.3. Cultural practices other than irrigation were similar to those recommended for commercial orchards in the Pacific Northwest (Washington State University, 2009).

Irrigation treatments during 2004 through 2007 were as follows: 1) full microjet sprinklers (FS). Trees with this microjet sprinkler system were irrigated at the full rate of ETc as described subsequently once a week during each growing season since 2002; 2) partial root-zone drying sprinklers (PRS). This system was similar to the full microjet sprinkler except that two microjet sprinklers were attached to two lateral polyethylene lines, each located either on the south (180°) or north side (180°) of the tree row. Applications alternated from side to side at every 2 weeks. At each irrigation time, trees in this treatment received 50% of the FS treatment; 3) full drip (FD). In this system, one drip line was installed in a 10-cm trench (subsurface), 30 cm away from and parallel to the tree row at each of the north and south sides of the tree row. Trees in this system received 100% of ETc but adjusted for canopy development (GS). Therefore, in this treatment, liters of water applied per tree = (ETc in mm/percent drip efficiency factor) × 1.52 × 4.27 m spacing × %GS. Efficiency for this system was considered to be 100%; 4) DD. The amount of water applied in this system was 65% of that applied to FD system; 5) PRD. At each biweekly irrigation cycle, trees were only irrigated by one of these drip lines, and in the next cycle, they were irrigated by the other line. In this way, partial root-zone drying was created in the trees. The amount of water applied through this system was 65% of FD.

Calculation of water requirement.

At the beginning of each season (approximately mid-May), the soil profile was watered to saturation in all irrigation systems. After the initial irrigation, water requirements were determined by calculating ETc (crop evapotranspiration) where ETc = ETr × Kc. In this equation, ETr (Penman-Monteith reference evapotranspiration) was calculated from the Agri-Met Parma Weather Station data (Allen et al., 1998) and Kc was the crop coefficient. Each year starting in 2002, the crop water use coefficient was calculated as: Kc = K_c base + %M × (mature K_c – K_c base). Percent canopy maturity (%M) was a measurement of canopy size and was calculated as: %M = 3.05 + 2.558 × (%GS) – 0.016 × (%GS)². Kc base was the base coefficient, calculated as the percentage area between the rows that was occupied by a cover crop. In our experiment, spacing between rows was 4.27 m and the herbicide strip extended 0.61 m on either side of the row. Thus, Kc base was [4.27 – (0.61 × 2)]/4.27 = 0.71. Percentage of GS was estimated as the area of orchard shaded by the tree canopy at different stages of growth. Ground shading reached 61.76% and tree maturity reached 100% on 1 Aug. 2005. Thus, Kc values for mature trees were used after 1 Aug. 2005. The value for mature Kc for each month was adopted from Proebsting (1994) for apples with cover crop, i.e., 0.71 in May, 0.96 in June, 1.04 in July and August, 1.0 in September, and 0.79 in October.

Fruit yield and quality attributes.

Yield and fruit quality data were collected during 2004 through 2007. However, in this report, yield and fruit quality attributes for only 2004 (young trees) and 2007 (mature trees) are reported. Methods for yield and quality measurements were similar to those described by Fallahi et al. (2001a).

Results of water use study.

Water use in all irrigation systems increased as trees matured (Tables 1 and 2). Trees used the highest amount of water in July and August in all years. Trees with FS treatment received a significantly greater volume of water than those with drip systems every year. Trees with a FS system received 72% and 56% more water than those with a FD system in 2003 (data not shown) and 2004, respectively (Table 1). These differences between FD and FS systems were consistent (38% to 41%) after 2005 (Tables 1 and 2) because the trees had reached the maximum ground shading (≈67%) and full maturity after 1 Aug. 2005. On average, mature trees with a FS system received 6461 L of water per tree (994 mm), whereas those with a FD system received 3996 L of water per tree (614 mm) over the 2006 and 2007 seasons (Table 2). Each tree with PRS received more water than those with any type of drip systems in 2004 and more than DD and PRD after 2004 (Tables 1 and 2). Although the volume of water applied to the trees with DD or PRD was only 65% of that applied to the trees with FD system, no major water stress symptoms were observed in the trees with DD or PRD systems. The only visible symptom was that trees receiving less than full levels of either sprinkler or drip irrigations had slightly early leaf senescence in late October, perhaps as a result of increased stress and ethylene production in the trees with lower irrigation.

Leib et al. (2006) compared three microsprinkler irrigation systems in mature ‘Fuji’ trees. The soil water content in the conventional irrigation (CI) was maintained close to field capacity, which was only 60% to 70% of estimated ETc for apple without cover crop. In that study, deficit irrigation (DI) and PRS were at ≈50% to 60% of the CI. They found that the 3-year average potential evapotranspiration was 991 mm and irrigation amounts applied were 706 mm, 570 mm, and 511 mm for CI, DI, and PRS irrigation regimes, respectively. We applied an average of 994 mm of water to our FS trees, which was ≈287 mm (≈29%) higher than the levels applied to the CI treatment in the Leib et al. (2006) report. This difference is the result of average ETr being ≈116 mm higher in Idaho (Table 2) and FS being applied at full ETc level (Table 2), whereas CI trees in their experiment received water at ≈70% of ETc. Rainfall in both experiments was comparable.

Fruit firmness.

In our long-term study, fruit firmness at harvest was unaffected by various irrigation regimes in 2007 (Table 3). Previous reports indicate that low water applications may result in a reduction in apple firmness because of the advanced maturity in fruits with water stress (Drake et al., 1981; Mills et al., 1994). However, other researchers showed that apples from non-irrigated plots were firmer than those from irrigated plots (Assaf et al., 1975; Guelfat'Reich et al., 1974; Guelfat'Reich and Ben-Arie, 1979). Assaf et al. (1975) indicated that fruit from trees subjected to water deficit were smaller than those from CI trees, which may account for the observed increase in fruit firmness. Leib et al. (2006) observed that ‘Fuji’ apple fruits from trees with DI and PRS treatments were firmer than those from trees with CI treatment. Talluto et al. (2008) have shown that fruit firmness was unaffected by DI in ‘Pink Lady’ apple.

Fruit color.

In our study, ‘Autumn Rose Fuji’ from trees receiving PRS treatment had slightly better (more uniform red) color than those from most other irrigation treatments (Table 3). Mills et al. (1994) reported that DI increased skin red color in ‘Braeburn’ apple. In contrast, DI did not affect fruit color in ‘Pink Lady’ in Australia (O'Connell and Goodwin, 2007; Talluto et al., 2008). The improved color development in apples with a DI system could be the result of the presence of less dense canopy and thus better light penetration (Lancaster, 1992) or could be the result of a reduced level of nitrogen (N) in the leaves and fruits (Mills et al., 1994). Lower N in the leaf and fruit tissues has been shown to improve fruit color in ‘Fuji’ apple (Fallahi et al., 2001a).

Fruit soluble solids concentrations, starch degradation, and titratable acidity.

In our study, trees receiving PRS treatment had significantly higher SSC than all other treatments when trees were young (in 2004), but trees with sprinkler systems had slightly lower soluble solids concentration (SSC) when trees were mature (in 2007) (Table 3). Other researchers reported that deficit irrigation increased SSC, including sucrose, glucose, fructose, and sorbitol in apple fruit, perhaps as a result of an increase in the concentration of dry matter (Kilili et al., 1996; Mills et al., 1994; Mpelasoka et al., 2000, 2001b; Naor, 2006). Leib et al. (2006) showed that SSC in fruits from trees receiving DI was higher than in fruits from trees receiving CI. A 2-year study by O'Connell and Goodwin (2007) on ‘Pink Lady’ in Victoria, Australia, showed that SSC tended to be higher in DI fruit than CI fruit for each of the 2 years. In contrast, Talluto et al. (2008) reported that ‘Pink Lady’ fruits from DI and CI treatments had similar SSC. Mills et al. (1994) and Marlow and Loescher (1984) reported that a high concentration of sorbitol will lead to the development of water core. Water core is not desirable in most apple cultivars, whereas it is considered as positive quality attributes for certain markets for ‘Fuji’. In our experiment, ‘Fuji’ fruits from FD system always had higher water core (data not shown).

Factors that lead to a higher hydrolysis of fruit starch can result in higher SSCs (Kramer, 1983) in apple. However, a simple fruit dip in iodine solution (SDP) may not always be a reliable measure of the sugar concentration of fruit. For example in our study in 2004, fruit from trees receiving PRD treatment had a significantly higher SDP than those from FS irrigation regime, whereas fruits in both treatments had a similar level of SSC (Table 3). This could be the result of conversion of simple sugar to other metabolites.

Fruit sunburn.

Tree with FS or FD systems had lower sunburn incidence than those with other treatments every year and data for 2004 and 2007 are shown in Table 3. Trees from these two treatments had larger canopies, more foliage, and thus protected the fruit against direct heat.