Irrigation and Rootstock Influence on Water Use, Tree Growth, Yield, and Fruit Quality at Harvest at Different Ages of Trees in ‘Pacific Gala’ Apple (2024)

Orchard establishment

The experimental orchard was established at the University of Idaho Parma Research and Extension Center in spring and early summer of 2002. ‘Pacific Gala’ trees on four rootstocks were planted at 1.52 × 4.27-m spacing with an east–west row orientation. The rootstocks were: ‘Budagovsky 9’ (‘B.9’), ‘Nic9’ (‘RN29’), ‘Cornell-Geneva30’ (‘G.30’), and ‘Supporter4’ (‘Sup.4’). ‘Manchurian’ crab apple [(Malus baccata) mandshurica] on ‘RN29’ rootstock (C & O Nursery, Wenatchee, WA) was planted in each row as a pollenizer between every 10 ‘Pacific Gala’ trees. Trees were trained into a vertical axis central leader system during the dormant season in early March every year. Tree leaders were maintained at a height of ≈3.7 m.

The experimental site had a semiarid climate with an annual precipitation of ≈297 mm and a sandy loam soil of pH ≈7.3. Crested wheatgrass [Agropyron cristatum (L.) Gaertn.], which is a drought-tolerant grass, was planted as the orchard floor cover in all treatments. Weeds were controlled chemically to maintain a 1.22-m weed-free herbicide strip under trees.

Trees in all treatments were blossom-thinned at ≈80% bloom with 5% lime sulfur followed by one or two applications of post-bloom thinners. The first post-bloom thinner was a mixture of carbaryl (44.1% by weight a.i.; Sevin XLR; 1-naphthyl N-methylcarbamate; Bayer Crop Science, Research Triangle Park, NC) and ethephon [21.7% a.i.; Ethrel (2-chloroethyl) phosphonic acid; Bayer Crop Science] at a rate of 0.125% to 0.156% of formulation and was applied at petal fall. The second post-bloom thinner (when applied, depending on the cropload) was carbaryl (Sevin XLR) at 0.125% formulation that was applied when fruitlet diameter was ≈7 mm. Fruit were subsequently hand-thinned when fruits were ≈18 mm in diameter (around mid-June) to maintain a space of at least 12.5 to 15 cm between fruits. Kaolin (95% a.i.; Surround; Englehard, Iselin, NJ) was sprayed for sunburn protection at the rate of 56.8 kg·ha⁻¹ in early July followed by three 1-week interval applications, each at 28.4 kg·ha⁻¹ every year.

Cultural practices other than irrigation were similar to those recommended for commercial orchards in the Pacific Northwest (Washington State University, 2012).

Irrigation regimes

Two irrigation regimes were installed, FS and FD. Each irrigation regime was applied in one row, consisting of four rootstocks as described earlier. A row of guard trees was used between every two experimental rows. These trees received only drip irrigation to prevent any possible overspray from the sprinkler system in the experimental rows. Trees from the guard rows were not used for any part of the study. The two irrigation regimes (FS and FD) in this study were as follows.

Interaction

There was no significant interaction between year and irrigation or rootstock treatments for any of the amount of applied water, tree growth, yield, or fruit quality attributes (fruit size, color, firmness, SDP, SSC, stem-end cracking) in this study. Thus, in addition to the results in each year, results of overall years from 2004 through 2007 are reported for each of these attributes.

Water application

The average precipitation during the irrigation periods of 2004–05, when trees were not yet fully mature, was 66.0 mm, and the average for 2006–07 irrigation periods, when trees were mature, was 55.1 mm (Table 1). Water application in all irrigation regimes increased as trees matured (Table 1). As expected, trees used the most water in July and August in all years. Trees with FS treatment received a considerably greater volume of water than those with FD 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 (Table 1), respectively. The differences between water applications in FS and FD systems were consistent (38% to 41%) after 2005 (Table 2) because trees had reached the maximum ground shading (≈62%) and full canopy maturity after 1 Aug. 2005. On average, young trees with a FS system received an average of 872.3 mm (5616.8 L/tree), whereas those with FD received 448.9 mm (2921.1 L/tree). However, when trees were mature, trees with a FS system received an average of 994 mm (6461.7 L/tree), whereas trees with a FD received 614.1 mm (3996 L/tree) of irrigation water per growing season (Table 1).

Leib et al. (2006) compared three microsprinkler irrigation systems in mature ‘Fuji’ trees in Washington State. In that study, the soil water content in the CI was maintained close to field capacity, which was only 60% to 70% of estimated ETc for apple without cover crop. They estimated that irrigation scheduling based on soil-water measurements required 26% less water than what was predicted by the ETc model for an apple orchard without a cover crop. They found that the 3-year average potential evapotranspiration was 991 mm, ETc was ≈790 mm, and irrigation amounts applied were 707 mm for CI irrigation regimes. In our study when trees were mature (2006 and 2007), the 2-year average for ETr was 1106.7 mm and for ETc was 1050.3 mm (Table 2), and thus, these values were ≈11% and 25% higher than similar measurements in Washington, respectively. During 2006 and 2007, we applied an average of 994 mm of water to the FS trees, which was ≈287 mm (≈29%) higher than the levels applied to the CI treatment in Washington State (Leib et al., 2006). This difference is perhaps largely the result of the higher ETr and ETc values in Idaho than Washington. We used Kc for orchards with a cover crop, whereas they used Kc for orchards without a cover crop in Washington. The difference could also be in part because trees receiving FS were applied with water at full ETc level in our study (Table 1), whereas CI trees in the Washington experiment received water at ≈70% of ETc. Rainfall in both experiments was somewhat comparable.

Effects of rootstock on tree growth and yield

‘Pacific Gala’ trees on ‘B.9’ had significantly smaller TCA, whereas those on ‘Sup.4’ had larger TCA than those on other rootstocks all years of the study (Table 2). Trees on ‘RN29’ were smaller than those on ‘G.30’ when trees were young in 2004 and 2005, but the tree size differences between these two rootstocks became insignificant when trees were mature in 2006 and 2007. Based on the foliage density after tree canopy reached 100% maturity (≈62% GS as described before), it appeared that 1.52-m spacing between trees was too wide for trees on ‘B.9’ and could have been reduced to 0.9- to 1.2-m spacing for this rootstock. Even between-row spacing for this rootstock could have been reduced from 4.2 to 3.6 m or 3.9 m and thus tree density on ‘B.9’ could have been increased from its current density of 1536 trees/ha to 2069 to 2989 trees/ha. However, 1.52-m spacing was appropriate for trees on ‘RN29’ rootstock. ‘Pacific Gala’ trees on ‘Sup.4’ were too large for planting with the tree spacing of this study.

Trees on ‘RN29’ had consistently higher yield per tree every year, leading to a higher cumulative yield over the 2004–07 seasons (Table 2). Trees on ‘B.9’ and ‘RN29’ were more precocious and had higher yield per tree and yield efficiency than those on ‘Sup.4’ in 2004 through 2006 (2 to 4 years after planting) (Table 2). Thus, both of these rootstocks can be recommended for their yield under closed-space conditions of this study. ‘Sup.4’ was not a suitable rootstock for ‘Pacific Gala’ in this study because trees were too large with low yield efficiency.

Effects of rootstock on fruit quality attributes

Fruit from trees on ‘RN29’ had greater but trees on ‘B.9’ had lower fruit weight than did those on other rootstocks at least in 3 of 4 years of this study (Table 3). Smaller fruits in trees on‘B.9’ were perhaps the result of a lower leaf/fruit ratio and smaller leaf size in these trees (data not shown). Fruit in trees on all rootstocks was kept at ≈15-cm spacing at the time of thinning, leading to a lower leaf/fruit ration in ‘B.9’ rootstock because this rootstock had a smaller canopy (Table 2) and lower foliage density (data not shown). Usually lower yield is associated with larger fruit size. However, trees on ‘Sup.4’ had smaller fruit despite their lower yield and thus were not considered to be suitable for planting.

Rootstock did not have a major and consistent impact on the ‘Pacific Gala’ fruit color. Trees on ‘Sup.4’ had slightly less red color than those on ‘B.9’ rootstock in 2 of 4 years of this study (Table 3) and that could be the result of the larger canopy size and thus more shading of trees on ‘Sup.4’ rootstock.

Fruit from trees on ‘B.9’ and ‘G.30’ often had higher SSC (Table 3) and SDP (Table 4) than those from other rootstocks. Trees on ‘G.30’ also had lower firmness and higher percentage of stem-end cracking than those on other rootstocks (Table 4). The trends in SSC, SDP, firmness, and stem-end cracking in the fruit from trees on ‘G.30’ suggest that this rootstock advances fruit maturity in ‘Pacific Gala’. Based on the trends of SSC and SDP in the fruits from trees on ‘B.9’, this rootstock should also advance fruit maturity. However, the reason that fruit from trees on ‘B.9’ did not have lower firmness or higher stem-end cracking than those on most other rootstocks could be the result of the existence of higher concentration of calcium in these fruits and this subject requires further study.

Effects of irrigation on tree growth and yield

Trees grown with FS irrigation had significantly (11.3%) higher TCA (Table 5) and more new shoots (data not shown) than those with an FD system across all years of the study. Trees with FD system were more precocious and had higher yield per tree and yield efficiency than trees with FS system in 2003 (data not shown), 2004, and 2005 (Table 5). Water stress resulted in a higher production of fruiting spurs in trees with drip treatment, leading to a higher production in early years (2003–05). When trees were mature (2006 and 2007), yield and yield efficiency in both FS and FD systems were similar. Because trees with a FD system received less water (Table 1) and were more precocious with significantly higher yield per tree than those with a FS system during 2004 and 2005 (Table 5), we suggest that FD is a preferred method of irrigation over a FS system for ‘Pacific Gala’ apples as far as yield and water consumption factors are considered.

Effect of irrigation on fruit quality attributes

Fruit weights in 2004 and 2005 and average fruit weights over 2004–07 from trees with FD were significantly larger than those from FS treatment (Table 6). However, when trees were mature (2006 and 2007), fruit weight differences between trees with FS and FD were not significant. Larger fruit size in FD during early years after planting (2004 and 2005) was because roots in these trees were mostly concentrated where the irrigation was applied and thus causing larger fruits. In 2006 and 2007, roots of trees receiving an FS irrigation system were expanded and were able to uptake more of the water that was applied to a distance from the tree.

‘Pacific Gala’ fruit from trees with FS consistently had better color than those with an FD system every year (Table 6) that could be the result of the cooler temperatures that prevailed under trees with an FS system (data not shown). Also ‘Pacific Gala’ leaves from trees with FS irrigation always had significantly lower magnesium (Mg) than those on FD every year (data not shown). Because Mg is the central part of the chlorophyll structure, trees on a FS system could have had lower chlorophyll concentration, leading to higher red color, and this subject merits further study.

There was no consistent effect of irrigation system on fruit SSC (Table 6). However, fruit from trees with the FD system often had lower firmness and higher SDP than those from FS irrigation, suggesting that fruit with FD were slightly more mature as a result of water stress as compared with trees with an FS system. Nevertheless, this advanced maturity did not cause a major difference in fruit stem-end cracking between the two irrigation treatments (Table 7). Factors that lead to a greater hydrolysis of fruit starch can result in higher SSC in apples (Kramer, 1983). Despite being a useful test, a simple fruit dip in iodine solution (SDP) may not always be a reliable measure of the starch concentration of fruit. For example, in our study in 2005, fruit from trees receiving FD treatment had a significantly lower SSC than those from FS irrigation regime (Table 6), whereas fruits in FD had a higher level of SDP than those in FS treatment (Table 7). This could be the result of conversion of simple sugar to other metabolites.

Ebel et al. (1993) reported that ‘Delicious’ apple fruit from trees receiving early-season RDI treatment were smaller and had a higher SSC and lower SDP that those from well-watered trees. However, they did not observe any difference in the color or firmness of fruits from RDI and well-watered trees. Leib et al. (2006) showed that SSC in fruit from trees receiving DI was higher than in fruit 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. Differences in the volume of water applied in deficit irrigation treatments and method of calculation for water requirement (ETc vs. soil moisture content) could partially explain these contradictory reports.