Effects of different irrigation levels on plant water status, yield, fruit quality, and water productivity in a drip-irrigated blueberry orchard under Mediterranean conditions

Highlights

• Irrigation at 100% evapotranspiration (ETc) reduced water use without affecting yield and quality.

• Irrigation at 100% ETc reduced the water use between 950 and 1130 m³ ha⁻¹ season⁻¹.

• Irrigation at 100% ETc maintained midday stem water potential >−1.0 MPa.

• Irrigation at 50% ETc significantly decreased fruit weight and yield.

• Irrigation at 50% ETc increased soluble solids and the water stress integral.

Abstract

As blueberries are susceptible to water stress and their future cultivation in semiarid Mediterranean areas will be challenged by drought, irrigation management strategies will be needed to optimize water productivity and maintain sufficient levels of fruit yield and quality. This study aim was to evaluate the effect of different irrigation levels on plant water status, yield, fruit quality, and water productivity in a drip-irrigated rabbiteye blueberry (Vaccinium ashei Reade 'Tifblue') orchard. Four irrigation treatments based on crop evapotranspiration (ETc) were applied to blueberry plants during two consecutive growing seasons (2012/2013 and 2013/2014): 125 (farmers’ irrigation management, T₁), 100 (T₂), 75 (T₃), and 50 (T₄) % ETc. During the study, the average values of midday stem water potential (Ψ_stem) were −0.85, −0.86, −0.97 and −1.11 MPa for the T₁, T₂, T_3, and T₄ treatments, respectively. Fruit weight (FW), yield (Y), fruits per plant (FP), soluble solids (SS), and the water stress integral (WSI) were significantly affected by the irrigation treatments. The water productivity (WP), juice pH, and weight/volume ratio were statistically similar among the treatments. The highest values of Y, FP, and FW were observed in the T₁ and T₂ treatments, while the lowest values were found in the T₄ treatment. In addition, the Y, FP, FW and WSI in the T₁ and T₂ treatments were significantly similar, but the total water application in the T₂ treatment was between 20% and 27% lower than that in the T₁ treatment. For the T₁ and T₂ treatments, the values of Y were between 8.8 and 9.4 kg plant ⁻¹, and the Ψ_stem was >−0.85 MPa during the two growing seasons. The interaction between irrigation treatments and growing season was only significant for the FW, with the lowest values observed in the T₄ treatment during the 2012/2013 growing season.

Introduction

Blueberries are functional foods, and their consumption has increased because of their positive effects on people’s well-being and health (Romo-Muñoz et al., 2020). In Chile, blueberry production has increased during the last year concentrating 20% of worldwide production (Brazelton and Young, 2017). The total planted area was 19,000 ha in 2019 with mainly highbush and rabbiteye blueberries (CIREN, 2020). The blueberry orchards are mainly located in Mediterranean-climate areas having high atmospheric demand for water vapor and scarce precipitation during the spring and summer period. In addition, rainfall has significantly decreased since 2010 due to a mega-drought limiting the water availability for irrigation in Mediterranean regions (Garreaud et al., 2020). Under these water scarcity conditions, irrigation management strategies are required in blueberry production to avoid plant water stress that negatively affects yield and fruit quality (Carrasco-Benavides et al., 2020, del Pozo et al., 2019).

Rabbiteye blueberry is well adapted to semi-arid conditions, but it is sensitive to soil water deficit. It is characterized by its low water absorption capacity, shallow root system, lack of root hairs, and high water requirements for profitable production (Bryla et al., 2011, Gough, 1994). Soil moisture is the most important factor affecting blueberry root growth, and any water deficit negatively affects vegetative growth and fruit production (Bryla and Strik, 2007, Bryla et al., 2011, Holzapfel et al., 2004, Spiers, 1998a, Spiers, 1998b). Thus, irrigation management has become an important decision-making tool for improving water productivity (WP, kg m⁻³) without compromising yield or fruit quality (Ortega-Farias et al., 2012).

Evidence has demonstrated significant reductions in fruit size and yield losses under severe water stress conditions. Depending on the cultivar, less frequent irrigation and drought periods from one to three weeks decreased the leaf water potential (Ψ_Leaf) from −0.3 to −1.6 MPa, which significantly reduced the yield and dry weight of the plants (Mingeau et al., 2001, Bryla and Strik, 2007). Additionally, water stress induces stomatal closure and diminishes CO₂ assimilation, with consequent yield losses. For highbush blueberries, Lee et al. (2006) reported that the Ψ_Leaf and CO₂ assimilation rate (An) decreased from −1.07 to −1.79 MPa and from 8.84 to 4.6 µmol m⁻² s⁻¹, respectively, after withholding water for seven days. For lowbush blueberries, Glass et al. (2005) observed values of midday stem water potential (Ψ_stem) ranging from −1.45 to −1.41 MPa for stressed plants irrigated at 75% of crop evapotranspiration (ETc). For well-irrigated highbush blueberries at different plant densities, Bryla and Strik (2007) recorded Ψ_stem values ranging from −0.5 to −0.4 MPa at the end of spring, and from −1.6 to −1.4 MPa at harvest.

Blueberries are also susceptible to over-irrigation, which reduces root activity, increases soil erosion and nutrient leaching, and increases the prevalence of fungal diseases, such as phytophthora root rot (Bryla and Linderman, 2007, Bryla et al., 2008, Bryla et al., 2011, Starr et al., 2004). The effects of under- or overirrigation on yield, fruit quality, and water productivity depends on weather conditions, site, cultivar, age, spacing, and irrigation method, among other factors. For the young highbush blueberry (cultivar 'Duke’), Ehret et al. (2012) found that compared to moderate irrigation treatments (soil matric potential (Ψ_M) from −20 to −50 kPa), over-irrigation (Ψ_M from −20 to −25 kPa) significantly reduced fruit firmness and the soluble solids of the fruits. In addition, total yields were not significantly different between over-and moderately irrigated plants, but they were significantly greater than those in a rainfed treatment (Ψ_M from −20 to −90 kPa). In this case, yields were 6.3, 6.6, and 6.0 kg plant⁻¹ for the overirrigated, moderately irrigated and rainfed treatments, respectively. For six-year-old plants, Ehret et al. (2015) found no difference in berry weight, fruit firmness, or soluble solids for plants irrigated under “moderated irrigation” (total water application (TWA) between 150 and 170 L plant⁻¹ year⁻¹) and overirrigation (TWA between 300 and 340 L plant⁻¹ year⁻¹) treatments. In a two-year experiment carried out in nursery plants of the northern highbush blueberry (cultivar ‘Elliott’), Bryla et al. (2011) found that plants drip-irrigated at 100% ETc (TWA = 289 mm year⁻¹) had a superior yield and berry weight compared to plants irrigated at 150% ETc (TWA = 411 ± 163 mm year⁻¹), which showed a reduction in growth, new canes, and cane dry weight. In the same study, the WP for plants irrigated at 50% ETc (TWA = 104 mm year⁻¹) was 54% higher than those watered at 100% ETc, but the yield and berry weight were significantly reduced by water restriction. In summary, the scheduling of irrigation in blueberry orchards must be done to avoid over and under irrigation to optimize yield and fruit quality.

In Chile, for a seven-year experiment with highbush blueberries (cv. ‘Bluetta’), Holzapfel et al. (2004) reported no significant differences in berry weight among plants irrigated at 33 (TWA = 208.3 mm year⁻¹), 100 (TWA = 624.9 mm year⁻¹) and 133% ETc (TWA = 833.2 mm year⁻¹). These authors pointed out that yield was reduced when water was applied at a rate of 200 mm year⁻¹ (≈3.6 t ha⁻¹) and remained relatively constant when water was applied at a rate of 600 mm year⁻¹ (6,8 t ha⁻¹). For a commercial, drip-irrigated rabbiteye blueberry (Vaccinium virgatum ‘Bonita’) orchard, Ortega-Farías (2013) found that plants irrigated at 80% ETc (TWA ≈270 mm year⁻¹) presented the highest yield (12.3 t ha⁻¹) and WP (4.3 kg m⁻³). In this study, the water application of 40% and 120% ETc significantly reduced yield and WP, corroborating that under- or overirrigation had negative effects on blueberry yield. In a drip-irrigated highbush blueberry (cv. ‘Brigitta’) experiment, Lobos et al., 2016, Lobos et al., 2018 indicated that depending on the growing season, the application of 75% ETc (TWA between 180 and 200 mm) produced a similar yield (≈4.8 kg plant⁻¹), berry weight (≈1.6 g fruit⁻¹), and WP (≈6.9 kg m⁻³) as the application of 100% ETc. Plants irrigated at 50% ETc produced less yield (≈3.0 kg plant⁻¹) and berry weight (≈1.4 g fruit⁻¹) but higher WP (≈7.5 kg m⁻³). The same authors observed that the Ψ_stem values were −1.2 and −0.7 MPa for plants in 50% and 100% ETc treatments, respectively.

However, the effects of nonlinear interactions among soil, cultivar, and climate on irrigation scheduling are still poorly understood by the blueberry farmers. Thus, irrigation management strategies are required to optimize water productivity, yield, and quality of rabbiteye blueberry orchards growing under water scarcity conditions. In this context, the purpose of this study is to evaluate the effects of four irrigation levels on plant water status, yield, fruit quality, and water productivity in a drip-irrigated rabbiteye blueberry (Vaccinium ashei Reade, ‘Tifblue’) orchard growing under Mediterranean climate conditions.