Effects of Drip Irrigation Configuration and Rate on Yield and Fruit Quality of Young Highbush Blueberry Plants

A 4-year study was conducted to establish the effects of drip irrigation configuration and rate on fruit yield and quality of young highbush blueberry plants (Vaccinium corymbosum L. ‘Duke’). Plants were grown in a silt loam soil on raised beds and were non-irrigated or irrigated using either one or two lines of suspended drip tape. Each line configuration had in-line emitters spaced every 0.3 or 0.45 m for a total of four drip configurations. Water was applied by each drip configuration at two rates, a moderate rate of 5 L/plant per irrigation event, and a heavy rate of 10 L/plant. The frequency of irrigation was guided by measurements of soil matric potential. Irrigation was applied each year, and plants were cropped beginning the second year after planting. Rainfall was above normal in the first 2 years of the study, and differences in soil moisture were most evident in the last 2 years, in which soil matric potential increased with irrigation volume. Neither the number of irrigation lines nor emitter spacing had an effect on yield or fruit quality. Yield was unaffected by irrigation rate until the fourth year after planting and was only higher when 5 L/plant was applied. The yield increase was the result of differences in fruit weight during the second of two harvests and was associated with delays in fruit maturation. Irrigation affected plant mineral concentrations but leaves and berries responded differently; affected minerals tended to decrease in leaves but increase in the fruit. Many irrigation-induced changes in fruit quality were evident 1 or 2 years before changes in yield. Higher irrigation volume increased fruit size and water content but reduced fruit firmness and soluble solids. Irrigation reduced fruit water loss during storage and thereby promoted longer shelf life. Irrigation also resulted in a change in anthocyanin composition in the fruit but did not affect antioxidants or total anthocyanin content. Blueberry is a rapidly expanding component of the small fruit sector in the Pacific Northwest of North America. Between 1992 and 2003, acreage planted to highbush blueberry in Oregon, Washington, and British Columbia increased 124%, 78%, and 122%, respectively (Strik and Yarborough, 2005) and the trend continues today. Almost all blueberry growers irrigate through the summer in an effort to match water availability to plant water demand. Of the various types of irrigation systems available, drip is becoming the most widely used in blueberry. Drip irrigation guidelines are available for horticultural crops grown in British Columbia (Van der Gulik, 1999) but no information on expected yield or fruit quality with respect to irrigation is available for blueberry. Some recent information is available that compares blueberry irrigation methods and rates on a silt clay-loam soil in Oregon (Bryla, 2008; Bryla et al., 2009, 2011), but there is nothing specific to the cultivars, climatic conditions, and soil types of British Columbia. Despite a number of drip irrigation configurations used in the Pacific Northwest (e.g., one or two lines, suspended or ground level), no literature is available comparing them. The effects of cultural practices, including irrigation and water management, on blueberry fruit quality attributes such as berry size, firmness, and shelf life remain largely undefined. Blueberries contain high levels of anthocyanins and other flavonoids, which show anticarcinogenic properties and are used to treat such ailments as urinary tract infections, blood vessel disorders, and ophthalmological conditions (Kalt and Dufour, 1997). Information on the effects of irrigation on antioxidants and anthocyanins in blueberries is lacking. The purpose of this study was to determine the effects of different drip irrigation configurations and rates on blueberry production. Treatments were imposed beginning the first year after planting, and the effects on yield and fruit quality were followed as the plants matured. Materials and Methods Study site. A 0.15-ha field of northern highbush blueberry (Vaccinium corymbosum L. ‘Duke’) was established at the Pacific Agri-Food Research Center, Agassiz, British Columbia, Canada (lat. 49 14#33$ N, long. 121 45#35$ W) in Oct. 2006. Field preparation in the spring of 2006 involved ploughing, discing, and the application of 90S elemental sulfur (Terra Link Horticulture Inc., Abbottsford, British Columbia, Canada) at a rate of 1120 kg ha to lower the soil pH from 5.6 to 5.0. Soil at the site was a moderately well-drained Monroe series (eluviated eutric Brunisol) silt loam. The field remained fallow until September at which time it was subsoiled and raised beds, 1 m wide and 0.2 m high, were created in a north–south orientation. Twoyear-old plants obtained from a local commercial nursery (JRT Nurseries, Abbottsford, British Columbia, Canada) were transplanted into the beds at a spacing of 1 m apart within rows and 3 m apart between rows. Beds were topped with 8 cm of new Western hemlock (Tsuga heterophylla Sarg.) and douglas-fir (Pseudotsuga menziesii Franco) sawdust mulch every other year. All areas between and around beds were seeded to 30% fescue and 70% perennial rye grass (Alleyway Agricultural Mix; Richardson Seed, Abbottsford, British Columbia, Canada). Experimental design. Nine irrigation treatments were arranged in a randomized complete block design and included all combinations of one or two lines of drip tape (DLT Heavywall Dripperline; Netafim, Fresno, CA), two drip emitter spacings, and two different volumes of irrigation (moderate and heavy) plus a non-irrigated control. Each treatment plot consisted of one row of six plants used for measurements. The treatments were randomly arranged in the blocks in two sets of five rows. Two pairs of blocks were replicated three times for a total of six replicates per treatment; note that one plot of plants in each block (i.e., Plot 10) was not used in the study. A guard row of blueberry was planted on each side of the block pairs and a guard plant was planted on each end of the plot pairs. Each plot pair was separated by a 2-m wide walkway. The single drip irrigation line was suspended over the center of the planting bed, whereas the two lines of drip were located 19 cm from the center on each side of the bed. The drip lines were attached to catch wires suspended at a height of 0.6 m and had 1-L h in-line emitters spaced every 0.3 or 0.45 m. Water was applied by each drip configuration at rates of 5 and 10 L/plant per irrigation event in the moderate and heavy irrigation treatments, respectively. This translates into 17,935 and 35,870 L ha (7,260 and 14,520 L/acre) in the moderate and heavy irrigation treatments, respectively. Received for publication 6 Dec. 2011. Accepted for publication 30 Jan. 2012. We are grateful to the BC Blueberry Council and the AAFC Developing Innovative Agri-Products (DIAP) program for financial assistance; to Ben Frey, Elyse Hofs, Russel Warwick, Taylor Holland, Albert Tsou, and June Dawson for plot work; to Albert Tsou for technical assistance; and to Mark Sweeney for horticultural advice. To whom reprint requests should be addressed; e-mail [email protected]. 414 HORTSCIENCE VOL. 47(3) MARCH 2012 Irrigation first began on 15 July 2007 and was applied annually for the duration of the study. As a guide, irrigation was initiated when the average soil matric potential (Ym) in the most heavily irrigated treatments was 20–25 kPa. Irrigations were initiated manually and applied using a Harrow Fertigation Manager (Climate Control Systems, Leamington, Ontario, Canada). The treatments were not irrigated simultaneously but were all irrigated within a 7-h period on the same day. Depending on the weather, irrigations began in May, June, or July each year and continued through August or September. The total volume of water applied to the moderate and heavy irrigation treatments was 45 and 90 L/plant in 2007, 20 and 40 L/plant in 2008, 65 and 250 L/plant in 2009, and 220 and 440 L/plant in 2010, respectively. Hence, the heavy irrigation treatment received twice as much water annually as the moderate treatment, except in 2009 when it received 3.8 times as much as a result of additional irrigation events. The proportion of water applied up to the final harvest was 50%, 38%, and 59% in 2008, 2009, and 2010, respectively. Field management. Plants were fertilized with 15N–8P–11K (Berry Blend fertilizer; Terra Link Inc., Abbottsford, British Columbia, Canada) in two broadcast applications each spring. Annual rates applied in 2007, 2008, 2009, and 2010 were 40, 57, 93, and 153 g/plant, respectively, as recommended in the British Columbia berry grower guide (BCMAL, 2009). Soil pH was measured in 2 cm · 30-cm samples collected from the top 30 cm of the planting (after sawdust was removed) each fall in all treatments and was always within the 4.5 to 5.2 range recommended for blueberry (BCMAL, 2009). Flowers were removed in the spring during the first year after planting, and plants were cropped and harvested beginning the next year. At flowering each year, a honeybee hive was positioned at the north end of the plot area to facilitate pollination. Bird netting was installed above and around all six blocks during berry ripening. Plants were pruned annually each winter according to industry standards (BCMAL, 2009). In early spring of each year, the perimeter of the sawdust-covered plots was sprayed with Touchdown Total herbicide (Sygenta Crop Protection Canada Inc., Guelph, Ontario, Canada) for control of annual and perennial grasses and broadleaf weeds. In 2009 and 2010, plants were treated for Bruce spanworm (Operophtera bruceata) using Dipel 2XDF (Bacillus thuringiensis) (Valent Canada Inc., Guelph, Ontario, Canada) and in 2010 and 2011 plants were sprayed with Pristine (boscalid, pyraclostrobin) fungicide (BASF Canada Inc., Mississauga, Ontario, Canada) and Switch (cyprodinil, fludioxonil) fungicide (Sygenta, Plattsville, Ontario, Canada) for the control of green fruit botrytis. Measurements. Rainfall, photosynthetically active radiation, relative humidity, and air and soil temperature were recorded for 2007–2010 at a meteorological station (HOBO U30 Weather Station, Onset, Bourne, MA) positioned 20 m from the plots. Daily potential evapotranspiration (ETo) was measured with an atmometer (ETgage Model E; ETgage Company, Loveland, CO) fitted with a Style #30 diffusion cover. The atmometer was placed within the plots at a height equivalent to the top of the plant canopy. Blueberry evapotranspiration (ET) was calculated by multiplying ETo by a crop coefficient of 0.27 derived from a lysimeter study on highbush blueberries of similar age and planting density with manual watering (Storlie and Eck, 1996). Soil Ym was measured intermittently at a depth of 30 cm with tensiometers in 2007 and daily with granular matrix sensors (Watermark Model 900M Monitors; Irrometer, Riverside, CA) in 2008–2010. Two tensiometers or sensors were buried in two or three plots per treatment, respectively; one was located halfway between two plants in the center of the plot and in the middle of the bed, and the other was located 19 cm from the middle of the bed on either side of the plant row. Thus, whether plots were irrigated using one or two lines of drip, one tensiometer or sensor was always located under a drip line (between two emitters) and the other was not. Fruit from each plot were hand-picked in three harvests in 2008 and 2009 and two harvests in 2010 and weighed. In 2010, the amount of immature fruit in the second harvest was estimated by determining the weight of immature fruit as a percentage of the total. Fruit firmness was measured using a firmness meter (FirmTech 2 Fruit Firmness Tester; BioWorks, Wamego, KS). The Compression Force Threshold procedure with a fixed range of compression forces (selected by the operator) was used to measure the grams of force required to compress the fruit 1 mm. After load cell calibration and a reference size measurement, which is required for fruit diameter measurements, fruit were placed with the proximal end facing inward on a metal plate turntable. The instrument measured fruit firmness and diameter simultaneously at room temperature. Two runs of 50 fruit each were conducted per treatment in each block on every harvest date. Mean fruit fresh weight was also determined by counting and weighing a random sample of a minimum of 100 fruit from each plot and harvest. Fruit shelf life was estimated by determining percent weight loss in cold storage. One hundred fruit were randomly selected from each plot and harvest, placed in perforated plastic bags, and weighed. The bags were placed as a single layer in shallow trays in a cooler at 4 C. Final weights were recorded after 1 week. Fruit percent water content was determined by measuring fresh weight and dry weight (after oven-drying at 105 C for 1 week) of random samples of 12 previously frozen fruit from each plot and harvest and was calculated as [(fresh weight – dry weight)/ fresh weight] · 100. Titratable acidity and soluble solids were measured on previously frozen fruit. Fifty grams of frozen fruit from each plot were placed in a 400-mL beaker and allowed to thaw for 90 min. The thawed fruit was blended for 40 s using a hand blender. Five grams of the blended fruit were transferred to a 50-mL centrifuge tube and the sample was homogenized for 30 s using a benchtop Polytron homogenizer (Kinematica, Inc., Lucerne, Switzerland). The homogenized sample was quantitatively transferred to a 250-mL beaker and made up to volume using deionized water. The mixture was then titrated with 0.1 M NaOH to an end point of 8.1 using a TirtroLine Easy titration unit (Schott Instruments, Mainz, Germany). Titratable acidity was calculated as percent citric acid according to Eq. [1]: % Citric Acid = ðmL NaOH x M NaOH x 0:064 x100Þ=sample wt: ðgÞ