COLORED SHADING NETS INCREASE YIELDS AND PROFITABILITY OF HIGHBUSH BLUEBERRIES

Abstract

The highbush blueberry (Vaccinium corymbosum L.) develops naturally under storey of deciduous forests. In Chile, there is a need to produce fruit earlier in the season, which has forced growers to plant under greater environmental stresses (temperature, radiation, and relative humidity). In this context, colored shading nets can alleviate stresses and affect yield and fruit quality. This research was set up to study the effects of shading nets (color, shading degrees) on environmental conditions faced by plants, as well as vegetative growth, yield and quality of fruit produced, as well as economical performance in highbush blueberries. The experiment started in 2003 in an orchard cv. Berkeley planted in Miraflores, Chile (Lat. 36°04’S; Long. 72°47’E) in 1994 at 3 × 1 m. Black, white, gray and red nets and 35 and 50% shading plus a control treatment (no net) were tested. Nets were set each season at 3.5 m height at fruit set and remained until early leaf fall. With respect to control, white 35 and 50, gray 35 and 50 and red 35%, decreased PAR radiation in 29%; while red 50, black 35 and black 50 decreased PAR in 41, 47 and 53% respectively. Colored nets did not consistently affect air temperature, soil moisture and relative humidity. Yields (wt/plant) over control were increased in year 1 and 2 by 90.5 and 44.6% respectively for white 50%, 59.6 and 24.9% (gray 35%) and 84.2 and 31.9% (red 50%). Black net, commonly used by growers, had long term negative effects on yield; thus for black 35%, yields were 37.2 and -8.3, while for black 50%, yields were -3.2 and -28% of control, respectively. Higher yields were due to greater fruit number, since nets did not affect fruit size or soluble solids. In season 1, black nets increased shoot and internode length, and leaf size, while shoot number and leaf length/width ratio were unchanged. INTRODUCTION Blueberries are under storey fruit crops native from southern (Vaccinium ashei) or northern (V. corymbosum) areas of United States (Eck, 1988). Worldwide area planted with blueberries has increased greatly in the last decade, since benefits for health derived from their consumption have been demonstrated (Kalt et al., 2001). In Chile, plantings are increasing at a rate of 500-800 ha/year. Due to high fruit prices at the beginning of the export season in the southern hemisphere, a large proportion of the expansion in Chile has occurred in areas with high temperature and light intensity, which could reduce photosynthesis, carbohydrate accumulation, and fruit yields (Darnell, 1991). Colored shading nets have been investigated in ornamentals and fruit crops (OrenShamir et al., 2001; Shahak et al., 2004) in these crops, depending upon the degree of shading and the color used, they have altered the amount and quality of light received by plants, which has triggered changes in branching, shoot extension, fruit set and fruit quality. In this context, the use of shading nets could alleviate the environmental stress faced by blueberry plants and have effects on their growth, fruit yield, and economical performance (Oren-Shamir et al., 2001). MATERIALS AND METHODS The trial was initiated in October 2003 in a commercial orchard of highbush cv. Berkeley planted in 1994 at 3 × 1 m. The orchard is located in Miraflores (Lat. 36°04’S; Proc. XXVII IHC Cultiv. Utiliz. Asian, Sub-Trop., Underutilized Hort. Crops Eds.-in-Chief: Dae-Geun Oh and Chieri Kubota Acta Hort. 770, ISHS 2008 194 Long. 72°47’E). A completely randomized design with 3 replications of 12 plants per treatment was used. The area destined to each treatment was 108 m, which comprised a plot of three rows with two plants each. To allow complete effect of nets, only six central plants were selected for measurements and six equivalent plants were used as control (no nets). The combination of 4 colors of net (white, black, gray and red) and 2 intensities of shading (35 and 50%), plus control treatment (no net), generated 9 treatments. In each of the two seasons, nets (Polytex, Iquique, Chile), were placed on a wire frame at 3.5 m height at fruit set (October 8-10) and removed at the onset of leaf fall (March 30). Temperature, relative humidity (HOBO, H8 sensors, Onset Co., Bourne, MA), and PAR radiation (LI-190SA sensor, Licor Biosciences, Lincoln, USA) were recorded every 15 min. Soil moisture was measured weekly with tensiometers (Model R, Irrometer Co., Riverside, USA) set at 30 cm depth. Vegetative growth measurements (leaf length and width, shoot number and length, and internode length), were done on 6 central plants in each experimental unit on April 3, 2004. To determine yield per plant and fruit production curves, fruit were hand-picked twice per week between 12/13 and 01/24 each season. Fruit weight was established on a sample of 100 fruit collected in each replication and each harvest. Data were analyzed through ANOVA; mean separation was done using Duncan’s multiple range test. RESULTS AND DISCUSSION Black nets reduced PAR radiation to 50% of their levels in open field; there was no correlation between the level of shading and the PAR levels under the shade (Fig. 1). Treatments generated minor differences in temperature, relative humidity and soil moisture (data not shown). Black nets generated longer shoots and larger leaves (length and width) but had no significant effects on shoot number as compared to control plants (Table 1); similar effects of shading were observed by Baraldi et al. (1994), in peaches. The treatments white 50%, gray 35% and red 50%, had yields that were 26-91% greater than control plants; on the other hand, yields under black 50% ( a shade commonly used by growers in Chile), were similar to the control in the first season and significantly inferior in the second year (Table 2). Treatments had no significant effect on fruit soluble solids or weight (Table 3). Since the treatments were established after bloom each year, once flower induction had already occurred (Eck, 1988), the differences in yield must have been caused by the influence of shading on fruit set. In these regards, Shahak et al. (2004) have reported increased fruit set after shading 6 weeks before harvest apples cv. Smothee and cv. Top Red and peaches cv. Hermosa. In climates with high incidence of radiation (such as Central Chile), this positive effect of shading on fruit set and yield could be due to the reduction of stressful conditions in midday hours which would avoid supraoptimal light levels, plant heating and photosynthesis inhibition (Dale, 1992; Ort, 2001); greater partitioning of carbohydrates towards shoot in black nets (Table 1) would explain their failure to produce higher yields. In the days following fruit set in blueberries, shoots, roots and leaves are competing for carbohydrates, water and nitrogen (Darnell and Birkhold, 1996); in these circumstances, developing fruit under shade treatments would have more water availability and a greater supply of carbohydrates from young developing leaves (Raveh et al., 2003), this situation would allow more availability of these compounds to the fruit and would increase fruit set. The greater fruit set would influence photosynthetic rate through a feed back mechanism, as has been demonstrated in several species (Flore and Lakso, 1986; Birrenkott and Stang, 1990; Matthew and Pellny, 2003). Is it also possible that some of the effects of the nets are due to changes in the light quality of the transmitted light, since it has been demonstrated that colored shading nets alter the quantity and quality of light that reaches the plants (Oren-Shamir et al., 2001; Shahak et al., 2004). Plants can detect the quality, quantity and orientation of light and use it as a signal to optimize their growth and development in a given environment (Rajapakse et al., 1999).