PHYSIOLOGICAL RESPONSE OF VACCINIUM CORYMBOSUM 'ELLIOTT' TO SHADING NETS IN MICHIGAN

Abstract

Plants of the ‘Elliott’ highbush blueberry (16-year-old, planted at 3 x 1 m) were grown in a sandy loam soil under colored shade nets (white, red, black and no net) of different density (25, 50 and 75% of shading) in Gobles, Michigan, USA, during 2006-07. Treatments were assigned randomly with 3 reps, and established as a continuum of 4 rows with 6 plants in each row. Nets (provided by Polysack Plastic Industries, Israel) were established horizontally from July to October at 3 m height. Leaf chlorophyll content increased and the ratio Chl a/Chl b decreased as shading intensity increased, with the black nets giving the largest changes. Also, leaf photosynthesis, measured at the actual light intensity under the nets, decreased as shading intensity increased, with the red and black 70% having the lowest values. Fluorescence (Fv/Fm) was significantly lower in plants without shading (control) compared with plants under 70% shading (white, black or red). Nets retarded fruit development; thus, on August 10, the proportion of ripe fruit was 26.9% without shading, while less than 10% of the fruit were ripe under black 70%, black 50%, red 70% and red50%. On August 23, the separation between the controls and the most delayed fruit development (black 70%) was over 30%. In general, the various shading treatments increased fruit weight compared to the controls, but decreased fruit soluble solids, with the black 50 and 70% having the greatest impact. Thus it is clear that colored shade nets alter blueberry physiology and could be a valuable tool to displace peak harvest season in blueberries. INTRODUCTION Blueberries grow naturally in the understory. When plants are subject to open field conditions, they might be stressed due to excess radiation, temperature and moisture deficit. 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 color used, they have changed light quantity and quality received by plants, which has altered branching, shoot extension, time of harvest, fruit set and fruit quality. In this context, the use of shading nets could alleviate environmental stresses faced by blueberries, increase yields and delay harvest. MATERIALS AND METHODS The trial was conducted in Gobles, Michigan using a commercial 16-year-old field of ‘Elliott’ planted in a sandy loam soil at 3 x 1 m. A completely randomized design was used with 3 replications of each treatment. The treatments were applied to 4 rows of 6 plants each and data were collected from the 12 plants in the middle two rows. Combination of 3 colors of net (white, red and black) and 3 intensities (25, 50 and 75%) of shading nets (provided by Polysack Plastic Industries, Israel), plus control treatment (without net), generated 10 treatments (Fig. 1). The trial was established horizontally at 3.5 m height after fruit set (July 7, 2006) and removed at the onset of leaf drop.