PHOTOSYNTHETIC RATE, FLOWERING, AND YIELD COMPONENT ALTERATION IN HAZELNUT IN RESPONSE TO DIFFERENT LIGHT ENVIRONMENTS

In hazelnut (Corylus avellana L.), vigorous vegetative growth and trad itional orchard practices that include little or no pruning combine to produce a dense, shady canopy, A study designed to quantify the effec t of shade on reproduction and photosynthetic rate in this shade-toler ant species was undertaken to assess whether some degree of pruning mi ght improve productivity, Shade cloth was used to exclude 30%, 47%, 63 %, 73%, or 92% of ambient sunlight from whole 'Ennis' and 'Barcelona' trees from mid-May until harvest, Photosynthetic light response curves were obtained for leaves that had developed in full sunlight, deep in side the canopy of unshaded trees, or in 92% shade, Light-saturated ne t photosynthetic rates were 12.0, 6.1, and 9.3 mu mol . m(-2). s(-1) o f CO2 and dark respiration rates were 2.0, 1.1, and 0.7 mu mol . m(-2) . s(-1) of CO2, respectively, for the three light regimes, Light-satur ated photosynthetic rates of leaves from 30% or 63% shade differed lit tle from the control (0% shade), Area per leaf increased by 49% and ch lorophyll concentration (dry weight basis) by 157% as shading increase d from 0% to 92%,Shading to 92% reduced specific leaf weight (68%), st omatal density (30%), light compensation point (69%), and dark respira tion rate (63%) compared to controls, Female inflorescence density dec lined by about one-third and male inflorescence density by 64% to 74% in the most heavily shaded trees of both cultivars compared to control s, Shade was more detrimental to yield than flowering: yield per tree dropped by >80%,from 2.9 to 3.4 kg in full sun to 0.6 to 0.9 kg in 92% shade. Shade reduced yield primarily by decreasing nut number and sec ondarily by decreasing nut size, The incidence of several kernel defec ts increased as shade increased, Therefore, hazelnut leaves showed con siderable capacity to adapt structurally and functionally to shade, bu t improving light penetration into the canopy would probably increase orchard productivity.