379 Effect of 6-BA, NAA and Their Mixtures on Fruit Thinning and Fruit Yield in ‘Conference’ and ‘Blanquilla’ Pear Cultivars L. Asín 1 , P. Vilardell 2 , J. Bonany 2 and S. Alegre 1 1 IRTA-Estació Experimental de Lleida, Rovira Roure 177, E-28196 Lleida, Spain 2 IRTA-Estació Experimental Agrícola Mas Badia, E-17134 La Tallada (Girona), Spain Keywords: fruit growth, crop load, auxin, cytokinin, fruit abscission Abstract Trials on ‘Conference’ and ‘Blanquilla’ orchards were carried out in 2005 and 2007 in the Lleida basin (north-east Spain). Several thinning treatments involving 6-BA and NAA were applied either alone or in mixtures and the results were compared with hand-thinning. In all years, the experimental design was a randomized block. Both bioregulators showed a thinning effect, but this was not statistically significant in all cases. An improvement in fruit thinning was detected when both bioregulators were applied together. In some cases, this effect was too great and resulted in a decrease of around 60% in the final fruit yield, with BA at 150 ppm + NAA at 40 ppm in ‘Conference’, while in ‘Blanquilla’, the decrease was of around 30%. There was a direct relationship between fruit weight and crop load and a tendency for fruit weight to increase with 6-BA in ‘Blanquilla’. INTRODUCTION In some deciduous fruit crops, fruit thinning is necessary to obtain regular marketable crops. This situation is usual in apple and peach orchards, but not very common in pear orchards where it is more desirable to increase fruit set than fruit thinning. In fact, flower thinners have never been popular in pear crops because fruit set is less reliable than in apple (Wertheim, 1998). Under the pear growing conditions of north-east Spain, growers have not considered fruit thinning necessary for two reasons: poor fruit set due to the unfavourable weather conditions which occur in some years during the fruit set period and the slight difference in fruit prices relating to the distribution of fruit size in the 1 st pick. In recent years, the prices of fruits with calibres of up to 60 and 65 mm, in ‘Blanquilla’ and ‘Conference’ respectively, have increased. In this new scenario, the thinning of pear crops should be taken into account in orchards with high fruit loads and poor fruit sizes. NAA (Naphthalene acetic acid) and 6-BA (6-benzyladenine) have been cited as pear thinners by both Wertheim, 1983 and Bound and Mitchell (2002) respectively. This paper studies the efficiency of NAA and 6-BA alone or in combination as related to different concentration. MATERIAL AND METHODS In 2005 and 2007, different fruit thinning trials were conducted on ‘Blanquilla’ and ‘Conference’ cultivars at the IRTA-Estació Experimental de Lleida. In both orchards, the rootstock was quince M-A, the trees were planted in 1994 at a tree densities of 1,250 and 1,667 trees/ha for ‘Blanquilla’ and ‘Conference’, respectively. The dosages of the active ingredients and the times of application are listed in Table 1. The spray volume was 1,000 L/ha. Sprays were carried out for all treatments when fruit size was 8-10 mm. The experimental design was a randomized block with 4 repetitions. Each plot had 4 trees and all determinations were carried out in the 2 central trees. For the central trees of each experimental unit, data relating to fruit set (fruits/100 flower clusters) for the whole tree, production (kg/tree) and average fruit weight (g) were recorded. These data were then analyzed by the ANOVA procedure for each locality and year, using SAS software. Statistical significance was judged at p≤0.050. When an Proc. XI th IS on Plant Bioregulators in Fruit Production Ed.: G. Costa Acta Hort. 884, ISHS 2010