LAUSR

ABSTRACT Effect of 2,4-D and naphthalene acetic acid (NAA) on fruit drop reduction in pummelo cv. Thong Dee was investigated in the pummelo growing areas of Nakhon Pathom province, Thailand. Five similar sized and aged of pummelo trees were selected to set up the experiment. Ten mature branches with the same size from each pummelo tree were randomly selected around the canopy for 2,4-D (20 and 40 mg L−1), NAA (20 and 40 mg L−1) application and control. All treatments were applied to selected pummelo branches 2 times at full bloom and 2 months after fruit set. The results showed that 20 mg L−1 NAA (14.84%) and 40 mg L−1 NAA (12.26%) gave significantly higher percent of fruit retention at 6 months after fruit set. However, leaf total nonstructural carbohydrate concentration analysis showed that 40 mg L−1 2,4-D (104.86 mg g−1) and 20 mg L−1 2,4-D (96.55 mg g−1) gave significantly higher total nonstructural carbohydrate than those in control (78.44 mg g−1). For fruit quality, 40 mg L−1 2,4-D and 20 mg L−1 2,4-D gave the highest peel weight with 435.55 and 358.57 g, respectively, and 40 mg L−1 2,4-D gave the highest peel thickness with 20.25 mm, while 20 mg L−1 NAA gave statistically higher total soluble solid than those in 20 mg L−1 2,4-D and 40 mg L−1 2,4-D. Therefore, 20 mg L−1 of NAA sprayed 2 times at full bloom and 2 month after fruit set effectively reduced fruit drop and increased percentage of fruit retention in pummelo cv. Thong Dee.